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Merging of coherent structures in a separation bubble

J. W. Kurelek
Department of Mechanical and Mechatronics Engineering University of Waterloo 200 University Ave. W., Waterloo, ON, Canada, N2L 3G1

Serhiy Yarusevych
Mechanical and Mechatronics Eng. University of Waterloo 200 University Avenue West Waterloo, Ontario, N2L 3G1 Canada

Sinopsis

This work examines the spatio-temporal evolution and interaction of coherent structures in a laminar separation bubble (LSB) when left to develop naturally and excited acoustically. The investigation is carried out in a wind tunnel using a NACA 0018 airfoil model at a chord Reynolds number of 125000 and an angle of attack of 4°. Excitation is provided by an external acoustic source and planar, time-resolved Particle Image Velocimetry is used to characterize both the streamwise and spanwise flow development. It is shown that the separation bubble is receptive to acoustic disturbances applied at the first subharmonic of the most unstable disturbance frequency in the natural LSB, leading to the inception, growth, and decay of velocity fluctuations in the separated shear layer. These velocity disturbances are shown to manifest through periodic vortex merging − a process that otherwise occurs randomly in the natural flow. Assessment of the spanwise flow topology reveals that structures merge in a non-uniform manner along the span, with localized merging occurring away from where streamwise-oriented bulges develop in the two vortices involved in the pairing process.