When using the discrete phase model with two way turbulence coupling I find that the particles don't seem to change the fluid velocity. I am not sure what the cause of this is. I run the particles through every 30 fluid iterations. There is an initial affect on the residuals through the source term but then the fluid just seems to go back to it's converged single phase state. Is it may be the fact that the new source term doesn't remain for all the 30 fluid iterations, so doesn't get chance to affect the fluid?

What i understand is, momentum source term due to particle phase is always there with all the 30 iterations. Its magnitude certainly gets changed with every iterations since it's dependent on the slip velocities. Overall, solution has the impact of discrete phase evenif you have 30 iterations/DPM iteration. How much effect it should have depends on yr case and its conditions.