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-   -   Oscillating coefficient values (http://www.cfd-online.com/Forums/star-ccm/101474-oscillating-coefficient-values.html)

Breivik May 4, 2012 18:31

Oscillating coefficient values
 
Hi,
I'm currently doing a full scale simulation of a ship with a moonpool in an implicit unsteady state and have done som 8000 iterations. The problem is that the drag coefficient value is oscillating. Is this due to the vortex sheading? And if so, what is an acceptable difference between the peak an bottom of the fluctuation? Of course given that it is oscillating around a specific value.
I'm using a time-step of 0.05s

Any answer would be much appreciated

Best regard
Breivik.

calim_cfd May 5, 2012 11:56

i guess that would call for some research.. guess you need some exp data.. personally, i would like to have nothing greater than 10%... but then again it all boils down to scales.. velocity..lenghtscale..and so on... hard to tell.. try digging some exp data and let us knows :o

gl:D

abdul099 May 6, 2012 09:11

When there's no totally weird flow behaviour, I would agree that oscillations are due to vortex shedding or other instabilities in the flow. When that's the reason, you should be able to see it in a scene. Check that and you know quickly if you're on the safe side.

How much difference you can accept is up to you. First, it depends on the order of magnitude of the average value. Second, when looking on relative values, it depends on the reference value. And there's no rule saying "oscillations up to 10% is okay, but 10.0000001% or more is completely wrong". Of course mostly it's not acceptable to have oscillations 10 000% up and down, although it's oscillating around a constant value, but there might be situations when even that is okay.

Breivik May 6, 2012 13:14

Yes there is a big vortex inside the moonpool that i presume is a contributor to oscillating pattern.
The difference between the peak and the bottom is: 0.0044 to 0.0026

thanks for all the answers

Best regards
Breivik


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