CH
C.O. Hermans
info
Please Note
<p>This page displays the records of the person named above and is not linked to a unique person identifier. This record may need to be merged to a profile.</p>
2 records found
1
Estuaries are economically and ecologically important regions, hence understanding its fluid and sediment dynamics is essential. One factor that influences the dynamics is turbulence. This physical phenomenon occurs in the Navier-Stokes equations, which are notoriously difficult to solve. Various methods exist to include the effect of turbulence, one of them is to parametrize the effects of turbulence via a model parameter called the eddy viscosity. The objective in this thesis is to study the effect of the choice of eddy viscosity on the resulting velocity and concentration profiles. Three choices for the eddy viscosity formulations are examined: a prescribed viscosity model that is constant in space and time, a prescribed parabolic viscosity model that is constant in time and the k-ε model, which is the most advanced model that is widely used in computational fluid dynamics and depends on the local physics of the flow, and is not constant in time. Special emphasis is given to systems with high concentrations. To reproduce some of the complex dynamics in these systems, two processes are essential: hindered settling of particles and stratification, i.e. the distinct layering of water due to density differences. Hindered settling is included for each model, and stratification is studied separately by analyzing both stratified and non stratified cases. The effect of the three models is investigated by implementing a vertical one dimensional water column model and examining resulting velocity and concentration profiles. This is done for a constant water level gradient, resulting in steady-state profiles. Newton's method is implemented with a non-uniform grid to ensure converged solutions.
Neglecting stratification effects, the parabolic viscosity model and k-ε viscosity model yield similar velocity and sediment concentration profiles. In contrast, the constant eddy viscosity model produces weaker flow and higher sediment concentrations. When stratification is included, all eddy viscosity formulations give qualitatively the same results. The suppression of turbulence leads to damped eddy viscosity profiles and sediment settles more easily. When the concentration at the bed is higher than a critical concentration, the concentration where hindered settling attains its maximum, a lutocline may form. This is a sharp transition between a clear upper layer and dense lower layer. All three viscosity profiles show the presence of this lutocline for certain parameter regimes. If the lutocline is present, the location is near the region where the turbulence is most strongly suppressed. Furthermore, the concentration where the lutocline forms can be approximated by the value where the hindered settling term reaches its maximum. ...
Neglecting stratification effects, the parabolic viscosity model and k-ε viscosity model yield similar velocity and sediment concentration profiles. In contrast, the constant eddy viscosity model produces weaker flow and higher sediment concentrations. When stratification is included, all eddy viscosity formulations give qualitatively the same results. The suppression of turbulence leads to damped eddy viscosity profiles and sediment settles more easily. When the concentration at the bed is higher than a critical concentration, the concentration where hindered settling attains its maximum, a lutocline may form. This is a sharp transition between a clear upper layer and dense lower layer. All three viscosity profiles show the presence of this lutocline for certain parameter regimes. If the lutocline is present, the location is near the region where the turbulence is most strongly suppressed. Furthermore, the concentration where the lutocline forms can be approximated by the value where the hindered settling term reaches its maximum. ...
Estuaries are economically and ecologically important regions, hence understanding its fluid and sediment dynamics is essential. One factor that influences the dynamics is turbulence. This physical phenomenon occurs in the Navier-Stokes equations, which are notoriously difficult to solve. Various methods exist to include the effect of turbulence, one of them is to parametrize the effects of turbulence via a model parameter called the eddy viscosity. The objective in this thesis is to study the effect of the choice of eddy viscosity on the resulting velocity and concentration profiles. Three choices for the eddy viscosity formulations are examined: a prescribed viscosity model that is constant in space and time, a prescribed parabolic viscosity model that is constant in time and the k-ε model, which is the most advanced model that is widely used in computational fluid dynamics and depends on the local physics of the flow, and is not constant in time. Special emphasis is given to systems with high concentrations. To reproduce some of the complex dynamics in these systems, two processes are essential: hindered settling of particles and stratification, i.e. the distinct layering of water due to density differences. Hindered settling is included for each model, and stratification is studied separately by analyzing both stratified and non stratified cases. The effect of the three models is investigated by implementing a vertical one dimensional water column model and examining resulting velocity and concentration profiles. This is done for a constant water level gradient, resulting in steady-state profiles. Newton's method is implemented with a non-uniform grid to ensure converged solutions.
Neglecting stratification effects, the parabolic viscosity model and k-ε viscosity model yield similar velocity and sediment concentration profiles. In contrast, the constant eddy viscosity model produces weaker flow and higher sediment concentrations. When stratification is included, all eddy viscosity formulations give qualitatively the same results. The suppression of turbulence leads to damped eddy viscosity profiles and sediment settles more easily. When the concentration at the bed is higher than a critical concentration, the concentration where hindered settling attains its maximum, a lutocline may form. This is a sharp transition between a clear upper layer and dense lower layer. All three viscosity profiles show the presence of this lutocline for certain parameter regimes. If the lutocline is present, the location is near the region where the turbulence is most strongly suppressed. Furthermore, the concentration where the lutocline forms can be approximated by the value where the hindered settling term reaches its maximum.
Neglecting stratification effects, the parabolic viscosity model and k-ε viscosity model yield similar velocity and sediment concentration profiles. In contrast, the constant eddy viscosity model produces weaker flow and higher sediment concentrations. When stratification is included, all eddy viscosity formulations give qualitatively the same results. The suppression of turbulence leads to damped eddy viscosity profiles and sediment settles more easily. When the concentration at the bed is higher than a critical concentration, the concentration where hindered settling attains its maximum, a lutocline may form. This is a sharp transition between a clear upper layer and dense lower layer. All three viscosity profiles show the presence of this lutocline for certain parameter regimes. If the lutocline is present, the location is near the region where the turbulence is most strongly suppressed. Furthermore, the concentration where the lutocline forms can be approximated by the value where the hindered settling term reaches its maximum.
The sea and the shoreline form a complex ecosystem driven by tides. So far, studies often ignore the moving boundary caused by these tides. The focus of this thesis to incorporate this boundary by using a coordinate transformation and a time-explicit numerical method. To achieve this, first the one-dimensional shallow water equations are derived from the 3D Navier Stokes equations. Then these 1D equations are non-dimensionalized and the coordinate transformation is done. This results in a system of non-linear equations. The seabed is modelled as a straight line. At the seaward side there is a periodic forced wave and at the landward side the water depth is 0. The time-explicit numerical method of Lax-Friedrichs is used. This method is stable under a more restricted Courant-Friedrichs-Lewy condition and is convergent for refined grids. For the Ameland inlet system the water depth, velocity and length of the basin results are calculated and compared to a simplified model and complemented by a Fourier analysis. The results are realistic (constant in the beginning of the basin with visible non-linearities at the landward side). An analysis is done to understand how the model behaves for dierent physical parameters, such as: the amplitude of the periodically forced wave, the undisturbed water depth, the length of the basin and the resistance. The model remains stable and the results are realistic.
...
The sea and the shoreline form a complex ecosystem driven by tides. So far, studies often ignore the moving boundary caused by these tides. The focus of this thesis to incorporate this boundary by using a coordinate transformation and a time-explicit numerical method. To achieve this, first the one-dimensional shallow water equations are derived from the 3D Navier Stokes equations. Then these 1D equations are non-dimensionalized and the coordinate transformation is done. This results in a system of non-linear equations. The seabed is modelled as a straight line. At the seaward side there is a periodic forced wave and at the landward side the water depth is 0. The time-explicit numerical method of Lax-Friedrichs is used. This method is stable under a more restricted Courant-Friedrichs-Lewy condition and is convergent for refined grids. For the Ameland inlet system the water depth, velocity and length of the basin results are calculated and compared to a simplified model and complemented by a Fourier analysis. The results are realistic (constant in the beginning of the basin with visible non-linearities at the landward side). An analysis is done to understand how the model behaves for dierent physical parameters, such as: the amplitude of the periodically forced wave, the undisturbed water depth, the length of the basin and the resistance. The model remains stable and the results are realistic.