### 2.7.1. Mass and Momentum

The following options are available for modeling the influence of a wall boundary on mass and momentum:

- No Slip Wall

- Free Slip Wall

- Finite Slip Wall

- Specified Shear

- Counter-rotating Wall

- Rotating Wall

For additional details, see Wall in the CFX-Solver Theory Guide.

#### 2.7.1.1. No Slip Wall

This is the most common type of wall boundary condition implementation. The fluid immediately next to the wall assumes the velocity of the wall, which is zero by default. Non-zero wall velocities are created in two ways: by explicitly setting a wall velocity and/or by activating mesh deformation.

Cartesian or cylindrical wall velocity components can be set to emulate wall motion even when mesh motion has not been activated. When using `Cartesian` or `Cylindrical Velocity Components`, the velocity is always in relation to the local (relative) frame of reference (that is, relative to the rotating frame in a rotating domain). When using `Cylindrical Velocity Components`, an axis for the cylindrical coordinate system must be specified in one of two ways:

- You can select the axis of the cylindrical coordinate system for the boundary condition (called the Rotation Axis) as an axis of the global coordinate frame, Coord 0.

- You can specify Rotation Axis From and Rotation Axis To points. These are the two end points of the axis. The global coordinate system must be used to specify these points. The positive z component of the cylindrical coordinates is in the direction of the vector from the Rotation Axis From point to the Rotation Axis To point.

For rotating domains, the axis will default to the domain axis of rotation.

Note: Only the component of the specified wall velocity that is tangent to the wall is applied. Mesh motion should be used if the desired wall velocity is normal to the wall, or has a component normal to the wall.

Mesh motion can occur on wall boundaries in domains where Mesh Deformation has been activated for the domain. (See Mesh Deformation in the CFX-Pre User's Guide for information about activating mesh deformation for the domain.) If this is the case, the wall velocity (as set above) can be made relative to the boundary frame or to the mesh motion occurring on the boundary. When the wall velocity is made relative to the boundary frame, only the specified wall velocity is assumed by the flow. When the wall velocity is made relative to the mesh motion, the velocity due to mesh motion is super-imposed on the specified wall velocity. The default behavior is that the wall velocity is relative to the boundary frame when `Parallel to Boundary` or `Surface of Revolution` mesh motion boundary conditions are used, and relative to the mesh motion for all other boundary conditions types. For details, see Mesh Deformation.

For example, in a piston-cylinder simulation, a zero velocity, no slip condition would be applied to all walls. The wall velocity would typically be made relative to the mesh motion for the moving piston boundary, and relative to the boundary frame for the cylinder side-walls. This would ensure that the fluid is properly affected by the motion of the piston, and that it is not dragged by the motion of the mesh on the cylinder side-walls.

Note: In a multiphase case, if one fluid uses a No Slip Wall, other fluids must see the same wall velocity or use the Free Slip condition.

#### 2.7.1.2. Free Slip Wall

Free-slip, where the shear stress at the wall is zero ( = 0), and the velocity of the fluid near the wall is not retarded by wall friction effects. In a multiphase case, if one fluid uses a `Free Slip` wall, other fluids can use any wall influence condition.

#### 2.7.1.3. Finite Slip Wall

The Finite Slip Wall option is available only for laminar flows. This option causes the fluid to “slip” at the wall when the wall shear stress is greater than a critical stress, . A typical use of the finite slip wall option is to simulate the flow of non-Newtonian fluid. When the wall shear stress is greater than the critical stress, the wall velocity at the edge of the shear-thinning boundary layer is computed without resolving it directly. ANSYS CFX simulates the slip by using a moving wall with the wall speed computed as follows:

| (2–6)

where is the slip speed, is a normalizing stress, is a positive power, is a pressure coefficient, and is the pressure.

#### 2.7.1.4. Specified Shear

The `Specified Shear` option enables you to specify the shear stress components on the fluid. The free-slip option is the case of a specified shear equal to 0. The current implementation removes any normal component of the user-specified shear stress.

For turbulent flows, the specified shear is applied directly; no considerations about turbulent wall functions are taken into account.

#### 2.7.1.5. Counter-rotating Wall

For domains specified with a Rotating Frame of Reference, a `Counter-rotating Wall` can be specified. The wall boundary is assumed to be stationary with respect to the stationary frame, essentially in counter-rotation with the rotating fluid and uses a no slip condition. In a multiphase case, if one fluid uses a `Counter-rotating Wall`, other fluids must use the same condition or the `Free Slip` condition.

#### 2.7.1.6. Rotating Wall

This option applies to both stationary and rotating domains and enables the wall to rotate with a specified angular velocity. The angular velocity is always in relation to the local (relative) frame of reference (that is, relative to the rotating frame in a rotating domain). An axis must be specified in a stationary domain and can optionally be specified in a rotating domain. Axis specification follows the same rules as for cylindrical velocity component inlets. For details, see Cylindrical Velocity Components.

Release 18.2 - © ANSYS, Inc. All rights reserved.