Match of time step

Submitted by rqwang on Sat, 08/20/2011 - 06:13

Hi,

Could anybody tell me how to setup the various time step in cfd-dem, i.e.

CFD/constant/couplingProperties
DEMts
couplingInterval

CFD/system/controlDict
deltaT

DEM/in.xxxx
timeStep
fix cfd all couple/cfd/force couple_every 100 mpi

How to keep these time variables consistent?

And anything else has to be adjusted if deltaT in controlDict changed?

Thanks.

rq

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cgoniva's picture

cgoniva | Sun, 08/21/2011 - 22:39

Hi!

set the DEM TS in the in.xxx according to the needs of the pure DEM problem,

then set the same value in couplingProperties as "DEMts" (sorry for this redundance - it sould be simplified in former LIGGGHTS and CFDEM versions)

then set the coupling interval in couplingProperties as "couplingInterval" which refers to the DEM timesteps. Depending on the problem you will need to have a close (small couplingInterval) / loose coupling.
(e.g. if DEMts=0.00001s and couplingInterval=10 it will exchange data (=couple) every 0.0001s)

now choose the CFD timestep in the controlDict. it must be equal or smaller than the coupling time, otherwise you will get the error: "Error - TS bigger than coupling interval!".
(in above example this means that the CFD timestep must be <= DEMts * couplingInterval = 0.0001s)

Hope that clearifies things...

Cheers,
Chris

raguelmoon's picture

raguelmoon | Mon, 08/22/2011 - 09:04

Hi,
Would you please tell me about taking the dimension size of simulation box both in DEM and CFD? I mean both dimension size (in CFD & DEM) should match or what? For, example, in DEM the dimension of box is 0 1 0 1 0 1 then what will be dimensions in CFD: equal to, greater than or what?
Thanks in advance
Ram

Ram

cgoniva's picture

cgoniva | Mon, 08/22/2011 - 09:53

Hi!

in general you wish to do a simulation of the same domain with DEM and CFD - then the domains should match.
But this is not a rule - they can of course be of different size (e.g. DEM domain is just a small part of the CFD domain)

Chris

cgoniva's picture

cgoniva | Mon, 08/22/2011 - 10:38

One more comment:

with the next version update of CFDEM (2.0.5) manual setting of DEM timestep in the couplingProperties will no longer be necessary if MPI coupling model is used. (timestep information is gained directly from LIGGGHTS).

Cheers, Chris