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 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 .
When the yield stress is less than , there is adhesion of the fluid to the wall.
When the yield stress is greater than , there is a slip velocity at the wall like :

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 .

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.