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Phil November 5, 2002 07:51

I am trying to run a basic analysis of a NACA0012 foil at M=0.67, I cannot get the analysis to do a simgle iteration because of divergance. I have followed the steps that are used in the Fluent tutorial regarding flow round a foil. any ideas what to try would be greatfully recieved!

Rahul November 7, 2002 04:17

Re: Foil
You may have higher angle of attack or your mesh is not enough fine. I was also doing similar work and had some problem. Refining mesh near wall (Use of Boundary layer) and greater domain of flow solved my problem. Rahul

Rodge November 7, 2002 11:01

Re: Foil
Be more specific, what viscous model are you using?, explicit/implicit? time-dependant?...

Phil November 7, 2002 11:15

Re: Foil
SA viscous model steady state coupled implicit energy on NACA0012 profile pressure far field radius of 25 chord lengths 101325Pa M=0.67 4 degree angle of attack

Air ideal gas sutherland law for viscosity

operating pressure 0

I have now managed to get the model to run by starting with a low flow M=0.l and can get the model to converge, by stepping the velocity up i appear to be able to increase the velocity to M=0.67 however this is very time consuming. Currently I have got the model up to M=0.4 with 0.1 steps.

I had tried stepping up to M=0.67 from the converged M=0.1 model but it diverged.

I assume that this is not the most efficient way of modelling this problem and would appreciate any suggestions which may help. Would increasing the convergence criteria say setting the nut residual to 1e-6 for the first low flow model make it more likely i could then step directly to th M=0.67 condition without multiple increments?

Rodge November 7, 2002 15:16

Re: Foil
I did the same airfoil but M 0.25 c=1m and 7 chord domain. I started with a Courant of about 40 and it worked. If it diverges, try lowering this number. You should read about the Courant number in the manual. You should also start from a 1st order upwind and when it converges, switch up to second. For this kind of simulation you should at least have a 2nd order to trust your values ( That's what my teacher says)

The convergence criteria will not help very much because you have a big domain, these numbers are calculated from the change in values of the first iterations to the last ones, so if you have too many cells with undisturbed flow, the convergence numbers will not go under the 110-3 condition.

In the manual you will find a better description. Good luck

Phil November 11, 2002 04:46

Re: Foil
cheers Rodge,

I started with a courant number of 5 and subsequently increased it to 20 when it looked like the model was settling down. but was using second order simulation, ill try starting with first order and switching.

did you find the pressure far field gave a good result with only 7 chord domain?

Rodge November 11, 2002 13:18

Re: Foil
Yes, actually in a 3D model with 3-4 chords I got reasonable results. (The flow is undisturbed outside 3 chords for my speed).

I compared with the NACA information and the Lift is all right. I even predicted CLmax. For drag, Fluent over-estimates the values but the curves were similar.

Phil November 19, 2002 05:30

Re: Foil

I have just started trying to model the foil in 3d and i an having difficulty meshing the volumes, In particular i have a very large equiangle skew at end of the foil at the trailing edge, some suggestion as to how you meshed this would be really helpful

cheers phil

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