How to calculate velocity at a interior surface?
 

Im trying to get the velocity at a certain surface by udf.
First I try to use
F_U(f,t)
but when it runs, the fluent will stop and say there is an error. I further examined it and found out it can only be used for boundary surface
Then i try to calculate the surface velocity by averaging the cells before and after the surface. Then i can get results but it is different from the contour result. I think the contour results is calculated by staggered grid? So how can i get exactly the same results as in the contour by udf?

Thanks