*CFD

Keyword type: step

This procedure is used to perform a three-dimensional computational fluid dynamics (CFD) calculation.

There are seven optional parameters: STEADY STATE, TIME RESET, TOTAL TIME AT START, COMPRESSIBLE, TURBULENCE MODEL, SCHEME and SIMPLEC.

The parameter STEADY STATE indicates that only the steady state should be calculated. If this parameter is absent, the calculation is assumed to be time dependent and a transient analysis is performed.

The initial time increment and time step period are interpreted mechanical time increment and mechanical time step. For each mechanical time increment a CFD calculation is performed consisting of several CFD time increments. Therefore, the initial CFD time increment cannot exceed the initial mechanical time increment. CFD time increments are usually much smaller than the mechanical time increments. The CFD calculation is performed up to the end of the mechanical time increment (if the calculation is transient) or up to steady state conditions (if the CFD calculation is a steady state calculation).

The parameter TIME RESET can be used to force the total time at the end of the present step to coincide with the total time at the end of the previous step. If there is no previous step the targeted total time is zero. If this parameter is absent the total time at the end of the present step is the total time at the end of the previous step plus the time period of the present step (2nd parameter underneath the *CFD keyword). Consequently, if the time at the end of the previous step is 10. and the present time period is 1., the total time at the end of the present step is 11. If the TIME RESET parameter is used, the total time at the beginning of the present step is 9. and at the end of the present step it will be 10. This is sometimes useful if transient coupled temperature-displacement calculations are preceded by a stationary heat transfer step to reach steady state conditions at the start of the transient coupled temperature-displacement calculations. Using the TIME RESET parameter in the stationary step (the first step in the calculation) will lead to a zero total time at the start of the subsequent instationary step.

The parameter TOTAL TIME AT START can be used to set the total time at the start of the step to a specific value.

The parameter COMPRESSIBLE specifies that the fluid is compressible. Default is incompressible.

For 3D fluid calculations the parameter TURBULENCE MODEL defines the turbulence model to be used. The user can choose among NONE (laminar calculations; this is default), K-EPSILON, K-OMEGA and SST [49].

The parameter SCHEME is only important for compressible fluids. It selects the scheme to be used to calculate the facial quantities from the element center quantities for the convective terms in the governing equations. Right now, the user can choose between upwind difference (SCHEME=UD, default) and modified smart (SCHEME=MODSMART). Upwind difference has more false diffusion, however, the stability is better. Modified smart is a so called high resolution scheme [59]. It has less false diffusion but may lead to divergence in some cases. If modified smart does not diverge, it will give better results. For incompressible fluids a Gamma scheme [35] is used. This cannot be influenced by the user. It is also a high resolution scheme.

Finally, the parameter SIMPLEC indicates, if selected, that the SIMPLEC algorithm is to be used rather than the standard SIMPLE. Right now, the SIMPLEC scheme can only be selected for compressible flow. It usually leads to faster convergence [59].

First line:

- *CFD
- Enter any needed parameters and their values.

- Initial time increment. This value will be modified due to automatic incrementation, unless the parameter DIRECT was specified (default 1.).
- Time period of the step (default 1.).
- Minimum time increment allowed. Only active if DIRECT is not specified. Default is the initial time increment or 1.e-5 times the time period of the step, whichever is smaller.
- Maximum time increment allowed. Only active if DIRECT is not specified. Default is 1.e+30.
- Initial time increment for CFD applications (default 1.e-2). If left free, a proper initial increment is calculated.

Example: couette1

*CFD .1,1.,,,0.01

defines a CFD step. The second line indicates that the initial time increment is .1, the total step time is 1 and the CFD time increment is 0.01.

Example files: couette1per.