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ISSN Онлайн: 2642-0554

Effects of free-stream turbulence intensity on transition within a laminar separation bubble

Mark S. Istvan
Department of Mechanical and Mechatronics Engineering University of Waterloo 200 University Ave. W, Waterloo, ON, N2L3G1, Canada

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

Аннотация

The laminar separation bubble formed over the suction side of a NACA 0018 airfoil was studied experimentally for increasing levels of free-stream turbulence intensity. Experiments were performed in a wind tunnel facility at a chord Reynolds number of 80000 and an angle of attack of 4 degrees. The spatio-temporal flow development was characterized by means of time-resolved Particle Image Velocimetry. The results show that the height and streamwise extent of the separation bubble are reduced as the level of turbulence intensity is increased. The decrease in separation bubble length is the result of downstream and upstream shifts in the locations of mean separation and reattachment, respectively, with the later caused by an upstream shift in the location of mean transition. Velocity fluctuations in the shear layer reach significant magnitudes earlier upstream as the level of turbulence intensity is increased, leading to the upstream shift in mean transition. At the highest level of turbulence intensity investigated, significant changes occur in the flow topology and dynamics, suggesting an alteration of the transition process. Maximum spatial amplification rates of disturbances are shown to decrease as the bubble size decreases, indicating that the upstream shift in mean transition is attributed to the increase in the initial amplitude of disturbances upstream of the separation bubble. Spectra of the wall-normal velocity fluctuations show that, as the bubble size is reduced, the central frequency of disturbances remains approximately constant, but that the detectable band of unstable frequencies broadens as a result of the broader range of disturbances present in the free-stream.