Volume Of Fluid (VOF) Multiphase Model Reference

Volume of Fluid (VOF) Properties

Convection

Specifies the discretization scheme of the convective flux in the VOF transport equation.

Method	Corresponding Method Nodes
1st-Order Selects the first-order convection.	None.
HRIC The HRIC scheme is used in VOF simulations to maintain a sharp interface between the participating fluid phases. See High-Resolution Interface Capturing (HRIC).	HRIC The following properties are available: Sharpening Factor This factor is used to reduce numerical diffusion in the simulation. The valid values are 0.0 through 1.0. When the sharpening factor is set to 0.0, there is no reduction in numerical diffusion. This value is the default because, in most cases, the recommended 2nd-order discretization scheme is sufficient to achieve a sharp interface between the two phases. When the sharpening factor is set to 1.0, there is no numerical diffusion and, therefore, a very sharp interface is obtained. For some flows, such as surface-tension dominated flows or tank sloshing flows, it is recommended that you increase the sharpening factor to improve the resolution of the interface between the phases. However, this additional sharpening can result in a non-physical alignment of the free surface with the grid lines. If a non-zero value is specified, an additional term (Eqn. (2585)) is added to the VOF transport equation (Eqn. (2584)). This value is $C_{\alpha }$ in Eqn. (2585). Raising the sharpening factor increases the runtime necessary to preserve volume conservation. Angle Factor The angle factor $C_{\theta }$ in the HRIC convection discretization scheme for interface tracking. If the free surface is not smooth and not following the grid lines, increase its value. CFL_l The lower Courant number limit in the HRIC convection discretization scheme. This value is ${Co}_l$ in Eqn. (2594). CFL_u The upper Courant number limit in the HRIC convection discretization scheme. This value is ${Co}_u$ in Eqn. (2594). Enable HRIC Gradient Smoothing Uses a smoothed gradient to predict the upwind-upwind value in the HRIC scheme. This reduces the spurious oscillations that can appear in the volume fraction field In some situations, particularly when the mesh has a large aspect ratio. See HRIC Gradient Smoothing Properties. Adaptive Mesh Interpolation Available when the Adaptive Mesh model is selected Option Specifies the interpolation strategy for mapping the volume fraction field from coarse to fine cells. The following options are available: Injection: uses zero order interpolation to map the volume fraction field from coarse to fine cells. This is the default. If the Free Surface Mesh Refinement criterion is used, interface cells are not marked for refinement. Only the cells near the interface are marked. Sharp Reconstruction: uses sharp interpolation of the volume fraction field based on interface reconstruction. The volume fractions on the refined cells are obtained by calculating induced cut volumes. For more information, see Sharp Interface Reconstruction with Adaptive Mesh Refinement. If the Free Surface Mesh Refinement Criterion is used, both interface cells and the cells near the interface are marked for refinement. Example: The following images show the interpolation of a volume fraction field for refined interface cells using Injection or Sharp Reconstruction interpolation, respectively: Interpolation Strategy Original Volume Fraction Volume Fraction after AMR Refinement Injection Sharp Reconstruction
Modified HRIC The Modified HRIC is a composite scheme that uses the angle factor to blend the downwind, QUICK and upwind differencing schemes. This non-linear blend of differencing makes it less prone to (mesh-dependent) perturbations as it significantly reduces the contributions from the downwind scheme alone. As such, by default, the modified HRIC scheme, as a less compressive version of the HRIC scheme, will lead to smoother but also more diffusive solutions than the default HRIC scheme. However, it is also possible to adjust the level of compressiveness of the scheme and consequently, the sharpness of the interface. The modified HRIC scheme is beneficial for free-surface flow VOF simulations, with a particular focus on flows that are characterized by traveling or stationary waves. See High-Resolution Interface Capturing (HRIC).	Modified HRIC The following properties are available: CFL_l The lower Courant number limit in the Modified HRIC convection discretization scheme. This value is ${Co}_l$ in Eqn. (2594). CFL_u The upper Courant number limit in the Modified HRIC convection discretization scheme. This value is ${Co}_u$ in Eqn. (2594). Interface Sharpness Specifies the options for the level of compressiveness of the scheme in terms of the Downwind Limit above which the Modified HRIC scheme uses downwind differencing only. This is ${\xi }_D$ in Eqn. (2592). The following options are available: Default—the default modified HRIC differencing scheme, a less compressive version of the HRIC scheme. It is equivalent to a downwind limit value of ${\xi }_D=0.5$ . Low—low interface sharpness, that is, a large downwind limit equivalent to a value of ${\xi }_D=0.75$ . High—high interface sharpness, that is, a small downwind limit equivalent to a value of ${\xi }_D=0.25$ . This option is more compressive than defaults and leads to a sharper fluid interface. Specified—activates the Downwind Limit property. Use this parameter to specify the downwind limit value to increase or decrease the sharpness or diffusion of the fluid interface. Downwind Limit Available when the Specified interface sharpness method is selected. Specifies the downwind limit value to increase the sharpness or diffusivity of the fluid interface. The range is [0, 1]. Example: The following images show the volume fraction solution for different values of the downwind limit. Interface Sharpness Downwind Limit = 0.3 Downwind Limit = 0.7 Specified 注 The choice of ${\xi }_D$ depends on the specific physical system being modeled, and low values are not always appropriate. If the downwind limit is too low, the numerical simulation can become unstable and can require small time-steps or low under-relaxation factors for convergence.

Face Density Reconstruction

Sets the discretization scheme to reconstruct density at internal faces.

Method	Corresponding Method Nodes
1st-Order Uses first order face density reconstruction. This discretization scheme is computationally cheaper, but less precise. This option is the default selection. This option can be used to reduce the simulation time for simulations where a high level of consistency between different transport equations is less important.	None.
2nd-Order Uses second order face density reconstruction. This discretization scheme is computationally more expensive, but offers a high level of consistency between all transport equations. This option should be used for compressible simulations (that is, if the Mach number exceeds about 0.3).	None.

Interface Detection Properties

HRIC Gradient Smoothing Properties

When Enable HRIC Gradient Smoothing is activated, a smoothed gradient is used to predict the upwind-upwind value in the HRIC scheme.

See Interface Smoothing.

[Min, Max] Gradient Smoothing Steps

Controls the minimum and maximum number of smoothing steps that are applied to the volume fraction gradient. The default values are Min = 0 and Max = 20.

If Min = Max, exactly Min smoothing steps are performed. If both Min and Max are 0, no smoothing is performed and the standard HRIC method is retained. If Min < Max, a locally adaptive number of smoothing steps is performed.

Reference Length Scale Factor

Defines the reference length scale against which curvature fluctuations between adjacent cells are evaluated. The default value is 10. A smaller factor leads to fewer local smoothing steps, a larger factor leads to more smoothing steps.

Initial Conditions

Volume Fraction

Region Settings

The following setting applies to each phase and also to each fluid region.

Each phase momentum source is added to the region (the fluid mixture) momentum source (see Eqn. (2588)). The phase momentum sources are also taken into account, along with the gravity force and inertial force, when calculating the Drag-Based Slip Velocity (see Eqn. (2895)).

Momentum Source Option

Method	Corresponding Physics Value Nodes
None Specified Adds a specified momentum source to the momentum equation.	When the Specified method is selected: Momentum Source Momentum Source Velocity Derivative See Momentum Source Option.

The following settings apply to fluid regions.

Initial Condition Option

Lets you customize initial conditions for an individual region.

See Setting Initial Conditions for a Particular Region.

Turbulence Source Option

Available when a turbulence model is activated in the physics continuum.

Method

Corresponding Physics Value Nodes

Turbulence Source Option None Specified Ambient

When the Specified method is selected: Specific Dissipation Rate Source Available for the K-Omega turbulence model. See K-Omega Regions Reference. Turbulent Kinetic Energy Source Available for both the K-Omega and K-Epsilon turbulence models. Turbulent Dissipation Rate Source Available for the K-Epsilon turbulence model. Turbulent Dissipation Rate Source Derivative Available for the K-Epsilon turbulence model. Turbulent Kinetic Energy Source Derivative Available for the K-Epsilon turbulence model. See K-Epsilon Regions Reference. When the Ambient method is selected: Ambient Turbulence Specification

Volume Fraction Source Option

Method	Corresponding Physics Value Nodes
Phase Source Term When activated, user-specified volume fraction sources are enabled.	Volume Fraction Sources Volume Fraction Sources Pressure Derivative Volume Fraction Sources Volume Fraction Derivative

Field Functions

When the Volume of Fluid (VOF) model is activated, the following field functions become available:

Absolute Total Pressure of [phase]

The pressure that results from isentropically bringing the flow to rest in the absolute frame of motion, defined as $P_{t,abs}^i=p_{t,i}+p_{ref}$ , where $p_{t,i}$ and $p_{ref}$ are the Total Pressure and the Reference Pressure for each phase.

Free Surface Quality Indicator

The identification of sharp and smeared cells is based on the Interface Detection that is part of the Volume of Fluid (VOF) Multiphase model.

Total Pressure of [phase]

The absolute total pressure minus the reference pressure defined. The total pressure is specified per phase.

For incompressible fluid:

$$p_{\text{t,i}}=p+\frac{1}{2}{\rho }_i{v_{rel,i}}^2$$

For compressible fluid:

$$H_{t,rel}^i=H(T_0,p_{t,i})$$

and

$$s(T_0,p_{t,i})=s(T,p)$$

where

• $p$ is the mixture pressure
• ${\rho }_i$ is the density of the phase i
• ${\text{v}}_{rel,i}$ is the relative velocity of phase i
• $H_{t,rel}^i$ relative total enthalpy of phase i
• $H(T,p)$ enthalpy at temperature T and pressure p
• $s(T,p)$ entropy at temperature T and pressure p and pressure.

Total Temperature of [phase]

The temperature that is obtained from bringing the fluid to rest. The total temperature is specified per phase, using the equation of state of the phase under consideration.

For incompressible fluid:

$$H_{t,rel}^i=H(T_{\mathrm{t},i},p_{t,i})$$

For compressible fluid:

$$H_{t,rel}^i=H(T_{\mathrm{t},i},p_0)$$

and

$$s\left(T_{t,i}{,p}_0\right)=s(T,p)$$

where

• $T$ is the mixture temperature
• $T_{t,i}$ relative total temperature of phase i
• $H_{t,rel}^i$ relative total enthalpy of phase i