Index

Slip conditions

Ver: 1
 

DESCRIPTION

Allows to set a slip law on a given boundary set.

Three different slipping law can be selected :

 
The Generalized Navier's law :  
fs =  fslip * (vwall - vs) *  |vs - vwall | exslip  / (vwall - vs)

    where     vs is the tangential velocity of the fluid
                    vwall is the tangential velocity of the wall
                    fslip and exslip are materials properties

    Note :    full slip is obtained for fslip = 0
                   the law is linear when exslip = 0, corresponds to a power law when exslip < 0
 
 

 The Threshold law :

                     - fslip ( vs - vwall )                                       vs - vwall  < yc / fslip
            fs =
                      - yc - fslip2 ( vs - vwall - (yc/fslip) )            vs - vwall  >= yc / fslip

                    where fslip and fslip2 are two different slip coefficients.
 

The Asymptotic law :

            fs  = -fslip [ 1 - exp(  (vs - vwall ) / vc ) ]
 

 For non-isothermal flows, the total shear force is written :

                        fs = F(v) * H(T)

                            where F(v) is one of the 3 laws described here above
                                        H(T) can be the Arrhenius law or Arrhenius approximate law.
 

                        Arrhenius law :

                                H(T) = exp ( alpha/(T-T0) - alpha/(Talpha - T0) )

                                    where    alpha is the energy of activation
                                                    Talpha is the reference temperature at which H(T) = 1
                                                    T0 is the temperature offset, T0 = 0 by default

                        Arrhenius approximate law :

                                H(T) = exp (  - alpha/(T - Talpha) )
 

 OPTIONS
 
-2 Define constraints on wall normals
Allows for specifying the wall normals that will be used for the calculation of normal tangential components.
-1 Upper level menu
Accepts the current setup
0 Define v_wall, the velocity of the wall
Allows for specifying the magnitude of the tangential velocity of the wall
1 F(v) = Generalized Navier's law
Switches to Generalized Navier's slipping law
2 F(v) = Threshold law
Switches to Threshold slipping law
3 F(v) = Asymptotic law
Switches to Asymptotic slipping law
4 H(T) = 1  (temperature independent)
for non-isothermal flows, switches to temperature independent law
5 H(T) = Arrhenius approximate law
for non-isothermal flows, switches to Arrhenius approximate law
6 H(T) = Arrhenius  law
for non-isothermal flows, switches to Arrhenius  law

NOTES

For non-vanishing values of exslip, the problem is non-linear.

For many non-linear flow problems, it is interesting to increase the slip coefficient from a low to its nominal value.
This can be done by means of an evolution scheme. The current task should then be of the evolution type.

When the boundary set on which the slip condition is applied presents large discontinuities of the normal direction, the user should split it up into smaller parts in order to avoid 'leakages' around the corners. These leakages are due to the single normal and tangent defined at the corner; this mean tangent is not really tangent to the flow domain boundary and some fluid goes out of the flow region.
 

 SEE ALSO