Pressure gradient in particle simulation
 

Dear friends, I am dealing with a particle simulation program and facing a problem now. My model is a channel flow(poiseuille problem) while the fluid is enduring a pressure gradient. Instead of imposing pressure gradient on the whole flow field, I add a constant force on each particle. Are there some problems in that simplification? There are supposed to be some coz I failed several times in getting the value of stress. Thx!