Effects of offset jet width on periodic flow in a dual jet

Abstract

The primary aim of this work is to investigate the impact of the offset jet width on the unsteady flow characteristics of a turbulent dual jet, which consists of a wall jet and an offset jet. A computational fluid dynamics code is developed to solve the unsteady Reynolds-averaged Navier–Stokes (URANS) equations. The width of the offset jet is varied while keeping the width of the wall jet constant at the separation distance between the two jets. When the ratio of the offset jet width ($w$) to the separation distance ($d$) is $w/d=0.5$, the flow field exhibits a periodic vortex shedding phenomenon. Conversely, when $w/d=0.4$, the flow field remains steady. The shedding phenomenon is discernible even when $w/d=2$. The instantaneous velocity components display sinusoidal oscillations at $0.5\le w/d\le 2$. Applying the fast Fourier transform to these sinusoidal signals yields a distinct frequency peak at the vortex shedding frequency. Within the range of $0.5\le w/d\le 2$, the shedding frequency decreases as the width of the offset jet increases. This trend continues until it reaches a constant value at $w/d=1.4$. This indicates that the width of the offset jet has a notable influence on the shedding phenomenon within the range of $0.5\le w/d\le 1.4$. For $1.4<w/d\le 2$, the shedding frequency remains unaffected by the offset jet width variation. Depending on the value of $w/d$, the shedding phenomenon is characterized by three flow regimes: a steady flow regime (for$w/d\le 0.4$), an outer share layer-influenced shedding regime (for $w/d=0.5-1.4$), and an outer shear layer-free shedding regime (for $w/d>1.4$).