Laminar Separation Bubble Dynamics on a Finite Wing

Abstract

Laminar separation bubbles substantially influence the performance of finite wings at low chord Reynolds numbers. The objective of this study is to explore the influence of wingtip effects on three-dimensional laminar separation bubble topology and dynamics on a finite wing. An experimental investigation is conducted on a laminar separation bubble forming on the suction surface of a cantilevered rectangular NACA 0018 wing with a semi-aspect ratio of 2.5 at a chord Reynolds number of 125 000 and an angle of attack of 6 degrees. Surface pressure and particle image velocimetry measurements are employed to investigate the separation bubble flowfield. Using a two-dimensional airfoil of the same profile, the separation bubble on the wing is compared to a nominally two-dimensional separation bubble at similar effective angles of attack. On the portion of the wing where laminar boundary layer separation occurs, the separated shear layer rolls up into spanwise uniform vortices which develop similarly to the vortices observed on the two-dimensional airfoil, despite spanwise changes to the mean separation bubble structure along the wingspan. Whereas a decrease in the angle of attack of the two-dimensional airfoil causes a downstream shift in the locations of separation and reattachment and a reduction in the frequency of shear layer vortex shedding, spanwise variations of these parameters on the wing are much smaller than the variations expected due to the reduction in effective angle of attack near the wingtip. On the inboard portion of the wing, the location and vortex shedding frequency of the separation bubble are analogous to the separation bubble on the two-dimensional airfoil at the effective angle of attack of the wing root. Downwash from the wingtip vortex inhibits boundary layer separation in proximity to the wingtip, suppressing shear layer vortex shedding and causing a delay in transition near the wingtip. Unlike a canonical two-dimensional separation bubble, the separation bubble on the wing becomes an open separation near the wingtip, where the spanwise pressure gradient causes fluid to enter into the separation bubble, producing a substantial spanwise flow within recirculation region. A comparison with the results of previous studies suggests a similar bubble topology across different wing geometries and experimental conditions. The results of this investigation quantify the influence of wingtip effects on a laminar separation bubble, elucidating the three-dimensional changes to the bubble’s mean structure and dynamics along the wingspan.