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July 22, 2013, 09:04 
Meshing a s809 wing in Gambit

#1 
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Nikolaos R.
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Hello everybody,
Im new in CFD so my questions may seem a bit dumb but i cant help it so here it goes!Im currently trying to simulate a 3d multiphase (airstream+rain drops) flow over a s809 wing.So i attempted to mesh the wing and the farfield and the result was a mesh that kept the solution from converging in Realizable ke model of 2nd order.After reading many answers in similar situations in these forums i tried to make a 2nd mesh in order to get convergence.The thing is that although the solution did converge,the results seem to be a bit off the experimental ones plus there is a highly skewed element in my wing which i dont know how to smooth it. I am simulating a S809 wing (initially in single phase flowairand then in multiphase flow) in Re=2x10^6 using kw sst, realizable ke and spalart allmaras turb models. Here are the 2 meshes i used for this simulation: 

July 23, 2013, 02:53 

#2 
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Nikolaos R.
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I would appreciate it very much if someone could give me a hint on how to improve the 3.mesh because of its highly skewed element at the trailing edge.Here are the vertices that i used for the airfoil


July 23, 2013, 03:32 

#3 
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Maxime Perelli
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post picture of your mesh, and especially where you get skewed elements
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July 23, 2013, 04:00 

#4 
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Nikolaos R.
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thanks for the reply Max,I attach below some pictures


July 23, 2013, 04:04 

#5 
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Maxime Perelli
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from your picture 7, it seems you meshed also your wing (solid part).
why?
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July 23, 2013, 04:08 

#6 
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Nikolaos R.
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I am simulating the 3d flow over the wing so i thought that i should have the wing unmeshed for the part where there is the actual wing and have the wing meshed where there should only be the flowfield,in order to study the effects due to induced drag


July 23, 2013, 04:15 

#7 
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Maxime Perelli
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if you delete the wing volume, do you see your wing as hollow?
You should have substracted your outer domain with your wing.
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July 23, 2013, 04:22 

#8 
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Nikolaos R.
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Yes Max i deleted the volume of the meshed wing (and the mesh in the face of the wing where z=5/I meshed that face because i got a warning when i tried to export the mesh,concerning boundaries) and theres a hole in my mesh.As for substracting,i didnt substract the wing initially (in 2d mesh) i just created the hole out of its adjacent edges.


July 23, 2013, 04:24 

#9 
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Maxime Perelli
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then delete the wing volume, and compute on the rest of the grid
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July 23, 2013, 04:28 

#10 
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Nikolaos R.
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Initially i had my wing unmeshed for 0<z<5(theres the actual wing in the calculations) and had it meshed for 0>z>2(theres only air in calculations).If i delete the wing volume (which is 2<z<0) then theres a hole in the mesh and then we assume that the wing extends from the left to the right side of the flowfield,or?Excuse me for those dumb questions


July 23, 2013, 04:37 

#11 
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Nikolaos R.
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what im trying to say is that if i calculate the grid just like that,dont we assume that we have got a wing all over the width of the flowfield and as such we neglect the 3d effects?


July 23, 2013, 04:44 

#12 
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Maxime Perelli
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ok I understood
The problem comes from your wing extension. So you have to fix this skew element. You can try to split the trailing edge (sharp angle), and mesh this surface with only one tri element. Then propate this split in the zdirection Untitled.png
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July 23, 2013, 04:53 

#13 
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Nikolaos R.
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So i split the upper and lower edges at the secondtolast node and i create a triangle as you show in your picture.Then i mesh the 2 parts of the wing separately (the whole wing and the last trianglelike part which i have created with the split).For the latter i use 1 tri element.Is that correct?


July 23, 2013, 05:03 

#14 
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Maxime Perelli
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yes, it is.
But you will need another split on the right (next to the triangle), especially if you want hexa mesh. Hence the triangle "lock" the thickness of your "wing" with one element. With this second split, you can generate a transition from x elements to one element. Then, as you refined your mesh on the wing, you will also do this refinement on this area
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July 23, 2013, 05:18 

#15 
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Nikolaos R.
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I didnt understand the second split next to the triangle


July 23, 2013, 05:22 

#16 
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Maxime Perelli
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on my picture I represented this volume with a rectangle (inside this rectangle you can mesh with tri or pave)
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July 23, 2013, 05:30 

#17 
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Nikolaos R.
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So the wing is split into 3 parts:the first one to the left(the biggest),the 2nd one(the trapezoid one) and the third one (the triangle one)?


July 23, 2013, 05:35 

#18 
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Maxime Perelli
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yes:
hexa > tri or pave (transition) > tri (one element) Then another split along (xy) for refinement at the end of the wing
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July 23, 2013, 05:44 

#19 
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Nikolaos R.
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After trying to split the right most area (the triangle one) my geometry changes and the edges seem to be converging


July 23, 2013, 06:01 

#20 
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Maxime Perelli
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wait your angle will be to sharp and will generate skewness.
Question, is your wing section constant? on your picture 7 , the trainling edge is represented in light blue, right?
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