Computational Fluid Dynamics for Modeling Gravity Currents in the Presence of Oscillatory Ambient Flow

Rosaria Musumeci,Enrico Foti,Laura Stancanelli

doi:10.3390/w10050635

Abstract

Gravity currents generated by lock release are studied in the case of initially quiescent ambient fluid and oscillating ambient fluid (regular surface waves). In particular, the dynamics of the density currents are investigated by means of CFD numerical simulations. The aim is to evaluate the influence of the ambient fluid velocity field on the observed mixing and turbulent processes. Results of two different turbulence closure models, namely the standard k − ε turbulence model and the LES model, are analyzed. Model predictions are validated through comparison with laboratory measurements. Results show that the k − ε model is able to catch the main current propagation parameters (e.g., front velocity at the different phases of the evolution of the current, gravity current depth, etc.), but that a LES model provides more realistic insights into the turbulent processes (e.g., formation of interfacial Kelvin–Helmholtz billows, vortex stretching and eventual break up into 3D turbulence). The ambient fluid velocity field strongly influences the dynamics of the gravity currents. In particular, the presence of an oscillatory motion induces a relative increase of mixing at the front (up to 25%) in proximity of the bottom layer, and further upstream, an increase of the mixing process (up to 60%) is observed due to the mass transport generated by waves. The observed mixing phenomena observed are also affected by the ratio between the gravity current velocity v f and the horizontal orbital velocity induced by waves u w , which has a stronger impact in the wave dominated regime ( v f / u w < 1).

Highlights

Gravity currents are mainly horizontal flows moving under the influence of gravity and generated by buoyancy differences
The propagation of gravity currents under oscillatory wave regimes is quite relevant in coastal regions, especially to understand the processes acting during the continuous natural or artificial discharges of fluids having a different density than the ambient fluid, e.g., river plumes, desalination plant, industrial discharges, etc. [3,7]
The results showed that irreversible mixing is detected during the entire development of the flow, during self-similar phases, and during the slumping phase

Summary

Introduction

Gravity currents are mainly horizontal flows moving under the influence of gravity and generated by buoyancy differences. Gravity currents are phenomena of great interest in the field of engineering and geophysics with numerous important environmental and industrial applications [1,2]. These include: the outflow of brackish waters [3] referred to as viscous gravity currents, pyroclastic flows [4] referred to as particle-laden gravity currents and mud and debris flows [5,6] referred to as concentrated flows. The propagation of gravity currents under oscillatory wave regimes is quite relevant in coastal regions, especially to understand the processes acting during the continuous natural or artificial discharges of fluids having a different density than the ambient fluid, e.g., river plumes, desalination plant, industrial discharges, etc. Several geometries of the flow domain have been investigated, for example: Water 2018, 10, 635; doi:10.3390/w10050635 www.mdpi.com/journal/water

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