Airfoil: Laminar or Turbulent

Yap Wen Jiun

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