How to define particle time step

MFiX
#1

Hello everyone,

I am new user of Mfix and also new into this field. I am using Mfix18.1.

There are two time steps; Fluid time step and Particle time step. To the best of my knowledge we are only defining Fluid time step dt on the interface.

My question is How we define particle time (Do we or it just calculate on its own)

Note: I am solving nano-particle fluidisation in 2D bed using DEM.

I tried hard to find out the manual where detailed information about these things mentioned, If anyone of you can send me the link of document. I will be really thankful

#2

It is computed implicitly by the DEM code. Check out the documentation on MFIX DEM!

#3

Hey thanks for your reply,

Can you specifically tell which manual should i refer. A manual which have complete information about the MFix not only governing equation and theory of multiphase.

#4

Hi Alok on the mfix website there is a tab for documentation i.e. MFiX Documentation - Multiphase Flow Science Group at NETL
where you will find the user manual for release 19.1 and other documents.

#5

See section 2.2.2 of https://mfix.netl.doe.gov/download/mfix/mfix_current_documentation/dem_doc_2012-1.pdf

#6

you can also define a fixed particle time step using user-defined function (usr3_des.f).

#7

There it is only mentioned about the dependency of colllision time on mass and stiffness of the particle. Is there any other where the implementation of collision time in Mfix is mentioned.

#8

Thanks, but how is it used normally in mfix means without giving userdefined function how Mfix is implementing Tdem.

#9

The collision time scale, based on the material properties (spring stiffness, particle mass, etc.). That time is then divided by 50 to get the dt MFIX uses to advance particles in time. Since that time scale is usually much smaller than the fluid time step, DEM sub-cycles in time after the fluid advance to “catch up.”

#10

The dem time step is defined as 1/50 of minimum collision time (sorry not fluid time step).
If you do want to change it to for example 1/20, change in the code check_solids_dem.f, assign DTSOLID = TCOLL/20.d0

#11

jia it’s actually a fraction of the collision time as jordan stated above. the fluid time step is completely independent and decoupled from the dem time step and the collision time.

#12

Yes you are right. This statement can be found in Jeff’s link, page 10 the paragraph after eq 33.