criteria for guessing coupling interval

Submitted by evansmuts on Tue, 06/25/2013 - 15:26

Hi

Is there any official criteria for guessing the coupling interval size and/or CFD timestep size based on particle/fluid properties? (DEM has the Rayleigh and Hertz criteria based on particle properties for particle collision time steps)

Or is it just a matter of trial and error (experience) to find what works best?

Cheers
Evan

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evansmuts | Fri, 06/28/2013 - 12:31

Hi

To add to my previous post, there is a relation for the "response time" for particle (i.e. the time taken to respond to changes in flow velocity). The following relation for low Re flow (Stokes flow), is taken from the book "Multiphase Flows with Droplets and Particles" by Crowe, Sommerfeld and Tsuji:

v = u( 1 - e^(t/TauV) )

where

TauV = (rho * D^2) / (18 * mu)
u = fluid velocity
v = particle velocity
rho = particle density
D = particle diameter
mu = fluid viscosity
t = response time

TauV, often called "momentum response time" or "relaxation time", is the time taken for the particle to reach 63% of fluid velocity.

The CFD timestep should be smaller than this value. From there you can get the coupling interval.

Cheers
Evan