Study of the laminar-turbulent transition of a wake behind an airfoil by using a towing wind tunnel

This paper describes an experimental investigation of the natural transition mechanism of a wake generated behind a thin airfoil 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 of the airfoil trailing-edge. When the airfoil is towed in the tunnel, starting vortices are observed first in the wake. The experimental result shows that the feature of startig vortex sustains in the natural transition process due to a self sustained resonance in the absolute 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 quasisteady state sustaining in the saturation state grow gradually during the vortex street formation, which lead to the vortex street deformation downstream.