# [flapping wing]: Reverse flow

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 May 11, 2006, 15:35 [flapping wing]: Reverse flow #1 Astozzia Guest   Posts: n/a Hey there, (first, sorry for this rather long post...) I am modelling the motion of a flapping wing under Fluent6.2, 2ddp, laminar, unsteady, dynamesh. The domain has a velocity inlet in the upstream and top andbottom sides (uniform flow), and an outflow for the far downstream (20c behind the airfoil). I have set these parameters for the solver: 1. pressure under-relaxation factor = 1 2. momentum under-relaxation factor = 0.9 3. 2nd order upwind for the momentum, SIMPLE coupling scheme. The airfoil starts with a relatively high velocity (220m/s), but the time step is dt=5e-05s to 1e-04s. The problem is that: 1. I get HUGE values of Cd and Cl for the first time step (~5,000 !?), even though I reported the reference values to the upstream conditions. 2. and I get reverse flow when the wing starts to rotate. I have tried with a different BC for the downstream (pressure-outlet instead of outflow), still the same problem. Can anyone help/advise me, I am getting stuck? Many thanks in advance. Astozzia.

 May 12, 2006, 03:31 Re: [flapping wing]: Reverse flow #2 philipov Guest   Posts: n/a I am curious how you managed to reach convergency with SIMPLE scheme in this case!??!?!?!?! Think about PISO!

 May 12, 2006, 10:16 Re: [flapping wing]: Reverse flow #3 bharat Guest   Posts: n/a Hi, Check if you have set the reference right hope that helps bharat

 May 12, 2006, 20:11 Re: [flapping wing]: Reverse flow #4 Astozzia Guest   Posts: n/a Hi Philipov, Well, with PISO, the simulation was so slow and the iterations reached their maximum ceiling in each time step without any convergence. I tried to play with the under-relaxation factors because I am not that at ease with them (P=0.3-0.5, M=0.7-0.9) Is anyone savvy about under-relaction factors an can give me a good link to better understand the mathematical aspect of it? Thx in advance. Astozzia

 May 12, 2006, 20:19 Re: [flapping wing]: Reverse flow #5 Astozzia Guest   Posts: n/a Thx barhat! you're right. I am figuring out all these stuff!!

 May 17, 2006, 21:46 Re: [flapping wing]: Reverse flow #6 Peter Attar Guest   Posts: n/a What's the Mach number? Remember what happens to the lift in incompressible flow at t=0 for a sudden pitch up of a 2d airfoil....

 May 18, 2006, 19:44 Re: [flapping wing]: Reverse flow #7 Astozzia Guest   Posts: n/a hey Peter, Actually, the velocity is so small V(airfoil)= 2m/s and V_inlet=5m/s in the upstream that the Mach number would be epsilon. > Remember what happens to the lift in incompressible flow at t=0 for a sudden pitch up of a 2d airfoil.... Er... Would that justify the enormous value I got? I'd appreciate if you understand where the problem may come from... Thx in advance. Astozzia

 May 19, 2006, 20:43 Re: [flapping wing]: Reverse flow #8 Peter Attar Guest   Posts: n/a For truely incompressible flow the lift at t=0+ will be infinite since it is a function of the second derivative of alpha(t). Decreasing your timestep will just make it worse. Increasing your timestep for the first couple of characteristic times might help. You can then decrease it accordingly.

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