Experimental investigation of flow development and coherent structures in normal and oblique impinging slot jets

Abstract

The development of normal and oblique impinging slot jets is investigated experimentally using planar particle image velocimetry. The study is performed for two jet Reynolds numbers, $${Re} = 3000$$ and 6000, and four jet orientation angles relative to the wall ( $$\theta = 90^\circ , 60^\circ , 45^\circ , 30^\circ$$ ), with the nozzle-to-plate spacing fixed at four slot widths. Within the range of impingement angles considered, the flow is characterized by a stagnation region, followed by a region of flow reorientation into a wall jet. The development of the wall jet downstream of the impingement region is shown to be closely related to the evolution of coherent structures forming due to the Kelvin–Helmholtz instability. For normal jet impingement at $${Re} = 3000$$ , these shear layer rollers remain coherent past the reorientation region and induce the formation and shedding of wall-bounded vorticity; the shed vorticity pairs with the primary shear layer vortices and ejects from the wall, resulting in deflection of the mean wall jet from the surface. Both the primary and the induced structures break down farther downstream, marking final stages of transition to turbulence. For $${Re} = 6000$$ , breakdown of the shear layer vortices occurs in the reorientation region, leading to earlier transition into a turbulent wall jet. Consequently, wall-bounded vorticity roll-up and ejection are less significant, and the deflection of the wall jet away from the surface is reduced. The analysis presented quantitatively relates the development of coherent structures to salient changes in the time-averaged flow statistics.