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Amir November 1, 2008 01:00

drag coefficient

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.

Charles November 1, 2008 02:22

Re: drag coefficient
Well, if you are not modelling transition, chances are good that your Cd vs. Re curve won't look a lot like the experiments.

Amir November 1, 2008 03:01

Re: drag coefficient
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.

Charles November 1, 2008 03:23

Re: drag coefficient
One word: Hoerner

Sham November 2, 2008 10:39

Re: drag coefficient
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.

Amir November 4, 2008 13:04

Re: drag coefficient
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 ?

Pascal November 7, 2008 06:47

Re: drag coefficient
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:

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