# Simple vs piso

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August 21, 2013, 10:52
Simple vs piso
#1
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Dongyue Li
Join Date: Jun 2012
Location: Torino, Italy
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Hi guys,

Im working on the reseach regarding the difference between steady SIMPLE and steady PISO. Its tough to write in this forum. So see the image pls.

My reference is H K Versteeg's book: An introduction to .......

As we know, in PISO we have a second corrector step which make it different with SIMPLE, but my confusion is “why is this second corrector step instrumental to the solving?”

In H K’s book, I make a conclusion(IF I WAS WRONG, CORRECT ME PLEASE):
Attached Images
 PISO.jpg (62.3 KB, 57 views) SIMPLE.jpg (57.2 KB, 42 views)

 August 21, 2013, 11:25 #2 Member   Ren/Xingyue Join Date: Jan 2010 Location: Nagoya , Japan Posts: 44 Rep Power: 8 The corrective pressure computed from the SIMPLE method is not the real pressure, so the second step is to make sure divergence of the corrected velocity field is zero. That is why the projection method is more popular than the SIMPLE method in the computation of unsteady flows.

August 21, 2013, 20:05
#3
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Dongyue Li
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Quote:
 Originally Posted by hilllike The corrective pressure computed from the SIMPLE method is not the real pressure, so the second step is to make sure divergence of the corrected velocity field is zero. That is why the projection method is more popular than the SIMPLE method in the computation of unsteady flows.
Hi Xingyue

Quote:
 Originally Posted by hilllike The corrective pressure computed from the SIMPLE method is not the real pressure,
AFAIS, the corrective pressure computed from PISO is not the real pressure either. but which I think is divergence free.

Am I right?

So, as you had said, both of the first step in SIMPLE and the two steps in PISO are making sure the velocity field divergence free, Why does PISO make it divergence free twice?

If I was wrong above, correct me, Thanks in advance.

August 21, 2013, 21:42
#4
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Dongyue Li
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This will make it clearer.
Attached Images
 1.jpg (49.1 KB, 58 views)

August 22, 2013, 03:22
#5
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Ren/Xingyue
Join Date: Jan 2010
Location: Nagoya , Japan
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Quote:
 Originally Posted by sharonyue Hi Xingyue AFAIS, the corrective pressure computed from PISO is not the real pressure either. but which I think is divergence free. Am I right? So, as you had said, both of the first step in SIMPLE and the two steps in PISO are making sure the velocity field divergence free, Why does PISO make it divergence free twice? If I was wrong above, correct me, Thanks in advance.
What I said is based on my understanding and I haven't written a code of PISO.

In SIMPLE method, the approximated velocity is computed implicitly but the corrective pressure is computed explicitly. That can't be coupled with one iterative step.

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