Hi,

I would like to ask a qn. For flow past an airfoil, when there is leading edge separation, the propulsive efficiency will drop, is this always true? To be more specific, the Re is low, around 10,000. The airfoil can be static or heaving/pitching.

I am thinking along the line that if there is leading edge separation, the lift will decrease. This 'll result in higher efficiency. Is this correct? How do u relate to the change in pressure when there is separation?

Thanks an advance!

It is not always true. The generation of the leading edge vortex coupled with delayed stall can result in generation of high lift forces.

http://www.ae.metu.edu.tr/~mkaya/kopru/papers/ylkt.pdf

Generation of leading edge separation bubble will not always result in loss of performance. It depends on the size of the separation bubble and the effect of it on the overall Cp distribution. If the bubble is small enough not to have significant effect on Cp distribution, it will have no effect on lift characteristics.