I am simulating incompressible flow past an airfoil in Phoenics, I am using a LAMINAR model. I am having a convergent problem when the angle of attack exceed 10 degree. But the NACA 23012 airfoil shouldn't stall only after about 15 degree or more. 1. What actually cause this instability? 2. Is it the turbulent boundary layer which is a 3-D and irregular flow? 3. Or it is the boundary layer separation that already developed? (If so than why is the airfoil stalled at

10 degree.) 4. The boundary layer of the airfoil is Laminar or

Turbulent at RN = 3 to the power of 6 ? 5. Boundary layer separation for such airfoil is developed

gradually with increase of angle of attack or happen

instantly just after the angle of attack reach the maximum ? Please help. Thank you

When a boundary layer is laminar it detaches earlier than when it is turbulent. So the turbulent boundary layer is actually a source of stability, rather than instability, since it helps to keep the boundary layer attached to the arfoil. Once the boundary layer detaches the drag increases and that's when the airfoil stalls.

Have a look at a book on boundary layers, e.g. Schlichting

you can find most of the theory and experiments there, together with airfoil examples, etc...

PG

you can check if the boundary layer is turbulent or laminar by examining the momentum thickness reynolds number. There are all sorts of correlations. There is an excellent paper by Fraser Higazy and Milne, or altenatively look in the phoenics journal in years 1994-1997. I wrote a paper dealing with boundary layer transition in phoenics.

Are you having convergence problems, or is the airfoil stalling? The convergence problems is code dependent, you can get convergence by changing parameters in the code.

If the airfoil is stalling, but the code is converging, then what reynolds number are you running the flow at? You can find out if that Reynolds number is in the laminar or turbulent regime, it would be appropriate to solve this problem turbulent.