Different result with different machines?

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Tsubasa
Posts: 175
Joined: Fri Nov 20, 2020 4:09 am

Different result with different machines?

Post by Tsubasa »

Hello,

I wonder simulation results are different if machines are different.
I run two simulations with the completely same setting.
Surprisingly the results are not identical at all.

I run 12000step, and in the first 50steps the results are the same as each other.
Is it true?
I think a very very small difference (less than 0.0001) accumulated over the 12000step and caused a big difference finally.

You can see the same results in step1 but different results in step12000.

Step1
Machine1

Code: Select all

 INSTANT    0.100000000E+00   TIME STEP NUMBER               1
 ============================================================= 
 ** Valeurs calculées sur cellules
     ------------------------------

   criterion               minimum         maximum   moy. ensemble   moy. spatiale
   ---------------  --------------  --------------  --------------  --------------
   Courant/Fourier          3.1321          3967.8          204.92          10.034
Machine2

Code: Select all

 INSTANT    0.100000000E+00   TIME STEP NUMBER               1
 ============================================================= 
** Computed values on cells
     ------------------------

   criterion               minimum         maximum        set mean    spatial mean
   ---------------  --------------  --------------  --------------  --------------
   Courant/Fourier          3.1321          3967.8          204.92          10.034




Step12000
Machine1

Code: Select all

INSTANT    0.120000000E+03   TIME STEP NUMBER            12000
 ============================================================= 
 ** Valeurs calculées sur cellules
     ------------------------------

   criterion               minimum         maximum        set mean    spatial mean
   ---------------  --------------  --------------  --------------  --------------
   Courant/Fourier        0.013982           310.3          8.8389          2.4452
Machine2

Code: Select all

INSTANT    0.120000000E+03   TIME STEP NUMBER            12000
 ============================================================= 
** Computed values on cells
     ------------------------

   criterion               minimum         maximum   moy. ensemble   moy. spatiale
   ---------------  --------------  --------------  --------------  --------------
   Courant/Fourier       0.0090879          314.62          8.6604           2.369
Yvan Fournier
Posts: 4070
Joined: Mon Feb 20, 2012 3:25 pm

Re: Different result with different machines?

Post by Yvan Fournier »

Hello,

Yes, this is quite common, and due to numerical sensitivity (linked to handling of round-off errors). To determine if there is a problem or not, you need to look a things in more detail :

Do you have fluctuations in the solution ? Probes are useful for this, and in many cases, generating and outputting time averages can be quite useful for this (if you take the example of LES computations, the instant results on two machiens or 2 time steps will change, but the statistics should remain similar).

Regards,

Yvan
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