Evolution of Coherent Structures in a Two-Dimensional Impinging Jet

This paper reports the results of an experimental investigation of a normally impinging slot jet created by a nozzle of high aspect ratio. Time resolved planar particle image velocimetry is used to characterize the jet development at Reynolds numbers of 3000 and 6000, with a nozzle-to-plate distance of 4 nozzle widths. Primary vortices form due to the Kelvin-Helmholtz instability in the jet shear layer prior to flow impingement. Frequency and spatial energy distributions of the coherent structures are analyzed. The primary vortices are shed at a Strouhal number of 0.5, and are convected downstream at a velocity of 50% and 62% of the jet centerline velocity for Reynolds numbers of 3000 and 6000, respectively. Vortex merging occurs in the vicinity of the impingement zone. Secondary vortices form at a quarter of the primary vortex shedding frequency, originating from the interaction between the merged primary vortices and the wall.