Separated flow over single fowler flap attaches after increase in angle of attack
 

Hi all,
I've been studying flow over flaps (2d-case). The case i need help with is of a single fowler flap at deflection of 30 deg for landing configurations. My study was to optimize flap position.
But while generating polar I observed sudden rise in CL at 15 deg AoA. In post process I observed flow on the flap was separated till 14 deg AoA and is attached at 15 deg. I have not been able to reason the cause. I did observe this in almost all other case except few.
Please if someone can help me understand what happens or can refer me some paper to read on this topic.

It happens quite often in my CFD analysis too.
I believe it is caused by the fact that at higher angle of attack the recirculation zone above the flap get bigger and "pushes" the flow
towards the flap thus reattaching it. Look at the Mach number contours
around and mainly above the flap.
In good design, this should not happen so you should change the position of the flap.

Thanks for the reply...
I have run several cases for different positions and i have observed it in almost all positions, except when flow doesn't attach at all...
And I observed the increase in circulation of the main element wake... What I'm not sure of what does trigger it... The process of change in circulation is sudden even for increase of AoA by 0.1 deg...