The Mechanism of Lift and Pitching Moment Reversal

TL;DR

This study investigates the flow mechanisms behind lift and pitching moment reversal on a 2-D airfoil after a single-burst actuation, highlighting the role of vortices and flow modes.

Contribution

It identifies the flow structures and modes responsible for lift reversal and enhancement, providing insight into flow control mechanisms.

Findings

Vortices-induced downwash causes lift and moment reversal.

POD mode 2 is linked to the reversal mechanism.

POD mode 1 relates to lift enhancement.

Abstract

The experiment of separated flow response to a single-burst actuation over a 2-D NACA-0009 airfoil at $12^o$ angle of attack was conducted. The mechanism of the lift and pitching moment reversal following the single-burst actuation was studied. A spatially localized region of high pressure caused by a vortices-induced downwash is responsible for the lift and pitching moment reversal. Proper orthogonal decomposition (POD) of the flowfield shows that mode 2 shares a similar structure that produces the downwash and is responsible for the lift and pitching moment reversal. On the other hand, POD mode 1, which represents the direction and strength of the reverse flow on the suction side of the airfoil is responsible for the lift enhancement.