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-   -   3D mesh for transient flow around wing section? (http://www.cfd-online.com/Forums/main/82602-3d-mesh-transient-flow-around-wing-section.html)

Nixmich November 30, 2010 11:36

3D mesh for transient flow around wing section?
 
Hi all,
I want to simulate the flow around a wing section which was generated by extruding an airfoil. The airfoil has a blunt trailing edge so transient effects occur. Does anybody know if it makes sense to simulate the flow in 3d in order to better capture these transient effects and if yes, how high the mesh resolution in the direction of the extrusion should be (hexa mesh)?
Thanks in advance!

Nixmich December 16, 2010 04:26

ok now I just tried it, 2 weeks calculation obtaining asymmetric results, positive lift coeffecient on one side and negative on the other side, adding up to nearly zero which is far from reality...

bhatiadinesh December 21, 2010 07:11

What are the boundary conditions you have used at the ends which are perpendicular to the plane of airfoil, the discrepancy might be due to incorect boundary conditions !!

Nixmich December 21, 2010 08:03

Thanks for the answer!
Actually I simulated the whole wind tunnel as used for the measurements I want to compare the CFD results to, in order to see to what extent the wind tunnel walls influence the flow around the airfoil.
So this is the setup:
-rectangular inlet with constant normal speed
-empty wind tunnel section with free slip walls, ~1/3 of the overall length
-middle section of the wind tunnel containing the airfoil, no slip walls, ~1/3 of the overall length
-empty wind tunnel section with free slip walls, ~1/3 of the overall length
-opening with constant relative pressure (0Pa, reference pressure 1atm)
The whole setup is absolutely symmetric (I checked this about a thousand times:)), so I have no idea why the flow shouldn't be symmetric as well. Apparently Ansys gets something wrong about the transition point, as the flow separates from the airfoil at about 0.5c on the one and not at all on the other side of the tunnel...

bhatiadinesh December 22, 2010 01:26

Hi,
Have you checked your mesh geometry for symmetry ? ( I am confused whether u were mentioning the actual one or the model earlier ) , check for symmetry of boundary condtions as well, I mean u must have forgotten to provide free slip or any other paramter on some of the walls.

Also check the Inlet and Exit Pressure Pressure and velocity profiles for asymmetry.

Traverse from Inlet to the airfoil/wing/blade and view pressure and velocity profiles in planes parallel to the inlet to see where the uniformity starts generating, at the tunnel walls or between them.

YOU CAN DETECT PROBLEMS IN UR INPUTS BY ANALYSING THE RESULTS ESPECIALLY IF U HAVE SOME IDEA OF HOW THE RESULTS SHOULD BE.

I Hope ur tunnel section is not curved ( Pardon me if u have laready mentioned that, I couldnt read the whole stuff )

-Dinesh

Nixmich January 5, 2011 04:00

Quote:

Originally Posted by bhatiadinesh (Post 288062)
Have you checked your mesh geometry for symmetry ?

yeah thats what i meant by saying i'd checked it over and over again:)
boundary conditions should be ok as well.

the "problem" seems to start at one of the side walls around the area where the boundary condition changes from free slip wall to no slip wall. u decreases more rapidly in a certain area in front of the airfoil on one side than on the other (dont know whether this is due to the boundary layer at the side wall or to effects from the airfoil). this is the point i dont understand because the conditions are exactly the same on either side.

i'll try again with free slip tunnel walls as soon as i can spare another 2 weeks computation time, although i expect that the flow will stay attached in this case leading to far too high cl (this is what comes out if i run a 2d/1cell thickness simulation, and this is also why i initially started this thread, to find out if anybody knows if there is any difference between 2d and extrusion to 3d)


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