How to modify velocity with UDF

jddmsh

Hi, everybody! I need to modify axial velocity to be reversed immediately in a unsteady system. I used the coupled solver. So before flow velotity vector being reversed, I set the time step as 0.0000001s, and the number of time steps is 1, the max iteration per time step is 1. after this, I complied my udf, it is: #include "udf.h" DEFINE_ADJUST(x_v,d) { Thread *t; cell_t c; thread_loop_c (t,d) { begin_c_loop (c,t) C_U(c,t)=-C_U(c,t); end_c_loop (c,t) } } But after iteration,I found the velocity was not reversed. I changed C_U(c,t)=-C_U(c,t) with C_U(c,t)=-C_U_M1(c,t), but the result is not changed. Anany body?