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Old   December 2, 2014, 10:41
Default Nonreacting bluff body burner
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Kristijan Krapic
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Hello,

As part of a project I'm doing a simulation of a nonreacting flame (basically an air jet) and comparing to experimental data. For that purpouse, I'm using the rhoSimplecFoam solver since the air jet velocity is 61m/s (Ma=0.18). The jet orifice is 1.8mm in radius and there is a coflow of 20 m/s extending from the bluff body wall to the edge of the domain.

The velocity profile I am getting is OK close to the orifice (jet exit) but is consistently underpredicted further away from the exit which means that the air jet doesn't "reach as far" as it would in reality.

I know this is a very general and broad question, but does anyone have any ideas what might the problem be? I've been working on this for 3 weeks iteratively modifying boundary conditions etc. with no luck.

I can include my case file if you need it.

Thank you for your guys help.
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Old   December 4, 2014, 11:42
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Are you sure that your turbulence model is capable for these kind of flows? I think the results don't seem to be unphysical, just some slight difference between measurement and simulation. Which model do other people use (papers?) for such problems?
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Old   December 6, 2014, 10:23
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Quote:
Originally Posted by RodriguezFatz View Post
Are you sure that your turbulence model is capable for these kind of flows? I think the results don't seem to be unphysical, just some slight difference between measurement and simulation. Which model do other people use (papers?) for such problems?
Unfortunately, I am not very familiar with the CFD theory so I am not sure about my choice of turbulence model. There isn't a lot of literature on this topic and even the papers I did find don't mention which turbulence model they use. I tried running the simulation with both the kEpsilon and the kOmega turbulence model and the results I get are generally the same. I tried playing with the values of k, epsilon and omega on the boundaries etc. but with no luck.

I found recently a plot of the velocity contours for the same set-up and case as mine (picture attached). The biggest difference I can notice is the very large boundary layer on the bluff body wall in comparison with my case. What could be causing the difference?
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Old   December 6, 2014, 11:36
Default Possible cause of problems
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I might have found the problem. One paper states that the kEpsilon model overpredicts the decay rate and the spreading rate of round jet flows. The solution is to modify the constants C_mu, C_e1 and C_e2 of the kEpsilon model in order to make them functions of the velocity decay rate and the jet width.

I posted a new thread about it here:

http://www.cfd-online.com/Forums/ope...tml#post522737
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