Slip Velocity Model

The phases in a phase interaction can move at different velocities, which can be both different magnitudes and different directions. Many important phenomena (for example, phase separation) require the movement of the gas phase relative to the liquid phase. To model differences in velocity between two phases, use the Slip Velocity model.

The Slip Velocity model can be useful in the following applications:

Dispersed multiphase flow in pipelines
Bubble column reactor
Segregation of phases under the influence of gravity
Fuel cells

Note	The Slip Velocity model is not compatible with a porous baffle interface as a porous baffle cannot be set up with phase-specific resistance values.

Table 1. Slip Velocity Model Reference
Theory	See Slip Velocity.
Provided By	Phase Interactions > [phase interaction] > Models > Optional Models
Example Node Path	[phase interaction] > Models > Slip Velocity
Requires	In a VOF Multiphase simulation: A VOF-VOF Phase Interaction. In a Multiphase Mixture simulation: An MMP-MMP Phase Interaction.
Properties	None.
Activates	Physics Models	None.
	Region Settings	Phase Conditions > [phase interaction] > Physics Conditions > Slip Velocity Method Slip Velocity Region Settings
	Solvers	Phase Slip Velocity Solver
	Field Functions	Velocity of [primary phase] Velocity of [secondary phase] See Field Functions.

Slip Velocity Region Settings

Slip Velocity Method

The method used to calculate the relative velocity between two phases.

Method

Corresponding Method Nodes

No slip

Enforces a homogeneous velocity, in case slip is needed in some regions but not in others.

None.

User-defined slip

Use this model if you know the relative velocity between the two phases in the phase interaction.

Slip Velocity

The relative velocity between the two phases is specified as a constant, a field function, or with user-code.

Drag-based slip

The slip velocity is based on body forces.

Available in fluid regions, if the Interaction Length Scale Model is selected in the phase interaction.

The Schiller-Naumann drag coefficient method is used and is applied per phase regime. See Schiller-Naumann drag coefficient.

Slip Velocity Interaction Length Scale Limiter Option.
Limits the interaction length scale of the dispersed phase in Eqn. (2896).
The available options are:
- None
  Does not apply the slip velocity interaction length scale limiting.
- Cell Size
  Computes a limited interaction length scale of the dispersed phase, $d_{s}^{\lim}$ in Eqn. (2902), with the limiting interaction length scale $l_{s}$ being half the cell size. This limiter option avoids that a particle covers all of the cell and can no longer be considered as dispersed in that cell.
- Wall Distance (This is the default.)
  Computes a limited interaction length scale of the dispersed phase, $d_{s}^{\lim}$ in Eqn. (2902), with the limiting interaction length scale $l_{s}$ being half the wall distance. This limiter option avoids that unphysical particles touching the wall form a film instead of occupying part of the cell.
- Cell Size + Wall Distance is a combination of the two options above.
Slip Velocity Limiter Option.
Limits the slip velocity as calculated by Eqn. (2896).
The available options are:
- None
  Does not apply the slip velocity limiting.
- Specified
  Computes a limited slip velocity, $v_{p s}^{\lim}$ in Eqn. (2903).
  Activates the Slip Velocity Limiter node under Physics Values where you specify the limit for the maximum slip velocity magnitude, $l_{v}$ in Eqn. (2903). The default value is 0 m/s.
If you experience instabilities associated with drag-based slip, use the slip velocity limiter to stabilize the simulation.

Note	Not available in fluid regions that are selected for a porous phase from the Porous Media model.

Darcy's Law

A porous viscous resistance exists for fluid regions that are selected in a porous phase from the Porous Media model, when Porous Media Drag is also active.

Available for the porous media model and in porous regions, where a porous viscous resistance is defined.

See Phase Slip in Porous Media.

Slip Velocity Limiter Option.

Same as above.

Phase Slip Velocity Solver Properties

The Phase Slip Velocity solver controls the solution update for the phase slip velocity.

Under-Relaxation Factor: The phase slip velocity is computed as a linear combination of the old value and the computed value. Smaller under-relaxation values provide more stability, but also give slower convergence. The residual cannot be reduced by more than the under-relaxation factor per iteration.
Temporary Storage Retained: Activates additional slip velocity based field functions. See Field Functions.
Body Force Smoothing Iterations: If the instability is very localized (such as unnaturally high computed specific body forces in single cells), the computed specific body force can be smoothed to improve convergence. Each smoothing iteration smears out the body forces over one additional layer of cells in every direction. Higher values provide a more uniform distribution of specific body forces and therefore more stability, but also decrease the local resolution of body forces.

The Neglect Local Time Derivative child node appears only for an unsteady simulation that uses the Drag-Based Slip Velocity method.

This node has the following property:

Neglect Local Time Derivative: Stabilizes the computation of the inertial forces by neglecting the local time derivative in the force computation.

Field Functions

The following field functions are available immediately:

Velocity of [primary phase]: This value is $v_{p}$ in Eqn. (2889).
Velocity of [secondary phase]: This value is $v_{s}$ in Eqn. (2889).

When Temporary Storage Retained is activated in the Phase Slip Velocity solver, the following field functions become available:

Phase Slip Velocity of [Phase Interaction]: This value is $v_{p s}$ in Eqn. (2889).
Phase-Pair Reynolds Number of [Phase Interaction]: This value is ${R e}_{p s}$ in Eqn. (2896).
Applies to Drag-Based Slip Velocity only.
Resultant Specific Body Force: This value is $b$ in Eqn. (2895).
Applies to Drag-Based Slip Velocity only.