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Old   November 10, 2013, 16:22
Default Parallel computing error estimation
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Rui Sun
Join Date: Apr 2013
Location: Blacksburg, VA, U.S.A.
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Dear All,

I am a beginning user and programmer of OpenFOAM and now I have some problems with parallel computing.

For the damBreak case, when I use only one processor(serial), the velocity field (U) I got is:

Quote:
20 internalField nonuniform List<vector>
21 7700
22 (
23 (0.012614 -0.0077165 0)
24 (0.0161998 0.00274904 0)
25 (0.00985702 0.00360953 0)
26 (0.00423264 0.00402813 0)
27 (-0.00493139 0.00445166 0)
28 (-0.0142829 0.00484226 0)
29 (-0.0246771 0.00519022 0)
30 (-0.0353863 0.00544968 0)
31 (-0.0464376 0.00561797 0)
............
but when I use more 4 processors, as is told in the User Guide, the result goes:

Quote:
20 internalField nonuniform List<vector>
21 7700
22 (
23 (0.0125989 -0.00766445 0)
24 (0.0161874 0.00272086 0)
25 (0.00982447 0.00353503 0)
26 (0.00420407 0.0039803 0)
27 (-0.00496663 0.00443006 0)
28 (-0.0143201 0.0048252 0)
29 (-0.0247386 0.00517623 0)
30 (-0.0354804 0.00543816 0)
31 (-0.0465716 0.00560836 0)

............
The result seems quite agreeable with the previous one, but I increase the number of processors to 16, the result changes again:

Quote:
23 (0.0126039 -0.00767821 0)
24 (0.0160584 0.00272547 0)
25 (0.00958388 0.00355593 0)
26 (0.00387355 0.00401163 0)
27 (-0.00542264 0.0044613 0)
28 (-0.0148699 0.00485382 0)
29 (-0.0253798 0.00519895 0)
30 (-0.0362003 0.0054529 0)
31 (-0.0473606 0.00561507 0)
............
And the difference becomes approximately 10 times larger.

I know that this error may probably come from parallel computing, but how can we predict how much the error is and make it as small as possible?

Best,

Rui
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