On the
**Forward** tab you can select the solver for the forward simulation
and specify the parameters for the solving process.

A detailed description of parameters is given in Forward section.

The numerical parameters determine the way
the linear system equations are solved by *CrysMAS*.

*CrysMAS* uses several linear system equations solvers. The user can optionally
select which of them to be used in simulation.

Iterative solvers:

**BiCG**: Biconjugate gradient iteration.**STABBiCG**: Biconjugate gradient iteration stabilized.**CGS**: Conjugate gradient squared iteration.**IR**: Richardson iteration.

Direct solver:

**GSSV**

Two pre-conditioners can be used for an iterative solver:

**DIAG****ILU**(default)

A default solver is selected for any type of computation. For temperature calculation the direct GSSV solver is in some cases more stable than the conjugated gradients, also it is the only solver incorporated which can use multiple threads. The GSSV solver is advised for direct thermal computation always if the hybrid mesh is used. Credit for GSSV goes to Demmel et al. (see bibliography). If you are interested in the meaning of the details of the GSSV parameters, please consult the SuperLU documentation available on the web, usually the default parameters are sufficient.

In case of hybrid mesh the stacked iteration procedure is running,
whereby the solver of the global thermal problem from the list
above on both types of the meshes is running alternately with the other
solver which is running only on the block-structured mesh. The
solver for the block-structured mesh is the SIP solver (Strongly
Implicit Procedure), see the paper of Stone in
*Bibliography
*.

The setting of numerical parameters for the SIP solver takes place
not in the **Forward** dialog button but in the
dialog buttons created automatically after the structured mesh was
generated. In case if no convection is computed on the structured
mesh the SIP solver is not used. The enthalpy transport equation is
discretized on the structured mesh in the same manner as in case of
the SIP solver. The matrix resulted from the discretization is
passed to the selected solver from the above list for the solution
of the global heat transfer problem.

The SIP solver is extremely quick but is applicable only for rigorously diagonally occupied matrix equations which are resulted from the Finite Volume discretization on the block structured mesh. Another drawback of the SIP solver is, it works in the sequential mode, therefore its parallelization is possible only by domain decomposition and partitioning of the computational weights attached to each block.

Select

**Computation**>**Numerical parameter**>**Forward**tab.The default values were gained from experience and produce satisfactory results. Change the defaults only if inevitable.

If necessary change the defaults for the forward solver, the preconditioner, the numbers of allowed inner and outer iterations, the desired residuum, the residuum improvement factor and the forward relaxation factor.

Click on

**Apply**and**Close**.or

Click on

**OK**to apply the changes and to close the dialog.

The **front tracking** section allows to
enable fronttracking and to set corresponding parameters:

**Track interface**If this option is activated, in case of the unstructured mesh the enthalpy method will not be used for the phase transition. The mesh will be adjusted to the phase boundary in a two phase material when calculating temperature. The vertices next to the phase boundary are moved in order to match with the melting isothermal with a given accuracy, and latent heat is released at these vertices. Generally, the enthalpy method seems to be more robust, but only up to certain growth rates. The front tracking method on the unstructured mesh is stable up to higher growth rates, and more flexible in the sense that it can handel an arbitrary number and distribution of interfaces. However, sometimessituations arise where the tracking process hangs, producing strongly distorted meshes. In these case, use the

**reset mesh**button to reset the mesh, and try again.If the phase transition is modeled by means of the hybrid mesh, then the button

**Track interface**activates the phase tracking procedure. The region boundary and the meshes inside are deformed so that vertices of the region boundary between the crystal and the melt regions will coincide with the isotherm line crossing the triple point in the converged solution. The phase tracking method works in both Czochralski (crystal over the melt) and Bridgmann (crystal under the melt) configurations.**Underrelaxation:**The computed shift of the vertices at the region boundary is reduced by this factor, in the case of the unstructured mesh also the release of latent heat is under-relaxed by this factor. The deceleration of the phase boundary movement is necessary in order to stabilize the computation. To quick interface movement may lead to the degraded numerical mesh.**Start Residual:**This parameter works only in conjunction with the phase tracking procedure on the hybrid mesh. The phase tracking is activated only if the normed residual of the enthalpy equation becomes less than the prescribed value.