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-   -   wall slip law for newtonian fluid (http://www.cfd-online.com/Forums/fluent/100073-wall-slip-law-newtonian-fluid.html)

tsi07 April 19, 2012 12:13

wall slip law for newtonian fluid
 
Hi,

I am trying to input a wall slip law for a newtonian fluid in a pipeline.
The law has a yield stress limit \tau_0.
When the yield stress is less than \tau_0, there is adhesion of the fluid to the wall.
When the yield stress is greater than \tau_0, there is a slip velocity at the wall like :
u_{wall}=\mu*\frac{\partial u_z}{\partial r}

The udf is here :
Code:

#define Cf 1                        /* friction coefficient */
#define TAU0 0.2                        /* limit yield stress for slipping */

DEFINE_PROFILE(wall_velocity_x, thread, position)
{
        face_t f;
        Thread *t0;
        cell_t c0;
        double VC[ND_ND];
        double TAUxy, VC_MAG, vg, mu;
        double dudy;
        begin_f_loop(f, thread)
                {
                t0 = THREAD_T0(thread);                /* adjacent cell thread to f */
                c0 = F_C0(f, thread);
                dudy = C_DUDY(c0,t0);
                mu = C_MU_L(c0,t0);       
                VC[0]=-mu*dudy;
                VC[1]=0;                                /* stress vector's y componant*/
                if (NV_MAG(VC)==0)                /* stress vector's magnitude */
                        {VC_MAG = 0.00000001;}
                else {VC_MAG=NV_MAG(VC);}
                vg=VC[0]/Cf;
                if ( VC_MAG < TAU0)
                        {
                        F_PROFILE(f, thread, position)=0;
                        }
                else        {
                        F_PROFILE(f, thread, position)=vg;
                        }
                }
        end_f_loop(f, thread)
}

At first I run Fluent with a "wall" boundary condition. And after some steps, I change the wall to a "velocity inlet" boundary condition in order to put the udf as a component of the axial velocity.

But, the continuity residual goes to 10^{20}.

I think the reason is the fact that I use the derivative of the adjacent cell of the boundary to put it as the value of the face of the boundary. But I don't know how to make it in an other way.

Help me please !


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