Acceleration of a point using a UDF
 

I am trying to get the acceleration of a point using a UDF. I have called in the velocity of the desired cell using c_v(c,t) and use the simple relation (V2-V1)/dt. When I compare the value of the velocity obtained using the udf to the value I obtained using a point surface and surface monitors, there is a very large discrepency. I have tried setting the surface monitor to both area weighted averages and vertex weighted averages (both give very similar results). My results from the udf are on the order of 1e-11 and the results from the surface monitors are on the order of 1e-2. I have also tried averaging the values over 8 different cells. I also tried creating larger cells to see if it made a difference. It did. By increasing the size of my cells my velocity obtained by C_V(c,t) were also larger. The problem is that they will continue to change as I increase my cell size. What is the relationship between the C_V(c,t) and the cell size? The other problem is that I obtained both positive and negative values. Is there something that relates the velocity obtained with the direction of the face? Are there any other suggestions as to how to determine the acceleration of a point using a udf (I need the acceleration to perform other tasks in my udf)?