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Bad results when compared with Wind Tunnel

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Old   October 23, 2006, 00:05
Default Bad results when compared with Wind Tunnel
  #1
Luiz
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I am having very bad results in airfoils drag and lift forces for any kind of airfoil but specielly with hight attack angle and flaps deflection. Im comparing this results with obtained in wind tunnels and Im very disappointed.

I tried many turbulences models without results and meshes.

Im using CFX and fluent.

Any sugestions?

Thanks.
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Old   October 23, 2006, 07:03
Default Re: Bad results when compared with Wind Tunnel
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Aus
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How abouts did you go about computing your drag and lift forces, coefficients etc mate?
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Old   October 23, 2006, 08:09
Default Re: Bad results when compared with Wind Tunnel
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Robin
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How well does your CFD model reflect the wind tunnel configuration and operating conditions?

-Robin
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Old   October 23, 2006, 09:14
Default Re: Bad results when compared with Wind Tunnel
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jon
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SST models are pretty good but it is essential the Y+ is ok. Also domain size is really important as if the walls are too close it can really effect the flow and in extreme can prevent you foil stalling.
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Old   October 23, 2006, 12:16
Default Re: Bad results when compared with Wind Tunnel
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Luiz
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Well, the coefficients are very wrong... From te resulting forces FX and FY, flow direction and area its easy to turn it dimensionless and I am comparing it with the "Abbott and Doenhoff" text book values.
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Old   October 26, 2006, 22:32
Default Re: Bad results when compared with Wind Tunnel
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TB
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I'm not in your area, but I heard that flow is pretty unsteady & 3D at high attack angle. Could that be a source of error, if you're running your simulation in steady state?

Are you set up your problem in exactly the same manner as the Abbott and Doenhoff? Have you compared the assumptions they made with the assumptions you made in the CFD model?
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Old   October 27, 2006, 03:35
Default Re: Bad results when compared with Wind Tunnel
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jon
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Dimension error?
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Old   October 27, 2006, 09:35
Default Re: Bad results when compared with Wind Tunnel
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Ali
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Sometimes a correction for measurements from wind tunnel are made to correct the flow field due to the exsisting of walls. Actual flow doesn't have this type of walls! To over come this:

1- You have to simulate the flow and then use the same method of correction to calcluate forces.

1- simulate the flow with a boundary walls far away from the foil to simulat the case with correction.

Ali
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