Smoothed CFL Reference

The Smoothed CFL time-step provider is similar to the CFL time-step provider, but adds smoothing in space. This smoothing is to reduce the potentially detrimental effect of isolated and strongly locally-varying mesh length scales on the overall use of the Courant Number as the time-step size driver.

The Smoothed CFL time-step provider is available when the Eulerian Multiphase (EMP), Volume of Fluid (VOF) or the Mixture Multiphase (MMP) model is activated in the physics continuum.

The Smoothed CFL time-step provider carries out volume-weighted smoothing in the same way as the Free Surface CFL time-step provider. See Free Surface CFL Reference.

The Smoothed CFL time-step provider continues to be evident when used in simulations involving boiling phenomena.

Table 1. Smoothed CFL Reference
Provided By	Right-click Continua > Physics 1 > Models > Adaptive Time-Step > Time-Step Providers and select New > Smoothed CFL
Example Node Path	Continua > Physics 1 > Models > Adaptive Time-Step > Time-Step Providers > Smoothed CFL
Requires	Time: Implicit Unsteady or PISO Unsteady Material: Multiphase. Multiphase Models: Eulerian Multiphase (EMP), Volume of Fluid (VOF) or Mixture Multiphase (MMP). Material: Gas, Liquid, Multi-Component Gas, Multi-Component Liquid or Multiphase > Two-Phase Thermodynamic Equilibrium. Flow: Any.
Properties	Enable/Disable, Max Condition Limit, Smoothing Steps, Velocity Treatment, Cut-off Percentage. See Smoothed CFL Properties.
Activates	Field Functions	Smoothed CFL Proposal See Field Functions.

Smoothed CFL Properties

Enable/Disable

See Adaptive Time-Step Provider Common Properties.

Max Condition Limit

The maximum value of the time-step condition. This value is $Φ_{m a x, l i m i t}$ that is used in Eqn. (95). The default value for Eulerian Multiphase (EMP) is 20, and 0.8 for the other models.

Smoothing Steps

The number of smoothing steps. This value specifies the number of times that the volume-weighted smoothing operation (Eqn. (94)) is performed.

Velocity Treatment

Applies to Eulerian Multiphase (EMP) simulations only. Prescribes the velocity that is used in the computation of the CFL number. The available formulation options are:

Phase Maximum: Uses the maximum phase velocity $\max v_{i}$ in the computation of the CFL number.
Mixture: Uses the phase volume average velocity in the computation of the CFL number, and is defined as $\sum v_{i} α_{i}$ , where $α_{i}$ is the volume fraction of phase i. This is the default.

Cut-off Percentage

Applies to Mixture Multiphase (MMP) and Volume of Fluid (VOF) simulations only.

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 $t_{q} (p)$ , such that for a given $p$ :

Figure 1. EQUATION_DISPLAY

\frac{# [c e l l s : Δ t_{c} \leq t_{q} (p)]}{# [a l l c e l l s]} \approx p %

(101)

The time-step is then set as $t_{q} (p)$ 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

The following field function is available when the Smoothed CFL time-step provider is activated.

Smoothed CFL Proposal: The local Smoothed CFL time-step proposal for each individual cell.