Is the turbulence model really accurate,or it's just by chance?
The turbulence model may get pretty good results for one case,compared to the experiment. But if we switch to a problem with similar flow mechanism, the same turbulence model may get quite bad results. I think some times the error just cancel with each other and we may get a 'good' simulation result by chance. The problem is, how to define if the model really works well or it just happens to work well?
As for simulation of complexed problem, the case available may be quite limited. Sometimes we may even just simulate a single case. In this case I find it really hard to judge which model behaves better with such little imformation. Is there any advice?
While I don't have the answer to your question, it's very true that wrong answers do look like right answers, and that's more often than we'd like to think. Just with the mesh already you've got two primary mechanisms at work: mesh divergence and mesh dilution.
So you might have a concentration which makes a certain value jump really high but at the same time have the mesh to coarse so it spreads across a wider area. Both yank at the other, and you can have a seemingly valid answer. Dangerous stuff.
Before one trusts a code or the implementation of a turbulence model, the code and model should be verified and validated. Something along the lines of http://turbmodels.larc.nasa.gov/index.html.
One should understand the flow physics being modeled, the turbulence model itself, and the experiment.
One should also run many simple cases and compare to what is out there to gain experience. Try to break the model, then back off to see where it works.
Yes, RANS codes can give bad results. It is just a model. But many bad results are also due to bad usage or bad practices.
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