Defining a Wall

You can define a wall to describe the physical behavior at the boundary surface between the fluid and solid.

You can specify most of the values in the Wall dialog box as uniform values that are constant in space, as values that vary in space according to a spatially-varying mapped data, or as values that vary programmatically according to JavaScript user subroutines. You can also apply a scale factor that multiplies the value you specify, and you can specify an amplitude curve to define a value that changes during the step.

You can also customize the default wall behavior for surfaces on which no boundary conditions are specified. For more information, see Defining the Default Wall Behavior.

See Also
About Walls
Defining the Fluid Physics of a Flow Simulation
  1. From the Boundaries section of the action bar, click Wall .
  2. Optional: Enter a descriptive Name.
  3. Select a boundary surface of a fluid domain as the geometry support for the wall.
  4. Specify the fluid or solid Physics behavior that applies for this wall boundary.
  5. Choose the Wall type.
    Wall typeDescription
    No-Slip The fluid sticks to the wall with no relative velocity between the fluid and the surface. Use this wall type where viscous effects are significant.
    Slip The fluid does not stick to the wall. Use this wall type where viscous effects are negligible or when the mesh size at the wall is much larger than the boundary layer thickness. Slip walls are also appropriate for models with symmetry surfaces and zero shear.
    Moving Wall Simulates a wall moving in a linear fashion that imparts velocity to the fluid in contact with it. You could use a moving wall to simulate the effect of a piston that moves gas out of a chamber.
    Rotating Wall Simulates a wall that rotates about an axis and imparts velocity to the fluid in contact with it. You could use a rotating wall to simulate the effects of a fan blade.
  6. Optional: For a moving wall, specify the motion in space in all three axes and in your selected coordinate system.

    For more information, see Defining Motion for a Moving Wall.

  7. Optional: For a rotating wall, specify the axis of rotation and the rotational speed.

    For more information, see Defining Rotational Behavior for a Rotating Wall.

  8. Specify a Thermal condition to describe the heat exchange behavior between the wall and the fluid.

    Note: Thermal condition options are available only if you enable thermal effects in the fluid physics of the simulation.

    OptionDescription
    Adiabatic No heat exchanged between the wall and the fluid.
    Temperature No heat exchanged, but the wall surface is held at a constant temperature.
    Heat flux Heat exchanged at a specified flux rate.
    Film condition Heat exchange from convection between the wall and the fluid. Two parameters determine the level of heat flux:
    • Film coefficient, a constant that relates heat flux to the difference in temperature between the wall and the fluid
    • Far-field temperature, which is the temperature of the surrounding fluid
    The default values are appropriate for typical objects at room temperature.
    Radiation to ambient Heat radiation from the wall to the environment. The emissivity of the wall (that is, its ability to transmit thermal energy) and the ambient temperature determine the level of radiation.
  9. Optional: Select Enable wall roughness to include the effects of wall roughness and the resulting turbulent flow in the simulation. You can then adjust either or both of the following aspects of wall roughness:
    1. Adjust the average Roughness height of the components on the wall that contribute to its roughness. For example, this parameter might specify the average height of the individual grains of sand on a piece of sandpaper.
    2. Adjust the Roughness constant, a measure of the uniformity of roughness. Values range from 0 to 1, with higher values indicating less uniformity. A value of 0.5 represents a tightly packed, uniformly distributed roughness.
  10. Optional: If you are creating a multispecies flow simulation, expand the Multispecies Conditions options and then see Defining the Multispecies Conditions at a Wall.
  11. Optional: Specify the surface-to-surface radiation behavior of the wall.

    For more information, see Defining Surface-to-surface Radiation Behavior at a Wall.

  12. Optional: Specify the solar radiation behavior for the wall.

    For more information, see Defining Solar Radiation Behavior at a Wall.

  13. Click OK.