need help on 3D meshing of flat-back airfoil

LSC · May 13, 2009, 00:08

The airfoil profile that I am meshing does not end with a common vertex at the trailing edge. It has a equal finite thickness for both upper and lower surface. I have followed the tutorials from Cornell. At the trailing edge, I have 3 vertex with 2 being the end points at the trailing edge and 1 at the center(y=0). My problem is with the 2 separate edges connecting the domain and the edge on the trailing edges. I tried using grid spacing with y+ =1 but encountered problems running the solver..I am actually doing 2D analysis but CFX only solves 3D..How to tackle the region?

jchawner · May 16, 2009, 09:05

It seems to me that you need more than 3 points on the blunt trailing edge. By using only 3 points you have a huge spacing jump across the wake region thru the boundary layer mesh into the blunt-wake mesh.

My suggestion for the blunt trailing edge is to cluster points to the top and bottom using the same spacing as to the airfoil surface and put enough points on the TE to have a smooth variation in cell sizes.

Hope this helps.

LSC · May 16, 2009, 09:10

Thanks a lot for the suggestion..will try it out

A_Lyaskin · May 18, 2009, 05:27

And why do you aim for y+=1 at the trailing edge? This gives you a huge spacing jump also in the flow direction. IMHO it's better to take the length of the first cell in the wake equal to the length of the last cell at the surface

LSC · May 18, 2009, 05:40

I am trying to investigate whether a grid spacing corresponding to Y+=1 would give me more accurate results for Lift and Drag Coefficient.. The blunt trailing edge certainly posed a lot of problems and I am trying very hard to figure how to mesh the wake region..What I tried was to put more nodes in the wake region which grid spacing equal to the surface node which increasing grid size towards the centerline..not sure whether this is the best approach..kindly advice

A_Lyaskin · May 18, 2009, 06:26

Well, y+ at the trailing edge itself wouldn't have much influence at the results. However you'll probably see some difference for y+=1 and y+, say, 100, but just because of numerical error caused by spacing jump.

Actually from my experience I've found another tricky thing - if you have a "stripe" of fine mesh in wake, its alignment also influence the results (CL). That's because flow leaves the trailing edge more or less in the direction of airfoil midline and then it takes some distance to turn back to flow direction. This in turn means that somewhere close to the airfoil you have spacing jump in the flow direction again! To minimize the effect you have to align the mesh with the wake - make a run, find the zero streamline, align the block edges with it, probably make a couple of more iterations, and than repeat the process for the other angle of attack...

May 13, 2009, 00:08	need help on 3D meshing of flat-back airfoil	#1
LSC Member LSC Join Date: May 2009 Posts: 58 Rep Power: 16	The airfoil profile that I am meshing does not end with a common vertex at the trailing edge. It has a equal finite thickness for both upper and lower surface. I have followed the tutorials from Cornell. At the trailing edge, I have 3 vertex with 2 being the end points at the trailing edge and 1 at the center(y=0). My problem is with the 2 separate edges connecting the domain and the edge on the trailing edges. I tried using grid spacing with y+ =1 but encountered problems running the solver..I am actually doing 2D analysis but CFX only solves 3D..How to tackle the region?

May 16, 2009, 09:05		#2
jchawner Senior Member John Chawner Join Date: Mar 2009 Location: Fort Worth, Texas, USA Posts: 275 Rep Power: 18	It seems to me that you need more than 3 points on the blunt trailing edge. By using only 3 points you have a huge spacing jump across the wake region thru the boundary layer mesh into the blunt-wake mesh. My suggestion for the blunt trailing edge is to cluster points to the top and bottom using the same spacing as to the airfoil surface and put enough points on the TE to have a smooth variation in cell sizes. Hope this helps. __________________ John Chawner / jrc@pointwise.com / www.pointwise.com Blog: http://blog.pointwise.com/ on Twitter: @jchawner

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May 16, 2009, 09:10		#3
LSC Member LSC Join Date: May 2009 Posts: 58 Rep Power: 16	Thanks a lot for the suggestion..will try it out

May 18, 2009, 05:27		#4
A_Lyaskin Member Anton Lyaskin Join Date: Mar 2009 Location: Samara, Russia Posts: 66 Rep Power: 17	And why do you aim for y+=1 at the trailing edge? This gives you a huge spacing jump also in the flow direction. IMHO it's better to take the length of the first cell in the wake equal to the length of the last cell at the surface

May 18, 2009, 05:40		#5
LSC Member LSC Join Date: May 2009 Posts: 58 Rep Power: 16	I am trying to investigate whether a grid spacing corresponding to Y+=1 would give me more accurate results for Lift and Drag Coefficient.. The blunt trailing edge certainly posed a lot of problems and I am trying very hard to figure how to mesh the wake region..What I tried was to put more nodes in the wake region which grid spacing equal to the surface node which increasing grid size towards the centerline..not sure whether this is the best approach..kindly advice

May 18, 2009, 06:26		#6
A_Lyaskin Member Anton Lyaskin Join Date: Mar 2009 Location: Samara, Russia Posts: 66 Rep Power: 17	Well, y+ at the trailing edge itself wouldn't have much influence at the results. However you'll probably see some difference for y+=1 and y+, say, 100, but just because of numerical error caused by spacing jump. Actually from my experience I've found another tricky thing - if you have a "stripe" of fine mesh in wake, its alignment also influence the results (CL). That's because flow leaves the trailing edge more or less in the direction of airfoil midline and then it takes some distance to turn back to flow direction. This in turn means that somewhere close to the airfoil you have spacing jump in the flow direction again! To minimize the effect you have to align the mesh with the wake - make a run, find the zero streamline, align the block edges with it, probably make a couple of more iterations, and than repeat the process for the other angle of attack...