We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel ... more We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel flows at low magnetic Reynolds numbers (i.e. in the framework of the quasi-static approximation where the Lorentz force is treated as an explicit contribution to the momentum balance). The computations are performed using a pseudospectral and a second-order collocated finite volume method. Two eddy-viscosity type models are compared for different mesh resolutions: the dynamic Smagorinsky (DSM) and the Wall-Adapting Local Eddy-viscosity (WALE) model. We examine in detail the contributions to the kinetic energy budget of each term appearing in the Navier-Stokes equations. In particular, the results show that the subgrid-scale dissipation measured in the finite volume simulations is systematically much lower than in the spectral ones.
We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel ... more We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel flows at low magnetic Reynolds numbers (ie in the framework of the quasi-static approximation where the Lorentz force is treated as an ...
Fluid–structure interactions are modelled by coupling the finite element fluid/ocean model ‘Fluid... more Fluid–structure interactions are modelled by coupling the finite element fluid/ocean model ‘Fluidity-ICOM’ with a combined finite–discrete element solid model ‘Y3D’. Because separate meshes are used for the fluids and solids, the present method is flexible in terms of discretisation schemes used for each material. Also, it can tackle multiple solids impacting on one another, without having ill-posed problems in the resolution of the fluid’s equations. Importantly, the proposed approach ensures that Newton’s third law is satisfied at the discrete level. This is done by first computing the action–reaction force on a supermesh, i.e. a function superspace of the fluid and solid meshes, and then projecting it to both meshes to use it as a source term in the fluid and solid equations. This paper demonstrates the properties of spatial conservation and accuracy of the method for a sphere immersed in a fluid, with prescribed fluid and solid velocities. While spatial conservation is shown to be independent of the mesh resolutions, accuracy requires fine resolutions in both fluid and solid meshes. It is further highlighted that unstructured meshes adapted to the solid concentration field reduce the numerical errors, in comparison with uniformly structured meshes with the same number of elements. The method is verified on flow past a falling sphere. Its potential for ocean applications is further shown through the simulation of vortex-induced vibrations of two cylinders and the flow past two flexible fibres.
Far-offshore wind turbines are attractive in view of harnessing high-speed winds and reducing imp... more Far-offshore wind turbines are attractive in view of harnessing high-speed winds and reducing impact on population. When the sea is hundreds of metres deep, drilling wind turbines down to the seabed is too expensive. Today’s bottom-mounted foundations could be replaced by floating platforms, which can minimise the lateral wave-loads acting on the wind turbine and reduce the foundation cost in deep water. Computer models capable of calculating the motion of a full floating wind turbine are at an early stage of development. An efficient strategy to minimise the computational cost is also lacking. This contribution highlights how the motion of a floating wind turbine, and its interaction with the ocean, can be predicted by means of computer-model simulations.
We assess the performances of three different subgrid scale models in large eddy simulations (LES... more We assess the performances of three different subgrid scale models in large eddy simulations (LES) of turbulent channel flows. Two regimes are considered: hydrodynamic and magnetohydrodynamic (i.e. in the presence of a uniform wall-normal magnetic field). The simulations are performed using a second-order finite volume (FV) and a pseudo-spectral (PS) method. The LES results are compared with under-resolved results (obtained without model) and direct numerical simulations (DNS). We show that discretization errors affect the FV results in two ways: (1) the flow statistics differ from the spectral estimates in the absence of subgrid model; and (2) the eddy viscosity systematically underestimates the spectral value in the presence of a subgrid model. This is mainly because numerical errors affect the computation of the derivatives, and in particular, they lower the discrete strain rate appearing in the viscous term and the subgrid model. The magnitude of the numerical errors further varies with the mesh resolution and the intensity of the turbulent fluctuations. In this manuscript, a novel formulation of the discrete strain, which was proven successful in homogeneous isotropic turbulence, is used to compute the FV eddy viscosities. Although the average norm of the discrete strain is largely increased using this formulation, the effect on the flow dynamics is marginal. This is explained by analysing the contribution of each term of the discrete kinetic energy balance. It is shown how the underestimation of the discrete viscous dissipation inhibits the effect of the improved discrete strain.
We analyze the impact of discretization errors on the performance of the Smagorinsky model in lar... more We analyze the impact of discretization errors on the performance of the Smagorinsky model in large eddy simulations (LES). To avoid difficulties related to solid boundaries, we focus on decaying homogeneous turbulence. It is shown that two numerical implementations of the model in the same finite volume code lead to significantly different results in terms of kinetic energy decay, time evolutions of the viscous dissipation and kinetic energy spectra. In comparison with spectral LES results, excellent predictions are however obtained with a novel formulation of the model derived from the discrete Navier–Stokes equations. We also highlight the effect of discretization errors on the measurement of physical quantities that involve scales close to the grid resolution.
We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel ... more We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel flows at low magnetic Reynolds numbers (i.e. in the framework of the quasi-static approximation where the Lorentz force is treated as an explicit contribution to the momentum balance). The computations are performed using a pseudospectral and a second-order collocated finite volume method. Two eddy-viscosity type models are compared for different mesh resolutions: the dynamic Smagorinsky (DSM) and the Wall-Adapting Local Eddy-viscosity (WALE) model. We examine in detail the contributions to the kinetic energy budget of each term appearing in the Navier-Stokes equations. In particular, the results show that the subgrid-scale dissipation measured in the finite volume simulations is systematically much lower than in the spectral ones.
We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel ... more We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel flows at low magnetic Reynolds numbers (ie in the framework of the quasi-static approximation where the Lorentz force is treated as an ...
Fluid–structure interactions are modelled by coupling the finite element fluid/ocean model ‘Fluid... more Fluid–structure interactions are modelled by coupling the finite element fluid/ocean model ‘Fluidity-ICOM’ with a combined finite–discrete element solid model ‘Y3D’. Because separate meshes are used for the fluids and solids, the present method is flexible in terms of discretisation schemes used for each material. Also, it can tackle multiple solids impacting on one another, without having ill-posed problems in the resolution of the fluid’s equations. Importantly, the proposed approach ensures that Newton’s third law is satisfied at the discrete level. This is done by first computing the action–reaction force on a supermesh, i.e. a function superspace of the fluid and solid meshes, and then projecting it to both meshes to use it as a source term in the fluid and solid equations. This paper demonstrates the properties of spatial conservation and accuracy of the method for a sphere immersed in a fluid, with prescribed fluid and solid velocities. While spatial conservation is shown to be independent of the mesh resolutions, accuracy requires fine resolutions in both fluid and solid meshes. It is further highlighted that unstructured meshes adapted to the solid concentration field reduce the numerical errors, in comparison with uniformly structured meshes with the same number of elements. The method is verified on flow past a falling sphere. Its potential for ocean applications is further shown through the simulation of vortex-induced vibrations of two cylinders and the flow past two flexible fibres.
Far-offshore wind turbines are attractive in view of harnessing high-speed winds and reducing imp... more Far-offshore wind turbines are attractive in view of harnessing high-speed winds and reducing impact on population. When the sea is hundreds of metres deep, drilling wind turbines down to the seabed is too expensive. Today’s bottom-mounted foundations could be replaced by floating platforms, which can minimise the lateral wave-loads acting on the wind turbine and reduce the foundation cost in deep water. Computer models capable of calculating the motion of a full floating wind turbine are at an early stage of development. An efficient strategy to minimise the computational cost is also lacking. This contribution highlights how the motion of a floating wind turbine, and its interaction with the ocean, can be predicted by means of computer-model simulations.
We assess the performances of three different subgrid scale models in large eddy simulations (LES... more We assess the performances of three different subgrid scale models in large eddy simulations (LES) of turbulent channel flows. Two regimes are considered: hydrodynamic and magnetohydrodynamic (i.e. in the presence of a uniform wall-normal magnetic field). The simulations are performed using a second-order finite volume (FV) and a pseudo-spectral (PS) method. The LES results are compared with under-resolved results (obtained without model) and direct numerical simulations (DNS). We show that discretization errors affect the FV results in two ways: (1) the flow statistics differ from the spectral estimates in the absence of subgrid model; and (2) the eddy viscosity systematically underestimates the spectral value in the presence of a subgrid model. This is mainly because numerical errors affect the computation of the derivatives, and in particular, they lower the discrete strain rate appearing in the viscous term and the subgrid model. The magnitude of the numerical errors further varies with the mesh resolution and the intensity of the turbulent fluctuations. In this manuscript, a novel formulation of the discrete strain, which was proven successful in homogeneous isotropic turbulence, is used to compute the FV eddy viscosities. Although the average norm of the discrete strain is largely increased using this formulation, the effect on the flow dynamics is marginal. This is explained by analysing the contribution of each term of the discrete kinetic energy balance. It is shown how the underestimation of the discrete viscous dissipation inhibits the effect of the improved discrete strain.
We analyze the impact of discretization errors on the performance of the Smagorinsky model in lar... more We analyze the impact of discretization errors on the performance of the Smagorinsky model in large eddy simulations (LES). To avoid difficulties related to solid boundaries, we focus on decaying homogeneous turbulence. It is shown that two numerical implementations of the model in the same finite volume code lead to significantly different results in terms of kinetic energy decay, time evolutions of the viscous dissipation and kinetic energy spectra. In comparison with spectral LES results, excellent predictions are however obtained with a novel formulation of the model derived from the discrete Navier–Stokes equations. We also highlight the effect of discretization errors on the measurement of physical quantities that involve scales close to the grid resolution.
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