Segregated VOF Solver Reference

Controls the strategy for volume fraction transport in solver iterations. The options are:

• Single-Step

  When this solution strategy is selected, the Segregated VOF solver performs a single step per time-step. This is the default. See Single-Step Solver Properties.

• Explicit Multi-Step (Deprecated)

  Note	The Explicit Multi-Step solution strategy option is deprecated and will be removed in a future version.

  When this solution strategy is selected, the Segregated VOF solver performs multiple steps per time-step. This option applies temporal sub-cycling to the transport of volume fraction and can improve the resolution of the interface between two phases. The explicit scheme imposes a strict CFL condition in the free surface cells, where $\frac{CFL}{N_{\exp }}\le 1$ , as it requires to resolve all time scales. The explicit multi-step solver limits the sub-step time-step size to the HRIC limit to determine the number of required sub-steps. However, in some dynamic free surface simulations, the number or required sub-steps varies a lot, occasionally resulting in an excessive number, which affects the CPU time. Multi-stepping can be used with Overset Mesh, but is not compatible with Mixing Plane interfaces.

  Explicit multi-stepping can be used with Overset Mesh, but is not compatible with Mixing Plane interfaces.

  The corresponding child node lets you specify the appropriate settings. See Explicit Multi-Step (Deprecated) Solver Properties.

• Implicit Multi-Step

  When this solution strategy is selected, the Segregated VOF solver performs multiple steps per time-step.

  The implicit multi-step solver is not bound to a strict CFL condition and volume fraction transport does not limit the global time step size. With this option, a fixed user-specified number of implicit sub-steps are used for the solution of the volume fraction transport equation. Additionally, the implicit multi-step solver allows the interface to smear when the CFL number exceeds the HRIC limit.

  If the global time-step size is small enough to resolve physics sufficiently well, but too large to fulfill HRIC stability requirements, implicit sub-stepping with an appropriate number of sub-steps is a cheaper alternative to reducing the global time-step size. Since the methodology is not bound to a time-step size restriction, it is considered as a better alternative to the explicit multi-stepping approach when dealing with highly dynamic simulations.

  The corresponding child node lets you specify the appropriate settings. See Implicit Multi-Step Solver Properties.

  Multi-stepping is compatible with the Implicit Unsteady solver using first-order Temporal Discretization or with the PISO Unsteady solver. Multi-stepping is not compatible with second-order Temporal Discretization.

  Note	You are not advised to use multi-stepping with the Steady model. For steady-state simulations use the Single-Step solution strategy.

Controls the frequency of volume fraction transport in solver iterations. The options are:

• First And Last Iteration

  Available for PISO Unsteady simulations only. This option is the default setting.

• Last Iteration

  Available for PISO Unsteady simulations only. This option can be useful when the pressure, velocity, and volume fraction fields are very loosely coupled.

• Every Iteration

  For Implicit Unsteady and Steady simulations, this option is the only setting available. For PISO Unsteady simulations, this option can help to increase stability at the cost of increased computational runtime.

When the Implicit Unsteady model is selected, the default value is 0.9. When the PISO Unsteady model is selected, this value is always set to 1.

When the Implicit Unsteady model is selected, the default value is 0.95. When the PISO Unsteady model is selected, this value is always set to 1.

The minimum number and maximum number of sub-steps allowed. The default range is [2, 200].

If the number of sub-steps with a size computed from the specified value for the sub-step CFL number is insufficient to advance the volume fraction field about the global time-step size, a larger sub-step CFL number than specified is used. However, this may cause mass conservation issues. Corresponds to $N_{\exp }$ in Eqn. (2629).

The target CFL number for the sub-steps. This value is used to determine the sub-step time-step size. The valid range is 0.0 to 0.5. The default value is 0.15. This value is ${CFL}_{\max }^{*}$ in Eqn. (2634).

This option is relevant only if the maximum CFL number in the cells belonging to the interface is larger than the Max Number of Steps value times the Courant Number value. When this option is activated, the volume fraction field is advanced in time only about Max Number of Steps times the sub-step time-step size that is computed from the specified target value for the sub-step CFL number. This time-step is smaller than the global time-step size. You would activate this property only if a quasi-steady state solution is approached using an unsteady run.

At each iteration, this property governs the extent to which the newly computed solution supplants the old solution at each sub-step.

When the Implicit Unsteady model is selected, the default value is 0.9. When the PISO Unsteady model is selected, this value is always set to 1.

The specified fixed number of implicit sub-steps. The specified number of steps is not bound to a strict CFL number limitation near the interface. Hence, when the sub-stepping CFL number exceeds the HRIC limit, interface smearing is allowed without any adverse effects on mass conservation.