peritalsis problem using DEFINE_GRID_MOTION
 

hi all,
i am doing a sinusoidal peristalsis problem in which the radius of the wave is given by: R(x,t)=epsilon+(a/2)(1-cos(2*pi/lambda(x-ct) . the respective values are provided. the program is supposed to take time, and x as input.
i have used a cfd as follows:
#include "udf.h"
DEFINE_GRID_MOTION(peristalsis,domain,dt,time,dtim e)
{
Thread *tf = DT_THREAD(dt);
face_t f;
Node *v;
real NV_VEC(axis),NV_VEC(val);
real val1, val2;
int n;

/*** set deforming flag on adjacent zone ***/
/*SET_DEFORMING_THREAD_FLAG(THREAD_T0(tf));*/
/*NV_D(axis, =, 1.0, 0.0, 0.0);*/
begin_f_loop(f,tf)
{
f_node_loop(f,tf,n)
{
v=F_NODE(f,tf,n);

if (NODE_X(v) > 0.000667 && NODE_POS_NEED_UPDATE (v))
{
NODE_POS_UPDATED(v);*/

val1= 1-(cos((2*3.14/0.1)*(NODE_X(v)-0.0002*time)));
printf("%d",NODE_X(v));
val2= 0.001+((0.0004/2)*val1);
val[2] = val2;
printf("%d",val2);
NV_V(NODE_COORD(v), =, val);
/*NODE_Y(v) = val2;*/
}
}
}
end_f_loop(f,tf);
}

i have used very small time steps, but it is still giving negative cell volume as an error at the very first step. kindly help me.
hoping for a reply.
thanx a lot

kindly help as i am eagerly waiting for it