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Different results when running on different number of threads

User: "Josh Fontana"
Altair Community Member
Updated by Josh Fontana

Hi Everyone,

I am running an acusolve job on an HPC cluster, and I found that the results are different for the same test-case run on different numbers of threads. And if I run the job on 6 threads, or 10 thread, it gives me this warning:

*** WARNING: inflow at outflow boundary: Outlet_3 (mass/time: in=-1.487182e-09, out=3.392650e-03)

whereas it does not give that warning if I run on 3, 4, 5, or 8 threads.

 

Some output log files are attached. Does anyone know what the problem might be?

 

The case itself is a simple internal flow problem through an expanding circular duct.

 

Thanks,

Josh

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    User: "acupro"
    Altair Employee
    Accepted Answer
    Updated by acupro

    Ok, thanks for the info. I was told some variation can be expected in these sorts of codes, when running on multiple threads. Would you think this level of variation is OK?

    image

    The first table shows the maximum values of velocity, pressure, and eddy viscosity for each case. The second table shows the percent difference between the maximums for the 20 proc and 10 proc cases (2nd column), and between the 40 proc and 10 proc cases (3rd column) for each variable.

     

    I'll admit, my mesh may not be the best-quality mesh. Could this much variation be due to a poor mesh?

    The difference for the Z-Velocity value is the largest percentage.  But if you look at the actual value, the largest difference is about 0.6E-4 m/s - very small, indeed.  It's also likely the location of the reverse flow portions will not be stable, so could wander around a bit.  You could extend the outlet a bit, and maybe add a contraction, to avoid the reverse flow.  (I always try to avoid reverse flow if all possible.)

    It could certainly still be related to mesh, too.