Influence of the Reynolds number on the instant flow evolution of a turbulent rectangular free jet of air. Average Particle Image Velocimetry (PIV) visualizations and measurements

Abstract

The paper is aimed at investigating the influence of the Reynolds number on the instant flow evolution of a rectangular free jet of air in the range of Reynolds numbers from Re = 35,300 to Re = 2,200, where the Reynolds number, Re, is defined according to the hydraulic diameter, D, of a rectangular slot of height H, equal to about D = 2H. The Particle Image Velocimetry (PIV) technique allows obtaining the instant PIV visualizations on the central symmetry section of the rectangular jet. The visual inspection of the instant frames with one and two vortices, except for Re = 35,300 where only one vortex images are detected, shows that after the jet exit is present the Flow with Constant Instant Height, with a length LCIH which increases with the decrease of the Reynolds number, from a ratio LCIH/H equal to LCIH/H = 0.9 at Re = 35,300 to LCIH/H = 4.0 at Re = 2,200. The instant PIV measurements, carried out at several distances from the jet exit, show that the variations of the ratio U=U0 of the centerline instant velocity, U, to the exit average velocity, U0, remain below ±4% for a length LCIV, defining the Flow with Constant Instant Velocity on the centerline. The ratio LCIV/H increases from LCIV/H = 1.1 at Re = 35,300 to LCIV/H = 4.1 at Re = 2,200 and is quite similar to LCIH/H. The instant PIV measurements of the centerline turbulence intensity, Tu, show that its variations remain below ±4% for a length LCIT, defining the Flow with Constant Instant Turbulence on the centerline. The ratio LCIT/H is equal to LCIV/H. The instant PIV velocity profiles in the transverse direction, are practically unchanged for a length LUVP which defines the flow with Unchanged Velocity Profile. The four lengths LUVP = LCIH = LCIV = LCIT are considered equal, defining the Flow with Negligible Disturbances, with length LND, or first type of flow, L1, which is in agreement with the average length of the flow with Constant Height, LCH, in the turbulent and laminar regime, and with the average length of the Undisturbed flow, LU, in the turbulent regime. The instant PIV frames show the presence, after the flow with negligible disturbances, of the Flow with Small Variations of the Height, where the jet height varies slightly without forming the vortex. The length of the flow with Small Variation of the Height is LSVH, and the first vortex appears after the total length (LND + LSVH). The variations of the instant PIV measurements of the centerline turbulence intensity, Tu, increase after the length LCIT, but remaining below ±5–9% for a length LSVT, which defines the Flow with Small Variation of the Turbulence. The total length (LCIT + LSVT) is comparable to (LCIH + LSVH) in the turbulent and laminar regimes, and can be assumed equal to the sum of the lengths of the flows with negligible and small disturbances, LND + LSD = L2, which is in agreement with the average length of the undisturbed flow, LU, in the laminar regime. The instant PIV frames show the flow with Coherent Vortices after L2, with a length LCV, followed by the vortices breakdown. The instant PIV frames allow visualizing the end of the flow with Coherent Vortices, defining the total length of the flows with negligible and small disturbances, and coherent vortices, as LND + LSD + LCV = L3, which is in good agreement with the average length of the potential core, LP