We study entrainment in lock-release gravity currents using highly spatially resolved optical transmission experiments and quantitative analysis of the available potential energy of the flow. The principal results provide a resolution to the debate regarding the mechanism and degree of mixing in the head of a gravity current during the slumping phase. The nature of the complex internal mixing structure changes as a bore propagates from the tail to the head of the current during the slumping phase and overtakes its leading edge. We use quantitative methods to identify the connection between dynamics and entrainment and show that its manifestation as examined using different methodologies is the cause of previous contradictory experimental findings. Therefore, we conclude that the two main perspectives previously considered at odds are in accord.

We report theoretical and experimental investigations of the flow instability responsible for the mushy-layer mode of convection and the formation of chimneys, drainage channels devoid of solid, during steady-state solidification of aqueous ammonium chloride. Under certain growth conditions a state of steady mushy-layer growth with no flow is unstable to the onset of convection, resulting in the formation of chimneys. We present regime diagrams to quantify the state of the flow as a function of the initial liquid concentration, the porous-medium Rayleigh number, and the sample width. For a given liquid concentration, increasing both the porous-medium Rayleigh number and the sample width caused the system to change from a stable state of no flow to a different state with the formation of chimneys. Decreasing the concentration ratio destabilized the system and promoted the formation of chimneys. As the initial liquid concentration increased, onset of convection and formation of chimneys occurred at larger values of the porous-medium Rayleigh number, but the critical cell widths for chimney formation are far less sensitive to the liquid concentration. At the highest liquid concentration, the mushy-layer mode of convection did not occur in the experiment. The formation of multiple chimneys and the morphological transitions between these states are discussed. The experimental results are interpreted in terms of a previous theoretical analysis of finite amplitude convection with chimneys, with a single value of the mushy-layer permeability consistent with the liquid concentrations considered in this study.

A Bayesian approach is described for localizing sources and receivers in a randomly scattered sound field, for which the complex Wishart distribution represents the joint distribution of the signals at multiple, arbitrarily separated receivers. The signals are assumed to be narrowband and fully saturated by scattering from homogeneous turbulence (or similar scattering processes) as they propagate along line-of-sight paths to the receivers. Here, we consider a scenario in which there is an array of receivers at known locations, a source of opportunity with unknown amplitude at an unknown location, and a single receiver at an unknown location. The objective is to passively determine the unknown receiver location even though the source characteristics are not initially known. A slice-sampling strategy is found to be suitable for solving the problem. Simulations show that a small number of independent signal samples at the receivers initially enables the unknown receiver to be localized on a circle of constant amplitude around the source. With more samples, the correlation characteristics of the field enable full determination of the unknown receiver location.