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Session: Control of separated/unsteady flows I
Session starts: Wednesday 05 November, 15:40
Presentation starts: 15:40
Room: Commission roon 2

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Connor Toppings (University of Waterloo)
Theodoros Michelis (TU Delft)
Marios Kotsonis (TU Delft)
Serhiy Yarusevych (University of Waterloo)

Abstract:
The bursting of a laminar separation bubble subject to periodic forcing is studied experimentally on a NACA0018 airfoil at a chord Reynolds number of 70000 during a transient ramp increase in angle of attack past the static stall angle with a maximum non-dimensional pitch rate of 0.01. Forcing at a constant frequency, equal to the most amplified frequency in the separated shear layer at the initial angle of attack, is applied to the boundary layer by a spanwise-uniform alternating current dielectric-barrier-discharge plasma actuator at the leading edge of the airfoil. In the unforced flow, bursting of the laminar separation bubble begins before the maximum angle of attack is reached. Weak periodic forcing, although unable to prevent bursting, delays the onset of bursting past the end of the pitching motion. Periodic forcing enables a laminar separation bubble to persist for a longer period of time under a stronger adverse pressure gradient relative to the unforced flow. Although periodic forcing may delay bursting, the dynamics of the bursting process are similar between unforced and forced conditions, requiring a period of approximately 30 convective time units for the flow to settle to a massively separated state. The delay in the onset of bursting increases and becomes more random as the forcing amplitude increases, until the forcing amplitude is sufficient to prevent bursting entirely. Bursting is observed to be an irreversible process, with the flow always progressing to a state of massive separation after the initial cessation of reattachment.