Free Surface Explicit Multi-Step Reference

Limits the time-step size to allow for a sharp interface when using the VOF Explicit Multi-Step (Deprecated) solver. This time-step provider is appropriate for free surface flows when the VOF interface crosses meshes that have different CFL numbers, such as from coarse mesh to fine mesh, or has a rapidly-changing velocity field.

A specific time-step provider is used for explicit multi-stepping because the free surface can cross several differently-sized cells during a single time-step. The time-step size must be calculated to ensure that the free surface crosses the target number of cells in the time-step. In the general case of arbitrarily varying cell sizes, this calculation requires the transport of a marker, which is the fundamental feature of this time-step provider.

In the single-stepping case, such a calculation is not necessary because the CFL number needs to be taken into account only in cells that belong to the free surface. The free surface cells are identified using the volume fraction gradient, which also captures one cell upstream of the surface so, therefore, no marker transport is required.

The target number of sub-steps is calculated as:

Figure 1. EQUATION_DISPLAY

K_{T} = \frac{N_{T}}{{C F L}_{\max}^{*}}

(97)

where

$N_{T}$ is the number of cells that the interface travels through in one time-step. This value is the Target Interface Propagation (# of cells) setting.
${C F L}_{\max}^{*}$ is the target Courant number for sub-steps. This value is the Courant Number setting in the Explicit Multi-Step (Deprecated) Solver. See Explicit Multi-Step (Deprecated) Solver Properties.

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

Table 1. Free Surface Explicit Multi-Step Reference
Provided By	Right-click Continua > Physics 1 > Models > Adaptive Time-Step > Time-Step Providers and select New > Free Surface Explicit Multi-Step
Example Node Path	Continua > Physics 1 > Models > Adaptive Time-Step > Time-Step Providers > Free Surface Explicit Multi-Step
Requires	Time: Implicit Unsteady or PISO Unsteady Material: Multiphase. Multiphase Models: Volume of Fluid (VOF). Solvers: Explicit Multi-Step (Deprecated)
Properties	Enable/Disable, Target Interface Propagation (# of cells). See Free Surface Explicit Multi-Step Properties.
Activates	Field Functions	Free Surface Explicit Multi-Step Proposal See Field Functions.

Free Surface Explicit Multi-Step Properties

Enable/Disable

See Adaptive Time-Step Provider Common Properties.

Target Interface Propagation (# of cells)

The target number of cells, $N_{T}$ in Eqn. (97). This is the number of cells that the interface is expected to travel through in one time step.

The range is 1 to infinity, the default is 2.

Field Functions

The following field function is available when the Free Surface Explicit Multi-Step time-step provider is activated.

Free Surface Explicit Multi-Step Proposal: The Free Surface Explicit Multi-Step time-step proposal.