The Footprint of Laminar Separation on a Wall-Bounded Wing Section at Transitional Reynolds Numbers

TL;DR

This study investigates laminar separation on a wing at transitional Reynolds numbers, revealing flow separation, recirculation zones, and spanwise flows, with detailed measurements and simulations showing the flow's sensitivity to viscous effects at low Re.

Contribution

It provides detailed experimental and numerical analysis of laminar separation and flow behavior on a wing at low Reynolds numbers, highlighting the influence of wall effects and flow unsteadiness.

Findings

Flow separation and recirculation zones are characterized at low Re.

Spanwise flows influence the flow structure behind separation.

Good agreement between measurements and direct numerical simulation.

Abstract

When a chordwise Reynolds number (Re) falls below about $10^5$ the performance of wings and aerodynamic sections become sensitive to viscous phenomena, including boundary layer separation and possible reattachment. Here, detailed measurements of the flow inside the boundary layer on the suction surface are shown for an aspect ratio 3 wing with wall boundaries. The separation lines and recirculation zones are shown on the wing and on the wall junction as Re and angle of incidence, ($\alpha$) are varied. There is good agreement on the lowest Re case which has also been computed in direct numerical simulation. Though the flow at midspan may sometimes be described as two-dimensional, at $\alpha \leq 6^\circ$ it is unrepresentative of the remainder of the wing, and the influence of the wall is seen in strong spanwise flows aft of the separation line. The geometry of the NACA 65(1)-412 section, used here, promotes a substantial chord length for the development of the recirculating regions behind separation making it apt for their study. However, the phenomena themselves are likely to be found over a wide range of wings with moderate thickness at moderate $\alpha$.