divergence in AMG solver
 

Hi im trying to use the following equation in udf. it does compile it but when i use it as a boundary condition for wall it says : error : divergence detected in AMG solver : x-momentum i use it for modeling slip flow on a half circle and the term [ (8*x*y/(D*D) ] is for 2*sin(a)*cos(a) where sin(a)=y/(D/2) , .... after omiting other terms and trying the other ones i found that the problem is the above term . can any body help me?

F_PROFILE(f,thread,i)=(x/(sqrt(x*x+y*y)))*coef*kn*D*((8*x*y/(D*D))*((C_U_G(c,cell_thread)[0])+(C_V_G(c,cell_thread)[1]))+((C_U_G(c,cell_thread)[1])+(C_V_G(c,cell_thread)[0])));

regards