Investigations on controlled transition development in a laminar separation bubble by means of LDA and PIV

When a laminar boundary layer separates because of an adverse streamwise pressure gradient, the flow is subject to increased instability with respect to smallamplitude disturbances. Laminar–turbulent transition occurs under a rapid three-dimensional (3D) development within the separated shear layer. When the following turbulent boundary layer reattaches, a laminar separation bubble is formed. To allow controlled measurements, a small-amplitude Tollmien–Schlichting wave (TS wave) was introduced into the boundary layer without (case I) and with (case II) spanwise forcing of steady 3D disturbances. Combined application of laser-Doppler anemometry (LDA) and particle image velocimetry (PIV) demonstrates the suitability of both measurement techniques to capture the development of unsteady, periodic phenomena. The transition mechanism occurring in the flow field under consideration is discussed, and results obtained by controlled measurements are compared to direct numerical simulations (DNS) and predictions from linear stability theory (LST). Flow visualizations and stereoscopic PIV measurements give better insight into the 3D breakdown of the separated shear layer. Nomenclature a amplitude f0 fundamental frequency H12 boundary layer shape factor, H12=d1/d2 h wavenumber coefficient in time k wavenumber coefficient in the spanwise direction l liter m meter Rex Reynolds number based on streamwise distance of the leading edge Red1 Reynolds number based on the displacement thickness d1 s second t time u¥ freestream velocity ud velocity at the boundary layer edge u,v,w velocities u¢,v¢ velocity fluctuations x streamwise coordinate y wall-normal coordinate z spanwise coordinate d boundary layer thickness d1 boundary layer displacement thickness d2 boundary layer momentum thickness kz spanwise wavelength F phase angle