3D Calculation model

 

The CFD problem should be strongly simplified at the beginning, in order to ensure a solution is reached. Later, the final precise basis calculation can be set up. Only the end calculations should be executed and documented with the highest cell number.

Geometry preparation

The starting point of any flow analysis is to describe the geometry of the mechanical system; therefore an electronic copy of the 3D geometry is mandatory. I recommend generating it with a professional CAD tool. If the model has been generated by others, it is necessary to correct and to simplify it. Details like bolts shoud be suppressed as they would unnecessary prolongate the calculation time.
Each CAD program has its own file format; there is a Solidworks, a Sold Edge, a CATIA, a Siemens NX format … Each can however export in the universally readable STEP format. Nowadays CAD is generated as volumes. Some CFD Tools like CFX or OpenFoam still require surfaces as a geometry input; the conversion is made in additional software like Ansys Design Modeler or Ansa. The import and export of geometry data has been conveniently skipped in FloEFD as the CFD is embedded in the selected CAD tool.

Boundary conditions and parameters

After the calculation mesh has been generated, the calculation can be set up in the CFD tool:
The boundary conditions must be given, mostly as inlet mass flow or inlet pressure; the CFD Software FloEFD offers as an alternative the possibility to input fan characteristics. If the heat transfer is to be calculated as well, then heat sources or fixed temperatures must be input. For the simulation of a rotating region, only the definition of the rotating region and of the rotational speed are necessary; the program will add source terms in the equations of movements in order to take into account the effects of the Coriolis and centrifugal forces. Now the materials, numerical parameters and convergence criteria can be defined and the calculation started.

Convergence

The calculation is converged when the mass and energy conservation are reached and when the pressure and speed values hardly change as the calculation progresses. The standard defined numerical parameters of the calculation tool can be changed in order to reach a convergent solution more easily; however giving low order parameters will result in a widespread unphysical spread of the results. Some very demanding applications will need advanced CFD knowledge to be able to fine-tune the meshing and solver settings to converge on a solution.

 

 
     Pressure contours for a radial fan 

Recorded Webinar:
Cooling of a Wind Power Station IGBT 
You will learn: - •Why frequency converters need cooling; -•Why the thermal resistance of the Thermal Interface Materials matters; - •How to set-up a transient case for low frequencies ...  

 


New page available about transient calculations:

IGBT chips at low frequencies

 

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