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-   -   Courant number 1 and still stable (https://www.cfd-online.com/Forums/openfoam-solving/60649-courant-number-1-still-stable.html)

 Pei-Ying Hsieh (Hsieh) February 22, 2005 09:47

HI, I recently ran a case

HI,

I recently ran a case using Fluent:
VOF, 2 immiscible fluids (air and water), no phase change, axi-symmetric flow, double precision, surface tension constant and wall contact angle specified on two different walls (70 and 90 degree).

Dominant flow direction is in X-direction.
samllest element in X-dir is 2e-5 meter, specified inlet velocity is 50 m/sec (the highest speed in the domain is around 100 m/s). Specified delta T is 1e-5 (constant). Based on these values, CFL: is around 50. Under-relaxation factor for pressure was set to 0.5, for density, momentum, and body force were all set to 0.6. I still got a reasonable solution.

I built a OpenFOam case using the same parameters (except for the under-relaxation factors, I have no idea where these are set when using PISO scheme). If I appled auto-adjustable delta t, then, the delta t quickly dropped to 1e-12. If I force it the stay at constant delta t, then, solution quickly diverged.

Any suggestion? Thanks!

Pei

 Mattijs Janssens (Mattijs) February 22, 2005 12:44

The piso scheme does not allo

The piso scheme does not allow underrelaxation. There is no equivalent steady state solver for vof so you are bound by the time step limitation. However your value is much too low and probably the result of an incorrect setup. Check the discussion thread on extending the inlet.

Mattijs

 Pei-Ying Hsieh (Hsieh) February 22, 2005 21:05

Hi, Mattijs, This is a tra

Hi, Mattijs,

This is a transient problem.

I am just wondering why my fluent calculation did not diverge. The converged solution did match our experimental data though.

I am checking my OpenFOAM setup anyway.

Pei

 Henry Weller (Henry) February 23, 2005 07:12

If it is a transient problem

If it is a transient problem why did it converge?

 Pei-Ying Hsieh (Hsieh) February 23, 2005 09:08

Hi, Henry, In Fluent, it i

Hi, Henry,

In Fluent, it iterates within each time step until solution converges within that time step, then march to next time step. I did get a converge solution within each time step - this is what I meant by "converge".

I am puzzling why the solution did not quickly diverge when running Fluent - I am just asking possible reason(s) for this.

Pei

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