Combined Investigations on Disturbance Development in a Laminar Separation Bubble by Means of PIV, LDA and DNS

submitted for the Joint EWA and PivNet 2 Workshop on CFD, PIV and Experiments in Aerodynamics, May 10–11, 2005, Göttingen, Germany Combined Investigations on Disturbance Development in a Laminar Separation Bubble by Means of PIV, LDA and DNS M. Lang†, O. Marxen‡, U. Rist‡ ‡ Institut für Aerodynamik und Gasdynamik (IAG), Universität Stuttgart † current affiliation: Behr GmbH & Co. KG Introduction Laminar separation bubbles (LSB) are an essential feature of the flow field in many technical applications at low to medium Reynolds numbers, for example laminar-airfoil sections or high-lift devices. A LSB occurs if the laminar boundary layer separates due to a strong adverse pressure gradient. The separated boundary layer then undergoes transition featuring an unsteady disturbance development, normally leading to reattachment. The occurrence of LSBs can essentially affect the efficiency of a whole system. Several landmark experiments (e.g. [1]) and numerical simulations (e.g. [5]) have been conducted on laminar-turbulent transition within a LSB. However, the physically complex flow field and its unsteady behaviour leading to the breakdown of the separated boundary layer is not completely understood. Experimental Setup The experiments were carried out in a laminar water-tunnel which was especially designed to study laminar-turbulent transition. A flat plate with an elliptical leading edge is mounted in the free stream of the test section. The streamwise pressure gradient to generate a pressure induced LSB is imposed on the flat plate by a displacement body (Fig. 1). The experiment was performed under controlled conditions. Small amplitude 2-D TollmienSchlichting (TS) waves were excited upstream of the displacement body by an oscillating wire. Additionally, 3-D disturbances were imposed by placing thin metal plates (spacers) regularly underneath the wire [2]. The fundamental frequency (f0 = 1.1Hz), generated by the oscillating wire, corresponds to the frequency which is most amplified according to linear stability theory (LST). The spanwise wavelength of the 3-D disturbance input was set to λz = 58 mm, so that regular vortex structures can be seen to appear in the transition region (Fig. 2). All measurements were carried out phase-locked with respect to the disturbance source. With a 2-component LDA the velocity components in mean flow direction (u) and perpendicular to the flat plate (v) were measured. The applied (Mono/Stereo)-PIV system consisted of a double pulsed Nd:YAG laser (150 mJ/pulse), two double-frame CCD-cameras (PCO SensiCam, 1280 px× 1024 px) and an external sequencer (DLR). A detailed description of the experiment and results are given in [3, 4]. Results An essential part of this transition experiment was the measurement of disturbance oscillations and their streamwise development within the boundary layer. Therefore, a method will be presented to measure unsteady 3-D disturbances using Mono-PIV. It will be shown that the amplification of small amplitude perturbations (Au ≤ 2%uδ; uδ =velocity at the boundary layer’s edge) can be accurately measured for comparisons with LDA, LST and also numerical simulations (DNS). Due to the required spatial resolution [3, 4] for measuring 3-D disturbance waves, the light-sheet optics (illuminating the xy-plane) and the camera were mounted on a traverse system to move simultaneously in spanwise (z) direction. Phase-locked measurements