Free Surface Implicit Multi-Step Reference

The HRIC and MHRIC convection require a Courant number (CFL) condition to be fulfilled in order to maintain a sharp interface (see High-Resolution Interface Capturing (HRIC)). When implicit multi-stepping is used, the Courant number condition from Eqn. (2593) for flow time-step $\mathrm{\Delta }t$ is modified to the following:

where $\tau =\frac{\mathrm{\Delta }t}{N_{imp}}$ is the effective convective time-step for the $N_{imp}$ number of sub-steps.

The Free Surface Implicit Multi-Step time-step provider determines a time-step size $\mathrm{\Delta }t$ such that the above CFL condition in Eqn. (98) is achieved for all cells in the vicinity of the interface.

On meshes with very small cells, or with ill-converged or spurious velocity fields, the CFL condition given by Eqn. (98) can result in very small time-step predictions that increase the runtime of the simulation unnecessarily. To remedy this problem, you can adjust the Cut-off Percentage property, which makes use of a quantile-based filtering approach.

The Free Surface Implicit Multi-Step time-step provider is available when the Volume of Fluid (VOF) model is activated in the physics continuum.

Free Surface Implicit Multi-Step Properties

Enable/Disable

See Adaptive Time-Step Provider Common Properties.

Cut-off Percentage

Specifies a percentage $p$ of small/bad cells to be ignored by the Adaptive Time-Step model. The prescribed cut-off percentage is used to find the quantile $t_q\left(p\right)$ , such that for a given $p$ :

$$\frac{\#[cells:\mathrm{\Delta }{t}_c\le t_q(p\left)\right]}{\#[allcells]}\approx p\%$$

(99)

The time-step is then set as $t_q\left(p\right)$ and any local time-steps that the provider proposes below this value are ignored by the model. The range for this property is 0 (default) to 100.

Field Functions

Free Surface Implicit Multi-Step Proposal

The local Free Surface Implicit Multi-Step time-step proposal for each individual cell.