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12th Spring School on Transition and Turbulence
Evolution of Tollmien-Schlichting waves excited by harmonic point source
Victor Barcelos Victorino , SP
Co-Authors: Victor Barcelos Victorino, Christian Bresci, Matheus Maia Beraldo, Marcello Augusto Faraco de Medeiros
Presenter: Victor Barcelos Victorino
Tollmien-Schlichting (TS) wave is the dominant mechanism of transition in two-dimensional boundary layer. Its evolution are well modeled by Linear Stability Theory (LST), however non-linear effects are not predicted by LST and promote crucial role on transition. The current study experimentally investigates the spatial evolution of controlled artificial disturbances inserted by a harmonic point source in a boundary layer over a flat plate. A sine-like wave with a preset frequency excites a loudspeaker buried in a bore present in the opposite work-side of model. A small hole of 0.8 mm diameter concentric with the bore is responsible to introduce a blow and suction flow induced by the loudspeaker’s membrane into the boundary layer at work-side. The model consists of a plexiglass plate with 1.8 m chord and 1 m span vertically assembled inside the test section of the Low Acoustic Noise and Turbulence (LANT) wind tunnel at EESC-USP. A super-elliptical asymmetric leading edge is attached on the plate, in order to minimize the region of non-zero pressure gradient. A flap and tab are also installed on model for pressure distribution adjustment. Hot-wire anemometry technique was employed for velocity measurements and an acquisition device was responsible for data record. Previous characterization of the base flow was carried out, such as turbulence intensity of free-stream velocity and two-dimensional Blasius profile. The turbulence intensity at central region of test section records 0.1% within the frequency range 8 - 1024 Hz. Good agreement with Blasius was obtained in a 400 mm span range, along two different streamwise positions. Furthermore, the profile of fluctuation component of velocity (u') amplitude was measured at several positions downstream the disturbance source. The results displayed typical shape of TS amplitude profile. Analysis of the frequency spectrum of TS peak will be employed in order to investigate possible non-linear effects. Preliminary observation exhibits, at later downstream positions, an amplitude growth in a band of frequencies near Branch I of neutral curve stability diagram.
Tollmien-Schlichting Wave, Harmonic Point Source, Boundary Layer Transition, instability