Entrainment in a Dense Current Flowing Down a Rough Sloping Bottom in a Rotating Fluid

Abstract

Dense oceanic overflows descend over the rough topography of the continental slope entraining and mixing with surrounding waters. The associated dilution dictates the fate of these currents and thus is of fundamental importance to the formation of deep water masses. The entrainment in a dense current flowing down a sloping bottom in a rotating homogeneous fluid is investigated using laboratory experiments, focusing on the influence of the bottom roughness on the flow dynamics. The roughness is idealized by an array of vertical rigid cylinders and both their spacing and height are varied as well as the inclination of the sloping bottom. The presence of the roughness is generally observed to decelerate the dense current, with a consequent reduction of the Froude number, when compared to the smooth bottom configuration. However, the dilution of the dense current due to mixing with the ambient fluid is enhanced by the roughness elements, especially for low Froude numbers. When the entrainment due to shear instability at the interface between the dense current and the ambient fluid is low, the additional turbulence and mixing arising at the bottom of the dense current due to the roughness elements strongly affects the dilution of the current. Finally, a strong dependence of the entrainment parameter on the Reynolds number is observed.

Statistical characterisation of turbulence for an unsteady gravity current

“…Shear not only triggers the growth of three-dimensional turbulence at high Reynolds numbers, but also excites Kelvin–Helmholtz instabilities in the presence of flow stratification. The amplification of the Kelvin–Helmholtz instabilities leads to the roll up of the current's interface into two-dimensional Kelvin–Helmholtz billows which subsequently entrain ambient fluid into the current and drives the growth of an interfacial mixing layer (Britter 1974; Hacker, Linden & Dalziel 1996; Hallworth et al 1996; Härtel et al 2000; Ottolenghi et al 2016 a , b ; Ottolenghi, Cenedese & Adduce 2017 b ).…”

Impact of ambient stable stratification on gravity currents propagating over a submerged canopy

“…However, to the best of our knowledge, all existing studies in the literature have only focused on the current-canopy interactions in a homogeneous ambient (e.g. Zhang & Nepf 2011;Ozan, Constantinescu & Nepf 2016;Ottolenghi, Cenedese & Adduce 2017;Cenedese et al 2018). Therefore, a clear understanding of the impact of ambient stable stratification on such flows is lacking.…”

Axisymmetric three-dimensional gravity currents generated by lock exchange

“…Hallworth et al 1996). Recent experiments described by Cuthbertson et al (2012Cuthbertson et al ( , 2014 and Ottolenghi, Cenedese & Adduce (2017b) on continuous-inflow gravity currents in rotating tanks illustrate the complexity of the necessary experimental arrangement even in the case of planar flows. Direct numerical simulations (DNS) have been used to shed light on the dynamics and the instabilities of axisymmetric constant-volume gravity currents, as in Cantero et al (2007).…”