IF
I.V.I. Fransen
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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.