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Good boundary conditions for a NACA wind tunnel ?

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Old   September 7, 2017, 17:51
Default Good boundary conditions for a NACA wind tunnel ?
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Terence Annarelli
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Hi,
For an aeronautical personal project, I try to reproduce NACA 2418 airfoil lift and drag coefficients with a 3D "wing" which has this profile, no taper nor dihedra, and a winglength of 3.6 m.
I do not want to use 2D problem solving (like the 2D airfoil tutorial) ; I use instead the "motorbike" tutorial as a basis. I plan to "validate" the wind tunnel with this "straight wing" before to put in it the whole aircraft.

Using blockMesh, I set a box around the wing (11m long X positive front, 6m high Y positive up, but with the same exact width as the winglength, Z positive right). I used snappyHexMesh to mesh around the .stl wing, which is placed one third of height and one third of length.

With same boundary conditions as the motorbike tutorial (but setting the lowerWall with same BC that the upperWall), and an airspeed of 40 m/s, I obtain same drag and lift coefficients than those found in the NACA doc (0.2 Cl and 0.009 Cd for 0 of AoA).
However, in order not to have to redo the .stl and meshing to change the AoA, I want to change BC to set a different AoA.

I first had hard times to find BC in order to have a "tilted stream". I set the upperWall BC same as the outlet BC and the lowerWall BC same as the inlet, with a flowvelocity equal to (-39.90 2.79 0) for example of 4 of AoA.
Using the streamlines function, it seems my flow is tilted by 4 now (well, far from the wing).

However, I tried many combinations, as for example walls or not in blockMeshDict for "frontAndBack" (I'm inclined to think I must set the frontAndBack patches as walls because these are connected to wing tips), as well as fixedValue or not for outlet and upperWall (perhaps I forgot to test any possible combination). And I cannot achieve "matching points" with my lift and drag doc for this 4 of AoA. The latest result is a good 0.6 Cl but an bad negative Cd of -0.03...

As I am new to CFD and openFoam (but being a mechanical engineer and engineering teacher), I'm a bit lost on setting good boundary conditions for an aerodynamic study.
I apologize if the solution is evident or if it can be found easily in CFD Online forums. I searched many threads but couldn't find the information. I would appreciate a lot your help on boundary conditions for this "wind tunnel" in 3D.

Thank you !
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Old   September 24, 2017, 21:20
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The drag on the foil maybe nagative so you drag coefficient is below zero. And the domain should be large enough if you do not want to change the foil angle. The span is about 3m, the computation domain msut be five to ten times to prevent the field being effected by the inlet and the wall condition.
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Old   September 25, 2017, 00:53
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Terence Annarelli
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Quote:
Originally Posted by cjboy09 View Post
The drag on the foil maybe nagative so you drag coefficient is below zero. And the domain should be large enough if you do not want to change the foil angle. The span is about 3m, the computation domain msut be five to ten times to prevent the field being effected by the inlet and the wall condition.


Thanks for your reply.
I updated the liftdir and dragdir in forceCoeffs file (I forgot to do it...) and it's better. I'm also currently trying a spherical domain with the freestream BC, but I'll increase the relative size of sphere as you said.


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aerodynamic coefficients, boundary conditions, naca, openfoam, wind tunnel

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