The LES is the direct numerical solving of the full instationnary Navier-Stockes equations. It requires extremely high CPU resources and it is mostly used for the aerodynamics of planes and cars
k-ε Turbulence model
Combinated with the logarithmical wall law is the standard approximation. It allows to calculate with a coarse mesh, however it systematically underestimates the heat transfer
SST k-ω, Low-Reynolds turbulence model
The shear stress transport (SST) formulation is more accurate than the standard models regarding speed profiles close to walls, heat transfer and recirculation, it requires a very fine high quality hexahedral mesh close to the walls. For complex geometries the meshing set-up is tideous and the calculation time is long.
CFX, Fluent and STAR-CCM® are the most well known commercial software. They are comprehensive, can be used for most applications and offer several turbulence models. The body-fitted mesh generation is the most time consuming operation of the whole process. Easy geometries can be entirely meshed with hexahedrons. Meshes of complex geometries combine hexahedrons close to the walls with tetrahedrons in the rest of the computational domain.
Intensive CFD users such as the developpers of steam turbines, gas turbines or jet engines always prefer to use their own programs, which allow faster designs, more adapted turbulence models or a better integration is the company design process. A senior CFD Engineer can usually use different programs and knows which ones should be used to solve the problem.
FloEFD®, developed by Mentor Graphics, combines cartesian mesh generators and solver capabilities, The
standard version of FloEFD is offered embedded in the CAD program Solidworks®;
other versions include an embeddment in CATIA.v5, Siemens NX, Solid Edge, Creo
or Inventor ®.
FloEFD can work with poor quality CAD, for example when two parts collide with each other. Thanks to the integration in the CAD system and the bypass of the mesh generation, it is possible for draughtsmen and engineers without CFD experience to start-up simple calculations.
The ability to quickly simulate a fan is a strong advantage, for which only the input of the dependency between mass flows and pressure generation, inside and outside diameter, and rotational speed are required.
The mesh is interpolated to the surfaces through automatic generated cut cells; the imprecision of the interpolation decreases rapidly as the number of cells is increased. It is highly recommended that the end calculations be performed with a higher number of cells in order to increase the precision. FloEFD offers the k-ε turbulence model with wall functions derived from the profile of Van Driest. For this approximation, the boundary layer region close to the wall is either handled as a thin or as a thick boundary layer. The Van Dreist profile is a reliable way to approximate the boundary layer, with which the regular user should be satisfied.
COMSOL® is an extensive software for the coupled calculation of the electromagnetism, heat transfer, flow, acoustics, and stress. The Multiphysics capabilities is a strong advantage, however users usually use coarse mesh unstructured with tetrahedrons cells: an acceptable precision for the flow and the heat transfer cannot be expected.
Power Electronics Cooling Calculation at Room Level
you will learn: - what does Room Level mean, - how to asses the working point of a fan, - how to asses the cooling and heating without calculating the heat transfer with the flow.
Article about E-Cooling in Mai 2017
periodical Engineering Edge
(article on pages 18-19)