Transition of a Separated Boundary Layer past a Semi-circular Leading-edge Aerofoil Model for Varying Angles of Attack: An Experimental Investigation
The features of a geometrically induced separated boundary layer has been documented for two angles of attack (α), 0° and 3° using a planar Particle Image Velocimetry (PIV) and Hotwire anemometry. Experiments are carried out on a thick flat plate with a semi-circular leading-edge aerofoil model for a Reynolds number (Re) of 25000, based on leading-edge diameter (D) and tunnel inlet velocity (U∞), where the inlet free-stream turbulence (fst) is 1.2%. The instantaneous field measurements from PIV reveal a significant unsteadiness in the shear layer that grows spatially, leading to breakdown and turbulent reattachment. The onset of separation and transition along with the reattachment locations identified with Hotwire data are complemented very well with PIV measurements. The separation bubble formed under α = 3° is 18.2% longer, compared to the bubble length at α = 0°. Further, the mathematical tool, S-transform is used to carry out the spectral analysis of streamwise fluctuating velocity component to study the growth of disturbances along the separated boundary layer. The distinct feature of this tool is that it captures the amplification of frequency even in a very small interval of time and hence, the structure of the incoming signal is revealed better compared to conventional tools.