Temperature evolution in the presence of anisotropic stresses

In a recent paper by Das et al. (Astrophys. Space Sci. 361:99, 2016) it was shown that the sign of the anisotropy parameter plays a pivotal role in determining the time of formation of the horizon of a collapsing radiating star. Spurred on by this observation we investigate the impact of the (an)isotropy featured in the (Das et al. in Astrophys. Space Sci. 361:99, 2016) collapsing model on the temperature profiles of the evolving system. We show that the temperature within the stellar interior is increased as the anisotropy in the pressure grows. Relaxational effects due to heat dissipation within the core further enhances the temperature at each interior point of the stellar distribution.     

Trophic functioning of the St. Lucia estuarine lake during a drought phase assessed using stable isotopes

The St. Lucia Estuary is Africa’s largest estuarine system and is currently experiencing the stress of prolonged freshwater deprivation, manifested by extremely low water levels and hypersalinity. These unprecedented conditions have raised questions regarding the trophic functioning of the ecosystem. Despite the substantial amount of research previously undertaken within this system, no studies of food web structure and function have yet been documented. This study therefore aimed to examine the food web structure of the St. Lucia estuary system through the use of carbon and nitrogen stable isotope analysis. Analysis of carbon isotope ratios indicates that benthic carbon sources are most utilised at sites with low water levels and generally higher salinity (Catalina Bay, Charter’s Creek). Conversely, the estuarine region of the mouth and Narrows, with its elevated water levels and lower salinity, still sustains a viable pelagic food web. Analysis of δ¹⁵N ratios indicates that the number of trophic transfers (food chain length) might be related to water levels. Overall, the study provides a greater understanding of the ecological processes of this complex estuarine lake, which may allow for future comparisons of trophic functioning under drought and normal/wet conditions to be made.     

Causal temperature profiles in horizon-free collapse

We investigate the causal temperature profiles in a recent model of a radiating star undergoing dissipative gravitational collapse without the formation of a horizon. It is shown that this simple exact model provides physically reasonable behaviour for the temperature profile within the framework of extended irreversible thermodynamics.     

Relaxational effects in radiating stellar collapse

Relaxational effects in stellar heat transport can in many cases be significant. Relativistic Fourier–Eckart theory is inherently quasi-stationary, and cannot incorporate these effects. The effects are naturally accounted for in causal relativistic thermodynamics, which provides an improved approximation to kinetic theory. Recent results, based on perturbations of a static star, show that relaxation effects can produce a significant increase in the central temperature and temperature gradient for a given luminosity. We use a simple stellar model that allows for non-perturbative deviations from staticity, and confirms qualitatively the predictions of the perturbative models.     

On radiating stellar collapse with shear

Relativistic models of radiating stellar collapse with shear require careful checking of all the equations and quantities involved. We illustrate this by showing that, in a recently proposed model, the metric ansatz is in fact too restrictive to allow any radial evolution to occur, and only tangential evolution is possible. Furthermore, the assumption of pressure isotropy is shown to reduce the model to the static case without radiation or shear.     

Study on the particle breakage of ballast based on a GPU accelerated discrete element method

Breakage of particles will have greatly influence on mechanical behavior of granular material(GM)under external loads,such as ballast,rockfill and sand.The discrete element method(DEM)is one of the most popular methods for simulating GM as each particle is represented on its own.To study breakage mechanism of particle breakage,a cohesive contact mode is developed based on the GPU accelerated DEM code-Blaze-DEM.A database of the 3D geometry model of rock blocks is established based on the 3D scanning method.And an agglomerate describing the rock block with a series of non-overlapping spherical particles is used to build the DEM numerical model of a railway ballast sample,which is used to the DEM oedometric test to study the particles’breakage characteristics of the sample under external load.Furthermore,to obtain the meso-mechanical parameters used in DEM,a black-analysis method is used based on the laboratory tests of the rock sample.Based on the DEM numerical tests,the particle breakage process and mechanisms of the railway ballast are studied.All results show that the developed code can better used for large scale simulation of the particle breakage analysis of granular material.     

Video DCIV measurements of mass and momentum fluxes and kinetic energies in laboratory waves breaking over a bar

The experimental investigation of water waves breaking over a barred beach in a glass walled flume is presented. The time series of the water levels along the flume were measured using wave gauges and video imaging of the water surface, providing estimates of the mean wave-heights and water levels at various positions along the flume. The instantaneous velocity fields were measured using digital correlation image velocimetry for positions beyond the initial breaking point. The mass (/velocity) and momentum fluxes, and kinetic energies were computed from the velocity flow fields by means of phase ensemble averaging and then time averaging of relevant quantities derived from the instantaneous velocity fields. These parameters reached a maximum value some distance shorewards of the break point on the bar. The relative density of the fluid in the crest of the wave, estimated using the forward and reverse velocity fluxes, was found to be in the range from 0.4 to 0.8. Preliminary analysis of the momentum fluxes and mean water levels indicates a decreasing dynamic pressure shorewards of a peak that occurs shortly after wave breaking.