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-   -   How to set reference values to find Cl, Cm... (3D) (http://www.cfd-online.com/Forums/fluent/41512-how-set-reference-values-find-cl-cm-3d.html)

 Cyril June 25, 2006 13:35

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

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 !!

 Jason June 25, 2006 15:11

Re: How to set reference values to find Cl, Cm...

A quick google search shows that the projected frontal area (at 0deg AoA) is a common reference area for model rockets, but most of the data I found only talked about drag. The only thing that matters is that you're using a consistent measurement for your comparisons. You don't know if the answers you are getting are going to be reasonable until you have something to compare them to. Once you have something to compare them to, then you have to use the same reference so that you're comparing the same thing. A lot of drag calculations are done using frontal area, but a lot of lift (angle of attack) calculations would use the planar area of the body (like wing analyses, or flat plates, etc). The selection of the reference area is dependent on what you're comparing your data to!

CD values are hard to get on long slender objects. They're highly dependent on mesh, turbulence model, BCs, fluid properties, etc... A poor mesh towards the front of the model could be sending a poor approximation of the BL down the body, which is going to throw off all of your drag results. I re-iterate. You need to have something to compare your results to (otherwise how can you even claim that your CD values are wrong? You say they're high, but high compared to what???). Only then is there any validation that your doing this right.

I'm not saying you need experimental values of your exact model. But you can find data on a simpler model rocket, then model that and compare your results. If you're getting the same results, then there's data supporting that your modeling technique is valid. But once again, that means you have data, and that data is recorded using a specific reference value, and that's going to fix what you pick for your reference values!

I hope this helps, and good luck, Jason

 Cyril June 26, 2006 01:36

Re: How to set reference values to find Cl, Cm...

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...

 Jason June 26, 2006 07:34

Re: How to set reference values to find Cl, Cm...

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

 Cyril June 26, 2006 07:59

Re: How to set reference values to find Cl, Cm...

okok...

So if I understood well, you can't calculate both Cd and Cl at the same time as they "use" different ref area ??

 Jason June 26, 2006 11:22

Re: How to set reference values to find Cl, Cm...

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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