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

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

Inadequacy of fluvial energetics for describing gravity current autosuspension

“…The meso-scale flow structures are able to generate gravitational potential directly, by stirring the flow 45 , and through wave breaking which also generates TKE. The turbulence is, in turn, able to ‘ring’ density interfaces generating internal waves 39 , 40 , 46 , or uplift particles through the diffusive effect of vortices. The TKE and gravitational potential are slowly lost to heat though viscous effects.…”

Unconfined turbidity current interactions with oblique slopes: deflection, reflection and combined-flow behaviours

“…The three-dimensional nature of the present experiments and flow density values that are orders of magnitude lower than some previous experiments and more commensurate with those of natural flows, likely account for the absence of these solitons and internal waves, as discussed previously. This mechanism for velocity pulsing on slopes, might potentially be combined with velocity pulsing mechanisms intrinsic to flows such as Kelvin-Helmholtz or Holmboe waves (Kostaschuk et al, 2018), or internal waves (Marshall et al, 2021(Marshall et al, , 2023. Such pulsing mechanisms are likely at a higher frequency (Kostaschuk et al, 2018), and thus subsidiary to the slope induced pulsing.…”

Unconfined gravity current interactions with orthogonal topography: Implications for combined-flow processes and the depositional record.

“…Internal wave formation in ponded suspensions is hypothesised to exploit This is non-peer reviewed preprint submitted to EarthArxiv 47 the contrast between the velocity, and the concentration and grainsize layers (e.g., Patacci et al, 2015). From experimental modelling of 2D gravity currents, internal wave formation has also been observed to occur at a critical layer within the body of gravity currents, at the height of the maximum internal velocity, thus suggesting the 'steady' body of gravity currents has inherent instabilities in the form of internal waves and may not be as steady as first assumed (e.g., Marshall et al, 2021Marshall et al, , 2023. Whether the same mechanism for internal wave generation (e.g., Patacci et al, 2015;Marshall et al, 2021Marshall et al, , 2023 is applicable in 3D, unconfined settings is yet to be explored.…”