The CFD problem should be strongly simplified at the beginning 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.

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 Tools
like CFX or OpenFoam still require surfaces as a geometry input; the
conversion is made in additional software like Ansys Design Modeler,
Spaceclaim, Salome or 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.

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 exporting CAD geometry for CFD calculations.

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.

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 converge on a solution.