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Old   November 1, 2008, 02:00
Default drag coefficient
  #1
Amir
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Hi.

I'm modeling one cylinder without rotation in fluent with different wind speeds. I tried for 5 different speeds and I got almost same results for drag coefficient, however in one experimental test, they obtained different values with different speeds. They obtained that the drag coefficient is increasing with increasing of speed. My data will be compared to the experimental.

My question is, what can be the reason for this different results? Is the drag coefficients related to wind speed? Or maybe I'm doing something wrong with fluent... The speed range is 5m/s to 25m/s All the conditions are same, just the speed is changing.

I appreciate any help.

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Old   November 1, 2008, 03:22
Default Re: drag coefficient
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Charles
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Well, if you are not modelling transition, chances are good that your Cd vs. Re curve won't look a lot like the experiments.

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Old   November 1, 2008, 04:01
Default Re: drag coefficient
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Amir
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I didn't understand what you mean about modeling transition. Can you explain more!

The Re number varies between 1.10E+05 to 5.48E+05.

Thank you so much.
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Old   November 1, 2008, 04:23
Default Re: drag coefficient
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Charles
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One word: Hoerner

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Old   November 2, 2008, 10:39
Default Re: drag coefficient
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Sham
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That is within transition to turbulence regime (i.e. drag crisis region). You need to use turbulence model like LES or DES to get accurate results. Normal second order wall modelling like k-e or k-w will not capture the drag properly.
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Old   November 4, 2008, 13:04
Default Re: drag coefficient
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Amir
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Thanks for your recommendation.

By the way I used RSM model also. what's your idea about it.?

Is this one also weak on predicting the drag ?

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Old   November 7, 2008, 06:47
Default Re: drag coefficient
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Pascal
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Let's say that LES is the less weak to predict drag!

But to give you a clue, in this kind of problem, drag will be very dependent of your transition point (or line, or area). Don't neglecte your study with RSM, but try to understand it (and why not compare it with LES).

Your transition point should move according to Re number.

Check this out:

http://www.eng.fsu.edu/~alvi/EML4304...escription.doc

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