Problem with drag coefficient calculation

Crystal · June 12, 2009, 21:55

Hi,

I'm quite new to Fluent and lately I've been facing a rather strange problem relating to drag coefficient calculation. I'm trying to simulate external flow over a sphere at Re = 100, 300 and 1.1 X 10^6. My problem is: when I select air as my fluid, my results closely matches the reported values. However, when I select water as my fluid, my results are totally off, and the values are close to zero.
I've checked that I've keyed in the corresponding velocities correctly. I even tried changing the dimensions of the sphere and running on double precision solver to ensure it is not caused by limit in precision. But to no avail.
What could be the problem? Would really appreciate any help on this!

Nectanebo80 · June 15, 2009, 06:09

Hello

Be careful when calculating coefficients (drag, lift, moment, etc) with Fluent. It depend on your reference values. The defaults values are for air. Length=1m, Surface = 1m2, Viscosity of air, etc.. You'll have to re-multiply with your fluid and geometry values in order to obtain the correct values (report/ reference values)

Ervin

Crystal · June 18, 2009, 06:44

Hi Ervin,

Thanks for your reply. Anyway, I've found out that the cause was due to Fluent somehow not being able to accept the change in the Material Properties values. I exported the mesh file from Gambit again, and repeated the setup process in Fluent. This solved the problem :-)

June 12, 2009, 21:55	Problem with drag coefficient calculation	#1
Crystal New Member Join Date: Jun 2009 Posts: 10 Rep Power: 17	Hi, I'm quite new to Fluent and lately I've been facing a rather strange problem relating to drag coefficient calculation. I'm trying to simulate external flow over a sphere at Re = 100, 300 and 1.1 X 10^6. My problem is: when I select air as my fluid, my results closely matches the reported values. However, when I select water as my fluid, my results are totally off, and the values are close to zero. I've checked that I've keyed in the corresponding velocities correctly. I even tried changing the dimensions of the sphere and running on double precision solver to ensure it is not caused by limit in precision. But to no avail. What could be the problem? Would really appreciate any help on this!

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June 15, 2009, 06:09		#2
Nectanebo80 New Member Ervin Join Date: Mar 2009 Posts: 4 Rep Power: 17	Hello Be careful when calculating coefficients (drag, lift, moment, etc) with Fluent. It depend on your reference values. The defaults values are for air. Length=1m, Surface = 1m2, Viscosity of air, etc.. You'll have to re-multiply with your fluid and geometry values in order to obtain the correct values (report/ reference values) Ervin

June 18, 2009, 06:44		#3
Crystal New Member Join Date: Jun 2009 Posts: 10 Rep Power: 17	Hi Ervin, Thanks for your reply. Anyway, I've found out that the cause was due to Fluent somehow not being able to accept the change in the Material Properties values. I exported the mesh file from Gambit again, and repeated the setup process in Fluent. This solved the problem :-)