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Wall Boundary Condition

In SimScale, a wall boundary condition is assigned to a face by primarily defining the behavior of the flow velocity at that face. In a CFD simulation, a wall can be an internal surface or an external surface.

There are four ways to define the flow velocity at the surface assigned with a Wall boundary condition. These are:

  • Moving Wall: Used for surfaces in motion, where the tangential velocity component is specified.
  • No-slip: Recommended for viscous flow and real-world surfaces where velocity is zero at the wall. If any surfaces in the CAD model are left unassigned, then a No-slip wall boundary condition gets automatically assigned to them.
  • Rotating Wall: Used to specify the rotation of a wall surface about an axis. A point on the axis, the axis of rotation, and angular velocity of the rotation must be specified. The latter could be specified as a constant, or as a time-dependent variable by either uploading a CSV file or directly entering the values in the table.
    For general information on how to use the table feature in SimScale, refer to this document.
  • Slip: With Slip, a friction-less surface can be modeled. Mathematically speaking, it erases the normal component of the velocity and keeps the tangential components untouched at the assigned surface.

Following is a snap of Wall boundary condition settings in the SimScale Workbench:

wall boundary condition settings
Figure 1: Assigning a Wall boundary condition in SimScale. There are four options to define velocity at the wall surface.

Wall Turbulence

Additionally, to generate the boundary layer profile near the wall regions, two approaches are available. These can be defined under Turbulence wall as:

  • Wall Function: 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 in the near-wall regions.

Note

To achieve a good accuracy with wall functions place the first cell of the mesh in the logarithmic region (30 < \(y^+\) < 300).

  • Full Resolution: In this case, no functions are used and the flow profile is completely resolved. The mesh has to fulfill certain criteria and should be particularly of high resolution in the near-wall regions.

Note

For explicit resolution near the wall region, the first cell should lie in the laminar sub-layer region (\(y^+\) < 1). Such a wall is referred to as fully resolved.

Read more about how to calculate y-plus (\(y^+\)) value here.

Example

The best example to understand the different wall types is that of a moving car. For simulation, the car is stationary and the air is blown at it through the inlet face (red).

example of moving car for wall boundary condition
Figure 2: Example of a moving car domain ready for CFD simulation. The four different wall velocities are shown.

Here, the car body (grey) is assigned no-slip, tires (black) are rotating walls, ground (light blue) is a moving wall (since no relative motion between air and ground), and the top along with the outer face (not seen) are slip walls.

Defining Temperature

For certain analysis types, the temperature needs to be defined too. The assignment of temperature on the surfaces of your flow domain is again done through Wall boundary condition:

Figure 3: Define the temperature value on a surface under Wall boundary condition in four possible ways.

All the details about the available choices and their implementation are described here.

Unassigned Boundary condition

Faces which are left unassigned without any boundary condition type automatically get assigned as no-slip walls with temperature being adiabatic (zero-gradient).

Last updated: November 3rd, 2020

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