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-   -   [ICEM] Leading Edge Flow separation- O grid or C-grid? (http://www.cfd-online.com/Forums/ansys-meshing/91145-leading-edge-flow-separation-o-grid-c-grid.html)

AeroBharath15 August 1, 2011 18:31

Leading Edge Flow separation- O grid or C-grid?
 
HI all,

My thesis is on leading edge flow separation control on airfoil using plasma actuators. I would like to know whether to use O-grid or C-grid type of meshing would be better to catch the flow separation at the leading edge. I have tried C-grid now but the Lift coefficient value deviates a lot from the experimental one. Kindly give me your suggestions please.

PSYMN August 2, 2011 12:36

At the leading edge, CGrid and OGrid should be the same.

For a sharp trailing edge, CGrid is recommended.

For a square or rounded trailing edge, Ogrid is recommended.

AeroBharath15 August 10, 2011 11:08

Thank you very much Simon... Im using a sharp trailing edge... i get errors while smoothening... It says "max values cannot be equal to the min values"... and the determinant values of the cells are between .98-1. how do i rectify this? Thanks for your help

bluelc August 18, 2011 09:22

trailing edge of naca0018
 
Quote:

Originally Posted by PSYMN (Post 318537)
At the leading edge, CGrid and OGrid should be the same.

For a sharp trailing edge, CGrid is recommended.

For a square or rounded trailing edge, Ogrid is recommended.

hi, simon

In your opinion, whether the trailing edge of naca0018 is sharp or not?

Furthermore, how to avoid these high aspect ratio grids in the wake flow when choosing CGrid? I always get some numerical errors in these high aspect ratio grids in wake flow?

PSYMN August 20, 2011 23:44

BlueLC...

Any airfoil can be sharp or not... If you just follow the NACA coordinates, you are probably heading toward a sharp trailing edge (a single point) at the trailing edge of the top and bottom of the airfoil.

However, any airfoil can be truncated slightly to give a small flat trailing edge. There is lots of data to show that this doesn't actually reduce the aero properties very much, and you could argue that in real life, the trailing edge always has some thickness, if only a few millimeters.

Some engineers truncate it with a small flat trailing edge, others use a rounded trailing edge... Either way, it allows for an ogrid mesh topology instead of a CGRID...



If you have high aspect ratio elements behind your trailing edge, it is because they are long in one direction and short in another... Typically, they are short perpendicular to the wing because you want to capture your boundary layer (Y+) and you can't really increase that to reduce the aspect ratio (Actually, I guess you can a little since the flow has usually slowed significantly by the time it reaches the trailing edge, but lets assume you want to keep it the same ;)). So the remaining solution is to reduce the edge distribution behind the wing so they are relatively short in that direction also. You can bias this so the edge size will grow as it moves away from the trailing edge (downstream)... Hopefully, you understand what I mean...

Best regards,

Simon

PSYMN August 21, 2011 00:00

AeroB

Sorry, Almost missed yours... Not sure what the problem is or how you got that error message. Maybe post more info, or maybe just skip the smoothing. The quality is probably good enough anyway.


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