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-   -   CFD Sensitivity Check Method (http://www.cfd-online.com/Forums/main/92632-cfd-sensitivity-check-method.html)

zephyrus17 September 20, 2011 09:09

CFD Sensitivity Check Method
 
I know it's important to do a sensitivity check with grid refinement or turbulence model, but which do I do first? Should I:

1) Stick to a turbulence model first, then refine the mesh, then compare each turbulence model suitable at that mesh setting?

2) Or refine the mesh individually for each turbulence model chosen, then compare them?

I've read the Wiki on Accurate results and "Why won't it converge", this is mostly more of a theoretical methodology-based question

mettler September 20, 2011 09:16

I always do a grid check. Also, it is important to know that it is not always a 'refinement' that you are after. I usually do a 'double' rule in which I will cut the number of cells in half and see how my answer compares. I will also double the number and see how it compares. Sometimes you will find that you have way too many cells and you can speed the computational time by trimming that down. That might only be the case for GIANT grid numbers, so I guess it depends on how many you have to begin with. But, you do have to show that you have the mesh correct.

zephyrus17 September 20, 2011 09:30

Yes, I always do a grid check by doubling the number of nodes, then checking the results, then doubling again if the results change a lot. But I'm wondering which I should do first, like I said above.

mettler September 20, 2011 09:33

I'd do grid first - if your mesh is bad then it won't matter what kind of turbulence model you pick.

zephyrus17 September 20, 2011 20:40

All right. That's good. Thanks! :)

Dorit September 26, 2011 07:04

Hi, I've done quite some research on that for flow over different airfoils. My conclusion is that since different turbulence models can potentially result in different flow features, you should strictly speaking do a mesh study on every model that you intend to use. Now since that is a lot of work I found that similar simulation results can be obtained for each group: all one equation models, fully turbulent 2 equation models and 2 equation models with wall damping functions. Different transitional models I found behave quite different to one and another.
Also, I know that it may sound contradictory but don't always expect to get an asymptotic curve of a certain variable as you increase/decrease your mesh density. For instance a very fine near-wall mesh may result in divergence from the asymptotic region. Another example is the SA model which seemed to produce more scattered results than say for example the RNG k-epsilon model.
Hope that helps.

Rophys September 26, 2011 10:11

Hello all
 
Hi all,

I am new in the CFD area and I have been reading a lot of things that is necessary to spend a good time preparing a good CFD mesh. A good mesh will facilitate the code convergence and will give me accurate results. I also ready that the CFL number should be less than 0.2. In this way, which parameters should I take into account to generate a good mesh. Let's take the convergent divergent nozzle example with 500 mm of length and initial velocity equal 10 m/s.


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