Flow evolution of a turbulent submerged two-dimensional rectangular free jet of air. Average Particle Image Velocimetry (PIV) visualizations and measurements

Abstract

The paper presents average flow visualizations and measurements, obtained with the Particle Image Velocimetry (PIV) technique, of a submerged rectangular free jet of air in the range of Reynolds numbers from Re=35,300 to Re=2200, where the Reynolds number is defined according to the hydraulic diameter of a rectangular slot of height H. According to the literature, just after the exit of the jet there is a zone of flow, called zone of flow establishment, containing the region of mixing fluid, at the border with the stagnant fluid, and the potential core, where velocity on the centerline maintains a value almost equal to the exit one. After this zone is present the zone of established flow or fully developed region. The goal of the paper is to show, with average PIV visualizations and measurements, that, before the zone of flow establishment is present a region of flow, never mentioned by the literature and called undisturbed region of flow, with a length, LU, which decreases with the increase of the Reynolds number. The main characteristics of the undisturbed region is the fact that the velocity profile maintains almost equal to the exit one, and can also be identified by a constant height of the average PIV visualizations, with length, LCH, or by a constant turbulence on the centerline, with length LCT. The average PIV velocity and turbulence measurements are compared to those performed with the Hot Film Anemometry (HFA) technique. The average PIV visualizations show that the region of constant height has a length LCH which increases from LCH=H at Re=35,300 to LCH=4–5H at Re=2200. The PIV measurements on the centerline of the jet show that turbulence remains constant at the level of the exit for a length, LCT, which increases from LCT=H at Re=35,300 to LCT=4–5H at Re=2200. The PIV measurements show that velocity remains constant at the exit level for a length, LU, which increases from LU=H at Re=35,300 to LU=6H at Re=2200 and is called undisturbed region of flow. In turbulent flow the length LU is almost equal to the lengths of the regions of constant height, LCH, and constant turbulence, LCT. In laminar flow, Re=2200, the length of the undisturbed region of flow, LU, is greater than the lengths of the regions of constant height and turbulence, LCT=LCH=4–5H. The average PIV and HFA velocity measurements confirm that the length of potential core, LP, increases from LP=4–5H at Re=35,300 to LP=7–8H at Re=2200, and are compared to the previous experimental and theoretical results of the literature in the zone of mixing fluid and in the fully developed region with a good agreement.