Analysis of intermittent trailing edge vortex shedding using recurrence plots

Abstract

Time resolved particle image velocimetry is conducted in the wake of a trailing edge separated flow past a NACA4415 airfoil at a fixed angle of attack beyond the static stall angle to analyse the formation and temporal evolution of the wake coherent structures. Under the presented experimental circumstances, intermittent trailing edge vortex shedding occurs. A flow diagnostic analysis combining the proper orthogonal decomposition method with a recurrence plot and quantification analysis, reveals that the occurrence and duration of vortex shedding interruptions are random and that the duration of the interruptions are shorter that the intervals of regular shedding. Furthermore, a recurrence plot based conditional averaging strategy is used allowing for the spatiotemporal evolution of the wake centreline to be visualised and for the wake convection speed to be calculated. The latter was found to be distributed normally around 0.66 of the free stream velocity. Classic detection and tracking of the trailing edge vortices revealed a formation length of � 20% of the airfoil chord length downstream of the trailing edge before alternately shedding can occur through mutual interactions of the upper and lower shear layer.