Ansys Fluent offers a partial-slip boundary condition for rarefied gases. In the rarefied regime, the finite amount of friction is no longer sufficient to allow the full adhesion of gas particles onto solid walls. This discrepancy leads to a velocity component tangential to the solid wall, which may be computed with the Maxwell model [96]:
 | (17–3) |
where 
is the tangential projection of the normal-shear-stress tensor along the
wall. The mean-free-path of a gas particle, 
, is given by:
 | (17–4) |
and 
denotes the momentum accommodation coefficient describing the interaction of
gas particles with the solid surface.
The applicability of the partial slip assumption can be determined from the Knudsen
number, relating the mean-free-path to the physical length scale 
. Anderson [10] offers a useful classification
divided in three regimes:
| Knudsen Number | Regime | Wall Boundary Treatment |
|---|---|---|
 < 0.01 | Continuum | No-slip wall |
0.01 <  < 0.2 | Transitional | Partial-slip wall |
 > 0.2 | Free molecular | Continuum hypothesis violated |
The density-based solver offers a Knudsen number calculator to help determine the regime of your flow. This utility computes the Knudsen number based on a characteristic physical length and the area-averaged flow quantities along an incoming-flow boundary. It is invoked with the following text command:
/define/boundary-conditions/knudsen-number-calculator
To allow for partial slip at solid walls, select Partial Slip for Rarefied
Gases under Shear Condition as shown in Figure 17.19: The Wall Dialog Box for Partial Slip Shear Condition. You can enter the value of the momentum accommodation
coefficient (
in Equation 17–3) in the Momentum
Accommodation Coefficient field.
Note: The partial slip boundary condition is only available with the density-based solver, and for laminar flows only.
