In the Species tab the parameters and discretization schema for species transport are set.
It is possible to define a reference concentration for the Boussinesq-approximation the solutal buoyancy term in the Navier-Stokes equations. By default this value is 0. In the most cases this is convenient. Only changes this value if you no what you are doing.
In CrysMAS two alternative discretization schema are available for species transport: A central schema and a up-winding technique with deferred correction. For details of the used numerical methods please refer to the text book J.H. Ferziger and M. Peric: "Computational methods for fluid dynamics", Springer, 1999 (ISBN 3-540-42074-6) or in the PHD-thesis of M.Hainke (reference given in the Bibliography ). The usage of the parameters is described in the following table:
Parameters | Description |
---|---|
upwinding (donor cell) | If the discretization schema is switched to upwinding, this stable but less accurate (discretization-error of first order) technique is used. This is the recommended setting for convection dominated problems with high solutal Peclet numbers (Pe>10). Use this setting whenever unphysical fluctuations in the species field occur during the calculation of the convective/diffusive transport. In order to improve accuracy for moderate solute Peclet numbers the solution obtained with the upwinding schema can be combined with the one of central difference technique (see below). The weight factor is given by the parameter Deferred correction . |
central schema | The central schema is more accurate alternative (discretization-error of second order) but suffers from stability problems in convection dominated flows (Pe >10). If unphysical fluctuations occur during calculations, there are two possibilities: Refine the mesh or switch to the upwinding schema (details described above). If central differences are used the parameter Deferred correction is ignored. |
Deferred correction | This represents the weight factor for combining the central difference and the upwinding solution. 0 corresponds to the full-upwind solution while 1 is equivalent to swith to the central schema with radio bottom. Please note: Not only the accuracy is improved with the a growing fraction of the central difference solution in the results, but also the susceptibility to instabilities is increased. |
Select Computation > Numerical parameter > Species tab.
The default values were gained from experience and produce satisfactory results. Change the defaults only if inevitable.
If necessary, change the discretization schema, deferred correction parameter etc..
Click on Apply and Close.
or
Click on OK to apply the changes and to close the dialog.
mesh. The solution of the transport problem can succeed on both structured and unstructured types of the mesh. The same single linear solver is applied to equations resulted from both types of the mesh. The solver parameters are passed from the considered dialog to the solver selected in the Forward dialog tab (temperature solver).