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 Jitendra March 16, 2002 10:59

planar mixing layers

I am trying to simulate compressible planar mixing layers after a splitter plate. One supersonic stream M 1.5 is on one side while other side is stagnant.(very small velocities to avoid numerical difficulties) I tried with standard k-e model and got good results. But when I applied compressibility corrections (sarkar and zeman models) results are converging to some absurd values... mixing layer is not developing ... has any body faced similar problems.

Any help would be appreciable

jitendra

 Doug March 16, 2002 16:27

Re: planar mixing layers

If you got good comparisons with experimental mixing rates with standard k-epsilon, it wouldn't be too surprising to see the compressibility corrected models predict too slow a mixing rate. As I'm sure you know, that's what the compressibility-corrected models effectively do - they lower the Mu_t values to reduce the mixing rates.

In our applications (which have mostly involved using wall jets for window cooling of high speed interceptors), standard k-epsilon often worked well and we didn't need to use the Sarkar or other corrections.

However, for some other cases, the standard k-epsilon gave trash and the compressibility corrections gave much better results.

The important thing from an application point of view is to anchor (i.e. read calibrate) your codes for the class of problems of interest using data for similar configurations and understand the limitations of the models.

One other thought - you could be running into a numerical instability based on your initial conditions or a bug in your coding of the turbulence models.

One way to check that out we be to take the "good" solution you get with the standard k-epsilon and then "restart" your calculation with the compressibility corrections turned on. What should happen then is that the mixing for the compressibility-corrected solution should then slow down, and the mixing will simply take longer. If you get convergence back to the "absurd" values, I'd get guess you have a bug.

Hope some of this is helpful. Best wishes in getting to the bottom of all this.

 Quain Tchew March 17, 2002 03:43

Re: planar mixing layers

Hi!

I'm study a code, which uses a method to do compressibility correction. I don't understand it, but I guess it is a kind of SEHK compressibility correction.

I want to know where can I get some materials about SEHK compressibility correction and sarkar compressibility correction.

Thanks

 Jitendra March 18, 2002 05:21

Re: planar mixing layers

Thanks doug,

>>If you get convergence back to the "absurd" values, >>I'd get guess you have a bug.

I am working on these lines now, as i am getting absurd results even after giving initial conditions of the solution from standard k-e.

something for Quain too....

here are some references on SEHK and zeman corrections,

Sarkar et. al. " The analysis and modelling of dilatational terms in compressible turbulence", ICASE Report 89-79 Zeman, O.(1990) "Dilatational dissipation - the concept and application in modelling compressible mixing layers", Physics of fluids, Vol 2, no. 2, pp 178-188.

one other paper which u may want to read is,

Viegas & Rubensin, "A comparitive study of several compressibility corrections to turbulence models applied to high speed shear layers" AIAA 91-1783.

As I understand from your previous posting, you are working on NSC2KE code

>In the code, if nearwall tubulence MACH is greater >than 0.25, the modification is made.

The compressibility correction in your code by most probability is zeman dilatational term ( I have given the ref.) which has a typical charateristic of providing a threshold turbulent mach number to apply corrections....

hope it is useful,

Regards, Jitendra

 Jitendra March 19, 2002 01:02

Re: planar mixing layers

Hi Doug,

I got the bug... now getting good results... in the process of reproducing the famous langley curve for planar mixing layer with my code...

thanks

jitendra

 Doug March 19, 2002 08:59

Re: planar mixing layers