Viscous Flow Model Reference

Viscous Flow Properties

Freeze Flow

When On, the Viscous Flow solver ignores the hydrodynamic equations and solves only the remaining equations of the system. For instance, when the Chemorheoloy model is selected, it only solves the Kamal and Sourour model Eqn. (1090) in conjunction with energy conservation for the static (no flow) condition. This property is Off by default.

Petrov Galerkin

(Also called PSPG for Pressure Stabilization Petrov Galerkin.) A non-dimensional coefficient, ${\beta }_{\text{pspg}}$ in Eqn. (1049), that controls numerical stabilization of incompressibility. The default value is 1.0.

Inertia

When On, this property has Simcenter STAR-CCM+ simulate the inertial mass of the viscous fluid. If the flow is in the creeping flow regime, Reynolds number $\text{Re}\ll 1$ , set the property Off to lower computational costs. This property is On by default.

Viscous Flow Boundary Settings

Free Stream

Represents the outer surface of an extrudate in a free surface simulation. The position of that boundary is computed as part of the solution by an Arbitrary-Lagrange-Eulerian (ALE) approach. The vertices of the boundary are moved by the morpher according to the selected morpher settings. See Free Stream.

Pressure

The pressure of the surrounding fluid. When simulating an extrusion, this is the air pressure.

Pressure Outlet

Defines the working pressure across the outlet.

Pressure

Specifies the exit pressure as a Dirichlet condition. Represents $p_{\text{outlet}}$ in Eqn. (1057).

Mass Flow Inlet

Specifies mass flow inward across the boundary.

Mass Flow Rate

The Mass Flow Rate represents the total mass per unit time (kg/s) for the whole boundary. You can use field functions and tables to describe a dependence on iteration or time-step, but the mass flow rate cannot vary spatially across the boundary. The total mass flow is distributed over all of the faces of the inlet. Represents $\dot{m}$ in Eqn. (1055).

Stagnation Inlet

Describes the resting conditions of the fluid.

Reference Frame Specification

Specifies the reference frame in which the fluid is at rest.

Method	Corresponding Physics Value Nodes
Lab Frame	Total Pressure The working pressure of the fluid.
Region Reference Frame	Relative Total Pressure The pressure that is obtained from isentropically bringing the flow to rest in the relative frame of motion. For fluids with low Reynolds numbers, such as polymers, there is no distinction between total pressure and working pressure.
Local Reference Frame	Boundary Reference Frame Specification Applies the chosen reference frame to the containing boundary. Relative Total Pressure The pressure that is obtained from isentropically bringing the flow to rest in the relative frame of motion. For polymers (always with low Reynolds numbers), there is no distinction made between total pressure and working pressure.
Part Reference Frame	Relative Total Pressure The pressure that is obtained from isentropically bringing the flow to rest in the relative frame of motion. For polymers (always with low Reynolds numbers), there is no distinction made between total pressure and working pressure.

Velocity Inlet

An inflow condition where the average velocity is known.

Average Velocity

The velocity of the fluid coming through the inlet, averaged over all the cell faces of the inlet. Represents $v_{\text{avg}}$ in Eqn. (1060).

Wall

Represents an impermeable surface that confines fluid regions.

Reference Frame Specification

Allows you to apply a rotating reference frame to the boundary, and the source of the reference frame.

Option	Corresponding Physics Value Nodes
Lab Frame Uses the Laboratory reference frame (default). No rotation.	None
Region Reference Frame Uses the reference frame of the parent region. When the region has a rotating reference frame attached, a physically valid setup using this condition puts the boundary in the reference frame of the region. This implies that the boundary is rotating with the region at the same RPM, and has the same rotation axis and origin as the region.	None
Local Reference Frame This option allows the boundary to have its own frame (only rotating frame) attributes. For example, this can be used 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.
Part Reference Frame Uses the direct reference frame of the associated part, as specified in the part subgroups for the parent region. This option is only available when you activate Specify by Part Subgroup under the Direct Rotating Reference Frame node. See 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

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 The relation between the slip velocity and the shear force at the wall is given by a power-law expression.	Slip Coefficient The slip coefficient $k_s$ in Eqn. (1062). Slip Exponent The slip exponent $n_s$ in Eqn. (1062).

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.

Symmetry Plane

Represents a plane of symmetry in the simulation. The symmetry plane boundary can be used as a planar "slip wall" with the shear stress being zero.

Viscous Flow Region Settings

Applies to fluid regions.

Momentum Source Option

Specifies whether you want to enter a momentum source term, and of which type.

Momentum Source Option	Corresponding Physics Value Nodes
None	None
Specified	Momentum Source Specifies the momentum source term, using standard profiles or as a Composite of X and Y components. Represents ${\mathbf{f}}_b$ in Eqn. (1034). Momentum Source Velocity Derivative The derivative of the momentum source velocity with respect to the x-, y-, z- components of the velocity as a tensor value, in any of the following forms of tensor: Axisymmetric Tensor Composite Symmetric Tensor Composite Tensor Isotropic Tensor Principal Tensor It is highly recommended to specify the derivative in order to improve the stability and convergence rate of the solution.

Viscous Flow Solver

The Viscous Flow Solver is exposed when you select the Viscous Flow physics model.

This solver has the Sparse Direct Solver as a child node.

Properties

Relaxation over # iterations

The number of iterations over which relaxation reaches full effect. The default value is 10.

If the Deborah number > 1, non-linear effects dominate and the system does not converge. In such cases, raise the value above 10, experimenting to find the lowest value that gives convergence. Do not lower the value below 10 for any viscoelastic constitutive models.

Solve in double precision

When On, the solver works in double precision; when Off, the solver works in single precision. This flag is independent of the precision mode in which Simcenter STAR-CCM+ is started; the precision mode only influences how variables are computed. Mixed modes can lead to truncation errors. By default, this property is On.

Verbose

When On, provides more output for this solver while the simulation is running, which can be useful for debugging. By default, this property is Off.

Solver Frozen

When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.

Viscous Flow Solver Controls

Viscoelastic Mode Coupling

Available only when the Viscoelastic model is selected. The Mode Coupling property determines whether the Viscous Flow Solver solves the system of equations in a coupled or decoupled manner. Controls the coupling of viscoelastic modes with pressure and velocity.

Method	Corresponding Value Nodes
Coupled The solver determines all unknowns simultaneously for the system of equations associated with the flow based on a full Newton scheme. This is the default.	None
Decoupled The solver determines the unknowns sequentially; therefore execution is faster and memory consumption is smaller.	Decoupled Option Number of Coupled Viscoelastic Modes Sets the number of viscoelastic modes to be coupled with the momentum and continuity equations. This option increases the robustness of the decoupled solver when dealing with multi-mode viscoelastic flows at high Weissenberg numbers. The default value is 0, which means that no viscoelastic modes are coupled with the momentum and continuity equations. Unsteady Explicit Stress In Momentum Formulation This flag is only relevant in transient simulations. When it is On, the viscoelastic extra stress is explicitly substituted in the momentum equation which increases the robustness of the decoupled solver in high Weissenberg number simulations. The default value is Off.