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Old   February 5, 2010, 04:21
Default averaging in les
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Kapa Lilla
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Dear All,

I made a compressible LES simulation and used the averaging function to get the mean and r.m.s. pressure. A bit disturbing, but the r.m.s. pressure contains negative values in some cells. How can this be?

Thanks,

Lilla
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Old   February 9, 2010, 05:02
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Eugene de Villiers
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I don't think the Prime2Mean values are being calculated in the way you think, i.e

Field_rms = sqrt(Mean(sqr(Field-FieldMean)))

Remember, this is a running average, so the initial value for FieldMean is not accurate. As a result the mean squares are calculated as:

Field_ms_new = alpha * Field_ms_old + beta*sqr(Field) - sqr(FieldMean_old) + sqr(FieldMean_new)

where alpha = (N-1)/N and beta = 1/N, which can lead to negative values for small N.

It could also happen, that you restart the calculation with a FieldMean present, but with no Field_ms. This could lead to large initial negative values, as the initial Field_ms is calculated as:

Field_ms_init = sqr(Field) - sqr(FieldMean)

A better initialisation would probably be:

Field_ms_init = sqr(Field - FieldMean)
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Old   February 10, 2010, 03:29
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Kapa Lilla
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Dear Eugene,

thanks for your answer. Indeed, I checked the implementation as well, and the problem can arise from my test case. I have a pulsating source around a constant flowfield. The source term I guess makes harder to compute the mean, so I decided to fix the mean flow by myself and than calculate p^2 for noise computation.

Best regards,

Lilla
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