The surface between a fluid and a wall in a flow analysis exhibits the
following behaviors by default:
- The surfaces are no-slip,
meaning the fluid sticks to the walls as it flows past.
- The surfaces are stationary
throughout the simulation with no translation or rotation.
- There is no heat exchange
between the fluid and the surface.
You can define a wall boundary condition to override these behaviors
for selected surfaces.
Wall Type
The wall type describes either the sticking behavior at the wall or
the movement or rotation of the wall.
| Wall Type
|
Description
|
| 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 wall
|
The fluid does not stick to the wall. Use
this wall type where viscous effects are negligible or when 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.
|
Thermal Condition
The thermal condition describes the heat exchange behavior between the
wall and the fluid. You can characterize this behavior in any of the following
ways:
| Thermal Condition
|
Description
|
| 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 due to convection between the
wall and the fluid. The level of heat flux is driven by the film coefficient, a
constant that relates heat flux to the difference in temperature between the
wall and the fluid; and the 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 level of radiation is driven by the emissivity of the wall
(that is, its ability to transmit thermal energy) and the ambient temperature.
|
Roughness
Enabling wall roughness introduces more turbulent flow into the
simulation. You can also adjust the average height and uniformity of rough
components on the wall.
Multispecies Behavior
If your simulation includes a multispecies section, you can define the relative concentration
of each species next to the wall. You can define fully catalytic wall behavior,
constant mass flux, or absorption at the wall.
Radiation
Walls can exhibit surface-to-surface radiation or they can interact
with solar radiation in a simulation.