# Wall boundary condition¶

The wall boundary condition defines a bounding face/surface. The flow velocity at the wall can be described by the following types:

## Wall Types¶

1. No-slip: This type is recommended for viscous flow and real wall surfaces. All velocity components are set to zero value at the wall.
2. Slip: This condition models a frictionless surface. For scalars the gradients are zero. For vectors normal components have zero value, while tangential components are calculated with gradients set to zero.
3. Moving: This condition is used for Translating surfaces where the tangential velocity component is specified. The normal components are same as Slip condition.
4. Rotating: This condition specifies rotation of a wall surface about an axis. The origin, axis of rotation, and angular velocity of the rotation must be specified. The latter could be specified as a constant, or by uploading a CSV file, as a time-dependent variable.

The pressure values on the wall are then calculated, while the pressure gradients are fixed to zero. For Incompressible flows the temperature properties are not required, while for Compressible flows the temperature at the wall must also be defined.

The temperature can therefore be a known or unknown value and can be defined by the following types:

• Fixed value: The temperature is known and specified by a constant value input from the user.
• Set gradient to zero: The values are not known with gradients set to zero value.
• Turbulent heat flux temperature: The boundary condition specifies the temperature gradient on the boundary by defining a fixed input heat source. This source could be specified either by ‘Heat flux’ or ‘Power’. In both cases, an initial temperature could be supplied.
• Wall heat transfer: This boundary condition allows a wall heat condition in terms of wall temperature and thermal diffusivity.

Additionally, to generate the boundary layer profile near wall regions, two approaches are available for all four types:

### Wall Functions¶

In this case appropriate functions are used to model the velocity profile. Thus, the boundary layer profile is not completely calculated and a relatively coarse mesh can be used near wall regions.

Important

For ‘Wall-functions’, the y-plus for the first grid point must be in range $$30<y^+<200$$ to ensure that the correct implementation.

### Full Resolution¶

In this case no functions are used and the profile is completely resolved. For this approach the mesh has to fulfill certain criterion and should be particularly of high resolution near wall regions.

Important

For ‘Full-resolution’,the y-plus criterion of $$y^+<1.0$$ is highly recommended to capture the correct flow physics.