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-   -   Airfoil in stall (http://www.cfd-online.com/Forums/fluent/27516-airfoil-stall.html)

 Park Geun Hong February 23, 2000 10:41

Airfoil in stall

Hi! everyone. I have some question in airfoil. In every case (RNG k-e, Real~ k-e, Standard k-e, S-A...)of airfoil in low AOA, Fluent predicts nearly good result with experimental data. But! In stall, no case predicts good result. Have anyone ever tested this case(airfoil in stall)? Please response me! A.S.A.P!

 John C. Chien February 23, 2000 11:38

Re: Airfoil in stall

(1). You should not get good results, because it is a very difficult problem to solve when an airfoil stalls. (2). You are dealing with transient, turbulent, separated flows. (3). You are still very lucky to get good results for low AOA cases. (4). It is not just the change in the angle of attack, when the airfoil stalls, the complete physics of flow also changes.

 John C. Chien February 23, 2000 18:45

Re: Airfoil in stall, thanks for the e-mail

(1). I know you are anxious to get good results for the stalled airfoil flows. (2). It is perfectly all right to use the code you are using. There is nothing wrong with it. (3). The difficulties are in two areas, namely, a) the accurate turbulence model for separated flow, which can predict the separation point and the large open wake behind. In this area, all you can do is to try out various models available in the code. If you can't get good results, then the turbulence model has to be improved, and this is no easy task. (4). The other difficulty is b). the separated wake flow can become transient (not just turbulent). So, you may have to solve the un-steady flow equations, instead of the steady-state equations. So, this is another possibility for you to try, that is to run some transient flow calculations. (5). The other possibility is to creat a show like " who wants to be a millionaire by solving the stalled airfoil flow problem". I think, it is likely to bring in many potential solutions that way. Just keep trying, don't give up.

 John C. Chien February 24, 2000 10:51

Re:random characters in your second e-mail, thanks

(1). Your second e-mail received. I can read only English portion. (2). There are also some random coded characters in the e-mail, sorry, I can't read those. It could be sent in other language. (3). If you need answers to CFD questions, please use plain English next time and check the language and font options. I think, there is no automatic multi-language translation on e-mail yet. Anyway, thank you for the e-mail.

 Sung-Eun Kim February 24, 2000 22:00

Re: Airfoil in stall

You may want to try transient calculations as Dr. Chien suggested. Unfortunately, you will have to be prepared to spend some time trying different turbulence models, different time step sizes, etc. My rule of thumb with respect to time step size is that your time step is largely considered to be adequately small if the solution converges within 10 iterations per time step. Better use one of the more recent models FLUENT offers, i.e., RNG, RKE, or RSM Model. I suggest you monitor the lift/drag and one of velocity components in the near-wake.

Don't blame too much if FLUENT can't predict stall angle accurately and the lift and drag coefficients do not match the measurements beyond the stall angle. But I'll be interested in hearing if there are better alternatives to what FLUENT offer.

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