# How to set reference values to find Cl, Cm... (3D)

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 June 25, 2006, 13:35 How to set reference values to find Cl, Cm... (3D) #1 Cyril Guest   Posts: n/a I'm trying to simulate a small experimental rocket (1,5m tall) in order to get its aerodynamic coeff (Cd, Cl, and Cm at a low angle of attack : 1°). The problem is that I have a big Cd. And that comes form the reference values... Here is my mesh : http://img145.imageshack.us/img145/9...illage15dv.png What sould I fill in area ? (Cross area or total area ?) And for lenght... ? I compute from my environment, (a half cylinder, in 'pressure far filed' bc). Any help is welcome !! Thank you !!

 June 26, 2006, 01:36 Re: How to set reference values to find Cl, Cm... #3 Cyril Guest   Posts: n/a Thank you Jason, for your answer. In fact, I'm more interested by the Cn (=lift). When I said "high" I meaned something like 3000 ! ;-) In fact, rockets I usualy build don't have air inlet like this one. So I know the values of Cd, Cl... of a "common" rocket. What I need to know now is the coefficient of this one, in order to be able to demonstrate that it is stable. I don't have any experimental data, or real references. The only comparison I can make is with other common rockets. I knew that the usual refence area was the fontal area (for Cd). But I didn't know that was that one Fluent was asking. I thought fluent was able to calculate this area itself in 3D...

 June 26, 2006, 07:34 Re: How to set reference values to find Cl, Cm... #4 Jason Guest   Posts: n/a Fluent won't make any assumption of the area because the choice of area is up to you. For an airraft a common reference area is the wing area, but for a tear drop in drag it would be the cross section at the maximum diameter. How is Fluent supposed to know which area to use? If it applied the same technique every time, it would grab the frontal area of the aircraft, not the wing area. Besides, Fluent doesn't even make any assumptions of flow direction, so it wouldn't really know which axis is the flow direction. Another downfall would be that if Fluent did use the projected area based on the flow direct, the reference area would change with AoA. That would make your data extremely difficult to follow because your reference areas would be constantly changing, so that now you have a CL that's a funcion of a changing reference area and a changing lift force. If you already know the values of Cd and Cl of a "common" rocket, then you must know what they're using for reference areas. If you know what they're using, use the same thing!!!! Jason

 June 26, 2006, 07:59 Re: How to set reference values to find Cl, Cm... #5 Cyril Guest   Posts: n/a okok... So if I understood well, you can't calculate both Cd and Cl at the same time as they "use" different ref area ??

 June 26, 2006, 11:22 Re: How to set reference values to find Cl, Cm... #6 Jason Guest   Posts: n/a No, what I'm saying is that you already know Cd and Cl data for a "Common Rocket"... so if you know that, then you MUST know the reference area they used. If you know what they used, then use the same thing (i.e. if they used the frontal area, you should use the frontal area... if they used the cross section of the body, you should use the cross section of the body). Hope this helps, Jason

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