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-   -   Drag coefficient & Lift coefficient?? (http://www.cfd-online.com/Forums/cfx/68019-drag-coefficient-lift-coefficient.html)

mehrdadeng September 3, 2009 09:24

Drag coefficient & Lift coefficient??
 
HI
how can calculate Drag coefficient & Lift coefficient that apply to an airplane in cfx?

thanks

cbr September 4, 2009 04:27

hi, try to find it in the forum, I remember an analogicaly thread

paulo September 4, 2009 10:36

Quote:

Originally Posted by mehrdadeng (Post 228420)
HI
how can calculate Drag coefficient & Lift coefficient that apply to an airplane in cfx?

thanks

Hi,

If you make your airplane as a 'wall' boundary condition, CFX calculates all the forces acting on it, and writes in the OUT file, at the end of the simulation.

Best Regards,

Paulo Rocha

mehrdadeng September 6, 2009 05:27

hi
there is a beter way.
you can use calculate tab in cfx 12.here you can calculate force or whatever simply.

best regard

mehrdadeng September 6, 2009 05:27

thanks dears!!

ghorrocks September 6, 2009 09:00

My preferred way is to have a CEL expression which evaluates the area using a projected area, density and velocity using an averaging function (if it is not known beforehand) and a force_x/y/z command to get the force. Then write it to a monitor point and you can see how it evolves as the simulation progresses. This way you can assess whether your simulation is converged as you go and using "edit run in progress" update things as the simulation progresses. This makes it much faster to get to a converged solution.

But really, whatever. All the ways work so just do what makes sense to you.

ckleanth September 6, 2009 15:48

Quote:

Originally Posted by ghorrocks (Post 228655)
My preferred way is to have a CEL expression which evaluates the area using a projected area..

how do you do that mate?

ghorrocks September 7, 2009 08:38

For most designs the projected area is known - length * diameter for a cylinder, chord * span for a wing. I have never calculated the projected areas in CEL but it should be possible - you can do an area integral of (area vector.projection vector) (that's a dot product - the area vector and projection vectors are components so the dot product needs to be written long-hand)

mehrdadeng September 7, 2009 09:20

thanks, glen
your idea is good but lift force apply either airfoils and body of airplane.and body of air plane is consist of many unknwon areas.

ghorrocks September 7, 2009 09:30

??? That's why you work out the projected area as I describe above.

mehrdadeng September 7, 2009 09:35

Quote:

Originally Posted by ghorrocks (Post 228655)
My preferred way is to have a CEL expression which evaluates the area using a projected area, density and velocity using an averaging function (if it is not known beforehand) and a force_x/y/z command to get the force. Then write it to a monitor point and you can see how it evolves as the simulation progresses. This way you can assess whether your simulation is converged as you go and using "edit run in progress" update things as the simulation progresses. This makes it much faster to get to a converged solution.

But really, whatever. All the ways work so just do what makes sense to you.

som question:
i dident your idea yet.so...
-where i do your idea?cfx-pre or cfx-post?
-how i write force and area in monitor point.

ckleanth September 7, 2009 14:16

Quote:

Originally Posted by ghorrocks (Post 228727)
For most designs the projected area is known - length * diameter for a cylinder, chord * span for a wing. I have never calculated the projected areas in CEL but it should be possible - you can do an area integral of (area vector.projection vector) (that's a dot product - the area vector and projection vectors are components so the dot product needs to be written long-hand)

hmmmmm :D

thanks

ghorrocks September 7, 2009 21:53

Oh yes, and you probably need a step function to only integrate over the positive areas if you have not separated the front and back surfaces already. Otherwise the front surface will give a positive value, the back face will give a negative value and the integral over the whole surface will be zero.

mehrdadeng September 12, 2009 06:05

thanks glen

sanchezz December 9, 2009 22:08

Hi,

I have a similar problem - calculating drag and lift coefficient, but no clue at all what I'm supposed to do. Could you give a more specific guide on what to do?

I can calculate the forces using the function calculator, but I did not find out how to calculate the front area of the car (the geometry is complicated so I really need CFX to calculate this for me). Doing this using a projected area sounds good, but how to approach this?

I also have to do this for lift, and on top of that, for a series of simulations. Therefore it would be really, really good if I could just safe a calculation (like a macro in excel) that gives me drag and lift coefficient directly without any further calculations by hand once I have set it up.

I know there are many people asking this stuff here and I did find a lot on that topic but there are always the same kind of vague answers, that are probably really easy to understand if you are skilled, but not, if you aren't. So I hope if there is a good and detailed answer from one of you skilled CFX people, it will help not only me, but a lot of other people who just want to calculate this seemingly easy task.

Thank you very much in advance!

sanchezz December 9, 2009 22:49

Oh, I forgot: Of course I would like to not only see it in post processing, but use it as a convergence criteria like suggested by glen. that would help me a lot making sure that my mesh is suitable etc. thanks!


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