Experimental Investigation of the Structure and Dynamics of Laminar Separation Bubbles

Abstract

This work is an experimental investigation of the dynamics of the laminar separation bubbles, which are typically present on the suction side of lifting surfaces at a large angle of attack. The separation bubble was generated on a flat plate by an adverse pressure gradient induced by The adverse pressure gradient was generated by using an inverted wing with a NACA 643-618 airfoil mounted above the flat plate. Using Particle Image Velocimetry (PIV), a parametric study of the effect of the upstream flow velocity and the induced pressure gradient on the mean flow topology and the unsteady behavior of the separation bubble was carried out in the low-speed water tunnel of the Hydrodynamics Laboratory at the University of Arizona. The topology of the laminar separation bubble, and in particular the unsteady flow dynamics, were found to be strongly dependent on these parameters. For certain conditions, strong vortex shedding near the reattachment region of the bubble was observed, which is a characterisc behavior of short bubbles. High-resolution spatio-temporal PIV measurements were made to analyze the formation and breakdown of these flow structures. The frequency of vortex shedding was determined from Fourier analysis of the time series of the velocity fluctuations. The non-dimensionalised frequencies were found to be nearly independent of the Reynolds number for the range of Reynolds numbers investigated here.