3D CFD Calculation model

During the concept simulation phase, the CFD model can be strongly simplified to ensure a converged solution is reached. The simulation of the preliminary design might requires more details and an increase of the computational domain. Only the calculations of the final design should be executed and documented with the highest precision and the highest cell number.

STEP and Parasolid format

Each CAD program (Solid Edge, Siemens NX, CATIA, Solidworks, Autodesk Inventor …) has its own data format. Any program can export to the universally readable STEP format. Errors often occur when converting 3D parts to the neutral STEP format. All commands are no longer available and can therefore neither be changed nor suppressed. Parasolid is a modeling kernel for 3D CAD systems that was taken over by Siemens PLM Software. There are hundreds of Parasolid based programs, the best known being Solid Edge, Solidworks and NX Siemens. The Parasolid "x-t" format is the best choice for exchanging CAD models between supporting CAD systems.

Import correction and simplification of the CAD Model

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 such as screws, small holes and fillets must be suppressed in order to avoid errors when building the model and to reduce the computing time.
Nowadays CAD is generated as volumes. Some CFD Toolslike CFX or OpenFoam still require surfaces as a geometry input; the conversion is made in additional software like Ansys Design Modeler, Spaceclaim, Salomeor Ansa. Some CFD tools are integrated in a CAD tool, so the tedious import and export of CAD data is skipped. Both Solidworks Flow Simulation and Flotherm XT are fully embedded in Solidworks, but not natively integrated in other 3D CAD systems. FloEFD is a better choice if a CAD system other than Solidworks is used, as FloEFD can also be integrated with PTC Creo, CATIA v5, Pro/ENGINEER Wildfire, Autodesk Inventor, Solid Edge and Siemens NX.

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 of the CFD calculation

The calculation is converged when both the mass and the 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 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 reached a converged solution.
Pressure contours for a radial fan