A.J.L.L. Buchner
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9 records found
1
Stratified Turbulent Flow Past a Monopile
Designing an Experimental Setup
Offshore Wind Farms (OWFs) are increasingly being placed in deeper, seasonally stratified parts of the sea, where the foundations supporting the turbines interact with density gradients in ways that aren't well understood.
The current knowledge primarily comes from Computational Fluid Dynamics (CFD) simulations and a limited number of field measurements.
Laboratory experiments, capable of providing high-resolution, repeatable data, remain scarce due to the complexity of the facilities required to create stable density stratification in a boundary layer.
This thesis addresses this knowledge-gap by developing and evaluating a new method for generating a two-layer, stably stratified turbulent boundary layer in an existing water tunnel.
This setup is then used to investigate the two-way interaction between such a boundary layer and the wake of a monopile.
A stratified turbulent boundary layer was created by combining Irwin spires, to thicken the boundary layer, with a gravity-driven injection system that introduced salt water into the flow through the sides of the spires.
Simultaneous velocity and density fields were measured using Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF), respectively, in the wake of a monopile model, over a range of bulk Richardson numbers.
Results show that the adapted facility successfully produced a stable density gradient while preserving most of the characteristic features of a turbulent boundary layer; the injection method did generate a gravity current and increased turbulence in the upper part of the flow, however.
This distinguishes the resulting boundary layer from the canonical flat-plate case.
In the wake of the monopile, stratification enlarged the recirculation region near the pycnocline, delayed the recovery of the streamwise velocity, suppressed vertical motion, and reduced the turbulent kinetic energy in the denser lower layer.
Only minor variations were observed across the tested Richardson numbers.
Conversely, the monopile wake was found to redistribute density, lowering it near the original pycnocline and raising it further up, while increasing both the turbulent buoyancy flux and the dissipation of scaled buoyancy variance, indicating increased irreversible mixing induced by the wake.
Although uncertainties in the upstream density evolution and in the Richardson number binning procedure limit a fully quantitative interpretation, the qualitative trends provide convincing evidence for a bidirectional interaction: stratification modifies the wake, and the wake in turn alters the local density distribution.
These findings demonstrate the feasibility of the developed method and provide a first idea of the wake dynamics, while also highlighting practical considerations for future facilities aiming to replicate environmentally relevant stratified conditions. ...
The current knowledge primarily comes from Computational Fluid Dynamics (CFD) simulations and a limited number of field measurements.
Laboratory experiments, capable of providing high-resolution, repeatable data, remain scarce due to the complexity of the facilities required to create stable density stratification in a boundary layer.
This thesis addresses this knowledge-gap by developing and evaluating a new method for generating a two-layer, stably stratified turbulent boundary layer in an existing water tunnel.
This setup is then used to investigate the two-way interaction between such a boundary layer and the wake of a monopile.
A stratified turbulent boundary layer was created by combining Irwin spires, to thicken the boundary layer, with a gravity-driven injection system that introduced salt water into the flow through the sides of the spires.
Simultaneous velocity and density fields were measured using Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF), respectively, in the wake of a monopile model, over a range of bulk Richardson numbers.
Results show that the adapted facility successfully produced a stable density gradient while preserving most of the characteristic features of a turbulent boundary layer; the injection method did generate a gravity current and increased turbulence in the upper part of the flow, however.
This distinguishes the resulting boundary layer from the canonical flat-plate case.
In the wake of the monopile, stratification enlarged the recirculation region near the pycnocline, delayed the recovery of the streamwise velocity, suppressed vertical motion, and reduced the turbulent kinetic energy in the denser lower layer.
Only minor variations were observed across the tested Richardson numbers.
Conversely, the monopile wake was found to redistribute density, lowering it near the original pycnocline and raising it further up, while increasing both the turbulent buoyancy flux and the dissipation of scaled buoyancy variance, indicating increased irreversible mixing induced by the wake.
Although uncertainties in the upstream density evolution and in the Richardson number binning procedure limit a fully quantitative interpretation, the qualitative trends provide convincing evidence for a bidirectional interaction: stratification modifies the wake, and the wake in turn alters the local density distribution.
These findings demonstrate the feasibility of the developed method and provide a first idea of the wake dynamics, while also highlighting practical considerations for future facilities aiming to replicate environmentally relevant stratified conditions. ...
Offshore Wind Farms (OWFs) are increasingly being placed in deeper, seasonally stratified parts of the sea, where the foundations supporting the turbines interact with density gradients in ways that aren't well understood.
The current knowledge primarily comes from Computational Fluid Dynamics (CFD) simulations and a limited number of field measurements.
Laboratory experiments, capable of providing high-resolution, repeatable data, remain scarce due to the complexity of the facilities required to create stable density stratification in a boundary layer.
This thesis addresses this knowledge-gap by developing and evaluating a new method for generating a two-layer, stably stratified turbulent boundary layer in an existing water tunnel.
This setup is then used to investigate the two-way interaction between such a boundary layer and the wake of a monopile.
A stratified turbulent boundary layer was created by combining Irwin spires, to thicken the boundary layer, with a gravity-driven injection system that introduced salt water into the flow through the sides of the spires.
Simultaneous velocity and density fields were measured using Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF), respectively, in the wake of a monopile model, over a range of bulk Richardson numbers.
Results show that the adapted facility successfully produced a stable density gradient while preserving most of the characteristic features of a turbulent boundary layer; the injection method did generate a gravity current and increased turbulence in the upper part of the flow, however.
This distinguishes the resulting boundary layer from the canonical flat-plate case.
In the wake of the monopile, stratification enlarged the recirculation region near the pycnocline, delayed the recovery of the streamwise velocity, suppressed vertical motion, and reduced the turbulent kinetic energy in the denser lower layer.
Only minor variations were observed across the tested Richardson numbers.
Conversely, the monopile wake was found to redistribute density, lowering it near the original pycnocline and raising it further up, while increasing both the turbulent buoyancy flux and the dissipation of scaled buoyancy variance, indicating increased irreversible mixing induced by the wake.
Although uncertainties in the upstream density evolution and in the Richardson number binning procedure limit a fully quantitative interpretation, the qualitative trends provide convincing evidence for a bidirectional interaction: stratification modifies the wake, and the wake in turn alters the local density distribution.
These findings demonstrate the feasibility of the developed method and provide a first idea of the wake dynamics, while also highlighting practical considerations for future facilities aiming to replicate environmentally relevant stratified conditions.
The current knowledge primarily comes from Computational Fluid Dynamics (CFD) simulations and a limited number of field measurements.
Laboratory experiments, capable of providing high-resolution, repeatable data, remain scarce due to the complexity of the facilities required to create stable density stratification in a boundary layer.
This thesis addresses this knowledge-gap by developing and evaluating a new method for generating a two-layer, stably stratified turbulent boundary layer in an existing water tunnel.
This setup is then used to investigate the two-way interaction between such a boundary layer and the wake of a monopile.
A stratified turbulent boundary layer was created by combining Irwin spires, to thicken the boundary layer, with a gravity-driven injection system that introduced salt water into the flow through the sides of the spires.
Simultaneous velocity and density fields were measured using Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF), respectively, in the wake of a monopile model, over a range of bulk Richardson numbers.
Results show that the adapted facility successfully produced a stable density gradient while preserving most of the characteristic features of a turbulent boundary layer; the injection method did generate a gravity current and increased turbulence in the upper part of the flow, however.
This distinguishes the resulting boundary layer from the canonical flat-plate case.
In the wake of the monopile, stratification enlarged the recirculation region near the pycnocline, delayed the recovery of the streamwise velocity, suppressed vertical motion, and reduced the turbulent kinetic energy in the denser lower layer.
Only minor variations were observed across the tested Richardson numbers.
Conversely, the monopile wake was found to redistribute density, lowering it near the original pycnocline and raising it further up, while increasing both the turbulent buoyancy flux and the dissipation of scaled buoyancy variance, indicating increased irreversible mixing induced by the wake.
Although uncertainties in the upstream density evolution and in the Richardson number binning procedure limit a fully quantitative interpretation, the qualitative trends provide convincing evidence for a bidirectional interaction: stratification modifies the wake, and the wake in turn alters the local density distribution.
These findings demonstrate the feasibility of the developed method and provide a first idea of the wake dynamics, while also highlighting practical considerations for future facilities aiming to replicate environmentally relevant stratified conditions.
Data driven reduced order modelling of transonic buffet
Using invariant manifolds
On aircraft wings operating in the transonic regime, self-sustained shock oscillations brought on by shock wave–boundary layer interactions give rise to the phenomenon known as transonic buffet. The operational flying envelope is limited by the ensuing unstable aerodynamic loads, which, if unchecked, may cause structural fatigue or even catastrophic failure. The non-linear dynamics of Hopf bifurcations and limit cycle oscillations, which control buffet start and progression, are not sufficiently captured by traditional linear reduced-order models (ROMs), despite the fact that they have been used with varying level of success for buffet suppression, with the use of controllers. The current thesis builds a data-driven, non-linear, reduced order modelling framework for describing the transonic buffet dynamics, based on spectral submanifold (SSM) embedding, in order to get over this restriction.
The methodology implements the use of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations, which simulate an n-dimensional dynamical system, combined with the use of an invariant manifold in n-dimensions, to construct compact and data-driven ROMs. First, steady and unsteady flow simulations are performed for a supercritical aerofoil across different Mach number–angle of attack combinations to identify the buffet onset boundary. Linear (eigenmode) analysis and Proper Orthogonal Decomposition (POD) are applied to extract eigenmodes and POD modes from the flow field, which are used to define a two-dimensional tangent space, to the n-dimensional manifold. The n-dimensional system dynamics, stored as flowfield data, can be projected onto this tangent space, creating a set of reduced coordinates through an encoder function. The dynamics of the system in the reduced coordinates are then captured with the help of an ordinary differential equation, obtained via polynomial regression. The pair of encoder function and the ordinary differential equations obtained, results in a reduced order model for the system. Both the linear and non-linear behaviour of the original system are captured and combined with a Taylor series expansion of the reduced coordinates to form a decoder function, which reconstructs the n-dimensional system output in the flowfield form. The identified ordinary differential equation is used to predict the evolution of reduced coordinates in the tangent space, while the decoder function enables the evaluation of the full flow field from the predicted reduced coordinates.
The validation of the obtained ROM is carried out against independent URANS simulations with alternate initialization condition, for each flow condition evaluated. The results show that the SSM-based ROM is capable of accurately reproducing limit cycle oscillations and reconstructing full flowfields with good accuracy. The reduced order model predictions and the reconstructed flowfields obtained from it show good agreement with actual features of the buffet phenomena like shock location at various points in time.
This work establishes the use of spectral submanifold embedding as a promising tool for constructing non-linear ROMs of transonic buffet. The developed framework provides new opportunities for real-time prediction and control of buffet phenomena. Beyond aerospace applications, the methodology has broader potential for analysing other non-linearizable fluid–structure interaction problems.
...
The methodology implements the use of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations, which simulate an n-dimensional dynamical system, combined with the use of an invariant manifold in n-dimensions, to construct compact and data-driven ROMs. First, steady and unsteady flow simulations are performed for a supercritical aerofoil across different Mach number–angle of attack combinations to identify the buffet onset boundary. Linear (eigenmode) analysis and Proper Orthogonal Decomposition (POD) are applied to extract eigenmodes and POD modes from the flow field, which are used to define a two-dimensional tangent space, to the n-dimensional manifold. The n-dimensional system dynamics, stored as flowfield data, can be projected onto this tangent space, creating a set of reduced coordinates through an encoder function. The dynamics of the system in the reduced coordinates are then captured with the help of an ordinary differential equation, obtained via polynomial regression. The pair of encoder function and the ordinary differential equations obtained, results in a reduced order model for the system. Both the linear and non-linear behaviour of the original system are captured and combined with a Taylor series expansion of the reduced coordinates to form a decoder function, which reconstructs the n-dimensional system output in the flowfield form. The identified ordinary differential equation is used to predict the evolution of reduced coordinates in the tangent space, while the decoder function enables the evaluation of the full flow field from the predicted reduced coordinates.
The validation of the obtained ROM is carried out against independent URANS simulations with alternate initialization condition, for each flow condition evaluated. The results show that the SSM-based ROM is capable of accurately reproducing limit cycle oscillations and reconstructing full flowfields with good accuracy. The reduced order model predictions and the reconstructed flowfields obtained from it show good agreement with actual features of the buffet phenomena like shock location at various points in time.
This work establishes the use of spectral submanifold embedding as a promising tool for constructing non-linear ROMs of transonic buffet. The developed framework provides new opportunities for real-time prediction and control of buffet phenomena. Beyond aerospace applications, the methodology has broader potential for analysing other non-linearizable fluid–structure interaction problems.
...
On aircraft wings operating in the transonic regime, self-sustained shock oscillations brought on by shock wave–boundary layer interactions give rise to the phenomenon known as transonic buffet. The operational flying envelope is limited by the ensuing unstable aerodynamic loads, which, if unchecked, may cause structural fatigue or even catastrophic failure. The non-linear dynamics of Hopf bifurcations and limit cycle oscillations, which control buffet start and progression, are not sufficiently captured by traditional linear reduced-order models (ROMs), despite the fact that they have been used with varying level of success for buffet suppression, with the use of controllers. The current thesis builds a data-driven, non-linear, reduced order modelling framework for describing the transonic buffet dynamics, based on spectral submanifold (SSM) embedding, in order to get over this restriction.
The methodology implements the use of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations, which simulate an n-dimensional dynamical system, combined with the use of an invariant manifold in n-dimensions, to construct compact and data-driven ROMs. First, steady and unsteady flow simulations are performed for a supercritical aerofoil across different Mach number–angle of attack combinations to identify the buffet onset boundary. Linear (eigenmode) analysis and Proper Orthogonal Decomposition (POD) are applied to extract eigenmodes and POD modes from the flow field, which are used to define a two-dimensional tangent space, to the n-dimensional manifold. The n-dimensional system dynamics, stored as flowfield data, can be projected onto this tangent space, creating a set of reduced coordinates through an encoder function. The dynamics of the system in the reduced coordinates are then captured with the help of an ordinary differential equation, obtained via polynomial regression. The pair of encoder function and the ordinary differential equations obtained, results in a reduced order model for the system. Both the linear and non-linear behaviour of the original system are captured and combined with a Taylor series expansion of the reduced coordinates to form a decoder function, which reconstructs the n-dimensional system output in the flowfield form. The identified ordinary differential equation is used to predict the evolution of reduced coordinates in the tangent space, while the decoder function enables the evaluation of the full flow field from the predicted reduced coordinates.
The validation of the obtained ROM is carried out against independent URANS simulations with alternate initialization condition, for each flow condition evaluated. The results show that the SSM-based ROM is capable of accurately reproducing limit cycle oscillations and reconstructing full flowfields with good accuracy. The reduced order model predictions and the reconstructed flowfields obtained from it show good agreement with actual features of the buffet phenomena like shock location at various points in time.
This work establishes the use of spectral submanifold embedding as a promising tool for constructing non-linear ROMs of transonic buffet. The developed framework provides new opportunities for real-time prediction and control of buffet phenomena. Beyond aerospace applications, the methodology has broader potential for analysing other non-linearizable fluid–structure interaction problems.
The methodology implements the use of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations, which simulate an n-dimensional dynamical system, combined with the use of an invariant manifold in n-dimensions, to construct compact and data-driven ROMs. First, steady and unsteady flow simulations are performed for a supercritical aerofoil across different Mach number–angle of attack combinations to identify the buffet onset boundary. Linear (eigenmode) analysis and Proper Orthogonal Decomposition (POD) are applied to extract eigenmodes and POD modes from the flow field, which are used to define a two-dimensional tangent space, to the n-dimensional manifold. The n-dimensional system dynamics, stored as flowfield data, can be projected onto this tangent space, creating a set of reduced coordinates through an encoder function. The dynamics of the system in the reduced coordinates are then captured with the help of an ordinary differential equation, obtained via polynomial regression. The pair of encoder function and the ordinary differential equations obtained, results in a reduced order model for the system. Both the linear and non-linear behaviour of the original system are captured and combined with a Taylor series expansion of the reduced coordinates to form a decoder function, which reconstructs the n-dimensional system output in the flowfield form. The identified ordinary differential equation is used to predict the evolution of reduced coordinates in the tangent space, while the decoder function enables the evaluation of the full flow field from the predicted reduced coordinates.
The validation of the obtained ROM is carried out against independent URANS simulations with alternate initialization condition, for each flow condition evaluated. The results show that the SSM-based ROM is capable of accurately reproducing limit cycle oscillations and reconstructing full flowfields with good accuracy. The reduced order model predictions and the reconstructed flowfields obtained from it show good agreement with actual features of the buffet phenomena like shock location at various points in time.
This work establishes the use of spectral submanifold embedding as a promising tool for constructing non-linear ROMs of transonic buffet. The developed framework provides new opportunities for real-time prediction and control of buffet phenomena. Beyond aerospace applications, the methodology has broader potential for analysing other non-linearizable fluid–structure interaction problems.
Aerodynamic wing-wake interaction during mosquito hovering
Investigating the effect of deviation angle of the figure-of-eight wing tip trajectory on the aerodynamic wake capture forces during mosquito hovering
Mosquitoes of the species Aedes Aegypti exhibit a particularly distinctive figure-eight shaped wingtip trajectory, which includes a vertical component of motion (characterized by deviation angle) that is hypothesized to influence aerodynamic force generation through interactions with the wing’s own wake, referred to as wake capture. While standard quasi-steady (QS) aerodynamic numerical models can effectively predict the forces generated by translational, rotational, coupling and added mass effects during wing motion, they do not capture the additional forces arising from wake interaction.
This study aims to leverage this known limitation of quasi-steady models, to investigate the influence of deviation angle on wake capture forces during hovering flight. Aerodynamic forces were measured using a robotic flapping-wing apparatus programmed to replicate biologically plausible kinematics of a mosquito, across a range of deviation angles (0° - 6°) (including an outlier at 7.5°). By subtracting forces predicted by a validated quasi-steady model from the experimentally measured forces, the residual unsteady component associated with wake capture was isolated and quantitatively analyzed.
The results of this study demonstrated that wake capture forces play a beneficial aerodynamic role in the mid-to-high deviation angle range, specifically between approximately 4° and 6°. Within this interval, the wake capture lift and drag forces exhibit relatively elevated mean values, and the experimentally observed lift-to-drag ratio surpasses quasi-steady predictions, indicating enhanced aerodynamic efficiency. Correspondingly, power utilization increases, and flapping efficiency peaks near 5° to 6°, collectively highlighting an optimal regime where unsteady wake interactions benefit aerodynamic performance. This favorable trend could be attributed to the pronounced out-of-plane wing motion at these deviation angles, which intensifies three-dimensional wake structures and possibly promotes more effective wing-wake vortex interactions that augment lift and overall flapping efficiency.
In spite of the high uncertainty in the force sensor readings, this study successfully established a kinematically accurate, high-fidelity setup for examining the influence of deviation angle on wake capture phenomena. The elevated noise from the inbuilt force sensor and structural vibration in the wing assembly were identified as the primary limitations, which could only be fully resolved through hardware replacement beyond the present scope. Nevertheless, the findings provide novel insights into the role of deviation angle in shaping wake capture forces, with both scientific implications for advancing flapping-wing micro air vehicle (FWMAV) design and societal relevance given the connection between mosquito flight and public health. ...
This study aims to leverage this known limitation of quasi-steady models, to investigate the influence of deviation angle on wake capture forces during hovering flight. Aerodynamic forces were measured using a robotic flapping-wing apparatus programmed to replicate biologically plausible kinematics of a mosquito, across a range of deviation angles (0° - 6°) (including an outlier at 7.5°). By subtracting forces predicted by a validated quasi-steady model from the experimentally measured forces, the residual unsteady component associated with wake capture was isolated and quantitatively analyzed.
The results of this study demonstrated that wake capture forces play a beneficial aerodynamic role in the mid-to-high deviation angle range, specifically between approximately 4° and 6°. Within this interval, the wake capture lift and drag forces exhibit relatively elevated mean values, and the experimentally observed lift-to-drag ratio surpasses quasi-steady predictions, indicating enhanced aerodynamic efficiency. Correspondingly, power utilization increases, and flapping efficiency peaks near 5° to 6°, collectively highlighting an optimal regime where unsteady wake interactions benefit aerodynamic performance. This favorable trend could be attributed to the pronounced out-of-plane wing motion at these deviation angles, which intensifies three-dimensional wake structures and possibly promotes more effective wing-wake vortex interactions that augment lift and overall flapping efficiency.
In spite of the high uncertainty in the force sensor readings, this study successfully established a kinematically accurate, high-fidelity setup for examining the influence of deviation angle on wake capture phenomena. The elevated noise from the inbuilt force sensor and structural vibration in the wing assembly were identified as the primary limitations, which could only be fully resolved through hardware replacement beyond the present scope. Nevertheless, the findings provide novel insights into the role of deviation angle in shaping wake capture forces, with both scientific implications for advancing flapping-wing micro air vehicle (FWMAV) design and societal relevance given the connection between mosquito flight and public health. ...
Mosquitoes of the species Aedes Aegypti exhibit a particularly distinctive figure-eight shaped wingtip trajectory, which includes a vertical component of motion (characterized by deviation angle) that is hypothesized to influence aerodynamic force generation through interactions with the wing’s own wake, referred to as wake capture. While standard quasi-steady (QS) aerodynamic numerical models can effectively predict the forces generated by translational, rotational, coupling and added mass effects during wing motion, they do not capture the additional forces arising from wake interaction.
This study aims to leverage this known limitation of quasi-steady models, to investigate the influence of deviation angle on wake capture forces during hovering flight. Aerodynamic forces were measured using a robotic flapping-wing apparatus programmed to replicate biologically plausible kinematics of a mosquito, across a range of deviation angles (0° - 6°) (including an outlier at 7.5°). By subtracting forces predicted by a validated quasi-steady model from the experimentally measured forces, the residual unsteady component associated with wake capture was isolated and quantitatively analyzed.
The results of this study demonstrated that wake capture forces play a beneficial aerodynamic role in the mid-to-high deviation angle range, specifically between approximately 4° and 6°. Within this interval, the wake capture lift and drag forces exhibit relatively elevated mean values, and the experimentally observed lift-to-drag ratio surpasses quasi-steady predictions, indicating enhanced aerodynamic efficiency. Correspondingly, power utilization increases, and flapping efficiency peaks near 5° to 6°, collectively highlighting an optimal regime where unsteady wake interactions benefit aerodynamic performance. This favorable trend could be attributed to the pronounced out-of-plane wing motion at these deviation angles, which intensifies three-dimensional wake structures and possibly promotes more effective wing-wake vortex interactions that augment lift and overall flapping efficiency.
In spite of the high uncertainty in the force sensor readings, this study successfully established a kinematically accurate, high-fidelity setup for examining the influence of deviation angle on wake capture phenomena. The elevated noise from the inbuilt force sensor and structural vibration in the wing assembly were identified as the primary limitations, which could only be fully resolved through hardware replacement beyond the present scope. Nevertheless, the findings provide novel insights into the role of deviation angle in shaping wake capture forces, with both scientific implications for advancing flapping-wing micro air vehicle (FWMAV) design and societal relevance given the connection between mosquito flight and public health.
This study aims to leverage this known limitation of quasi-steady models, to investigate the influence of deviation angle on wake capture forces during hovering flight. Aerodynamic forces were measured using a robotic flapping-wing apparatus programmed to replicate biologically plausible kinematics of a mosquito, across a range of deviation angles (0° - 6°) (including an outlier at 7.5°). By subtracting forces predicted by a validated quasi-steady model from the experimentally measured forces, the residual unsteady component associated with wake capture was isolated and quantitatively analyzed.
The results of this study demonstrated that wake capture forces play a beneficial aerodynamic role in the mid-to-high deviation angle range, specifically between approximately 4° and 6°. Within this interval, the wake capture lift and drag forces exhibit relatively elevated mean values, and the experimentally observed lift-to-drag ratio surpasses quasi-steady predictions, indicating enhanced aerodynamic efficiency. Correspondingly, power utilization increases, and flapping efficiency peaks near 5° to 6°, collectively highlighting an optimal regime where unsteady wake interactions benefit aerodynamic performance. This favorable trend could be attributed to the pronounced out-of-plane wing motion at these deviation angles, which intensifies three-dimensional wake structures and possibly promotes more effective wing-wake vortex interactions that augment lift and overall flapping efficiency.
In spite of the high uncertainty in the force sensor readings, this study successfully established a kinematically accurate, high-fidelity setup for examining the influence of deviation angle on wake capture phenomena. The elevated noise from the inbuilt force sensor and structural vibration in the wing assembly were identified as the primary limitations, which could only be fully resolved through hardware replacement beyond the present scope. Nevertheless, the findings provide novel insights into the role of deviation angle in shaping wake capture forces, with both scientific implications for advancing flapping-wing micro air vehicle (FWMAV) design and societal relevance given the connection between mosquito flight and public health.
Aerodynamics of a Vertical-Axis Wind Turbine Blade in Sinusoidal Inflow Conditions
An Experimental Investigation
Climate change motivates the transition from fossil fuels to low-carbon energy sources,and wind power is a key option. Among wind-energy technologies, vertical-axiswind turbines (VAWTs) offer advantages for offshore and urban applications dueto their omnidirectional wind acceptance and simpler structural requirements.In offshore floating platforms, the motion of the structure can induce periodicvariations in the apparent wind velocity, which may influence turbineperformance. This thesis also investigates whether such periodic (sinusoidal)wind velocities can alter the energy production of VAWTs, and examines theinfluence of the tip-speed ratio (λ), the dimensionless wind velocity amplitude(A∗), and the dimensionless wind-velocityfrequency (ω∗) on these effects, without seeking their optimization. The work followsthree stages: (1) characterization of the wind-tunnel test section, (2) designand construction of a single-bladed H-Darrieus VAWT with adjustable solidity,and (3) wind-tunnel experiments under steady and sinusoidal inflow conditions.Because the tunnel could not reliably reproduce the desired sinusoidalprofiles, a hexapod was used to oscillate the turbine and simulate unsteadyinflow. Measurements show that periodic inflow induces phase-dependent bladeloading and subtle torque fluctuations, but produces no statisticallysignificant increase in period-averaged power. Efficiency re mained primarilycontrolled by λ, with only minor, inconclusive deviations near peak operatingconditions due to ω∗. Recommendations include utilizing alow-turbulence-intensity wind tunnel, employing precision-aligned bearings, andconducting tests at higher tip speed ratios (λ > 3.0) to better resolve potentialunsteady effects. ...
Climate change motivates the transition from fossil fuels to low-carbon energy sources,and wind power is a key option. Among wind-energy technologies, vertical-axiswind turbines (VAWTs) offer advantages for offshore and urban applications dueto their omnidirectional wind acceptance and simpler structural requirements.In offshore floating platforms, the motion of the structure can induce periodicvariations in the apparent wind velocity, which may influence turbineperformance. This thesis also investigates whether such periodic (sinusoidal)wind velocities can alter the energy production of VAWTs, and examines theinfluence of the tip-speed ratio (λ), the dimensionless wind velocity amplitude(A∗), and the dimensionless wind-velocityfrequency (ω∗) on these effects, without seeking their optimization. The work followsthree stages: (1) characterization of the wind-tunnel test section, (2) designand construction of a single-bladed H-Darrieus VAWT with adjustable solidity,and (3) wind-tunnel experiments under steady and sinusoidal inflow conditions.Because the tunnel could not reliably reproduce the desired sinusoidalprofiles, a hexapod was used to oscillate the turbine and simulate unsteadyinflow. Measurements show that periodic inflow induces phase-dependent bladeloading and subtle torque fluctuations, but produces no statisticallysignificant increase in period-averaged power. Efficiency re mained primarilycontrolled by λ, with only minor, inconclusive deviations near peak operatingconditions due to ω∗. Recommendations include utilizing alow-turbulence-intensity wind tunnel, employing precision-aligned bearings, andconducting tests at higher tip speed ratios (λ > 3.0) to better resolve potentialunsteady effects.
Laminar Separation Bubbles (LSBs) typically form in a low Reynolds number boundary layer flow involving a strong pressure gradient. These bubbles adversely impact the aerodynamic performance of a body by increasing the drag, susceptibility to stall, and noise emission. Since a large number of unmanned aerial vehicles and small wind turbine blades operate in a similar Reynolds number range, it is crucial to have an understanding of the bubbles' behavior to improve aerodynamic performance in these operating conditions. The frequency at which this bubble sheds due to amplification of disturbances is termed the natural, harmonic, or fundamental shedding frequency. The way in which the shed vortices break down impacts the behavior of the bubble, and hence it is crucial to study this breakdown of the vortices as well. Though previous works have shown that varying external forcing frequency alters the bubbles' behavior, there is no study investigating the variation of bubble topology and the breakdown of vortices in a forcing frequency regime varying from impulsive to fundamental forcing.
An experimental study is performed using a NACA 0018 airfoil in the Anechoic Tunnel at TU Delft. Under the chosen conditions, a short LSB forms on the suction side and is chosen as the baseline bubble for the current study. Using a spanwise uniform Alternating Current-Dielectric Barrier Discharge (AC-DBD) plasma actuator placed upstream of the separation point, two-dimensional forcing is applied. The actuator geometry and voltage are kept fixed while only the actuation frequency is varied from a low frequency regime, where the bubble has enough time to recover close to its natural state between the actuations, up to the regime where the forcing is close to the fundamental shedding frequency of the bubble. Surface pressure and velocity fields are measured using pressure taps and Particle Image Velocimetry (PIV), respectively. PIV measurements are done in both time averaged and phase averaged manner to gain insight into the vortex dynamics and breakdown characteristics. While the surface pressure taps along the airfoil chord provide time-averaged pressure measurements, PIV data captures the bubble topology in the wall normal plane, and the breakdown characteristics of the actuated wave packet in the wall parallel plane.
The LSB topology quantified in terms of bubble area shows a monotonic decrease in area with an increase in forcing frequency. The decrease in area is steep at lower forcing frequencies and gradual at higher forcing frequencies. The displacement thickness moves closer to the wall, and the maximum shape factor value shifts upstream, indicating increased stability of the free shear layer. Analysis of the breakdown characteristics shows distinct breakdown behavior across the forcing frequency ranges. At a lower forcing frequency, the breakdown shows features of the unforced bubble breakdown. In the subharmonic and fundamental forcing frequency range, the wavelength variation of the convected wave packet shows a distinct U-shape, indicating changed stability of the bubble as compared to lower forcing frequency actuation. The results provide valuable insights into the bubble topology and the breakdown characteristics under the considered forcing frequency range and can be useful in designing an active control system in the future for similar low Reynolds number range. ...
An experimental study is performed using a NACA 0018 airfoil in the Anechoic Tunnel at TU Delft. Under the chosen conditions, a short LSB forms on the suction side and is chosen as the baseline bubble for the current study. Using a spanwise uniform Alternating Current-Dielectric Barrier Discharge (AC-DBD) plasma actuator placed upstream of the separation point, two-dimensional forcing is applied. The actuator geometry and voltage are kept fixed while only the actuation frequency is varied from a low frequency regime, where the bubble has enough time to recover close to its natural state between the actuations, up to the regime where the forcing is close to the fundamental shedding frequency of the bubble. Surface pressure and velocity fields are measured using pressure taps and Particle Image Velocimetry (PIV), respectively. PIV measurements are done in both time averaged and phase averaged manner to gain insight into the vortex dynamics and breakdown characteristics. While the surface pressure taps along the airfoil chord provide time-averaged pressure measurements, PIV data captures the bubble topology in the wall normal plane, and the breakdown characteristics of the actuated wave packet in the wall parallel plane.
The LSB topology quantified in terms of bubble area shows a monotonic decrease in area with an increase in forcing frequency. The decrease in area is steep at lower forcing frequencies and gradual at higher forcing frequencies. The displacement thickness moves closer to the wall, and the maximum shape factor value shifts upstream, indicating increased stability of the free shear layer. Analysis of the breakdown characteristics shows distinct breakdown behavior across the forcing frequency ranges. At a lower forcing frequency, the breakdown shows features of the unforced bubble breakdown. In the subharmonic and fundamental forcing frequency range, the wavelength variation of the convected wave packet shows a distinct U-shape, indicating changed stability of the bubble as compared to lower forcing frequency actuation. The results provide valuable insights into the bubble topology and the breakdown characteristics under the considered forcing frequency range and can be useful in designing an active control system in the future for similar low Reynolds number range. ...
Laminar Separation Bubbles (LSBs) typically form in a low Reynolds number boundary layer flow involving a strong pressure gradient. These bubbles adversely impact the aerodynamic performance of a body by increasing the drag, susceptibility to stall, and noise emission. Since a large number of unmanned aerial vehicles and small wind turbine blades operate in a similar Reynolds number range, it is crucial to have an understanding of the bubbles' behavior to improve aerodynamic performance in these operating conditions. The frequency at which this bubble sheds due to amplification of disturbances is termed the natural, harmonic, or fundamental shedding frequency. The way in which the shed vortices break down impacts the behavior of the bubble, and hence it is crucial to study this breakdown of the vortices as well. Though previous works have shown that varying external forcing frequency alters the bubbles' behavior, there is no study investigating the variation of bubble topology and the breakdown of vortices in a forcing frequency regime varying from impulsive to fundamental forcing.
An experimental study is performed using a NACA 0018 airfoil in the Anechoic Tunnel at TU Delft. Under the chosen conditions, a short LSB forms on the suction side and is chosen as the baseline bubble for the current study. Using a spanwise uniform Alternating Current-Dielectric Barrier Discharge (AC-DBD) plasma actuator placed upstream of the separation point, two-dimensional forcing is applied. The actuator geometry and voltage are kept fixed while only the actuation frequency is varied from a low frequency regime, where the bubble has enough time to recover close to its natural state between the actuations, up to the regime where the forcing is close to the fundamental shedding frequency of the bubble. Surface pressure and velocity fields are measured using pressure taps and Particle Image Velocimetry (PIV), respectively. PIV measurements are done in both time averaged and phase averaged manner to gain insight into the vortex dynamics and breakdown characteristics. While the surface pressure taps along the airfoil chord provide time-averaged pressure measurements, PIV data captures the bubble topology in the wall normal plane, and the breakdown characteristics of the actuated wave packet in the wall parallel plane.
The LSB topology quantified in terms of bubble area shows a monotonic decrease in area with an increase in forcing frequency. The decrease in area is steep at lower forcing frequencies and gradual at higher forcing frequencies. The displacement thickness moves closer to the wall, and the maximum shape factor value shifts upstream, indicating increased stability of the free shear layer. Analysis of the breakdown characteristics shows distinct breakdown behavior across the forcing frequency ranges. At a lower forcing frequency, the breakdown shows features of the unforced bubble breakdown. In the subharmonic and fundamental forcing frequency range, the wavelength variation of the convected wave packet shows a distinct U-shape, indicating changed stability of the bubble as compared to lower forcing frequency actuation. The results provide valuable insights into the bubble topology and the breakdown characteristics under the considered forcing frequency range and can be useful in designing an active control system in the future for similar low Reynolds number range.
An experimental study is performed using a NACA 0018 airfoil in the Anechoic Tunnel at TU Delft. Under the chosen conditions, a short LSB forms on the suction side and is chosen as the baseline bubble for the current study. Using a spanwise uniform Alternating Current-Dielectric Barrier Discharge (AC-DBD) plasma actuator placed upstream of the separation point, two-dimensional forcing is applied. The actuator geometry and voltage are kept fixed while only the actuation frequency is varied from a low frequency regime, where the bubble has enough time to recover close to its natural state between the actuations, up to the regime where the forcing is close to the fundamental shedding frequency of the bubble. Surface pressure and velocity fields are measured using pressure taps and Particle Image Velocimetry (PIV), respectively. PIV measurements are done in both time averaged and phase averaged manner to gain insight into the vortex dynamics and breakdown characteristics. While the surface pressure taps along the airfoil chord provide time-averaged pressure measurements, PIV data captures the bubble topology in the wall normal plane, and the breakdown characteristics of the actuated wave packet in the wall parallel plane.
The LSB topology quantified in terms of bubble area shows a monotonic decrease in area with an increase in forcing frequency. The decrease in area is steep at lower forcing frequencies and gradual at higher forcing frequencies. The displacement thickness moves closer to the wall, and the maximum shape factor value shifts upstream, indicating increased stability of the free shear layer. Analysis of the breakdown characteristics shows distinct breakdown behavior across the forcing frequency ranges. At a lower forcing frequency, the breakdown shows features of the unforced bubble breakdown. In the subharmonic and fundamental forcing frequency range, the wavelength variation of the convected wave packet shows a distinct U-shape, indicating changed stability of the bubble as compared to lower forcing frequency actuation. The results provide valuable insights into the bubble topology and the breakdown characteristics under the considered forcing frequency range and can be useful in designing an active control system in the future for similar low Reynolds number range.
It Takes Two To Tango
Advancing Flow Field and Emission Rate Estimation from Space: Insights from the TANGO mission
Master thesis
(2024)
-
K.V. Reerink, A.J.L.L. Buchner, J. Westerweel, D. Fiscaletti, Tobias Borsdorff
The TANGO mission represents a significant leap forward in the global monitoring of greenhouse gas emission plumes, particularly CO2 and NO2. Comprising two satellites, TANGO-Carbon and TANGO-Nitro, the mission is designed to operate from 2027 to 2031, offering unprecedented capabilities through stereo (temporally separated) and high spatial res- olution emission plume images. These capabilities create a unique framework that facilitates the development of novel estimation methods for emission rates of greenhouse gas emitters. Conventional methods for estimating emission rates of CO2 and NO2 rely on combining gas concentration measurements with wind velocity fields derived from meteorological data, often introducing substantial uncertainties due to the inherent inaccuracies in wind velocity estimation. To address this challenge, this research explores alternative methods enabled by TANGO’s innovative framework, which allows for the direct measurement of emission plume velocity from concentration data. By eliminating the dependency on uncertain me- teorological inputs, these methods promise to reduce the uncertainty in emission rate estimates.
Through detailed simulations and analyses, this research demonstrates that the TANGO mission can effectively establish a framework for directly measuring emission plume velocities. By simulating the data products of the TANGO satellites using Large-Eddy Simulations and applying advanced methods such as traditional Correlation Image Velocimetry (CIV) and Computer Vision Correlation Image Velocimetry (CVision-CIV), wind velocity fields were successfully estimated from plume imagery. These estimates were found to be of promising precision across a range of conditions, including varying wind velocities, emission rates of greenhouse gasses, and levels of measurement noise in the simulations. Results revealed that the CVision-CIV method outperforms the traditional CIV method, especially in scenarios with low signal-to-noise ratios. Wind velocity fields directly estimated from plume imagery were implemented in the Cross-sectional Flux Method to estimate CO2 emission rates. The emission rate estimates indicate that direct plume velocity measure- ments provide a more accurate estimate of emission source rates than conventional methods, which rely on indirect wind velocity estimates from meteorological data. The use of wind velocity fields obtained through the CVision-CIV method resulted in CO2 emission rate estimates with ±20% accuracy in most scenarios, particularly under optimal SNR conditions. Additionally, the study highlights the impact of mission parameters such as image resolu- tion and measurement noise on the accuracy of wind velocity estimations. It was found that higher image resolution and longer time intervals between measurements enhance the precision of wind velocity field estimates by reducing the relative effects of measurement noise.
In summary, this research demonstrates that direct estimation of wind velocities from emission plume imagery, as enabled by the TANGO mission’s advanced capabilities, can accurately be performed and significantly enhance the accuracy of emission rate estimates. The improved wind velocity estimation methods proposed in this thesis offer a promising advancement in remote sensing techniques for greenhouse gas monitoring. ...
Through detailed simulations and analyses, this research demonstrates that the TANGO mission can effectively establish a framework for directly measuring emission plume velocities. By simulating the data products of the TANGO satellites using Large-Eddy Simulations and applying advanced methods such as traditional Correlation Image Velocimetry (CIV) and Computer Vision Correlation Image Velocimetry (CVision-CIV), wind velocity fields were successfully estimated from plume imagery. These estimates were found to be of promising precision across a range of conditions, including varying wind velocities, emission rates of greenhouse gasses, and levels of measurement noise in the simulations. Results revealed that the CVision-CIV method outperforms the traditional CIV method, especially in scenarios with low signal-to-noise ratios. Wind velocity fields directly estimated from plume imagery were implemented in the Cross-sectional Flux Method to estimate CO2 emission rates. The emission rate estimates indicate that direct plume velocity measure- ments provide a more accurate estimate of emission source rates than conventional methods, which rely on indirect wind velocity estimates from meteorological data. The use of wind velocity fields obtained through the CVision-CIV method resulted in CO2 emission rate estimates with ±20% accuracy in most scenarios, particularly under optimal SNR conditions. Additionally, the study highlights the impact of mission parameters such as image resolu- tion and measurement noise on the accuracy of wind velocity estimations. It was found that higher image resolution and longer time intervals between measurements enhance the precision of wind velocity field estimates by reducing the relative effects of measurement noise.
In summary, this research demonstrates that direct estimation of wind velocities from emission plume imagery, as enabled by the TANGO mission’s advanced capabilities, can accurately be performed and significantly enhance the accuracy of emission rate estimates. The improved wind velocity estimation methods proposed in this thesis offer a promising advancement in remote sensing techniques for greenhouse gas monitoring. ...
The TANGO mission represents a significant leap forward in the global monitoring of greenhouse gas emission plumes, particularly CO2 and NO2. Comprising two satellites, TANGO-Carbon and TANGO-Nitro, the mission is designed to operate from 2027 to 2031, offering unprecedented capabilities through stereo (temporally separated) and high spatial res- olution emission plume images. These capabilities create a unique framework that facilitates the development of novel estimation methods for emission rates of greenhouse gas emitters. Conventional methods for estimating emission rates of CO2 and NO2 rely on combining gas concentration measurements with wind velocity fields derived from meteorological data, often introducing substantial uncertainties due to the inherent inaccuracies in wind velocity estimation. To address this challenge, this research explores alternative methods enabled by TANGO’s innovative framework, which allows for the direct measurement of emission plume velocity from concentration data. By eliminating the dependency on uncertain me- teorological inputs, these methods promise to reduce the uncertainty in emission rate estimates.
Through detailed simulations and analyses, this research demonstrates that the TANGO mission can effectively establish a framework for directly measuring emission plume velocities. By simulating the data products of the TANGO satellites using Large-Eddy Simulations and applying advanced methods such as traditional Correlation Image Velocimetry (CIV) and Computer Vision Correlation Image Velocimetry (CVision-CIV), wind velocity fields were successfully estimated from plume imagery. These estimates were found to be of promising precision across a range of conditions, including varying wind velocities, emission rates of greenhouse gasses, and levels of measurement noise in the simulations. Results revealed that the CVision-CIV method outperforms the traditional CIV method, especially in scenarios with low signal-to-noise ratios. Wind velocity fields directly estimated from plume imagery were implemented in the Cross-sectional Flux Method to estimate CO2 emission rates. The emission rate estimates indicate that direct plume velocity measure- ments provide a more accurate estimate of emission source rates than conventional methods, which rely on indirect wind velocity estimates from meteorological data. The use of wind velocity fields obtained through the CVision-CIV method resulted in CO2 emission rate estimates with ±20% accuracy in most scenarios, particularly under optimal SNR conditions. Additionally, the study highlights the impact of mission parameters such as image resolu- tion and measurement noise on the accuracy of wind velocity estimations. It was found that higher image resolution and longer time intervals between measurements enhance the precision of wind velocity field estimates by reducing the relative effects of measurement noise.
In summary, this research demonstrates that direct estimation of wind velocities from emission plume imagery, as enabled by the TANGO mission’s advanced capabilities, can accurately be performed and significantly enhance the accuracy of emission rate estimates. The improved wind velocity estimation methods proposed in this thesis offer a promising advancement in remote sensing techniques for greenhouse gas monitoring.
Through detailed simulations and analyses, this research demonstrates that the TANGO mission can effectively establish a framework for directly measuring emission plume velocities. By simulating the data products of the TANGO satellites using Large-Eddy Simulations and applying advanced methods such as traditional Correlation Image Velocimetry (CIV) and Computer Vision Correlation Image Velocimetry (CVision-CIV), wind velocity fields were successfully estimated from plume imagery. These estimates were found to be of promising precision across a range of conditions, including varying wind velocities, emission rates of greenhouse gasses, and levels of measurement noise in the simulations. Results revealed that the CVision-CIV method outperforms the traditional CIV method, especially in scenarios with low signal-to-noise ratios. Wind velocity fields directly estimated from plume imagery were implemented in the Cross-sectional Flux Method to estimate CO2 emission rates. The emission rate estimates indicate that direct plume velocity measure- ments provide a more accurate estimate of emission source rates than conventional methods, which rely on indirect wind velocity estimates from meteorological data. The use of wind velocity fields obtained through the CVision-CIV method resulted in CO2 emission rate estimates with ±20% accuracy in most scenarios, particularly under optimal SNR conditions. Additionally, the study highlights the impact of mission parameters such as image resolu- tion and measurement noise on the accuracy of wind velocity estimations. It was found that higher image resolution and longer time intervals between measurements enhance the precision of wind velocity field estimates by reducing the relative effects of measurement noise.
In summary, this research demonstrates that direct estimation of wind velocities from emission plume imagery, as enabled by the TANGO mission’s advanced capabilities, can accurately be performed and significantly enhance the accuracy of emission rate estimates. The improved wind velocity estimation methods proposed in this thesis offer a promising advancement in remote sensing techniques for greenhouse gas monitoring.
3D Tracking of Motile Microorganisms
An Experimental Investigation of the Kinematics and Interactions of C. Reinhardtii
This thesis presents a novel 3D tracking system for studying microorganism locomotion in complex fluid environments, addressing the limitations of current 2D and 3D techniques . The multi-camera microscopy system developed allows for high-resolution tracking of microswimmers like Chlamydomonas reinhardtii, providing new insights into their helical swimming patterns and interactions. The work includes the study of pairwise interactions and the impact of non-Newtonian fluids on motility, revealing key differences in swimming behavior and fluid dynamics in different media type.
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This thesis presents a novel 3D tracking system for studying microorganism locomotion in complex fluid environments, addressing the limitations of current 2D and 3D techniques . The multi-camera microscopy system developed allows for high-resolution tracking of microswimmers like Chlamydomonas reinhardtii, providing new insights into their helical swimming patterns and interactions. The work includes the study of pairwise interactions and the impact of non-Newtonian fluids on motility, revealing key differences in swimming behavior and fluid dynamics in different media type.
Laminar vortex shedding past a flat plate at 90°
An experimental study on the effects of acceleration and viscoelasticity
Fluid flows such as rowing and animal flight involve both acceleration and vortex shedding. Some of such flows also involve complex fluids, as in blood flow from the left atrium into the left ventricle in a human heart. Among the complexities of such fluids, we are interested in the viscoelastic nature. The effect of acceleration in vortex dynamics and forces has been studied for a long time. However, there have been discrepancies in the origin of the unsteady forces and modelling them. It is still not very well understood how unsteady forces scale in laminar, highly separated flows? On the other hand, the presence of viscoelasticity alters the vortex dynamics and hence the forces. It has been well studied how the viscoelasticity may affect steady state forces. However, the change in vortex dynamics and forces due to presence of viscoelasticity during acceleration is not well understood. This experimental study aims to add to the literature on unsteady force modelling, effect of viscoelasticity in unsteady forces and vortex dynamics.
To begin with, shear and extensional rheological experiments were performed to identify a weakly elastic fluid. An experimental setup was built with a linear traverse, PIV system and force sensor. The experiments were performed with an accelerating flat plate (aspect ratio of two), in the identified viscoelastic fluid and also in a viscosity matched Newtonian fluid for one-one comparison. To understand the vortex dynamics, we use FTLE fields, Lamb-Oseen model, and Q-criteria. We quantify the vortex formation time and other properties of a vortex ring.
Overall, we observe that both the vortex growth rate and the decay rate are enhanced by the presence of viscoelasticity. We extend the idea of optimal vortex formation to two time-scales instead of one. One, when the plate no longer provides energy and the other, when the vortex is filament free without any more addition of coherent fluid parcels. Furthermore, a limit for optimal vortex formation is proposed to indicate a completely different type of vortex dynamics at lower accelerations. The drag reduction and enhancement due to the presence of viscoelasticity qualitatively agrees well with the trend in literature. In terms of unsteady forces, we try to interpret an equivalent mass for potential flow's added mass in separated Newtonian flows. We propose a model using wake's mass, and it reasonably agrees with our experimental results in Newtonian cases. We use FTLE ridges and vortex-frame streamlines to estimate wake mass, along with a literature driven model for third dimension. Furthermore, we also propose that the added mass in separated flows is time varying and eventually reaches a constant value. Moreover, the effect of viscoelasticity is primarily observed in unsteady forces for acceleration less than optimal vortex formation limit.
...
To begin with, shear and extensional rheological experiments were performed to identify a weakly elastic fluid. An experimental setup was built with a linear traverse, PIV system and force sensor. The experiments were performed with an accelerating flat plate (aspect ratio of two), in the identified viscoelastic fluid and also in a viscosity matched Newtonian fluid for one-one comparison. To understand the vortex dynamics, we use FTLE fields, Lamb-Oseen model, and Q-criteria. We quantify the vortex formation time and other properties of a vortex ring.
Overall, we observe that both the vortex growth rate and the decay rate are enhanced by the presence of viscoelasticity. We extend the idea of optimal vortex formation to two time-scales instead of one. One, when the plate no longer provides energy and the other, when the vortex is filament free without any more addition of coherent fluid parcels. Furthermore, a limit for optimal vortex formation is proposed to indicate a completely different type of vortex dynamics at lower accelerations. The drag reduction and enhancement due to the presence of viscoelasticity qualitatively agrees well with the trend in literature. In terms of unsteady forces, we try to interpret an equivalent mass for potential flow's added mass in separated Newtonian flows. We propose a model using wake's mass, and it reasonably agrees with our experimental results in Newtonian cases. We use FTLE ridges and vortex-frame streamlines to estimate wake mass, along with a literature driven model for third dimension. Furthermore, we also propose that the added mass in separated flows is time varying and eventually reaches a constant value. Moreover, the effect of viscoelasticity is primarily observed in unsteady forces for acceleration less than optimal vortex formation limit.
...
Fluid flows such as rowing and animal flight involve both acceleration and vortex shedding. Some of such flows also involve complex fluids, as in blood flow from the left atrium into the left ventricle in a human heart. Among the complexities of such fluids, we are interested in the viscoelastic nature. The effect of acceleration in vortex dynamics and forces has been studied for a long time. However, there have been discrepancies in the origin of the unsteady forces and modelling them. It is still not very well understood how unsteady forces scale in laminar, highly separated flows? On the other hand, the presence of viscoelasticity alters the vortex dynamics and hence the forces. It has been well studied how the viscoelasticity may affect steady state forces. However, the change in vortex dynamics and forces due to presence of viscoelasticity during acceleration is not well understood. This experimental study aims to add to the literature on unsteady force modelling, effect of viscoelasticity in unsteady forces and vortex dynamics.
To begin with, shear and extensional rheological experiments were performed to identify a weakly elastic fluid. An experimental setup was built with a linear traverse, PIV system and force sensor. The experiments were performed with an accelerating flat plate (aspect ratio of two), in the identified viscoelastic fluid and also in a viscosity matched Newtonian fluid for one-one comparison. To understand the vortex dynamics, we use FTLE fields, Lamb-Oseen model, and Q-criteria. We quantify the vortex formation time and other properties of a vortex ring.
Overall, we observe that both the vortex growth rate and the decay rate are enhanced by the presence of viscoelasticity. We extend the idea of optimal vortex formation to two time-scales instead of one. One, when the plate no longer provides energy and the other, when the vortex is filament free without any more addition of coherent fluid parcels. Furthermore, a limit for optimal vortex formation is proposed to indicate a completely different type of vortex dynamics at lower accelerations. The drag reduction and enhancement due to the presence of viscoelasticity qualitatively agrees well with the trend in literature. In terms of unsteady forces, we try to interpret an equivalent mass for potential flow's added mass in separated Newtonian flows. We propose a model using wake's mass, and it reasonably agrees with our experimental results in Newtonian cases. We use FTLE ridges and vortex-frame streamlines to estimate wake mass, along with a literature driven model for third dimension. Furthermore, we also propose that the added mass in separated flows is time varying and eventually reaches a constant value. Moreover, the effect of viscoelasticity is primarily observed in unsteady forces for acceleration less than optimal vortex formation limit.
To begin with, shear and extensional rheological experiments were performed to identify a weakly elastic fluid. An experimental setup was built with a linear traverse, PIV system and force sensor. The experiments were performed with an accelerating flat plate (aspect ratio of two), in the identified viscoelastic fluid and also in a viscosity matched Newtonian fluid for one-one comparison. To understand the vortex dynamics, we use FTLE fields, Lamb-Oseen model, and Q-criteria. We quantify the vortex formation time and other properties of a vortex ring.
Overall, we observe that both the vortex growth rate and the decay rate are enhanced by the presence of viscoelasticity. We extend the idea of optimal vortex formation to two time-scales instead of one. One, when the plate no longer provides energy and the other, when the vortex is filament free without any more addition of coherent fluid parcels. Furthermore, a limit for optimal vortex formation is proposed to indicate a completely different type of vortex dynamics at lower accelerations. The drag reduction and enhancement due to the presence of viscoelasticity qualitatively agrees well with the trend in literature. In terms of unsteady forces, we try to interpret an equivalent mass for potential flow's added mass in separated Newtonian flows. We propose a model using wake's mass, and it reasonably agrees with our experimental results in Newtonian cases. We use FTLE ridges and vortex-frame streamlines to estimate wake mass, along with a literature driven model for third dimension. Furthermore, we also propose that the added mass in separated flows is time varying and eventually reaches a constant value. Moreover, the effect of viscoelasticity is primarily observed in unsteady forces for acceleration less than optimal vortex formation limit.
Considering the abundance of viscoelastic fluids in nature, growing attention has been received for the study of microorganisms in viscoelastic fluids. In Newtonian fluids, the flow of microorganisms is governed by the viscosity alone. But in viscoelastic fluids, the addition of the elastic component complicates the situation. The flow of the organism is now dictated by a combination of the two forces- viscous and elastic. Understanding the behaviour of these organisms is not only important because of the presence of such environments in nature, but also because of the potential pharmaceutical applications that such studies might lead to.
This thesis focuses on the experimental study of the swimming of Chlamydomonas reinhardtii, a model motile swimmer that swims at low Reynolds numbers. The main objective of the thesis is to characterize these differences in motility and hydrodynamic interactions of these cells in Newtonian and viscoelastic fluids of varying viscosities. This is accomplished by observing the motion of a dilute suspension of Chlamydomonas reinhardtii in 3D using four cameras. The cells are subsequently tracked using an in-house 3D particle tracking code that incorporates a recursive divide and conquer strategy to reconstruct the trajectories.
The experiments showed a drop in velocity in viscoelastic fluids as compared to its Newtonian counterparts, validating the results from existing literature. The cells also maintained their helical motion previously observed in TRIS, although with a drop in radius and pitch in viscoelastic fluids. The ratio of the radius to the pitch, however, remained constant for all the cases, indicating a tendency to retain its overall motion.
The study of cell-wall interactions is of prime importance because of the presence of confined surfaces encountered in nature. The concentration profile of the cells was observed to be similar in all the fluids, showing a non-uniform distribution with large concentration at the boundaries. The overall trajectories near the wall in TRIS revealed that the cells tend to arrive at steep angles in the contact region and leave at shallow angles, a phenomenon termed as asymmetric reflection. When the viscosity increased, however, this behaviour became less apparent as the incoming angles became less steep, while the outgoing angles remained shallow. For the viscoelastic case, the behaviour appears to become more symmetric with increase in viscoelasticity. Additionally, the data also points to a less steep drop in velocity in the contact region in solutions of higher viscosity as compared to the less viscous solutions. Though this might indicate a greater influence of hydrodynamics near the wall for more viscous fluids, more data is required to observe this behaviour deeply.
The obtained results show a clear effect of viscoelasticity on the motility and kinematics of the cells, confirming results and predictions from existing literature. For cell wall interactions, differences are certainly discerned, not only for the viscoelastic cases but also for the more viscous cases. More experiments on these fluids are recommended to give a clear indication of the change in wall interactions in fluids of higher viscosity and viscoelasticity. ...
This thesis focuses on the experimental study of the swimming of Chlamydomonas reinhardtii, a model motile swimmer that swims at low Reynolds numbers. The main objective of the thesis is to characterize these differences in motility and hydrodynamic interactions of these cells in Newtonian and viscoelastic fluids of varying viscosities. This is accomplished by observing the motion of a dilute suspension of Chlamydomonas reinhardtii in 3D using four cameras. The cells are subsequently tracked using an in-house 3D particle tracking code that incorporates a recursive divide and conquer strategy to reconstruct the trajectories.
The experiments showed a drop in velocity in viscoelastic fluids as compared to its Newtonian counterparts, validating the results from existing literature. The cells also maintained their helical motion previously observed in TRIS, although with a drop in radius and pitch in viscoelastic fluids. The ratio of the radius to the pitch, however, remained constant for all the cases, indicating a tendency to retain its overall motion.
The study of cell-wall interactions is of prime importance because of the presence of confined surfaces encountered in nature. The concentration profile of the cells was observed to be similar in all the fluids, showing a non-uniform distribution with large concentration at the boundaries. The overall trajectories near the wall in TRIS revealed that the cells tend to arrive at steep angles in the contact region and leave at shallow angles, a phenomenon termed as asymmetric reflection. When the viscosity increased, however, this behaviour became less apparent as the incoming angles became less steep, while the outgoing angles remained shallow. For the viscoelastic case, the behaviour appears to become more symmetric with increase in viscoelasticity. Additionally, the data also points to a less steep drop in velocity in the contact region in solutions of higher viscosity as compared to the less viscous solutions. Though this might indicate a greater influence of hydrodynamics near the wall for more viscous fluids, more data is required to observe this behaviour deeply.
The obtained results show a clear effect of viscoelasticity on the motility and kinematics of the cells, confirming results and predictions from existing literature. For cell wall interactions, differences are certainly discerned, not only for the viscoelastic cases but also for the more viscous cases. More experiments on these fluids are recommended to give a clear indication of the change in wall interactions in fluids of higher viscosity and viscoelasticity. ...
Considering the abundance of viscoelastic fluids in nature, growing attention has been received for the study of microorganisms in viscoelastic fluids. In Newtonian fluids, the flow of microorganisms is governed by the viscosity alone. But in viscoelastic fluids, the addition of the elastic component complicates the situation. The flow of the organism is now dictated by a combination of the two forces- viscous and elastic. Understanding the behaviour of these organisms is not only important because of the presence of such environments in nature, but also because of the potential pharmaceutical applications that such studies might lead to.
This thesis focuses on the experimental study of the swimming of Chlamydomonas reinhardtii, a model motile swimmer that swims at low Reynolds numbers. The main objective of the thesis is to characterize these differences in motility and hydrodynamic interactions of these cells in Newtonian and viscoelastic fluids of varying viscosities. This is accomplished by observing the motion of a dilute suspension of Chlamydomonas reinhardtii in 3D using four cameras. The cells are subsequently tracked using an in-house 3D particle tracking code that incorporates a recursive divide and conquer strategy to reconstruct the trajectories.
The experiments showed a drop in velocity in viscoelastic fluids as compared to its Newtonian counterparts, validating the results from existing literature. The cells also maintained their helical motion previously observed in TRIS, although with a drop in radius and pitch in viscoelastic fluids. The ratio of the radius to the pitch, however, remained constant for all the cases, indicating a tendency to retain its overall motion.
The study of cell-wall interactions is of prime importance because of the presence of confined surfaces encountered in nature. The concentration profile of the cells was observed to be similar in all the fluids, showing a non-uniform distribution with large concentration at the boundaries. The overall trajectories near the wall in TRIS revealed that the cells tend to arrive at steep angles in the contact region and leave at shallow angles, a phenomenon termed as asymmetric reflection. When the viscosity increased, however, this behaviour became less apparent as the incoming angles became less steep, while the outgoing angles remained shallow. For the viscoelastic case, the behaviour appears to become more symmetric with increase in viscoelasticity. Additionally, the data also points to a less steep drop in velocity in the contact region in solutions of higher viscosity as compared to the less viscous solutions. Though this might indicate a greater influence of hydrodynamics near the wall for more viscous fluids, more data is required to observe this behaviour deeply.
The obtained results show a clear effect of viscoelasticity on the motility and kinematics of the cells, confirming results and predictions from existing literature. For cell wall interactions, differences are certainly discerned, not only for the viscoelastic cases but also for the more viscous cases. More experiments on these fluids are recommended to give a clear indication of the change in wall interactions in fluids of higher viscosity and viscoelasticity.
This thesis focuses on the experimental study of the swimming of Chlamydomonas reinhardtii, a model motile swimmer that swims at low Reynolds numbers. The main objective of the thesis is to characterize these differences in motility and hydrodynamic interactions of these cells in Newtonian and viscoelastic fluids of varying viscosities. This is accomplished by observing the motion of a dilute suspension of Chlamydomonas reinhardtii in 3D using four cameras. The cells are subsequently tracked using an in-house 3D particle tracking code that incorporates a recursive divide and conquer strategy to reconstruct the trajectories.
The experiments showed a drop in velocity in viscoelastic fluids as compared to its Newtonian counterparts, validating the results from existing literature. The cells also maintained their helical motion previously observed in TRIS, although with a drop in radius and pitch in viscoelastic fluids. The ratio of the radius to the pitch, however, remained constant for all the cases, indicating a tendency to retain its overall motion.
The study of cell-wall interactions is of prime importance because of the presence of confined surfaces encountered in nature. The concentration profile of the cells was observed to be similar in all the fluids, showing a non-uniform distribution with large concentration at the boundaries. The overall trajectories near the wall in TRIS revealed that the cells tend to arrive at steep angles in the contact region and leave at shallow angles, a phenomenon termed as asymmetric reflection. When the viscosity increased, however, this behaviour became less apparent as the incoming angles became less steep, while the outgoing angles remained shallow. For the viscoelastic case, the behaviour appears to become more symmetric with increase in viscoelasticity. Additionally, the data also points to a less steep drop in velocity in the contact region in solutions of higher viscosity as compared to the less viscous solutions. Though this might indicate a greater influence of hydrodynamics near the wall for more viscous fluids, more data is required to observe this behaviour deeply.
The obtained results show a clear effect of viscoelasticity on the motility and kinematics of the cells, confirming results and predictions from existing literature. For cell wall interactions, differences are certainly discerned, not only for the viscoelastic cases but also for the more viscous cases. More experiments on these fluids are recommended to give a clear indication of the change in wall interactions in fluids of higher viscosity and viscoelasticity.