2D Low Speed Airfoil Problem when altering Inlet

mike wilson

I am having problems running a simple 2D mesh of a NACA aerofoil section through CFX 10.

I am trying to vary the angle of attack of the section to extract a lift polar, however if I use cartesian velocity components (U,V,W) to create an inlet flow angle of attack (ie rectangular domain with section lying in domain axis), the drags extracted are 100 times larger than expected, and the lift curve gradient is less than expected. (CASE 1)

Due to this I decided to create a number of geometries where the airfoil section was rotated (to allow the inlet velocity to be inputted using only one component ie. U). When running this through CFX I get a lift curve that matches experimental data and the drags are a factor of 10 to large 9which is better than 100!). (CASE 2)

I am having trouble explaining why this is happening, and was wondering if someone may be able to shed any light on the matter? Or how to solve it.

I understand that my meshes will be different do to the geometry rotating in the second case but the mesh spacing i used i kept constant and in the same relative positions. Results for both case converge to e-5 after about 60 iterations.

I have compared the locations of the max residuals and found that for both cases they are in roughly the same place (taking into account the rotation).

The domain is a rectangular one and is set up as follows and I have ran SST, SST + TT and KEpsilon all with the same trends:

CASE 1; Airfoil Section: Wall (no slip), Outlet: outlet Relative pressure 0, Domain Top: Wall (free slip), Domain Sides: Symmetry planes, Inlet and domain base: Inlets with the relevant cartesian velocities.

CASE 2; Airfoil Section: Wall (no slip), Outlet: outlet Relative pressure 0, Domain Top: Wall (free slip), Domain Sides: Symmetry planes, Domain Base: Wall (free slip), Inlet: Inlet with velocity in +U

Any help would be much appreciated! Thanks in advance