Propagation of perturbations and meandering in a free surface shallow water jet
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RESEARCH ARTICLE
Propagation of perturbations and meandering in a free surface shallow water jet Massimo Miozzi1 · Giovanni Paolo Romano2 Received: 30 March 2020 / Revised: 24 July 2020 / Accepted: 26 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract A shallow water free-surface turbulent jet is investigated at Froude numbers variable between about 0.15 and 0.6 and a Reynolds number equal to 5000, with a jet height-to-width aspect ratio variable from 0.5 to 1.1. In these experimental conditions, the flow is unstable and develops local transverse instabilities, which are amplified to a global meandering motion, forcing the jet to oscillate orthogonally to its axis. Instantaneous and averaged velocity fields are obtained by means of high-density, correlation-based time-resolved Particle Tracking Velocimetry. From the average point of view, the present jet configuration resembles a confined jet condition similar to that of wall jets, but with additional relevant three-dimensional effects, retaining self-similar properties, with a net co-flow and some ambient fluid entrainment. For increasing Froude numbers, such configuration is moved downstream and the jet spreading is delayed. The high resolution in space and time of present measurements allows to locally detect and following perturbed patterns and to derive amplitude and frequency of oscillation of the global meandering motion, the former increasing with axial distance and decreasing with Froude number, the latter doing just the opposite. The velocity of propagation of perturbed patterns is also investigated in comparison to the jet mean velocity, the local celerity of propagation of fluctuating velocity fields being derived. Results indicate that axial fluctuations propagate downstream similarly to transverse fluctuations along the orthogonal direction, thus showing a close coupling among the two motions and the onset of a self-sustained mechanism driven by the mean flow, responsible for the generation of the macroscopic meandering motion. This effect is in agreement with numerical predictions and is emphasised as the Froude number increases. Graphic abstract
* Massimo Miozzi [email protected] Extended author information available on the last page of the article
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1 Introduction Jet flow instabilities have been considered from a long time, due to the intrinsic interest for the onset of unsteady behaviours in a relatively simple geometrical configuration and to the consequent possibilities related to the development and control of engineering devices (Cohen and Wygnansky 1987; Lin 2003). In this framework, the present work focuses on the behaviour of a turbulent shallow water jet with free-surface effects. This scenario is of particular interest in understanding the flow configurations close to river mouths, as it can deeply influence the coastal dynamics of currents and sediments in marine environment and the interactions with marine streams and tidal motions (Watts 1983; Bower an
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