Lock-exchange release density currents over three-dimensional regular roughness elements

Abstract

A fundamental study has been conducted to understand the front characteristics and the mixing in the flow of density currents over rough surfaces. A large-eddy simulation (LES) has been performed for lock-exchange release density currents over rough walls to shed light on the unsteady mixing processes. A volume-penalization method, which is a special case of the immersed-boundary method, has been implemented to realize the bottom-mounted rough topology. In this study, the LES has been conducted in a channel with a lower wall covered with three-dimensional cube- and pyramid-shaped roughness elements, such that all cases have the same base area, but differences in the roughness solidity and volume fraction of roughness. Both cases of identical roughness elements and those with randomness in height have been considered. The maximum roughness height for all cases is kept at a constant fraction (10 %) of the total channel height. The study focuses on the instantaneous mixing processes in lock-exchange release currents over rough surfaces. An important contribution of the work is that qualitative and quantitative analysis has been conducted to demonstrate additional mixing mechanisms due to the presence of surface roughness that enhances dilution of the current. Enhanced mixing due to roughness is related to the strength of the shear layer resulting from the roughness, and hence depends on friction Reynolds number ($Re_{\unicode[STIX]{x1D70F}}$). The combined role of current characteristics and $Re_{\unicode[STIX]{x1D70F}}$ together dictate the mixing processes and extent of dilution in density currents over surface roughness.

Lock-release gravity currents propagating over roughness elements

“…The loss of definition of the frontal region is expected to be also dependent on the spatial arrangement of the bricks. Similar results on the 3D nature of the flow were found by [28] who, through 3D LES simulations, show how the current front loses its momentum locally crossing over and surrounding the bottom roughness and how this behavior exhibits strong coupling to the distribution, shape and height of roughness features.…”

“…When viscous effects overcome inertial effects, a third phase develops and the current front velocity decreases more rapidly following the theoretical power law t −4∕5 , with the front position advancing as t 1∕5 . Usually, to define the experimental front position of a 2D dense current, the spanwise-averaged density fields are used, adopting a density threshold value [5,[34][35][36] or identifying the inflection point along the front interface between the current and the ambient fluid through the evaluation of the maximum and the minimum density value [28]. In this work the front position, namely the foremost point of the current along the streamwise direction, x f , is defined as the point where the spatial derivative of the longitudinal velocity u in the streamwise direction u∕ x is maximum.…”

“…Gravity currents interacting with array of obstacles were investigated by [27] by LES. [28] studied gravity currents flowing over roughness elements by LES showing how bottom roughness significantly alters the front characteristics and the mixing within the current. Entrainment in rotating gravity currents flowing down a rough sloping bottom was analyzed by [29].…”

Numerical investigation of starting turbulent buoyant plumes released in neutral atmosphere

“…The radial expansion of the plume during the time interval is given as the change of the volume (change of the area times the distance travelled by the plume during this time), where the distance travelled by the plume is and the area of the initial release is . A similar approach of using the volume-based measure to quantify the entrainment has been conducted successfully by Bhaganagar & Pillalamarri (2017).…”

Gravity currents over fixed beds of monodisperse spheres

“…Nasr-Azadani & Meiburg (2014) employ direct numerical simulations to investigate the three-dimensional vortical flow structures generated when a turbidity current propagates over topographical features such as a Gaussian bump, along with their effects on mixing and entrainment. The recent LES simulations by Bhaganagar & Pillalamarri (2017) address lock-exchange currents over square and triangular roughness elements, with a special emphasis on the role of the friction Reynolds number. Ozan, Constantinescu & Hogg (2015) highlight the role of dilution in gravity currents propagating over arrays with horizontal axes.…”