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Structure and dynamics of a laminar separation bubble near a wingtip

Published online by Cambridge University Press:  28 October 2021

Connor E. Toppings  and
Serhiy Yarusevych Open the ORCID record for Serhiy Yarusevych [Opens in a new window]
Connor E. Toppings
Affiliation:
Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
Serhiy Yarusevych*
Affiliation:
Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
*
†Email address for correspondence: syarus@uwaterloo.ca
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Abstract

The three-dimensional flow topology of a laminar separation bubble forming on the suction surface of a semispan wing with an aspect ratio of $2.5$ and NACA 0018 airfoil section is characterised experimentally using surface pressure measurements and particle image velocimetry at a chord Reynolds number of $125\ 000$. In the inboard region of the wing, the separation bubble is essentially two-dimensional, and the transition process in the separated shear layer leads to periodic vortex shedding, which dominates the bubble dynamics, similar to two-dimensional separation bubbles. However, progressive spanwise changes in the mean structure and vortex dynamics occur near the wingtip, leading to an open separation and eventual suppression of the bubble. In the immediate proximity of the wingtip, the boundary layer remains attached, no vortex shedding occurs and the flow remains laminar, terminating separation bubble formation. Despite variations in the mean separation bubble topology and vortex dynamics along the span, the fundamental shedding characteristics remain nearly invariant across the portion of the wing where vortex shedding occurs, and the flow appears to lock onto a common instability mode across the span, leading to minimal changes in the mean bubble characteristics despite notable changes in the effective angle of attack along the span. A comparison with available surface flow visualisations from previous studies indicates that the observed changes to the mean bubble footprint along the span of the wing are similar across different geometries and flow characteristics, suggesting similarities in the three-dimensional bubble topology and dynamics on finite wings.

JFM classification

Boundary Layers: Boundary layer separation Vortex Flows: Vortex shedding Wakes/Jets: Separated flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 929 , 25 December 2021 , A39
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Copyright
© The Author(s), 2021. Published by Cambridge University Press

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Toppings and Yarusevych supplementary movie 1

Side-view phase averaged contours of spanwise vorticity and spanwise velocity, showing vortex shedding cycle at z/c=1.25.

Download Toppings and Yarusevych supplementary movie 1(Video)
Video 1 MB

Toppings and Yarusevych supplementary movie 2

Side-view phase averaged contours of spanwise vorticity and spanwise velocity, showing vortex shedding cycle at at z/c=2.00.

Download Toppings and Yarusevych supplementary movie 2(Video)
Video 1.2 MB

Toppings and Yarusevych supplementary movie 3

Side-view phase averaged contours of spanwise vorticity and spanwise velocity, showing vortex shedding cycle at at z/c=2.15.

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Video 770.2 KB

Toppings and Yarusevych supplementary movie 4

Side-view phase averaged contours of spanwise vorticity and spanwise velocity, showing vortex shedding cycle at at z/c=2.25.

Download Toppings and Yarusevych supplementary movie 4(Video)
Video 552.6 KB

Toppings and Yarusevych supplementary movie 5

Top-view phase averaged contours of streamwise velocity, showing vortex shedding cycle.

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Video 930.3 KB