Volume Fraction Solver for Multiphase

The discretized form of the continuity equation (Eqn. (1904)), is solved for each phase.

To make sure that Eqn. (1903) is satisfied after solving the volume fraction equations for all phases, the volume fraction of each phase in a cell is normalized by the sum of the volume fractions of all the phases in that cell.

Volume Fraction Solver Properties

The default settings use only the implicit under-relaxation factor for robustness. In some scenarios, such as a fine mesh, it can take a while for the Volume Fraction to propagate through the system. In these cases, you can split the under-relaxation property value between the two factors. For example, to specify an overall relaxation factor = 0.15, you can specify the combination Implicit Under-Relaxation Factor = 0.6, Explicit Under-Relaxation Factor = 0.25. This combination can help the volume fraction to converge faster without upsetting the overall convergence of the simulation.

The same approach can be used for S-Gamma under-relaxation. In general, it is good practice to use the same under-relaxation factors for the Volume Fraction and S-Gamma equations since their ratio is a key parameter in interphase interactions.

Implicit Under-Relaxation Factor

Improves stability and convergence of the linear system by using the relaxation factor to increase diagonal dominance of the matrix.

The default value is 0.5.

Explicit Under-Relaxation Factor

This property specifies the multiplier that is applied to the provisional increment of the solution. Sharing the relaxation between implicit and explicit factors can be used to improve performance.

The default value is 1.0.

Overall Relaxation Factor

The product of the two under-relaxation factors (that is, Implicit Under-Relaxation Factor times Explicit Under-Relaxation Factor). This (read-only) value is a guide to the combined effect of the relaxation.

Number of Steps

Available when the Large Scale Interface Detection model is activated.

The specified fixed number of implicit sub-steps. Corresponds to $N_{imp}$ in Eqn. (2628).

With the Number of Steps set to 1, which is the default, the volume fraction calculation is performed at the same time-step as the other calculations and the CFL number near the interface is limited to 1. The implicit multi-stepping feature eliminates this restriction on the CFL number near the large scale interface.

To enable the implicit multi-step solver, set Number of Steps to a value greater than 1. This way, sub-stepping is carried out for the volume fraction transport equation with a reduced time-step, without limiting the global time-step size. Hence, when the sub-stepping CFL number exceeds the HRIC limit, interface smearing is allowed without any adverse effects on mass conservation. Additionally, the implicit multi-step solver allows the interface to smear when the CFL number exceeds the HRIC limit.

Increasing the number of sub-steps increases memory requirements as well as computational efforts. The range allowed for number of steps is from 1 to 20.

Note	Implicit multi-stepping is compatible only with the Implicit Unsteady solver using first-order Temporal Discretization. Implicit multi-stepping is not compatible with second order Temporal Discretization or steady-state simulations.

See Setting the Solver Parameters.

The properties in the Expert category are for a temporary debug situation, at the expense of simulation accuracy and higher memory usage. Do not change these properties unless you are thoroughly familiar with the Simcenter STAR-CCM+ discretization techniques.

Reconstruction Frozen

When On, Simcenter STAR-CCM+ does not update reconstruction gradients with each iteration, but rather uses gradients from the last iteration in which they were updated. Activate Temporary Storage Retained in conjunction with this property. This property is Off by default.

Reconstruction Zeroed

When On, the solver sets reconstruction gradients to zero at the next iteration. This action means that face values used for upwinding (Eqn. (905)) and for computing cell gradients (Eqn. (917) and Eqn. (918)) become first-order estimates. This property is Off by default. If you turn this property Off after having it On, the solver recomputes the gradients on the next iteration.

Solver Frozen

When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Temporary Storage Retained

When On, Simcenter STAR-CCM+ retains additional field data that the solver generates during an iteration. The particular data retained depends on the solver, and becomes available as field functions during subsequent iterations. Off by default.