Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The underlying mechanism

TL;DR

This study uncovers the vortex triad mechanism behind the quasi-periodic bursting and reformation of the laminar separation bubble on a NACA-0012 aerofoil, revealing flow oscillation dynamics and potential control strategies.

Contribution

It identifies the vortex triad and flow oscillation mechanism responsible for LSB bursting and reformation, advancing understanding of flow dynamics over aerofoils.

Findings

Flow oscillates between attached and separated states periodically.

A vortex triad drives the flow oscillation and bubble dynamics.

Flow direction influences transition location and flow attachment.

Abstract

The present investigation shows that a triad of three vortices, two co-rotating vortices (TCV) and a secondary vortex lies beneath them and counter-rotating with them, is behind the quasi-periodic self-sustained bursting and reformation of the laminar separation bubble (LSB) and its associated low-frequency flow oscillation (LFO). The upstream vortex of the TCV (UV) is driven by the gradient of the oscillating-velocity across the laminar portion of the separated shear layer and is faithfully aligned with it. The secondary vortex acts as a roller support that facilitates the rotation and orientation of the TCV. A global oscillation in the flow-field around the aerofoil is observed in all of the investigated angles of attack, including at zero angle of attack. The flow switches between an attached-phase against an adverse pressure gradient (APG) and a separated-phase despite a favourable pressure gradient (FPG) in a periodic manner with some disturbed cycles. When the direction of the oscillating-flow is clockwise, it adds momentum to the boundary layer and helps it to remain attached against the APG and vice versa. The transition location along the separated shear layer moves upstream when the oscillating-flow rotates in the clockwise direction causing early-transition and vice versa. The best description of the mechanism is that of a whirligig. When the oscillating-flow rotates in the clockwise direction, the UV whirls in the clockwise direction and store energy until it is saturated, then the process is reversed. The present investigation paves the way for formulating a time-accurate physics-based model of the LFO and stall prediction, and opens the door for control of its undesirable effects.