Initial time step will have effects on LES result?

Jingxue Wang · May 21, 2018, 09:44

Hello,every one!

I use LES to calculate the wind pressure on roof-mounted solar panels. Before，I calculated the wind pressure results of LES using previous 6s(from 4s to 10s). The mean and STD statistics values are very close to wind tunnel test. I continue to calculate to get more longer data（from 10s to 20s）. But the results of next calculation is too bad and have much difference with experiment results.I am so curious about this. I can not figure out the reason. The only one probable reason that I can think is the initial time step will have effects on this? The second stage calculation starts at the latestTime time step of the first stage calculation.

I hope can get any suggestion and help!Thanks

piu58 · May 21, 2018, 10:51

If your simulation doesn't crash, you eventual get a pseudo steady state result. This result should not depend (very strongly) from the initial conditions. But it depends strongly on the boundary conditions.

Using a developed flow as starting point, the first time steps however, depend strongly on this flow but to a lesser degree on boundary conditions.

From my experience it is note easy (and often quite hard) to establish adequate b.c. I don't think that you may learn that to a high level from text books. You need experience, which is invaluable.

Santiago · May 21, 2018, 14:18

Quote:

Originally Posted by Jingxue Wang

Hello,every one!

I use LES to calculate the wind pressure on roof-mounted solar panels. Before，I calculated the wind pressure results of LES using previous 6s(from 4s to 10s). The mean and STD statistics values are very close to wind tunnel test. I continue to calculate to get more longer data（from 10s to 20s）. But the results of next calculation is too bad and have much difference with experiment results.I am so curious about this. I can not figure out the reason. The only one probable reason that I can think is the initial time step will have effects on this? The second stage calculation starts at the latestTime time step of the first stage calculation.

I hope can get any suggestion and help!Thanks

Different how?

May 21, 2018, 09:44	Initial time step will have effects on LES result?	#1
Jingxue Wang Member Jingxue Wang Join Date: Sep 2017 Posts: 58 Rep Power: 9	Hello,every one! I use LES to calculate the wind pressure on roof-mounted solar panels. Before，I calculated the wind pressure results of LES using previous 6s(from 4s to 10s). The mean and STD statistics values are very close to wind tunnel test. I continue to calculate to get more longer data（from 10s to 20s）. But the results of next calculation is too bad and have much difference with experiment results.I am so curious about this. I can not figure out the reason. The only one probable reason that I can think is the initial time step will have effects on this? The second stage calculation starts at the latestTime time step of the first stage calculation. I hope can get any suggestion and help!Thanks

May 21, 2018, 10:51		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	If your simulation doesn't crash, you eventual get a pseudo steady state result. This result should not depend (very strongly) from the initial conditions. But it depends strongly on the boundary conditions. Using a developed flow as starting point, the first time steps however, depend strongly on this flow but to a lesser degree on boundary conditions. From my experience it is note easy (and often quite hard) to establish adequate b.c. I don't think that you may learn that to a high level from text books. You need experience, which is invaluable. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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