# how to calculate mean value of lift and drag

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December 10, 2010, 10:21
how to calculate mean value of lift and drag
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Hello all....
if i am running unsteady simulation, i will get values of lift and drag that will show time periodic curve when the solution is converged. How can i precisely get the mean value of lift and drag from this converged solution...

one cl curve is attached. this curve is showing time periodic convergence from flow time of 40 and onwards. now how can i calculate mean value from it in FLUENT
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 Cl convergence.jpg (31.3 KB, 48 views)

 December 10, 2010, 21:44 #2 New Member   Pano Join Date: Apr 2010 Posts: 12 Rep Power: 9 hi there,, this is not a proper way but, anyways, you can do it and you can present it unless you come up with a more elegant way. You can export the data for the lift coefficient in an excel spreadsheet, isolate the values for which the simulation has "converged" and get an average value... if you can get Fluent to plot Cl with time or if you can export your data to Matlab, then you can say mean Cf = (1/t)*integral(Cl*dt) where t is the total time you want to take into account, i.e. the time for which the periodic behaviour of Cl is fairly steady. But anyways, beware!! why do you really want the average Cl in an unsteady problem? are you sure it makes sense?? I presume you can always run a steady flow simulation just to get these notional values even though your problem is unsteady essentially.

 December 11, 2010, 06:07 #3 Member   Aamer Shahzad Join Date: Mar 2010 Posts: 58 Rep Power: 9 Hi pano..... thanks for the reply...... your suggestions regarding the use of excel or matlab seems valid. but i didn't understand how can i get these values from a steady simulation. i am in search of one specific value so that i can compare it with my experimental data... in my case, if i run the problem for steady case, it doesnt converge to give me a straight line either for Cl or for Cd. so then, how can i find the value through steady simulation....

 December 11, 2010, 06:36 #4 Super Moderator     Praveen. C Join Date: Mar 2009 Location: Bangalore Posts: 259 Blog Entries: 6 Rep Power: 11 In the initial time, there may be some transients. Choose some t0 so that for t > t0, the Cd seems periodic. If you have data until time T, then compute an average as Alternately you can also do a fourier transform and the constant term in the fourier expansion will be the time average value.

 December 12, 2010, 13:37 #6 Member   Aamer Shahzad Join Date: Mar 2010 Posts: 58 Rep Power: 9 Dear Pano... your comments on the subject were very helpful. i am actually validating experimental results of corrugated airfoil at reynolds number of 34000. my case is inherently unsteady and turbulent. but i still started with laminar to check how my simulation goes. the solution was not converging with steady runs. but when i switched to SA model, the solution started to converge for low angles of attack with steady runs, but the solution was not converging for higher angles of attack due to flow separation from the surface. so i had to switch to unsteady calculations. as far as rechecking of mesh is concerned, i am already onto it......

 December 13, 2010, 03:29 #7 Member   Aamer Shahzad Join Date: Mar 2010 Posts: 58 Rep Power: 9 Dear Pano..... i am getting correct Cd values for almost all angles of attack but i am getting slightly higher Cl than the experimental values . For example at 5 degree angle of attack, experimental Cl is 0.43-0.46 but my computational value is 0.55. What all things to look for, if you are getting correct values of drag Cd but higher values of lift Cl.

 December 18, 2010, 21:29 #9 New Member   sam13 Join Date: Jul 2009 Location: St. John's, NL, Canada Posts: 23 Rep Power: 10 Just wondering can any one tell me how to do the fourier transform to get the average, amplitude and phase ? Can any one provide me any references? I will appreciate any help. Thanks, Shafi

 December 21, 2010, 01:23 #10 Member   Aamer Shahzad Join Date: Mar 2010 Posts: 58 Rep Power: 9 Dear Pano... thanks for enlightening me on the subject with very relevant technical details. Most of the things that you have asked me to try have already been done, like , change of boundries (i have tried both outflow and pressure outlet), wall treatment i.e the use of wall functions with K-e model, type of grid (2 different type of grids have been tested, although both were o type) but as i have to proceed to 3D later, so i foresee terrible problems if i go for C-type grid, so i am avoiding that. the values remain the same more or less. Honestly speaking, i dont have any idea about algos like Xfoils, i will look into that. by the way, my airfoil is not a conventional one, it has corrugations on the surface like that of a dragonfly wing.

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