Viscous Regime Models Reference

Reference Frame Specification

Allows you to specify a reference frame to which the boundary is associated with.

Depending on the boundary type, you define one or more of the following properties with respect to the specified reference frame:

• Flow Direction Specification
• Velocity
• Velocity Magnitude
• Flow Direction Specification
• Total Pressure
• Total Temperature
• Tangential Velocity Specification

Option	Corresponding Physics Value Nodes
Lab Frame Uses the Laboratory reference frame. This option is the default for Velocity Inlet and Stagnation Inlet boundaries.	None.
Part Reference Frame Uses the direct rotating reference frame of the region with respect to its associated part(s). This option is only available when you activate Specify by Part Subgroup under the Direct Rotating Reference Frame node. For more details, refer to Defining a Direct Reference Frame. You can visualize the reference frame specification within the [region] > Physics Values > Direct Rotating Reference Frame > [subgroup] > Direct Rotating Reference Frame Values node.	None
Region Reference Frame Uses the reference frame of the parent region. When the region is defined in a rotating reference frame, the boundary uses the reference frame of the region. You can see the region axis within the [region] > Physics Values > Axis node. If this node does not exist, the motion assigned to the region is automatically providing a region axis. For example, if you assign a rotation motion defined within the Tools > Motions node to the region, the region axis is the axis of the rotation motion.	None.
Local Reference Frame Allows you to specify a local reference frame for the boundary. You can only select a rotating reference frame. You can use when both the boundary and the region are rotating at a different RPM but have the same axis and origin. A setup where the region and boundary have different values for the RPM, axis, and origin is also possible.	Boundary Reference Frame Specification Applies the chosen Reference Frame to the containing boundary.

Shear Stress Specification

Defines how a wall surface acts on a fluid passing across it.

Method	Corresponding Physics Value Nodes
No-Slip Relative fluid velocity tangential to the wall is set to zero. When you choose No-Slip, Simcenter STAR-CCM+ adds the Tangential Velocity Specification.	None.
Slip Fluid velocity at the wall is extrapolated from the parallel component of the velocity in the cell next to the wall. When simulating non-Newtonian fluid flow using the Viscous Flow model, the relation between the slip velocity and the shear force at the wall is given by a power-law expression.	Available only with the Viscous Flow model: Slip Coefficient The slip coefficient $k_s$ in Eqn. (1062). Slip Exponent The slip exponent $n_s$ in Eqn. (1062).
Specified Shear Allows you to model partial slip at the wall using a known wall shear force. This option is recommended for situation where there is some degree of slippage between the fluid and the wall, such as for polymer/rubber flow simulations or hydrophobic/hydrophilic fluid flow simulations.	Wall Shear Force Sets the wall shear force ${(\mathbf{T}\cdot \mathbf{a})}_f$, where $T$ is the viscous stress tensor and $a$ is the surface normal vector, as a vector profile value in the chosen Coordinate System. The wall shear force enters the momentum equation in the form of a source term in the near-wall cell. Wall Shear Velocity Derivative For a Wall Shear Force that is a function of velocity, sets the derivative of the wall shear force with respect to the x-, y-, [z-] components of velocity as a tensor profile value. The derivative is used in the linearization of the momentum equation. It is highly recommended to specify the derivative in order to improve the stability and convergence rate of the solution.

Tangential Velocity Specification

When a Tangential Velocity Specification is used at a wall $f$, only the tangential component $v_{\tau }$ of the specified velocity ${\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}$ is used. If you specify a velocity with a component normal to the wall, the normal component of ${\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}$ is ignored, since the velocity ${\mathbf{\text{v}}}_f$ at the face is computed as:

$${\mathbf{\text{v}}}_f={\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}-({\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}\cdot \mathbf{\text{a}}-G_f)\frac{\mathbf{\text{a}}}{a^2}$$

()

where $\mathbf{\text{a}}$ is the face area vector and ${\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}$ is the velocity in the laboratory frame. The normal component contribution at the face $f$ can only come from the grid flux $G_f$ that is given by Eqn. (4868). That is, if the wall itself is moving at a velocity with a non-zero component in the wall normal direction.

Note that:

$${\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}={\mathbf{\text{v}}}_{\text{spec}}^{\text{mesh}}+{\mathbf{\text{v}}}_{\text{mesh}}^{\text{ref}}+{\mathbf{\text{v}}}_{\text{ref}}^{\text{lab}}$$

()

where:

• ${\mathbf{\text{v}}}_{\text{spec}}^{\text{lab}}$ is the specified velocity measured in the laboratory frame.
• ${\mathbf{\text{v}}}_{\text{spec}}^{\text{mesh}}$ is the specified velocity measured relative to the mesh.
• ${\mathbf{\text{v}}}_{\text{mesh}}^{\text{ref}}$ is the velocity of the mesh relative to the reference frame. This quantity is in Region > Physical Values > Motion Specification..
• ${\mathbf{\text{v}}}_{\text{ref}}^{\text{lab}}$ is the velocity of the reference frame relative to the laboratory frame.

The net tangential velocity in the laboratory frame is the vector sum of the tangential velocity with respect to the reference frame selected under the condition Reference Frame Specification and the velocity of the selected frame with respect to the laboratory frame.

Unless specified in an Axis node, axis of rotation is defined by information in the Motion Specification node under the Physics Values node.

Method	Corresponding Physics Value Nodes
Fixed Tangential velocity is zero with respect to the applicable reference frame.	None.
Vector Tangential velocity is specified by rotation rate with respect to the applicable reference frame.	Velocity Relative Velocity Sets the velocity vector in the chosen Coordinate System. When relative, the vector is defined within the reference frame applied to the boundary.
Rotation Rate Tangential velocity is specified as a vector with respect to the applicable reference frame.	Wall Rotation Wall Relative Rotation Sets the wall rotation rate around the axis of the reference frame applied to the containing boundary.
Local Rotation Rate Tangential velocity is defined by the rotation rate around the axis specified in the Axis node of the boundary and with respect to the applicable reference frame.	Local Axis Specifies a vector whose Origin and Direction define an axis of rotation. The local axes can be defined By Surface Subgrouping if the following prerequisites are fulfilled: Allow Per-Surface Values of the boundary is activated. Specify by Part Subgroup of the Local Axis node is activated. For more workflow of Applying Quantities by Subgroup, refer to Defining Subgroups. Wall Rotation Wall Relative Rotation As for Rotation Rate.

A version of this field function is created for each phase in a multiphase continuum. This field function is volume fraction weighted for each phase.

This field function is volume fraction weighted for each phase.