With global greenhouse gas emissions continuing to rise, there is a growing need to reduce the environmental impact of industrial processes. One area of interest is the development of alternative fuel sources, such as hydrogen; however, the characteristics of hydrogen flame propagation are not yet fully understood. Modern combustion research relies on CFD modelling, which depends on physical parameters such as the turbulent burning velocity. Tomographic particle image velocimetry (PIV) was chosen as the most effective method to study turbulent burning velocity. This technique was employed to simultaneously capture the flame front location and the flow velocity field. The results indicate that both flame front detection and velocity field measurement can be performed concurrently on varying flame conditions. This study concludes that tomographic PIV is a viable approach for investigating flame front propagation and particularly the turbulent burning velocity, in premixed combustion.

This thesis presents an innovative approach to investigating automotive underbody aerodynamics through the development and application of an on-site 3D Lagrangian Particle Tracking (LPT) system. Automotive performance, particularly in high-speed racing applications, is significantly influenced by the aerodynamic efficiency of vehicle diffusers. The work addresses the challenges associated with accurately capturing complex three dimensional flow structures beneath a moving vehicle, where traditional flow measurement techniques struggle to capture underbody flows, especially in an experimental setting.

The research builds upon a previous study on a diffuser equipped radio-controlled car, which uses a measurement technique known as the Ring of Fire. By improving the camera setup, as well as creating better seeding and illumination, the setup allowed for successful particle tracking of neutrally buoyant Helium Filled Soap Bubbles underneath a car model, driving at around 7.5 m/s. The particle tracks captured in a measurement domain the size of (300 x 150 x 200) mm3 allow for the reconstruction of a velocity field around three tested car geometries, by combining data from multiple runs of the car driving through the measurement domain. These geometries are a flat floor car model, a car model fitted with a 15◦ planar diffuser, and a car model with the same diffuser, but also an additional strip of vortex generator fins placed ahead of the diffuser leading edge.

Using a pressure gradient integration method, a pressure field around the car models was obtained. Looking at both the velocity and pressure distribution around the models, the setup was able to capture the difference in peak velocity underneath the car, where the diffuser equipped model showed a maximum velocity of around 1.4 times the freestream velocity. Velocity and pressure coefficient profiles measured along the car’s centerline closely match those reported in the literature, confirming that the diffuser primarily impacts the rear region of the vehicle.

Streamwise vortices introduced into the diffuser by the vortex generator strip showed to be primarily moving high momentum flow closer to the diffuser surface, while potentially resolving a laminar separation bubble near the diffuser leading edge, observed for the plain diffuser case. Difference in local velocity magnitude and pressure coefficient measured at the diffuser leading edge between the flat floor and diffuser equipped models proved to be large enough to be statistically significant. An estimated 25 runs was needed to reach a velocity convergence inside the diffuser within 1% of the mean car velocity, where only 4 or 5 runs would be enough for the flat floor regions upstream of the diffuser.

This work shows the improvement made to the Ring of Fire setup developed to measure on-site automotive underbody aerodynamics. It proves the capabilities of applying 3D LPT to quantify underbody flows, and the potential to apply this setup on larger and faster vehicles.

Aerodynamic drag plays a critical role in high-speed sports, including running, where small performance margins can be decisive for victory in elite competitions. Despite its importance, research on running aerodynamics remains underexplored, with most studies relying on stationary mannequins or simulations that do not capture the dynamic behavior of flow around a moving runner. In other words, there is a significant research gap in experimentally visualizing the wake flow and accurately measuring the drag in real-world running scenarios. This study aims to address this research gap.

In recent years, the Ring of Fire measurement technique has emerged as a feasible option to visualise and analyse flow structures of transiting objects based on particle image velocimetry.
The technique has already been proven in sports like cycling and ice skating, but has not yet been applied to running. This study adapts the Ring of Fire measurement method for sprinting athletes, and the raw images are processed by Shake-the-Box (STB) Lagrangian Particle Tracking. This results in a three-dimensional, time-resolved velocity field in the wake of a runner with an uncertainty of less than 5\% of the runner's speed. This velocity field is used for qualitative flow visualisations, as well as for drag estimations, which are computed from a control volume approach by utilising the flow field before and after the passage of the athlete.

The experimental methodology involved nine junior athletes, wearing both standard sprint suits and aerodynamic suits, sprinting at constant speeds through a measurement setup including three high speed cameras, four LED arrays and Helium Filled Soap Bubbles (HFSB).

The results of the investigation include the visualisation of the full wake,
quantification of the velocity deficit, a qualitative vorticity analysis, some lateral velocity findings and the measured values for drag of one of the athletes.

There are multiple interesting findings about the flow around a dynamic runner in this report. One of them is the splitting of the wake into two side-by-side stream tubes in the far wake. The velocity deficit in a relevant region of the dynamic wake has also been quantified, and it is shown to vary with the inverse of the distance behind the runner, which is useful information for trailing runners.
Another interesting observation is that the vorticity field in the near wake generally follows the rules of finite cylinder flow, where the body parts of the athlete are finite cylinders with variable diameter.
The hypothesis that the flow should have a lateral oscillation related to the frequency of the runner's steps has also been confirmed.

This study demonstrates the applicability of the Ring of Fire system to running aerodynamics and offers the full visualisation of the three dimensional flow in the wake of a moving runner, bridging the research gap to pave the way for further advancements in the field.

Wings with leading-edge (LE) tubercles have gained increasing attention over the past decade. Despite their impressive aerodynamic performance, the underlying flow control mechanisms of tubercles remain controversial. In this thesis, both experimental and theoretical approaches are employed to investigate the flow patterns of a tubercled wing at pre-stall and post-stall angles of attack (AoAs).

In the experimental study, 2D Particle Image Velocimetry (PIV) was used to measure flow patterns at cross-flow planes along the chord. At a pre-stall AoA, high-vorticity regions generated by the tubercles appear in an alternating pattern near the LE. A quantitative comparison was conducted to examine the similarities between a tubercle and a delta wing. The results show that tubercles cannot be regarded as small delta wings in terms of vortex generation. The leading-edge vortex (LEV) sheets are convected downstream, where they interact with laminar separation bubbles (LSBs), creating complex flow patterns in the downstream regions. At a post-stall AoA, stall cells (SCs) appear along the span, with their formation dependent on both Reynolds number (Re) and tubercle amplitude. However, the spacing of SCs is relatively independent of AoA, Re, and amplitude, consistently ranging between 5 to 7 tubercle wavelengths.

In the theoretical study, the lifting line theory (LLT) approach was first used to predict the LEV strength but proved ineffective due to the absence of thickness effects. A subsequent analysis using the panel method in xflr5 showed that the Kutta condition should also be applied to the leading edge (LE) rather than only to the trailing edge (TE). Crow’s model was adapted by taking LEVs into consideration. However, a global description of the instability was not obtained due to difficulties in representing LEVs and related mathematical challenges.

This thesis contributes to a further understanding of the tubercle’s role in flow control. The LEVs generated by the tubercles are identified as key factors influencing flow evolution, yet these effects are not captured by LLT-based models or a conventional panel method. Future reduced-order models (ROMs) should account for the influence of LEVs to provide accurate representations of tubercled wing flow dynamics.

This thesis investigates the role of winglet slots in reducing tip vortices, inspired by bird wingtips. The effects of slotting and wingtip flexibility were analyzed using multiple bio-inspired models tested in a low-speed wind tunnel at Reynolds numbers from 2.9 × 10⁴ to 8.8 × 10⁴. Three experimental techniques were employed: force balance for aerodynamic force estimation, motion tracking of wingtip movement, and stereo PIV analysis to measure cross-flow planes in the winglet wake. Non-intrusive methods were used to estimate drag components based on PIV data and wake integral approaches. The slots significantly reduced induced drag by breaking down the tip vortex into smaller, weaker vortices. This breakdown was visualized using s-PIV results and analyzed through velocity component contours. The streamwise vorticity breakdown was examined in relation to wingtip flexibility, and the vortex cores' location and characteristics were analyzed, along with the phenomenon of vortex wandering.

By the end of the thesis, it was concluded that although slotted winglets reduced induced drag, they led to a significant increase in profile drag, resulting in higher overall drag compared to unslotted winglets. Additionally, the slots caused a loss in lift. The overall aerodynamic performance, measured by the lift-to-drag ratio, was also higher for the
unslotted winglet. Among the slotted winglets, static bending was common, while wingtip vibrations were minimal. The most flexible slotted winglet had the lowest induced drag. However, the results indicate that at very low Reynolds numbers, slotted winglets deteriorate the wing’s aerodynamic performance compared to unslotted ones. Among the slotted winglets, those with intermediate flexibility demonstrated the best aerodynamic performance, highlighting the importance of optimizing wingtip flexibility.

Blades in rotor systems such as helicopters and wind turbines experience highly dynamic inflow conditions such as rapid pitching that can lead to dynamic stall. In turn, that causes large force fluctuations which translate into fatigue for the blade structure and controllability issues for the vehicles. Biomimetics brings a potential solution in the shape of leading edge tubercles inspired from the flippers of humpback whales. These possess impressive manoeuvrability for the size and their flippers pitch up and down rapidly. While the effect of this bio-inspired leading edge shape is well-documented in steady conditions, being known to delay stall and make its character more benign, there are very few studies investigating the effect in unsteady conditions. Therefore, the scope of this study is to analyse the effect of leading edge tubercles on the flow structure evolution throughout sinusoidal oscillations of finite wings and on their tip vortex strength.

This thesis combined two methodologies: one experimental and another computational. The experimental study focused on the wind tunnel analysis of two different rectangular finite wings with leading-edge tubercles against a straight leading edge finite wing subjected to sinusoidal pitching oscillations. The study uses 3D Particle Tracking Velocimetry (PTV) technique, with the state-of-the-art Shake-The-Box (STB) algorithm which uses Helium-filled soap bubbles as flow tracer particles in order to obtain the complex instantaneous flow field around the wings throughout the oscillation cycles. After obtaining the particle tracks, phase-averaging was used to improve the quality of experimental results by making use of the multiple oscillation cycles recorded. The computational study replicates the wind tunnel conditions. U-RANS simulations were run using the Ansys CFX solver and using a deforming mesh to model the pitching motion of the airfoil. Both the CFD and the experiment were conducted at a Reynolds number of Re = 3.3 × 10⁴.

The results of both the experimental and the CFD correlate well in terms of the flow structure, allowing for an interesting comparison between the two methods. These show that the wings with leading edge tubercles do show a more benign stall behaviour and quicker reattachment on the downstroke of the oscillations thanks to the compartmentalisation effect of the streamwise vortices shed by the tubercles. However, surprisingly the angle of attack at which the maximum lift coefficient is produced is not increased or delayed compared to the straight leading edge (SLE) wings. Furthermore, the tubercle wings exhibit reduced tip vortex strength throughout the oscillation cycle, thanks in part to the destructive effect of the streamwise vortices near the wing tip on the tip vortex. Finally, in close correlation with the tip vortex circulation, the tubercle wings have a lower induced drag coefficient compared to SLE wings, with a better span efficiency thanks to the compartmentalisation effect of the spanwise flow.

Laser light reflection mitigation in Particle Image Velocimetry (PIV) is crucial for accurate flow
field measurements. While numerous methods exist for planar PIV, fewer have been developed for volumetric PIV systems, especially for coaxial setups like Robotic PIV. Light reflections in volumetric PIV experiments result in high-intensity regions that corrupt particle detection and analysis.

This study presents three novel approaches for treating light reflections in Robotic PIV experiments. The first and second methods use image filtering and masking techniques in the
wavenumber space to separate particle images from reflection regions. The first technique called Spatial Fourier Filter involves decomposing the image signal into low- and high-wavenumber components using the 2D discrete Fourier transform (DFT). A high-pass filter is then applied to attenuate the intensity of reflection regions. Then, the second methodology Spatial Fourier Filter + Mask takes the resulting image from the first method and performs a step of automated adaptive masking to remove residual reflection areas that the filtering approach is not able to eliminate. The third methodology named 3D-based Particle Concentration Mask acts in a later stage of the processing pipeline, creating a 3D mask on the instantaneous processed Shake-the-Box data by analysing the particle concentration distribution over the flow domain.

The proposed methods are tested on experimental data obtained from experiments performed with Robotic PIV on three different geometries: a side-view mirror, Formula 1 car and a propeller. The tests were conducted at one of TU Delft Aerospace Engineering Faculty’s facilities, the W-tunnel in the High-Speed Laboratory (HSL). Comparison between raw and pre-processed images, as well as particle tracking results, is presented.

The results from this data comparison show unsatisfactory outcomes from both Spatial
Fourier Filter and 3D-based Particle Concentration Mask, which fail to fully remove the spurious regions. Nevertheless, the results confirm the successful removal of reflection-induced artifacts in instantaneous images by using the spatial Fourier filter automated masking approach. The developed image pre-processing strategy effectively removes reflection regions in Robotic PIV images, preventing the appearance of spurious particle tracks. The method shows promising results mitigating unsteady light reflections in Robotic PIV, improving the accuracy of flow field measurements. Additional attention is required in the PIV sequence creation step to ensure an adequate level of overlap between measurement volumes. This facilitates addressing the spatial gaps introduced by the masking procedure, that have been proven to robustly be filled in by the multi-view advantage offered by Robotic PIV.

Inspired by the leading-edge protuberances found on the flippers of humpback whales, tubercles have drawn significant interest for improving airfoil aerodynamic performance. In static conditions, airfoils with tubercles exhibit a softer onset of stall and increased lift in post-stall regime, though with a reduced maximum lift coefficient. However, their impact under dynamic conditions is less understood, particularly how they affect the formation and convection of the dynamic stall vortex (DSV), which is crucial to the dynamic stall process. Therefore, this study aims to investigate how tubercles affect the development and behavior of the DSV during dynamic stall conditions. To deliberately trigger the formation of the DSV and thereby force the airfoil into dynamic stall conditions, this study employed a pitch-up and hold motion starting at a zero angle of attack and increasing to final angles of 30 and 55 degrees. Two pitching rates of k=0.05 and 0.1 were investigated, providing a comprehensive analysis of how these conditions influence the aerodynamic characteristics of the airfoils across a wide range of dynamic stall scenarios.

To assess the impact of leading-edge tubercles on airfoil aerodynamics under dynamic stall conditions, a series of wind tunnel experiments was conducted, involving two tubercled airfoils and a smooth leading- edge airfoil. These experiments were performed at a Reynolds number Re=3.3x10⁴, determined by the chord length and free-stream velocity. Employing particle tracking velocimetry (PTV), the velocity field on the suction side of each airfoil was measured, while ensuring precise airfoil positioning within the flow-field through the use of white tracking markers. Detailed monitoring of the DSV core location and circulation was conducted using the normalized angular momentum (NAM) criterion, also referred to as Γ₁ method. This approach provided quantitative insights into the influence of the tubercles on the DSV. Due to the experimental setup limitations in directly measuring aerodynamic forces, computational fluid dynamics (CFD) simulations were conducted using OpenFOAM. These simulations employed a sliding mesh technique, which facilitated the consistent application of the same computational framework for various tubercle geometries and pitching conditions. The accuracy and reliability of the simulations were rigorously validated against the experimental data.

The results reveal that leading-edge tubercles significantly influence the aerodynamic performance of airfoils under dynamic stall conditions. It has been found that tubercles modify the onset and severity of dynamic stall by reducing the strength of the DSV and shifting its formation closer to the trailing-edge. These changes result in a weaker and shorter lift overshoot, facilitating a quicker transition to the deep stall regime where tubercles enhance the lift provided by the airfoil. This alteration in the dynamic stall process has been consistently observed across all tested pitching motions, with the effects of the tubercles found to be proportional to their amplitude. These findings suggest that tubercles serve as dynamic stall mitigation devices, potentially benefiting applications where dynamic stall frequently occurs and can compromise structural integrity.

This study investigated the effect of tangential strain on the stability of perturbed laminar lean premixed hydrogen flames in a counterflow reactants-to-products configuration. Laminar premixed flames are highly susceptible to intrinsic instabilities, including hydrodynamic and thermodiffusive instabilities, which can cause perturbations in the flame front to grow. However, the impact of tangential strain, introduced by velocity gradients along the flame front in a counterflow setup, had not been fully explored in the context of flame stability. The present work bridged this gap by combining the counterflow setup used in previous studies of NOx reduction with the perturbation analysis commonly employed in flame stability research.

Direct Numerical Simulations were performed to study the behaviour of sinusoidal perturbations imposed on the flame front, focusing on the growth rates of these perturbations under two different strain rates (2000 1/s and 4000 1/s). The results indicated that the tangential strain improved the stability of the flame front, as the strain rate led to a reduction in both the maximum observed growth rate and the growth rate after the perturbation reached its peak. The growth rates observed were in the non-linear regime, characterised by continual variation over time. At higher strain rates, the flame front stabilised more quickly, suggesting a strong correlation between strain rate and perturbation growth dynamics. The strain-induced velocity gradients displaced the flame front, reducing its effective curvature and wavelength, further contributing to the stabilisation process.

In addition to the strain rate, the study investigated the effects of varying the amplitude and wavelength of initial perturbations. It was found that the amplitude of the initial perturbation had little impact on the maximum growth rate, but variations in the initial wavelength significantly influenced both the growth dynamics and the maximum growth rate of the perturbations.

Overall, the results provided a comprehensive understanding of how tangential strain affected flame stability and highlighted the importance of wavelength variations in determining perturbation growth rates. These findings provide insights into potential strategies to improve flame stability in practical combustion systems aimed at reducing emissions, such as in lean hydrogen combustion.

As the world grapples with the challenges of global warming, there has been a significant push for the development of new methods of harnessing renewable energy. Energy harvesters have been a new avenue to generate electrical power locally for remote applications. An energy harvester is a device that taps into aeroelastic phenomena to convert vibrational energy into electrical energy. Additional benefits of harvesters include being lower in complexity, ease of maintenance, and the resulting cost-effectiveness.

The splitter plate represents a wake control device that alters the wake to make the energy capture more efficient and practical when attached to a cylinder. However, the flow mechanisms under different conditions have not been fully understood. The motivation of the present work is to bridge this gap by characterizing the mechanisms and wake dynamics of a cylinder with a flexible splitter plate. This will help advance the understanding of the wake behavior of these devices to make safe and efficient devices. To characterize the behavior of the wake, measurements are made at different flow speeds. A preliminary analysis of the splitter plate motion showed that increasing the Reynolds number produced four distinct regimes to be studied further: crossover, baseline, reduced amplitude, and chaotic regimes. The initial analysis also revealed a condition named burst, defined as a period of resonant, two-dimensional, high amplitude oscillations of the splitter plate. All regimes are compared to a bare cylinder with no splitter plate to ascertain differences in wake behavior.

The use Helium Filled Soap Bubbles (HFSB) in flow diagnostic tools such as Particle Image Velocity (PIV) experiments has permitted large-scale measurements. A Lagrangian Particle Tracking (LPT) experiment was performed in which the HFSB is tracked individually with the help of an algorithm. The present study employs Shake-The-Box (STB), a novel LPT tracking algorithm that uses time-resolved experimental data to predict future tracer particle locations. The benefit of such an algorithm includes reduced ghost particle detection and computational efficiency. The details of this experimental method have been covered extensively in this thesis. The raw experimental data was processed to perform statistical analysis, proper orthogonal decomposition, and spectral analysis to provide insights to meet the thesis objective.

This study explores the flowfield around a pusher propeller-wing configuration using Lagrangian Particle Tracking (LPT). It assesses LPT’s capability to characterise propeller-wing interactions, emphasising its ability to resolve complex flow dynamics.

The experiment successfully achieves high particle concentrations downstream of the propeller, with propeller rotation enhancing the uniformity of the particle distribution. Velocity measurements are precise and feature low uncertainty, and vortex dynamics shed by the propeller are effectively resolved.

Furthermore, the study confirms LPT’s robustness in characterising wing-propeller flow characteristics, particularly in thrust and torque calculations, highlighting its non-invasive nature for performance measurements.

The investigation reveals how the wing influences the slipstream’s vortex structure and identifies the pylon’s effects. The method captures the propeller-induced effects on the wing, providing insights into upwash, downwash, and angle of attack variations.

In conclusion, this research demonstrates the utility of LPT in comprehensively characterising propeller-wing interactions, offering valuable insights for aviation and propeller innovations.

This work provides a foundation for designing aerodynamic mudflaps used in heavy-duty vehicles, which will help these vehicles become greener. First, a simulated moving ground consisting of a leading edge, roller setup and a novel tangential blowing system was devised as an alternative to the expensive conveyor belts necessary for pertinent experiments. The crucial design parameters, such as rounded, smaller support walls and uniform pressure inputs, were determined using two initial blowing slot designs. Next, a boundary layer analysis and tests on an isolated wheel using planar PIV demonstrated its ability to drastically reduce the effects of the boundary layer and accurately emulate wheel flow. Finally, mudflaps of different solidity and solidity locations were tested using stereoscopic PIV measurements on a simplified setup. Drag estimates showed that more solidity led to higher drag and wake skewing, and having lesser solidity on the top gave 15% lower drag than the bottom.

The near wake of the Ahmed reference body in the presence of steady cross-wind is investigated experimentally by means of large-scale three-dimensional Particle Image Velocimetry (PIV). A novel experimental technique is employed which exploits coaxial imaging and illumination while relying on Helium Filled Soap Bubble (HFSB) seeding as well as on robotic actuation. A 50% replica of the Ahmed body with a 25° slant angle is, therefore, considered in different configurations, among which the headwind condition and two different yaw angles, β1 = +4° and β2 = +8°. A height-based Reynolds number of 1.15 × 10^5 is chosen together with an airspeed of 12 m/s and the Open Jet Facility (OJF) Wind Tunnel of TU Delft is employed to perform the experiment.The relevance of the proposed work lies in the importance of experimental data concerning the Ahmed reference body as well as in the shortage of it with respect to the cross-wind case. The Ahmed body, indeed, once studied to improve the understanding of the flow field generated by road vehicles and to achieve a substantial drag reduction, is nowadays used as Computational Fluid Dynamics (CFD) benchmark and validation tool for automotive applications.To the best of the author’s knowledge, no work is present in the literature which investigates the flow field around the Ahmed body by means of three-dimensional quantitative measurement techniques such as the employed robotic volumetric velocimetry. The proposed study is, therefore, expected to be a significant contribution to the body of knowledge, supplying new valuable data to be used for validation purposes and to inspire future works.

In the European Union greenhouse gas emissions of heavy duty vehicles make up 30% of the total caused by road transport. By placing vehicles in a platoon configuration the aerodynamic drag can be heavily reduced. The effect of platooning has been studied on both American and European type vehicles. However many of these studies only disclose the drag reductions and do not fully explain the flow behaviour between the models which in the end is what is causing the reduction in drag. Next to this studies done on European type vehicles mostly used time-averaged simulations so a next step is understanding the effect of unsteady flow on the vehicles. In this study the flow field between two vehicles in a platooning configuration is analysed numerically and experimentally for varying intervehicle distance and front- and rear drag reduction devices. The GETS model was used in this study which is a simplified model of a European heavy duty vehicle. For the numerical analysis Exa PowerFLOW is used which is a transient CFD package based on the LBM. Turbulence is modelled using VLES together with a RNG of the κ - ϵ equations. A study of the mesh size showed the sensitivity on the drag coefficient of the single model.
The experimental analysis was performed in the OJF of the TU Delft at a Reynolds number of 3.9 x 105 based on the square root of the model. Next to balance measurements CVV measurements in the gap between the two models were taken in order to visualize the flow field.
The single model showed a toroidal recirculation region in which a vortex ring can be seen. The drag force fluctuates at a Strouhal number of 0.073, side and lift force at 0.058 and 0.102 and 0.130 and 0.160. These fluctuations are caused by pressure fluctuations in the wake where the highest magnitudes are seen in the shear layer. The models with a smaller front radius saw a rise in drag with flow separation occurring for the smallest radius for the numerical results and for both smaller radii in the experiment. The addition of a boat tail lowered the drag of the models. With increasing tail angles the drag reduction also increases which is caused by the increase in base pressure. The tails also decrease the force fluctuations due to decreased pressure fluctuations in the shear layer.
At the closest spacing of 0.10 times the vehicle length all tested configurations benefit from the platoon. The flow between the two models is made up of a toroidal recirculation region much smaller in size compared to the single model. In general the flow takes an S-shaped path between the two models. From the underbody of the leading model it either stagnates on the front of the trailing model or moves into the gap where it ends up in the upper of lower vortex or stagnates on the front or rear of one of the models. When a sharper front edge radius is applied to the trailing model more flow is deflected into the gap leading to lower pressure vortices and a higher stagnation pressure. A small vertical misalignment of the trailing model, which was seen during the experimental campaign, leads to a higher stagnation pressure on the bottom of the trailing model, also here more flow is deflected into the gap. At this spacing the side-force fluctuations are a bit higher due to the vortices that leave the domain passing over the rounded edges of the model.
At the middle spacing of 0.45 times the vehicle length not all configurations benefit from the platoon. The trailing models with the baseline frontal radius saw an increase in drag due to the lack of thrust generated by the rounded edges. The other models did see a drag decrease. At this distance the wake of the leading model is quite similar to that of the single model. Flow enters from the top, bottom and sides of the model and ends up in the recirculation region or it stagnates on the rear of the model before it leaves the wake and stagnates on the front of the trailing model or flows over the rounded edges where it accelerates and leaves the gap. The effect of a sharper radius applied to the trailing model has little effect on the flow field. When a tail is applied to the leading model the recirculation region has been reduced as was seen for the single models. Due to the upwash the stagnation pressure on the bottom front has been increased but the drag has decreased compared to the platoon without a tail due to the increased suction of the rounded edges. At this distance the force fluctuations are much higher compared to the single model. The magnitude of drag and side force can go up to 2 and 6.5 times the values of the single model depending on the configuration. This is due to the stronger vortices from the leading model which pass over the trailing model.
For the last vehicle spacing of 0.91 s/L all the configurations benefit from the platoon again. For the baseline models the gains are only a few percent but for the trailing model with a sharper front radius the gains are higher than those at the middle intervehicle distance. This is due to the reduced stagnation pressure and almost undisturbed suction coming from the rounded edges. At this distance any effect on the wake of the leading vehicle has vanished. The unsteady forces still show an increased amplitude compared to the single model however compared to the middle intervehicle distance they are much lower.
The comparison between the results from the numerical and experimental analysis is quite good. The drag and flow field results are very similar. For the pressure and the unsteady forces this is less the case. The mismatch seen in the pressure comparison can be caused by alignment errors that were observed during the experiment. Next to this while reconstructing the flow field from the experimental measurements the resulting coordinates were outside the expected range. A manual coordinate shift was applied which may have resulted in additional errors. For the unsteady forces it is assumed that the ground plate and its interaction with the model as well as additional vibrations are the cause of the much higher fluctuations seen from the experimental data.

The continuous pursuit of reducing drag in many speed sports, such as cycling and ice-skating, demands novel approaches to gain further insight into flow phenomena around athletes. In recent years, the Ring of Fire measurement technique has emerged as a feasible option to visualise and analyse flow structures of transiting objects based on particle image velocimetry. The accuracy of this on-site measurement technique has not yet been validated under equal test conditions.

This master's thesis aims to compare drag area values of a cyclist from Ring of Fire measurements to simultaneously acquired power meter data. Tests with the cyclist in upright and time-trial posture, as well as different helmet types and various drafting distances, are envisaged to assess the correlation between the two measurement techniques in multiple drag area regimes, and to gain insight into large distance drafting above 3m, which, to the best knowledge of the author, has not yet been studied in academic research. In addition, the campaign plans to remove any user operations during the test, which would be another step towards a fully autonomous Ring of Fire system, as envisioned in the future. A spacious indoor facility is suggested as the testing site to minimize environmental effects and to allow for the continuous motion of the cyclist. The Ring of Fire method shows great potential, as the measurements are conducted under simulated racing conditions and wake visualisation allows the operator to locate origins of drag. Validating the drag area results could further attest to the Ring of Fire's viability as an optimisation tool in the upcoming years.

The conducted campaign, within the framework of this thesis, indicates good agreement between the power meter and Ring of Fire techniques when assessing the relative drag area delta of a small-scale helmet change and a large-scale posture change. In terms of absolute values, the power meter model shows a high dependency on underlying model constants. Using literature-based coefficients, the absolute CdA values are within 5% of the Ring of Fire derived values.

Furthermore, the feasibility of evaluating long distance drafting effects with the Ring of Fire system is demonstrated. Measurable drag area savings of 15% are obtained by the trailing cyclist at front wheel to front wheel distances of 7-9m. Due to non-uniform inflow conditions in front of the trailing cyclist, a wake contouring algorithm needed to be employed to satisfy mass preservation within the control volume by resizing the inlet and outlet plane. In addition, enclosure of the wake structure and contouring of a representative inlet plane is achieved. The flow topology in the wake of the trailing cyclist is acquired by a stereo-PIV system. Primary wake structures, as well as in-plane velocity fields, are comparable to those observed behind isolated riders.

In the context of the ongoing research regarding boundary layer transition, this project aims to study the effects of isolated cylindrical roughness elements on the transition from laminar to turbulent flow in a swept wing boundary layer. The project entails experimental campaigns: the 45 degrees swept wing model is placed inside the test section of the Low Turbulence Tunnel (LTT) and millimeter-sized roughness elements are attached to it, so that their interference with the boundary layer flow can be studied. The main goal is to investigate the flow features generating aft the roughness elements and their development into turbulent streaks. Moreover, this project aims to understand and quantify the effects of different roughness sizes on transition mechanisms, in the specific case of a three-dimensional boundary layer. This is achieved by means of different experimental techniques, such as PIV, HWA and thermographic flow visualization.
The obtained results lead to a comprehensive description of this type of flows, with several phenomena changing their characteristics due to a change in parameters (freestream speed, roughness diameter and roughness height). Velocity field, spectral analysis of the signal, wedge width evolution, instability modes and velocity fluctuations in time were investigated with different parametric conditions, in order to understand how the flow was affected by
them. Furthermore, the study allowed to evaluate the use of non-dimensional parameters such as roughness Reynolds number and aspect ratio for the prediction of the flow topology. The complexity of the phenomena involved is underlined in the conclusions of this research, with the interaction of several flow features making it a broad and interdisciplinary field of research.

Laser diagnostic techniques such as Particle Image Velocimetry and Particle Tracking Velocimetry to measure the flow of a fluid around an object have been in prevalence for a few decades. Typically, the fluid, whose motion is of interest, is seeded with micron scale particles and illuminated with a laser. Due to scattering inefficiencies of these micron scale particles the maximum measurement volumes were about a few litres. Development of the neutrally buoyant sub-millimetre scale Helium-Filled Soap Bubbles allows for much larger scattering cross-sections thus enabling large-scale measurements in air (Caridi et.al. 2015).

Large scale Particle Tracking Velocimetry has been used to study the flow in the wake of a cyclist at typical time-trial speeds. A novel PTV algorithm known as Shake-The-Box (STB) (Schanz et.al. 2013) has been used to obtain the particle tracks from the images acquired in a thin volume in the cyclist’s wake using the wake-scanning method. The Lagrangian particle tracking technique was found to be more accurate than the Time Resolved Tomographic PIV through another experiment. Particularly for large field of views involving scanning at different positions, it produces whole-field results free from the boundary effects that arise between different positions of the scans. This yields accurate velocity components and their gradients that are crucial for pressure reconstruction.

Flow-field results from 4D-PTV show that the wake structure is similar to those found in the literature. Two large regions of momentum deficit are present, one behind the thighs and one near the lower legs and the wheel axis. Furthermore, they reveal the presence of some vortices which were not reported previously. Based on the observed signs of the vortices the points of origination of these vortices are identified based on separation mechanisms. There is a good agreement in the flow-field results with the literature. Pressure fields reconstructed from the velocity flow-field show low pressure pockets in the regions of the vortex cores and separated flow over the lower back.

Wake flow-field is utilised to compute the drag using the control volume approach (van Oudheusden 2007) and compared with the drag forces obtained from an external balance measurement to obtain the accuracy of large scale 4D-PTV. In total, five measurements at different velocities in the range of 13m/s to 15m/s are analysed. The accuracy of the drag estimates from 4D-PTV is obtained within 5%. Out of the three terms of the drag force, the momentum term contributes approximately 95% and the contribution from the pressure term is less than 0.3%. Rest is contributed by the Reynolds stresses in streamwise direction. Almost all the variation in the drag force comes from the momentum term.

Junction flow denotes the fluid phenomenon where the flow on a flat surface encounters a protuberance. This type of flow, which is highly three-dimensional, turbulent and unsteady, usually leads to the generation of a Horseshoe vortex at the obstacle's leading edge. Existing extensive experimental and numerical studies focus on the inception of such vortex, as it exhibits a bi-stable behaviour which persists downstream in the vortex legs. In a practical situation such as wing-fuselage juncture, the created vortex interacts with the flow around the aircraft. Consequently, the interference drag increases and the wake can reduce the effectiveness of the stabilizers. To tackle these effects, control devices are implemented to alleviate the strength of the vortex, being leading edge fairing the most widely used device.

In this research project the junction flow is subjected to the influence of different control devices: leading edge fairing, vortex generators and the novel antifairing are tested. The flow field is captured with the state-of-the-art large-scale tomographic PTV technique. The aim of this project is to characterize the vortical structures associated with each passive control device.

The time-averaged flow field results show the fairing is the most efficient device in reducing the presence of the Horseshoe vortex, but it is also the one which contaminates a larger wake area with the vortex wake. The vortex generators are the worst performer both in the mitigation of the vortex and the turbulence level in the wake. Nevertheless, the performance of such control devices highly depends on the location of the vortex generators. Thus, it is speculated that an optimal position exists which can yield better results. The antifairing shows minimal differences in the velocity field and the vortical structures compared with the reference case, but with a slightly lower turbulence level.

An auxiliary experiment with stereoscopic PIV is performed to the wake of the junction, in which the momentum deficit, a good indicator of the drag, is calculated for the different configurations. The leading edge fairing shows minimal drag reduction associated with the HSV of just about 1-2%. The VG once again have proven not being very effective with an increment of 4%. A reduction of more than 15% is measured for the antifairing case. Although the associated flow topology assimilates to the reference case.

This study investigates the HSV system around the junction as a volume. In comparison with most of the previous experimental studies on the subject, performed mostly with planar PIV and limited to just a few planes, the volumetric measurement can deliver more information with a single measurement. Therefore, it can better define the flow topology and vortical features and facilitate the understanding of the control devices working mechanisms.

Drag measurements on bluff-bodies such as bobsleighs using force balance systems have been in prevalence for a few decades. However these studies do not reveal anything on the flow behaviour around the body. In the recent past, various flow visualization techniques have been applied to investigate the flow behaviour around bobsleighs. Even though these studies have highlighted the qualitative flow features, there is a lack of a deep insight into the wake flow topology and the behaviour of the flow structures that are formed in the highly three-dimensional flow. Additionally, when the bobsleigh moves in a curvilinear path, the front cowling rotates with respect to the rear cowling, causing an increase in the frontal area exposed to the wind and in turn a higher drag. However, this perception is not quite straightforward and requires a thorough understanding of the mechanisms and sources of drag generation, in particular at different front cowling angles.

Stereoscopic Particle Image Velocimetry has been used to study the near-wake flow topology of a scaled bobsleigh model. From the velocity field information obtained from PIV, the pressure field in the wake of the bobsleigh model is determined from the Pressure Poisson Equation (PPE) using the methodology outlined by Oudheusden [41]. With the knowledge of the velocity and the pressure fields, the aerodynamic drag is computed using the control volume approach on a wake plane which is at approximately two diameters from the model. The drag obtained from PIV is compared to that obtained from balance measurements. The effects of different bobsleigh configurations on the aerodynamic drag is observed. Secondly, uncertainty quantification of the aerodynamic drag is performed. Finally, a Proper Orthogonal Decomposition (POD) analysis is performed for the velocity fluctuations in order to gain an insight on the motion of the flow structures formed in the wake.

The investigation of the near wake flow topology of the bobsleigh reveals the presence of two counter-rotating vortices with a significant downwash between them in case of the reference configuration. The motion and behaviour of these vortices is obtained by performing the PIV analysis on multiple vertical wake planes. The investigation of the effect of the front cowling misalignment on the drag reveals that there is a reduction of drag for smaller misalignment angles (upto 5°) and a marginal increase in drag for further increase of misalignment (from 5° to 20°). From the analysis of aerodynamic drag conducted on a vertical plane that is approximately two diameters behind the model, it can be seen that the momentum deficit is the highest contributor to the aerodynamic drag (88%) followed by the Reynolds stresses (9%) and pressure (3%). It is also observed that the aerodynamic drag obtained is in good agreement with the results obtained from the balance measurements with a variation of only 2 to 3%. The uncertainty analysis of drag shows that among the different flow variables which affect the drag, the average pressure has the maximum uncertainty. Finally, the POD analysis of the velocity fluctuations shows that the first two fluctuating modes which are the most energetic are not perfectly correlated to each other due to the highly three-dimensional nature of the flow.

One of the most commonly used flow control techniques to trim the aerodynamic performance of wind turbine blades are vortex generators. This passive device, which does not require any active energy, is gaining an increasing interest in this sector especially due to the trend of increasing wind turbine size. Vortex generators are a flow control technique used to prevent or at least lessen the severity of flow separation. The streamwise vortices shed at the free tips of the vortex generators increase the mixing between the high-energy flow in the outer part of the boundary-layer with the low energy regions near the walls. The shed vortices are defined by the strength of the vortices just downstream of the device, the streamwise decay of the vortex strength and the vortex trajectory. In order to assess and optimise the use of vortex generators, there is a need to accurately model the effect of this device in a cost and time efficient way. This could be achieved by modelling vortex generators using integral boundary-layer codes. XFOIL and RFOIL are two well-known integral boundary-layer codes. These two codes rely on Prandtl’s theory stating that a flow field around an object can be divided into two areas: the inviscid outer flow and the viscid inner flow. The inviscid part of the flow is solved using a streamfunction panel method while the viscid boundary-layer solution is prescribed by the boundary-layer equations and additional closure relations. Both solutions are strongly interconnected to each other. A suitable methodology to model the effect of vortex generators into integral boundary-layer codes is the source term approach. This approach was already initiated by Kerho and Kramer1 but is revised carefully in this thesis. The introduction of the streamwise vortices results in an increase in dissipation. To simulate this, the shear-lag equation is modified by introducing an additional source term to the equilibrium shear stress coefficient. The strength of this additional source term is varied over the chord to mimic the downstream decay of the shed vortex circulation. Implementing the source term into XFOIL and RFOIL allows to capture the expected effect of vortex generators on the boundary-layer properties. The source term shape function is prescribed by three parameters: the source term strength, the decay rate and the location of the vortex generators. Because the source term depends on the vortex generator geometry and the local flow properties, its value is calibrated for different airfoil/vortex generator configurations using reference data. Afterwards, these calibrated source terms are related back to the vortex generator parameters and local boundary-layer properties to establish an empirical relation for the source term integral. To do so, a multiple variable linear regression is applied in which the independent variables are the vortex generator height, vortex generator length, inflow angle, local boundary-layer momentum thickness and edge velocity. The source term empirical relation is implemented in XFOIL and RFOIL. Implementing the source term empirical relation led to the development of the foundations of the design tools XFOILVG and RFOILVG. These two codes are extensively validated to evaluate the performance and identify code limitations. Validating the source term empirical relation and its implementation, is realised by comparing the codes’ predictions with reference data from the data base that was already used to set up the source term relation. To further validate the robustness and generality of the code with respect to airfoil selection and Reynolds number, external data sets adopted from literature are used. Based on the validation results, it is believed that the source term approach to model vortex generators within the integral boundary-layer theory is very promising. To further elaborate on this method, recommendations for further work are made. These recommendations concern the data base used to set up the source term empirical relation, the effect of vortex generators on the friction coefficient and the role and value of adding a transition model.