[akbar.mech]

hello every body
i am trying to simulate particle tracking using DPM method but i quite dont understand some parameters about it :
1-what is exactly "particle time step size" and how should it be determined?
2-in the injection section if you select surface injection method you need to specify some different parameters and total flow rate is one of them . what i cant understand is that when number of particles at each time step is already known ( for each cell one particle is injected at every time step) and the density and diameter of particles is already determined . using these parameters gives the flow rate of particles (kg/s) so why do we need to enter flow rate again ?
i would appreciate your help