Theory for the Slip Wall Boundary Condition
The Slip Wall boundary condition is a linearized version of the Slip Velocity option in the Wall boundary in the CFD interface. See the section Theory for the Wall Boundary Condition in Theory for the Single-Phase Flow Interfaces in the CFD Module User’s Guide.
The Slip Wall boundary is useful in microscale systems where the boundary condition is rarely strictly no slip or perfect slip. There are two main effects due to noncontinuum effects: a slip velocity either due to a viscous stress or a thermal gradient along the boundary and a temperature jump due to a thermal gradient perpendicular to the boundary.
The linearized slip velocity is given as:
where σs is the viscous slip coefficient, λ0 is the mean free path, σT is the thermal slip coefficient, utherm is the thermal creep. There are two contributions to the slip velocity uslip: the contribution from the acoustics stress tensor τ and the thermal creep utherm induced by the thermal gradient tangential to the boundary. The linearized temperature jump between the fluid and the boundary is given as,
where ζT is the temperature jump coefficient and λ1 is the acoustic perturbation of the mean free path.