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failed validating Cd for 2D airfoil

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Old   January 24, 2019, 10:52
Question failed validating Cd for 2D airfoil
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Sereff
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Dear Foamers:

The case I was simulating is air flow around a 2D airfoil with Re = 1e6, with following:
  • Solver: pimpleFoam
  • Model: k-Omega SST
  • Meshing: snappHexMesh

I've been getting pretty covincing results for Cl, but not for Cd. I've read from some source that Cd is far more sensitive to mesh quality than Cl does. I assume that 'high mesh quality' means more refined mesh as in more cells and finer boundary layers?

And talking about boundary layers, I took a closer look at the yPlus value for my simulations on different mesh, please correct me if I was using wrong BCs,
case 1,30<y+<70, with boundary conditions on airfoil:
  • U: type noSlip;
  • turbulence properties: type xxWallFunction; value $internalField
case 2,0.09<y+<1.2, with boundary conditions on airfoil:
  • U: type noSlip;
  • turbulence properties: type fixedValue; value uniform 1e-15
But all my simulations are having trouble with Cd. Any idea?

Kind regards,

Last edited by Sereff; January 24, 2019 at 15:04.
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Old   January 24, 2019, 11:50
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Attached are my Cd and Cl results for different aoa.

Last edited by Sereff; January 28, 2019 at 05:09.
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Old   January 25, 2019, 02:41
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Drag is a difficult property to get right in airfoil simulations. It is very dependent on the wall boundary layer, and so using a wall function will likely not work.


What is meant by a high mesh quality is details such as density, minimal grading near the walls, low non-orthogonality and skewness, and cells aligned to the direction of flow as much as possible.


Also, as the drag is so dependent on the boundary layer, accurate turbulence modelling at the boundary is also very important. Unfortunately, all RANS formulations are simply an approximation of turbulence effects, so this introduces enhanced difficulty in getting a correct boundary layer
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Old   January 25, 2019, 03:44
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Hi, thank you for your anwer.

So I assume with k-Omega-SST, it is expected to have a rather bad Cd prediction. But do you know what boundary condition should I use for velocity on the surface of airfoil? I've been told if wall function were to apply then I should use 'zeroGradient' for airfoil surface instead of 'noSlip'.

Kind regards,
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Old   January 25, 2019, 05:10
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AFAIK if you are interested in the effect of the wall, a no slip condition is always needed
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2d airfoil, boundary conditions, drag coefficient, validation, yplus

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