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Old   March 31, 2015, 07:20
Default Reason for different results
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Hi all,

I'm doing a report on pipe in pipe heat exchangers and I'm comparing the results I have to a previous student's results. We draw the same conclusions however the results and calculation we obtained are slightly different and I would like to talk about why this is.

I was wondering if anyone could give a brief explanation to the reasons that could affect this?

I believe it is something along the lines of different meshs resulting in different mesh quality. Also to do with the convergence of the residuals as my residuals were kinda all over the place but my supervisor recommended this is ok and it is occurring because the physics are still working (didn't really understand)

Help would be appreciated

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Old   March 31, 2015, 17:03
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When you use a different mesh, you don't expect the same results. You must perform a mesh convergence study to see how the results are affected by the mesh. And regarding convergence, a lot depends on the geometry/mesh/flow physics. What was your convergence criteria? Provide some info about your simulation and post pictures of the residuals, mesh and geo.

A very good thread for convergence:

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Old   March 31, 2015, 23:53
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I always doubt the quality of CFD done by others unless its someone I am really familiar with and can trust.

For example, one thing I always look out for is how the other person defined "convergence". Many will define convergence as residuals below a certain threshold, which is a terrible definition and can lead to wildly different results (especially if you are comparing local results).

Mesh quality is also very important, a few skewed cells being in the wrong place is all that it takes to skew the result one way or the other.

Of course, if they choose to model the problem differently (i.e ideal gas instead of constant density) then you should expect results. But even when all options are the same, it's interesting to note the difference in what should be the same result.
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