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

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.

Numerical investigation of unsteady effects in oscillatory sheet flows

“…Turbulence modulation and formation of a concentration plateau show similarities with the two-layer structure separated by a lutocline well known for fine-sediment transport (Ozdemir, Hsu & Balachandar 2010; Salinas et al. 2021). However, the mechanism at play is fundamentally different.…”

“…For configuration F512, the development of the fluid Reynolds stresses is inhibited by the formation of the plateau around flow reversal (150 • , 0 • and 30 • ) suggesting a strong modulation of turbulence by the particles. Turbulence modulation and formation of a concentration plateau show similarities with the two-layer structure separated by a lutocline well known for fine-sediment transport (Ozdemir, Hsu & Balachandar 2010;Salinas et al 2021). However, the mechanism at play is fundamentally different.…”

Slope dependence of self-similar structure and entrainment in gravity currents

“…(ii)In panels (d, f,h) (subcritical current), two horizontal surfaces of zero turbulent production marked as z| P=0 are shown. The layer of fluid between these planes is termed 'destruction layer', since turbulence production is negative, and is the result of the interaction between near-wall hairpins ('HP' in zoom-in view I) and counter-rotating vortices ('CV') (see Salinas et al 2021b). (iii)Comparing the different terms of the momentum equation, it can be seen that in the interface layer (above the maximum of velocity), the balance is mainly between ∂(ũ w)/∂ z (zoom-in view IV) and ∂( ũ w )/∂ z (zoom-in view III) in the supercritical current, while it is between ∂(ũ w)/∂ z (zoom-in view VI) and ∂ 2 ũ/∂ z2 /Re in (zoom-in view V) in the subcritical current.…”

“…Depending on the slope of the bed, three different flow regimes have been identified (Sequeiros 2012;Salinas et al 2020Salinas et al , 2021bSalinas, Balachandar & Cantero 2021a). At steeper slopes of S 0.05, the current evolves to a near-self-similar supercritical state, which is characterized by a turbulent near-wall layer close to the bottom boundary and a turbulent interface layer where the current vigorously mixes with the ambient (Salinas et al 2021a).…”

“…At shallow slopes of S 0.01, the current evolves to a near-self-similar subcritical state, which is also characterized by a turbulent near-wall layer but the interface layer remains turbulent-free without active mixing with the ambient. In fact, the near-wall and interface layers are separated by an intermediate destruction layer of negative turbulence production, which acts as a lid and prevents near-wall turbulence from penetrating into the upper layer (Salinas et al 2021b). Of particular importance is the appearance of a lutocline, or a very sharp density gradient, at the top of the subcritical current.…”