CFD Online Discussion Forums - Difference between C_U and F

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Difference between C_U and F_U

Hi everyone,

I'm currently setting a boundary condition for the shear stress on the walls, hence all faces i'm dealing with are interfaces.

My model is just a stationary 3-d cylindrical pipe with a velocity inlet and 0-pa outlet.

Why do I get different values for F_U and C_U? Aren't they supposed to be the fluid velocities of the same part/cell?

Code:

begin_f_loop(f,thread){

        v[0]=F_U(f, thread);

        v[1]=F_V(f, thread);

        v[2]=F_W(f, thread);

        cv[0]=C_U(F_C0(f,thread), THREAD_T0(thread));

        cv[1]=C_V(F_C0(f,thread), THREAD_T0(thread));

        cv[2]=C_W(F_C0(f,thread), THREAD_T0(thread));

}

F_X variables are those on the boundary, defined at the centroid of the cell's face residing on the boundary. C_X variables are defined in the cell centers. If they were equal you wouldn't get any stress at all.