How to
Hello,
I am trying to get the drag force experienced by particle through post-processing. When I run the MFiX-DEM simulations, I monitored and output the drag_coefficients of particles in the GUI Output-Vtk. The procedure to get drag force in my way is:
1st, determine which particle is studied;
2nd, determine which mesh cell the particle is in;
3rd, calculate the slip velocity (velocity difference between particle velocity and cell / gas velocity)
4th, read the drag_coefficients in the particle data file;
5th, get the drag force through drga_cofficients, particle diameter, slip velocity and gas density.
However, the drag force got from the above procedure is quiet unreasonable, because it is much lower than the particle weight. In fact, the drag_coefficients output are on the orders of magnitude of -7, which is much lower than expected.
I don’t know what is going wrong. Are the drag coefficients reasonable ?
Please attach your project files if you want us to look at your case. Go to the main menu, select “Submit bug report” and upload the ZIP file here.
Thanks for your reply! After look up the source file, I found the drag_coefficient defined in source codes is different from the textbook in chemical engineering. In MFiX code, the drag coefficient was defined as :
" Note F_GP obtained from des_drag_gp subroutine is given as:
f_gp=betavol_p/eps where vol_p is the particle volume.*
The drag force on each particle is equal to:
- beta(u_g-u_s)vol_p/eps.
- Therefore, the drag force = f_gp*(u_g - u_s) "*
After revised the formular, the calculated drag force is comparable to particle’s weight. However, the drag force is about only 0.45 times of particle weight, even in conditions where particle bed is fluidized. Is that proper?
All the files output by mfix is packed here.
0.096m_s.zip (31.1 MB)
Thanks for your advice. In fact, I have tried some other drag models, such as Gidaspow model, and the results are similar. The following is the results with Gidaspow. Therefore, I think it’s not caused by drag models.
0.2m_s - Gidaspow.zip (1.4 MB)
As you can see, the particles have been fluidzed. In my opinion, the Drag force shoule be roughly equal to gravity. However, the drag force computated through post-process is much smaller than expected.
At the same time, I try to output the drag force particle by particle in MFiX-DEM through user-defined-scalar. The files complied well. However, the output Usr_Defined_Var_1 is on the magnitude of 10^(-8). The particle weight is about 2.6e-7 N. I think the drag force output is also unreasonable.
The *drag_gs_des1.*f changed was included in the uploaded files.
test_1113.zip (28.6 MB)