What causes the bumps in E387 polar diagram

[Salto]

I've often wondered what causes the rapid rise in drag, then reduction, at low Reynolds numbers for airfoils like the E387. Is this the effect of seperation bubbles forming on the top surface? If so, then can anyone explain to me why the drag returns to a lower value at higher angles of attack?

Here's a link to the airfoil. http://www.nasg.com/afdb/show-airfoil-e.phtml?id=159

Graham.

[HELModels]

I've wondered the same exact thing many moons ago. I believe that the flow seperates and then reattaches as a turbulent layer or something which is draggier but better than full seperation.

[Purdue Aero Man]

If that is the case, it would probably be due to a laminar separation bubble, as you have both alluded to. But I am not certain.

[raptor22]

I've wondered the same exact thing many moons ago. I believe that the flow seperates and then reattaches as a turbulent layer or something which is draggier but better than full seperation.

All true. it is spoken of on the martin hepperle site.

Not sure why it reduces in drag again, I beleive that higher specific AOA's the forward portions of the airfoil tend to force turbulent flow. Hepperle does state that the answer to seperation bubbles is a turbulator or other device to force turbulent flow transition.

--Alex