Calcuationg mass flux of a UDS?
 

I'm modeling mass diffusion of a solid "tablet" mounted along one wall in a channel with water as the flowing fluid. I have a diffusivity coefficient and surface concentration for the tablet. Currently I'm using a UDS with specified values for surface concentration on my defined tablet surface. I need to calculate the mass flux along the tablet surface for a variety of flow rates through my channel. Do I need a UDF to do this? If so, could anyone direct me to where I might find the necessary UDF?

Alternatively, is there a different, better way to set up my scenerio, or is UDS the best way? I'm questioning the UDS because when looking at the concentration gradient for a wide range of velocities, the concentration boundary layer is nearly identical. Thanks