How to write udf of slip wall condition
 

hello: i am computing a micronozzle, it's wall condition is slip,it is to say that the velocity of fluid on the wall is not zero. and fluent is need to acquire the shear stress of x and y component , so i should know the dU/dx or dU/dy. and the slip velocity is U=K*du/dx, here k is a constant, and du/dx and be know from udf Micros. My question is how can i know dU/dx after i know U(here U is present velocity and u is velocity of previous time step). and is there any other Micro in udf can finish my work?? thanks