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Cyril June 20, 2006 05:45

Calculate Cn and Cm of a tube+air inlet ?
Hi !

I'm trying to calculate aerodynamic coefficients of a rocket.

I need to calculate the Cn (normal drag coeff and the Cm (in 3D).

Which parameters do I need to set, and how to set them ??

(I use a pressure far field, Mach 0.49, and alpha=1)

Thanks !!!


Jason June 20, 2006 08:40

Re: Calculate Cn and Cm of a tube+air inlet ?
Set the reference values (Report->Reference Values). You can tell it to calculate the air properties by selecting your pressure far field BC in the Compute from box. Then enter your geometric values (Ref Length, Ref Area).

Then, when you want to get your forces, you can get them by going to Report->Forces.

If Cn and Cm are your main values of interest, you should be monitoring them to make sure they converge (Solve->Monitors->Forces). Residuals can be misleading without this information.

Hope this helps, and good luck, Jason

Cyril June 20, 2006 08:52

Re: Calculate Cn and Cm of a tube+air inlet ?
For the ref. values... what should I set for the ref length and ref area ? (I've got a rocket, not a plane...)

And I found the Cm in the forces monitors... but how can I set the Cn (it's not in the drop list (only Cd, Cl and Cm))?

Thanks !

P.S : I don't realy know what is the Cn... ;-)

Jason June 20, 2006 12:16

Re: Calculate Cn and Cm of a tube+air inlet ?
There isn't any fixed rule for reference length and area. You should be comparing your results to some other data (published data or test data). If you have data you're comparing to, then use their definition for reference area and length. If I had to guess, I would think the reference length would be the diameter of the model and the area would be the (diameter*length). Instead, I've seen some data reported (typically for mortar rounds) where the reference length was the maximum diameter, and the reference area was the cross sectional area (at maximum diameter). Other people may have other definitions. I'd do some research online to see what other people got for predictions and what they used for reference values.

As for the force monitors... the Cd, Cl, and Cm are just labels given to the plots and/or files that you write the monitor data to. So the Cm uses some default value for the moment center and moment axis, but those may not be what you're looking for, so you'll have to change the moment axis and the center of rotation (only the primary axes are available in the monitors... there's been several comments to Fluent in regards to this, as well as to only allowing two forces and one moment instead of all 6 components). Cd and Cl are also just predefined forces and may have nothing to do with what you're looking for, so you can change the direction of the force vector to get what you are looking for.

Cd = drag coefficient (typically in the direction of the wind) Cl = lift coefficient (typically perpendicular to the wind, in a direction that is opposing gravity i.e. "up") Cm = pitching moment coefficient (typically defined about the CG and is defined so that a positive moment will cause the nose to go in a "positive lift direction"... so it'll cause the nose to title "up"). Cn = normal force coefficient (similar to the lift coefficient, but defined as perpendicular to the body instead of the wind)

So basically Cl and Cd are lift and drag in the wind axis, while Cn is lift in the body axis (and Ca "axial coefficient" is the drag in the body axis). Moment coefficients are typically reported in the body axis. Also, don't forget there's three forces and three moments (Cy is a typical side force coefficient... I'm used to seeing the three moments as Clm (pitch), Cln (yaw), and Cll (roll)... instead of adding the "l" some people use capital letters to represent forces and lower case to represent moments).

Those are pretty typical definitions in free body aerodynamics. Other industries may use terms a little different, and worse yet, some universities and/or companies may use terms a little different as well. Figure out what the precedence is where you are and follow that.

See section 26.19.3 of the users guide on defining force and moment monitors in Fluent.

Hope this helps, and good luck, Jason

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