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-   -   time step size too small and max. time steps is not enough (http://www.cfd-online.com/Forums/fluent/123573-time-step-size-too-small-max-time-steps-not-enough.html)

czhao86 September 16, 2013 11:31

time step size too small and max. time steps is not enough
 
Hi,

I am doing a steady state DPM, and the time step is always in the order of 1e-7s with Brownian force on, and it takes a lot of time steps for them to reach the outlet. I changed the 'Step Length Factor' from 5 to 1, and the time steps I assume only change by 5 times, and it is still too small. Anyone have some idea? Thanks.

flotus1 September 16, 2013 12:19

Its the Brownian forces again...;)

I assume you have rather small particles.
The usual integration mechanisms when Brownian forces are involved only work (because of stability AND accuracy constraints) if the time step size is in the order of or smaller than the particle relaxation time.
For small particle Reynolds numbers (c_d = 24/Re) the particle relaxation time is
\tau_p= \frac{d^2 \rho_p}{18 \eta_f}.
Here d is the particle diameter, \rho_p is the particle density and \eta_f is the dynamic viscosity of the fluid.
This relaxation time becomes small for small particles, leading to small time step sizes.

czhao86 September 16, 2013 12:23

So in another word, if the Brownian motion is on, I can not simulate several hundreds of seconds process, as the time step is 1e9...

flotus1 September 16, 2013 12:27

You could do this with implicit integration methods, but this will lead to huge errors in the particle velocity distribution.
I dont know if this is possible in Fluent.

czhao86 September 16, 2013 13:27

Thank you very much. Time for me to learn implicit theme then...


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