Airfoil Boundary Layer Bubble Separation and Transition in a Surging Stream

TL;DR

This study investigates how high-amplitude harmonic surging affects airfoil laminar separation bubbles through theoretical and experimental methods, revealing that bubble bursting can occur during early favorable pressure gradients, causing significant lift and drag oscillations.

Contribution

It introduces a novel generalized pressure coefficient for surging flows, enabling direct comparison between surging and steady conditions, and analyzes bubble dynamics under unsteady flow effects.

Findings

Bubble bursting occurs during early favorable pressure gradients.

Surging causes large oscillations in lift and drag coefficients.

Generalized pressure coefficient reduces form-drag measurement errors.

Abstract

The effect of high-amplitude harmonic surging on airfoil laminar separation bubbles was investigated theoretically, and experimentally in a dedicated surging-flow wind tunnel. A generalized pressure coefficient was developed that accounts for local static pressure variations due to surging. This generalization, never previously implemented, facilitated direct comparisons between surging and quasi-steady pressure coefficients, and thus unsteady effects could be distinguished from Reynolds number effects. A momentum-integral boundary layer analysis was implemented to determine movement of the bubble separation point, and movement of the transition point was extracted from experimental surface pressure coefficients. The most significant finding was that bubble bursting occurs, counterintuitively, during early imposition of the favorable temporal pressure gradient. This is because the favorable pressure gradient rapidly drives the bubble aft, rendering it unable to reattach. Bursting resulted in large lift and form-drag coefficient oscillations, and failure to implement the generalized pressure coefficient definition resulted in temporal form-drag errors of up to 400 counts.