[fivos]

Hi to everyone,

Has anyone any explanation what the unsteady flux dissipation correction does exactly at transient compressible simulations? This is a new feature in v9.02, under the segregated flow (continua-> physics -> models -> segregated flow). In the documentation there are not much info, apart from:

Enable/disable mass flux dissipation corrections for unsteady simulations, modifying the coefficients in Eqn. (208), based on CFL

and

The limiting acoustic CFL, applied for unsteady simulations, to scale the mass flux dissipation. The default value is 1.0. Larger values damp more spurious oscillations, but at the expense of accuracy.

I have simulated two highly compressible case with a custom EOS (in all cases the time step is chosen in such a way that the acoustic courant is 0.5):

- A shock tube: without the unsteady flux dissipation correction the solution matches well with the theory; the characteristic regions are predicted correctly (L, R, L*, R*) and the wave speeds as well, while the solution is sharp at the shock and smooth at the rarefaction wave. With the unsteady flux dissipation correction (CFL = 1) the main characteristics of the solution are the same (wave speeds and region characteristics), but there are additional wiggles and oscillations introduced near both the shock wave and the rarefaction wave! This seems to contradict what is mentioned in the documentation..

- A foil: without the unsteady flux dissipation correction I get unphysical high values for pressure and also there is a checker-board pattern in the pressure field (!). With the unsteady flux dissipation correction (CFL = 1), I get a much clearer pressure field.

From the two cases there seems to be a contradicting behavior of this correction. Does anyone have any additional information?

Thanks in advance.
Regards

[yannaos]

You can find the detailed formulation for the Unsteady Dissipation Corrections within STAR-CCM+ help (v902) under:

User Guide > Modelling Physics > Modelling Flow and Energy > Fluid Flow Formulation > Segregated Flow Formulation > Continuity Equation in Discrete Form

and there, please look for the Unsteady Dissipation Corrections section.

[fivos]

Thanks yannaos for your reply. I have seen the exact formulation for the unsteady dissipation correction in the density correction, but, since it is entirely focused on formulas, I cannot understand its exact meaning and, most importantly, its effect (apart from that it damps oscillations, even though the shock tube test case showed the opposite..).

To be honest with you, its the first time I see this correction and I am not sure if I apply it correctly. I am going to search for it more thoroughly, but do you (or anyone else) know any relevant publications showing the idea of this corrector and maybe test cases?

Thanks in advance.
Regards.

[yannaos]

Within the STAR-CCM+ guide, under Release Notes>..> Enhancements, you can find also the following bit of information about the Unsteady flux dissipation corrections:
"Correction based on the local acoustic CFL number to reduce spurious sound wave reflections for unsteady acoustics simulations".
Strictly speaking, your case 1/ does not contradict this tool, because as you pointed out, you had a well behaving solution. So, why using corrections tools?
Your second one, is much more interesting: If confirmed, it would mean that this tool could provide a solution to checker-board problems.

[fivos]

Ok Yannaos, thanks for your time and responses. If I find anything new, I will post it here..