[gregorv]

I am trying to model 2D wing profiles with simpleFoam without using wall functions as I am mostly interested in laminar wing profiles. I am currently trying to compare the NACA 23015 profile (not a laminar profile in itself, of course) computations at 0 AOA with experimental data at the Reynolds number of 260000. I also tried with other profiles with very similar results.

I am using the Spalart-Allmaras turbulence model, with the value of nuTilda at both the inlet and the wing itself set to a small number (10^(-8)) in dimensionless units. I played about with combinations of inlets and outlets and the calculations are very insensitive to that.

The problem I have is that (using the liftDrag tool as recompiled for 1.4.1) drag is overpredicted by a factor of more than 2. The pressure and viscous contributions to drag are roughly the same in size. I understand that the liftDrag tool can be used as-is for my problem seeing that the turbulent viscosity should drop to 0 on the wing itself?

To create the mesh, I am using an own made tool to create scripts for gmsh, and the meshes I use are regular (hexahedral) for the distances well outside the boundary layer then unstructured outside. I checked with paraFoam that the velocity takes a number cells to change from the value of 0 to the freestram velocity and that the velocity in the first cell is practically 0 (well, blue in paraFoam ;) ). Could there nevertheless still exist artificial viscosity due to the mesh that bumps the drag in my case to a larger than expected value? Also, the aspect ratio of the cells just next to the wing is naturally quite large (at the order of 20), is that a problem for the boundary layer calculations and the cause of my error? The results do change somewhat with further refinement of the boundary layer discretization, but this is to the order of 10% and nowhere near the factor of 2 needed. The solutions always converge nicely and give very plausible looking flow fields.

I would be glad to hear from anyone who has experience obtaining successful drag predicitions for laminar profiles.