Study of Transition Mechanism in a Wake Behind an Airfoil with a Small Angle of Attack by Using a Towing Wind Tunnel

Abstract

This paper describes an experimental investigation of the transition mechanism of a wake generated behind a thin airfoil with a small angle of attack in a towing wind tunnel. A linear stability analysis shows that the wake is characterized by a change in the instability from locally absolute to locally convective at a downstream location (30 mm) of the airfoil trailing-edge. When the airfoil is towed in the tunnel, boundary layers develop on the upper/lower airfoil surfaces with different thicknesses. Since the asymmetric wake is generated, starting vortices of a single row are observed first in the wake, which is different from the wake Karman vortex street. The experimental result shows that time-harmonic fluctuations of the starting vortex sustain in the natural transition process due to a self sustained resonance in the absolutely unstable region behind the trailing edge. The wake profile in the saturation steady state yields the vortex street structure, where the fluctuation frequency is defined as the fundamental unstable mode by the final saturation steady state. The growth of the fundamental unstable mode in the convectively unstable region suppresses the high frequency fluctuations associated with the staring vortex generation. On the other hand, low-frequency fluctuations in the quasi-steady state sustaining in the saturation state grow gradually during the vortex street formation, which lead to the vortex street deformation downstream.