
[Sponsors] 
September 3, 2009, 08:24 
Drag coefficient & Lift coefficient??

#1 
Member
mohsen
Join Date: Mar 2009
Posts: 53
Rep Power: 17 
HI
how can calculate Drag coefficient & Lift coefficient that apply to an airplane in cfx? thanks 

September 4, 2009, 03:27 

#2 
New Member
Join Date: Aug 2009
Posts: 10
Rep Power: 16 
hi, try to find it in the forum, I remember an analogicaly thread


September 4, 2009, 09:36 

#3  
Member
Paulo Alexandre Costa Rocha
Join Date: Mar 2009
Posts: 71
Rep Power: 17 
Quote:
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 

September 6, 2009, 04:27 

#4 
Member
mohsen
Join Date: Mar 2009
Posts: 53
Rep Power: 17 
hi
there is a beter way. you can use calculate tab in cfx 12.here you can calculate force or whatever simply. best regard 

September 6, 2009, 04:27 

#5 
Member
mohsen
Join Date: Mar 2009
Posts: 53
Rep Power: 17 
thanks dears!!


September 6, 2009, 08:00 

#6 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,746
Rep Power: 143 
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. 

September 6, 2009, 14:48 

#7 
Senior Member
George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 18 
how do you do that mate?
__________________
Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials 

September 7, 2009, 07:38 

#8 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,746
Rep Power: 143 
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 longhand)


September 7, 2009, 08:20 

#9 
Member
mohsen
Join Date: Mar 2009
Posts: 53
Rep Power: 17 
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. 

September 7, 2009, 08:30 

#10 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,746
Rep Power: 143 
??? That's why you work out the projected area as I describe above.


September 7, 2009, 08:35 

#11  
Member
mohsen
Join Date: Mar 2009
Posts: 53
Rep Power: 17 
Quote:
i dident your idea yet.so... where i do your idea?cfxpre or cfxpost? how i write force and area in monitor point. 

September 7, 2009, 13:16 

#12  
Senior Member
George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 18 
Quote:
thanks
__________________
Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials 

September 7, 2009, 20:53 

#13 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,746
Rep Power: 143 
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.


September 12, 2009, 05:05 

#14 
Member
mohsen
Join Date: Mar 2009
Posts: 53
Rep Power: 17 
thanks glen


December 9, 2009, 21:08 

#15 
New Member
Join Date: Nov 2009
Posts: 26
Rep Power: 16 
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! 

December 9, 2009, 21:49 

#16 
New Member
Join Date: Nov 2009
Posts: 26
Rep Power: 16 
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!


Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
lift & drag coefficient on airfoil  n. natik  FLUENT  8  March 31, 2015 19:02 
Fluent Good Lift coefficient BAD drag coefficient  Rif  Main CFD Forum  4  March 9, 2010 10:52 
Automotive test case  vinz  OpenFOAM Running, Solving & CFD  98  October 27, 2008 08:43 
drag and lift coefficient  Noé  Siemens  5  July 13, 2004 10:21 
drag and lift coefficient of compressible cylinder  Bin Li  Main CFD Forum  1  March 7, 2004 09:49 