CFL Reference

Limits the time-step size to meet the specified target criteria for the Convective Courant Number. This time-step provider is appropriate for transient flow problems.

The CFL time-step provider sets the time-step based on the CFL number, the convective time scale. The time-step is adjusted so that the actual CFL number approaches either the specified maximum CFL number or the specified mean CFL number, whichever yields the minimum time-step. This time-step provider is available when the Segregated Flow model or the Coupled Flow model is activated in the physics continuum.

Table 1. CFL Reference
Provided By	Right-click Continua > Physics 1 > Models > Adaptive Time-Step > Time-Step Providers and select New > CFL
Example Node Path	Continua > Physics 1 > Models > Adaptive Time-Step > Time-Step Providers > CFL
Requires	Time: Implicit Unsteady or PISO Unsteady Material: Multiphase. Multiphase Models: 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, Target Mean CFL Number, Target Max CFL Number, Cut-off Percentage. See CFL Properties.
Activates	Field Functions	CFL (Max.) Proposal, CFL (Mean) Proposal See Field Functions.

CFL Properties

Enable/Disable

See Adaptive Time-Step Provider Common Properties.

Target Mean CFL Number

The default value is 0.5.

Target Max CFL Number

For coupled flow, this is the parameter

{C F L}_{m a x}^{t a r}_{}

in Eqn. (971). The default value is 5.0.

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 $t_{q} (p)$ 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 %

(93)

The time-step is then set as $t_{q} (p)$ and any local time-steps $Δ t_{c}$ 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 functions are available when the CFL time-step provider is activated.

CFL (Max.) Proposal: The local time-step proposal for the target maximum CFL condition, computed for each individual cell.

CFL (Mean) Proposal: The time-step proposal for the target mean CFL condition, computed for each individual cell.