Hi Will,
Thanks for your message.
I have only run external flows with a range of Reynolds numbers,
including 2D cylinder Re=200, turbulent jet Re=10,000, SD7003 Re=60,000.
All these cases converged ok.
In your previous email, you mentioned that you are using the
forward-euler pseudo-time scheme and "the pseudo time step is 1 over 10
of the physical time step. 10 iteration per physical time step." You
cannot expect any level of convergence with these numbers! I would
encourage you to use RK4 / tvd-RK3 and make the iteration count 10-20
times bigger. Using P-multigrid always helps.
Would you be able to share your case or at least the .ini file? I can
try to quickly run it or at least have a look.
Regards,
Niki
On 20/04/18 07:08, Will wrote:
Dear Niki,
If my understanding is right. It seems that for dual-time AC method in
pyFR, there are difficulties on the convergence of the diffusion term.
Based on my simulations, the pressure residual of the pseudo
convergence file increases with the intensity of the flow diffusion.
Very low pressure residual (less than 1) only happens when the flow is
only in convection dominance but not much diffusion, which makes the
flow behave laminar. When the turbulence intensity of the flow occurs,
the pressure residual is no longer small (around 10) and shows a
strong stiffness (decreases linearly with very small slope). When the
turbulence intensity of the flow increases the pressure residual
increases (larger than 10) and still shows a very strong stiffness. So
the pressure residual in pyFR is more like an indicator of diffusion
intensity but not convergence.
So I don't quite understand how do you converge and meet the mass
conservation (div u = 0) in AC method? It seems that pyFR has
difficulties on holding mass conservation in turbulence.
Have you ever tested the AC solver on diffusion equations? Does it
really show small residuals? As you said "The pressure residual (div
u), which is more stiff to converge than velocity residuals".
According to you, "is kind of an indicator how far the pseudo waves
which distribute the pressure have travelled". Actually, I don't know
the connection between the pseudo waves and mass conservation.
In your cases shown above, you have really small pressure residual.
How is the turbulence intensity in your cases? Is the flow behaving in
chaos or more laminar flow?
Best regards,
Will
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