Hi everyone, I’m working on MFIX-DEM simulations with user-defined particle locations and size. The particle locations are in the *particle_input.dat* file, and the diameters are read from a separate text file by using *usr0_des.f*. In addition, *usr1_des.f* is used to implement a fluid-to-solid drag force by looping through all the IJK cells and then calling the subroutine DES_DRAG_GP. The inputs to this subroutine are particle ID, particle and fluid velocity, and local fluid (or gas) volume fraction *i.e.*, fluid volume fraction in the IJK cell. Particle ID and velocity are MFIX-computed, and I have calculated the fluid velocity outside of MFIX. This known fluid velocity is defined in *usr1_des.f*. However, I’m concerned with the local fluid volume fraction calculation. I understand that this is calculated as (1 - solid volume fraction). When DES_INTERP_ON is “false”, the local solid volume fraction should be the total particle volume (sum of PVOL(NP), for all NP belonging to the IJK cell) divided by the cell volume (VOL(IJK)). I separately computed the total particle volume and cell volume and verified that the calculations match with that of MFIX. But their ratio does not match with that returned by the function used to calculate local solid volume fraction *i.e.*, EP_S(IJK, xxM), found in *fldvar_mod.f*. I’d be grateful if someone could help me figure out why the calculations do not match and a method to verify the local solid/fluid volume fraction, for my case of polydisperse particles

Thanks