CFD Online Discussion Forums - Momentum by force equation in UDF

Hello,

I am simulating a wall by momentum equations in Cell Zone Conditions. The flow is stationary.
I have the force given by equation like:
F=c*uxn
Where:
F - force vector
c - constant
u - velocity vector
n - wall normal vector
Force generally is F=momentum/time derivative.
So, my question is, how to provide time in UDF in steady/stationary flow?

For example for X direction of flow I would have something like:
Lx=c*ux*nx
and I have to convert the force into momentum, so I would need to derive by time:
Lx/dt=Mx=c*ux*nx/dt

Code:

#include "udf.h"



DEFINE_SOURCE(name,c,t,dS,eqn)

{

real c, x_vel, L, source, n;

c=5;

x_vel=C_U(c,t);

u=x_vel;

n=2;



L=c*u*n;

source=L/dt;



return source;



}

My question is how to write line:

Code:

source=L/dt;

in a proper way?

Thank you for help!