Abstract: Multi-layered submarine gravity currents, characterized by inner and outer shear layers, are of vital importance for global geochemical cycling and the safety of marine infrastructures. One of their remarkable features is that they can run for hundreds of miles with little dilution. The ultralong runout is attributed to the suppression of turbulent entrainment of ambient fluid, achieved at sufficiently large Richardson numbers, but the underlying physics remains enigmatic.  Is the suppression caused by the ‘switching off’ of turbulence in the outer layer, as stated in the Nature paper [1] or by the reduction of turbulence intensity while still maintaining turbulence, as argued by van Reeuwijk et al. [2]? This enigma poses great challenges for modeling mixing and entrainment in submarine gravity currents, especially considering their multi-layered structure.

In this talk, we will give new insights into the suppression of entrainment and shear layer dynamics by conducting direct numerical simulations (DNS) of inclined gravity currents on multiple slope angles spanning a wide range of Richardson numbers. Based on the governing physics, we will derive a model to predict entrainment rate (E) and multiple mixing measures as functions of the bulk Richardson number (Ri) for both the inner and the outer layers. To achieve this, we will parameterize the budget of turbulent kinetic energy in both layers and connect the budget with Ri and E. The model exhibits good agreement with the DNS data across a wide range of Ri, contributing to a better understanding of fundamental dynamics in gravity currents.

References:

[1] Salinas, J.S., Balachandar, S., Shringarpure, M. Et al. Anatomy of subcritical submarine flows with a lutocline and an intermediate destruction layer. Nature Communications 12, 1649 (2021).

[2] Van Reeuwijk M, Holzner M, Caulfield CP. Mixing and entrainment are suppressed in inclined gravity currents, Journal of Fluid Mechanics, 873, 786-815(2019)