Investigations on Disturbance Amplification in a Laminar Separation Bubble by Means of LDA and PIV

Laminar-turbulent transition is an important feature in aerospace aerodynamics because of its influence on fuel consumption via the drag. Despite many years of research on fundamental mechanisms of the transition process in many generic configurations relevant for aircrafts, as well as other applications, there are situations which are less well understood and hence much more difficult to predict than certain “standard” cases, as e.g. zero-pressure gradient flat-plate boundary-layer transition. When a laminar boundary layer separates due to an adverse streamwise pressure gradient, the flow is subject to increased instability with respect to small-amplitude disturbances. Laminar-turbulent transition will occur, and in the considered case of a laminar separation bubble (LSB), the turbulent boundary layer will reattach. The experiments were carried out in a laminar water-tunnel. The low turbulence level allows investigations on the development of intentionally excited 2-D or 3-D dis turbance-wave combinations under controlled conditions. With respect to the disturbance source, the development of such perturbations within the separated boundary layer is measured by means of phase-averaged Laser-Doppler-Anemometry (LDA) and phase-locked Particle Image Velocimetry (mono-PIV and stereoscopic-PIV (SPIV)). The role of unsteady disturbances with (3-D) and without (2-D) controlled spanwise variation in the occurring mechanism of transition is examined in detail. The transition mechanism occurring in the flow-field under consideration will be discussed and observations for the development of small disturbances are compared to direct numerical simulations (DNS) and predictions from linear stability theory (LST). A method will be shown to measure wall-normal amplitude distributions and amplification rates of essentially small, unsteady (periodical) 3-D disturbance waves within the boundary layer by means of phase-locked mono-PIV. By sequentially shifting the phase angle with respect to the disturbance source over one fundamental period, instantaneous boundary-layer profiles were measured. In order to also capture spanwise wave lengths in case of 3-D disturbance input, the flow field was scanned automatically with the light sheet in this direction for each phase angle. A double Fourier transform in time (fundamental period) and spanwise direction then yields the amplitudes and phases of the disturbance modes. Results are compared in detail with corresponding LDA measurements and linear stability theory. With the possibility of measuring unsteady disturbance development by PIV the measurement time could be significantly decreased by at least a factor of ten in comparison to adequate phase-averaged LDA measurements. 11 InternationalSymposium on Applications of Laser Techniques to Fluid Mechanics, 8-11 July 2002, Lisbon, Portugal