Observations of large-scale coherent structures in gravity currents: implications for flow dynamics

C R Marshall,J Peakall,R M Dorrell,S M Tobias,G M Keevil

doi:10.1007/s00348-021-03217-4

Abstract

Density driven flows, also known as gravity currents, comprise a head, body, and tail. Yet whilst the body typically forms the largest part of such flows, its structure remains poorly understood. In this work, experimental data gathered using particle image velocimetry enables the instantaneous, whole-field dynamics of constant-influx solute-based gravity currents to be resolved. While averaged turbulent kinetic energy profiles are comparable to previous work, the instantaneous data sets reveal significant temporal variation, with velocity measurements indicating large-scale wave-like motions within the body. Spectral analysis and dynamic mode decomposition, of streamwise and vertical velocity, are used to identify the frequencies and structures of the dominant motions within the flow. By considering an idealised theoretical density profile, it is suggested that these structures may be internal gravity waves that form a critical layer within the flow located at the height of the maximum internal velocity. Irreversible internal wave breaking that has been postulated to occur at this critical layer suggests formation of internal eddy transport barriers, demonstrating that new dynamic models of turbulent mixing in gravity currents are needed.Graphic abstract

Highlights

Gravity currents, known as density currents, are a common class of geophysical flow that occur in many natural and man-made environments (Ungarish 2009; Simpson 1997)
It has been shown that for some of these waves the wave phase speed is approximately equal to the mean flow speed, indicating a potential critical layer within the gravity current body
The presence of internal waves in the gravity current body has been postulated by Dorrell et al (2019), who suggest that the gravity current body has a structure similar to that of a zonal jet (Fig. 1b) (Dritschel and Scott 2011; Rossby and Zhang 2001; Maxworthy 1984; Bower and Hogg 1996)

Summary

Introduction

Known as density currents, are a common class of geophysical flow that occur in many natural and man-made environments (Ungarish 2009; Simpson 1997) They are of particular relevance to the study of atmospheres and oceans, with examples including thunderstorm outflows, and sediment transport in lakes and oceans (Parsons and Garcıa 1998; Britter and Linden 1980; Simpson 1997; Bonnecaze et al 1993; Talling 2014).

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