Talia Tokyay

Rigorously derived shallow water equations (SWEs) are applied to results of large eddy simulation (LES) of a continuously fed gravity current in order to assess (1) sensitivity of current depth results to its definition; (2) coefficients in depth-averaged continuity and momentum equation due to the nonuniformity of density and velocity profiles; and (3) sensitivity of entrainment coefficient to definition of current depth. It is shown that using different definitions of the current depth may produce significantly different numerical results. The coefficients due to nonuniformity in the continuity equation are very close to unity, whereas the coefficients in the momentum flux and the pressure term in the momentum equation are different from unity by a margin that is very sensitive to the definition of current depth. The entrainment coefficient is more sensitive to the selected parameterization than to the definition of the current depth.

A three-dimensional numerical model of ANSYS, Fluent (2011) was employed for studying mid to high discharge supercritical two-phase flow over a single slope spillway with a single step for aeration of the flow. In this study 18 simulations were conducted using the Volume of Fluid (VOF) method for air-water interface tracking and simple k-ε model for turbulence closure. Submerged circular shaped pipes located at the bottom of the step were utilized as aerators. Analyses concentrate on the air-entrainment phenomenon and jet-length of the flow from the step to the re-attachment point. The variables considered in the study are discharge, aerator size, different aerator arrangements, Froude number of the flow, presence of a ramp before the step and its angle. Observed jet-length values in this study were compared with two sets of empirical formulae from literature for code validation. Cross-sectional average of air concentration due to bottom aeration was determined in the streamwise direction downstream of the re-attachment of the jet. The air concentration is observed to follow a logarithmic decay in the flow direction within the de-aeration zone. Keywords: jet-length, aerator, VOF, air-entrainment, ramp, computational fluid dynamic

High-resolution three-dimensional large-eddy simulations are used to investigate the effects of internal solitary waves (ISWs) breaking over a sloping boundary. The lock release method is applied in a two-layer stratified fluid system to generate three different breaking mechanisms (i.e., plunging, collapsing, and surging breakers). The different breaking types are investigated in terms of their effects on the dynamics of the ISW and the interaction of the ISW with the sloping boundary. During each breaking event, the pycnocline region entrains fresher water from the upper layer and saltier water from the lower one. The associated increase of the intermediate density layer also induces changes of the pycnocline water density. This process occurs with a velocity that can be evaluated using the bulk entrainment parameter. We show how the intermediate layer features depend on the ISW shoaling and breaking dynamics and we discuss entrainment in breaking ISWs. The instabilities induced by boundary layer separation allow entrainment of saltier water, while the run-up of the gravity current causes the decrease of the intermediate layer mean density. Simultaneously, the entrained water mixes into the pycnocline region. For all cases, the temporal evolution of the instantaneous mixing efficiency is discussed. The plunging breaker case shows the largest amount of mixing, which is mostly induced by rear-edge overturning in the onshore direction. The largest entrainment is observed in the surging breaker case in response to the large gravity current flowing upslope. The paper discusses how the different turbulent instabilities induced by the ISWs breaking affect the time delay between the times when entrainment of patches of salty and fresh water from the neighboring layers occurs and the time the density of the intermediate layers becomes fairly uniform via mixing. We finally point out that the entrainment parameter and the mixing efficiency describe two different effects of the turbulent instabilities occurring in a stratified fluid in terms of changes of the bulk density profile.

Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming‐induced variations in velocity, temperature, and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in situ measurements, laboratory experiments, and numerical simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3 to 1.2 m thick water layers at around 12 m water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large‐scale phenomena like algal blooms.

Planar, Boussinesq, compositional gravity currents formed by the release of a fixed volume of heavier fluid from a closed lock and advancing on an inclined no-slip bottom surface in a reservoir with a horizontal free surface are investigated based on 3-D large eddy simulation. The initial region containing the lock fluid has a rectangular shape. Simulations are conducted for bottom slope angles, θ, between 0° and 60°. The running length of the inclined bottom was sufficiently long to allow a detailed study of the evolution, front dynamics and structure of the current during the latter stages of the deceleration phase (front velocity reduces with time). Results show that currents advancing over inclined surfaces with θ > 10° are characterized by the formation of an intensified mixed vortex (IMV) at the back of the head. The IMV forms faster and its coherence, circulation, and size increase monotonically with increasing bottom slope angle. The paper discusses how the buoyancy in the head varies with varying bottom slope angle and with time. In particular, for θ ≥ 30°, the current reaches a regime where the total buoyancy of the head and IMV is close to a constant and the value of this constant increases with increasing θ. During this regime, the head mainly loses buoyancy to the IMV. For θ ≥ 40°, a close to linear decay of the head buoyancy with time is observed during the later stages of this regime. Simulation results show that, while for relatively small bottom slope angles most of the sediment is entrained beneath the head, for θ > 20° the IMV has a much larger capacity to entrain the sediment compared to the head region past the initial stages of the propagation of the current. This means that sediment entrainment patterns of currents propagating over highly inclined surfaces are qualitatively very different from the widely studied case of currents propagating over horizontal surfaces. The paper also discusses the different regimes observed in the temporal evolution of the front velocity and the applicability of theoretical models derived based on the data obtained for relatively small bottom slope angles and a relatively short evolution of the current to describe the evolution of currents propagating over large bottom slope angles and/or at large times after the start of the deceleration phase. While it is found that mixing increases monotonically with increasing θ, the largest total kinetic energy for a given front position is observed for θ = 30°-40°. Results also show that the largest magnitude of the bed friction velocity is induced for θ = 30°-40°, which means that the currents with the largest capacity to entrain sediment are those with the largest rate of increase of total kinetic energy with the propagation distance.