Anatomy of subcritical submarine flows with a lutocline and an intermediate destruction layer

Jorge S Salinas,J Fedele,M Shringarpure,S Zuñiga,S Balachandar,D Hoyal,M I Cantero

doi:10.1038/s41467-021-21966-y

Jorge S Salinas, J Fedele + Show 5 more

Open Access

https://doi.org/10.1038/s41467-021-21966-y

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Abstract

Turbidity currents are sediment-laden flows that travel over a sloping bed under a stagnant ambient fluid, driven by the density difference between the current and the ambient. Turbidity currents transport large amounts of carbon, nutrients and fresh water through oceans and play an important role in global geochemical cycling and seafloor ecosystems. Supercritical currents are observed in steeper slopes. Subcritical currents are observed in milder slopes, where the near-bed and interface layers are prevented from interacting across the velocity maximum. Past works show the existence of such a barrier to vertical momentum transfer is essential for the body of the subcritical current to extend over hundreds of kilometers in length without much increase in height. Here we observe the body of subcritical currents to have a three layer structure, where the turbulent near-bed layer and the non-turbulent interface layer are separated by an intermediate layer of negative turbulence production. We explain the mechanism by which this layer prevents the near-bed turbulent structures from penetrating into the interface layer by transferring energy back from turbulence to the mean flow.

Highlights

Turbidity currents are sediment-laden flows that travel over a sloping bed under a stagnant ambient fluid, driven by the density difference between the current and the ambient
In the bed-normal direction, a turbidity current can be broadly characterized by a near-bed layer, where the velocity increases from the no-slip condition at the bed to a maximum at the top of the layer, and an interface layer, where the velocity decreases from the maximum value back to zero at the boundary between the current and the ambient fluid
Subcritical currents (Fr < 1) are observed in milder slopes[17] where the damping effect of stable density stratification in the interface layer dominates over shear production of turbulence

Summary

Results and discussion

Consider a dilute supercritical turbidity current flowing down a sloping bed of θ = 2.86∘, with height 17.3 m along the body, driven by sediment of size 11 μm, specific gravity 2.65 and at a volumetric concentration of 0.1% This results in a mean velocity of 1.17 m s−1 and bulk Reynolds number of 1.56 × 107. A substantial layer of fluid where total TKE production is negative clearly separates the near-bed turbulent region from the interface layer. To further explore how mixing is hindered at the interface layer we present in Fig. 4a blown up views of different quantities at three different bed-normal locations: below the velocity maximum (z = 1), in the region of negative total TKE production (z = 1.4) and in the interface layer (z = 2). Following the works of Parker et al and others[15,30,50], the dimensionless mean streamwise momentum in the statistically stationary state simplifies to

Reτ u3τ

Reτ u4τ ε þ

Methods

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