Effects of thermal conduction in sonoluminescence

Current experimental results indicate that sonoluminescence is largely thermal in origin, with the spectra showing a direct relation between luminous intensity and the temper ature s generated inside the collapsing cavitation bubbles. In the present paper the strong dependence of the luminous intensity on the nature of the gas dissolved in the liquid is explained in terms of thermal conduction. Provided the cavitation bubbles are sufficiently small, loss of heat from the bubble into the liquid can significantly reduce the temperatures attained during collapse, so that there is a consequent reduction in the luminous intensity. This process is demonstrated analytically by means of a numerical solution of the equations of motion of a gas inside a collapsing cavitation bubble. The agreement between the theory and the observed luminous intensities for different dissolved gases is good

The collapse of a spherical cavity in a compressible liquid

This paper presents numerical solutions for the flow in the vicinity of a collapsing spherical bubble in water. The bubble is assumed to contain a small amount of gas and the solutions are taken beyond the point where the bubble reaches its minimum radius up to the stage where a pressure wave forms and propagates outwards into the liquid. The motion up to the point where the minimum radius is attained, is found by solving the equations of motion both in the Lagrangian and in the characteristic forms. These are in good agreement with each other and also with the approximate theory of Gilmore which is demonstrated to be accurate over a wide range of Mach number. The liquid flow after the minimum radius has been attained is determined from a solution of the Lagrangian equations. It is shown that an acoustic approximation is quite valid for fairly high pressures and this fact is used to determine the peak intensity of the pressure wave at a distance from the center of collapse. It is estimated in the case of typical cavitation bubbles that such intensities are sufficient to cause cavitation damage

Bounds on the local energy density of holographic CFTs from bulk geometry

The stress tensor is a basic local operator in any field theory; in the context of AdS/CFT, it is the operator which is dual to the bulk geometry itself. Here we exploit this feature by using the bulk geometry to place constraints on the local energy density in static states of holographic $(2+1)$-dimensional CFTs living on a closed (but otherwise generally curved) spatial geometry. We allow for the presence of a marginal scalar deformation, dual to a massless scalar field in the bulk. For certain vacuum states in which the bulk geometry is well-behaved at zero temperature, we find that the bulk equations of motion imply that the local energy density integrated over specific boundary domains is negative. In the absence of scalar deformations, we use the inverse mean curvature flow to show that if the CFT spatial geometry has spherical topology but non-constant curvature, the local energy density must be positive somewhere. This result extends to other topologies, but only for certain types of vacuum; in particular, for a generic toroidal boundary, the vacuum's bulk dual must be the zero-temperature limit of a toroidal black hole.Comment: 14+2 pages, 2 figures. v2: fixed equations (51) and (52

An Analysis of Echoes from a Solid Elastic Sphere in Water

It is well-known in sonar work that the pulse form of a direct echo from a target bears little relation to the form of the original signal. This is true even for regularly shaped bodies, such as a sphere. In this paper, the case of a homogeneous elastic sphere in water is examined theoretically and it is shown in comparison with experimental results, that the observed effects originate from vibrations induced in the sphere by the incident sound. Calculated results are presented for a variety of solid materials and it seems that echo forms could possibly provide information about the size and constitution of a sonar target

Uniform Distributions of Sound Sources on the Surface of a Rigid Sphere and Some Applications

A study is made of certain elementary source distributions on the surface of a rigid sphere for a fairly extensive range of frequencies. The elementary systems considered consist of a point source, uniformly vibrating caps and rings, and plane line sources. Results are given for typical cases of the far zone sound fields and acoustic impedances of caps and rings, and also for the far zone sound field of a particular plane line source. Combinations of the elementary rings and caps are examined with a view to producing desired directional patterns and the results for a particular directional beam are presented. Taking a sphere as a model for the human head, the results for a point source are used to examine possible mechanisms for the binaural localization of sources of sound. These results indicate that, due to non-linear variation of phase with frequency, a pulsed sound should appear in a somewhat different form at each ear. It is suggested that localization is achieved by the brain in reconciling such pulse forms and that time and intensity differences are elements in a much more general process. Another possible application of the results involves the particular line source considered which could be taken to represent a human mouth

Tackling Slippage

This issue of Frontiers of CLTS explores current thinking and practice on the topic of tackling slippage of open defecation free (ODF) status. It looks at how slippage is defined and identified, and at different patterns of slippage that are seen after ODF is declared. Although a considerable amount has been written on how to establish strong Community-Led Total sanitation (CLTS) programmes that prevent slippage from happening, this issue looks at how to reverse slippage that has already taken place. Note however, that at a certain level, strategies used to reverse slippage and those used in advance to set a programme up for success to prevent slippage occurring overlap.From the literature, there is little documented evidence on how slippage can be reversed; evidence and guidance tend to focus on prevention. This review begins to address this gap. Implementers are encouraged to use the proposed patterns of slippage framework and slippage factors section to understand the type and extent of slippage experienced, then use the examples in the section on tackling slippage to identify potential slippage responses.In addition to a review of current literature, in depth interviews were carried out with key informants at global, regional and country level. Key informants were selected purposively to identify experiences and innovations in tackling slippage from across the sector.Issue 14, September 201

News items : (i) Book Awards won by Peter Mayo and Leona English (ii) Flagging 2014 special issue ‘Knowledge production… Reflective dialogue between activists and academics’ with guest editor Nisha Thapyial.

These news items describe: (1) a book award won by Peter Mayo and Leona English (2) the theme of the next issue of 'Postcolonial Directions in Education', and (3) two conference visits made by the author: 'Quality in the classroom', in Kathmandu, Nepal, 2013, and the AERA conference in San Francisco, 2013