686 research outputs found
On imploding cylindrical and spherical shock waves in a perfect gas
The problem of a cylindrically or spherically imploding and reflecting shock wave in a flow initially at rest is studied without the use of the strong-shock approximation. Dimensional arguments are first used to show that this flow admits a general solution where an infinitesimally weak shock from infinity strengthens as it converges towards the origin. For a perfect-gas equation of state, this solution depends only on the dimensionality of the flow and on the ratio of specific heats. The Guderley power-law result can then be interpreted as the leading-order, strong-shock approximation, valid near the origin at the implosion centre. We improve the Guderley solution by adding two further terms in the series expansion solution for both the incoming and the reflected shock waves. A series expansion, valid where the shock is still weak and very far from the origin, is also constructed. With an appropriate change of variables and using the exact shock-jump conditions, a numerical, characteristics-based solution is obtained describing the general shock motion from almost infinity to very close to the reflection point. Comparisons are made between the series expansions, the characteristics solution, and the results obtained using an Euler solver. These show that the addition of two terms to the Guderley solution significantly extends the range of validity of the strong-shock series expansion
Foraging strategies of coexisting lacertid lizards in the arid Tankwa Karoo Basin of South Africa
The original publication is available at http://africanzoology.journals.ac.za/pubFour lacertid lizards, Pedioplanis laticeps, P. lineo-ocellata, Meroles knoxii and Nucras tessellata,
occur sympatrically on the arid plains of the Tankwa Karoo Basin in South Africa. The aim of
this study was to evaluate the significance of foraging strategy in resource partitioning among
the four species, allowing them to co-occur in a structurally simple system with a limited
number of potential niches. Previous workers already identified P. lineo-ocellata and M. knoxii
as sit-and-wait foragers and N. tessellata as an active forager. We recorded data on three
foraging variables: movements per minute, proportion of time spent moving, and proportion
of attacks on prey whilst moving, for juveniles and adults of P. laticeps. By comparing
the foraging data obtained for P. laticeps to those for other lacertid species, we were able to
demonstrate that adult P. laticeps are ambush foragers.We also noted a significant ontogenetic
shift in foraging behaviour in P. laticeps, and, due to a significantly higher frequency of short
brief movements, we classified juveniles as mixed foragers. The sharing of an ambush foraging
strategy by at least three of the four lacertid species co-occurring on the Tankwa plains,
suggests considerable overlap along the trophic dimension of ecological space. This overlap
presumably promotes occupation of separate microhabitats by the three ambush foragers in
the Tankwa Karoo Basin.Publisher's versio
Community structure and intertidal zonation of the macrobenthos on a macrotidal, ultra-dissipative sandy beach: summer-winter comparison
Community structure and intertidal zonation of the macrobenthos on a macrotidal, ultra-dissipative beach were studied. On the beach of De Panne, Belgium, six transects perpendicular to the waterline (each with five stations) were sampled in September 1995 (summer) and March 1996 (winter). The 30 stations were distributed across the continuum from mean high water spring to mean low water spring in order to sample the macrobenthos at different levels of elevation. The 39 species found had total densities up to 5,500 ind m-2 in summer and 1,400 ind m-2 in winter. The highest densities were found in the spionid polychaetes Scolelepis squamata and Spio filicornis, the nephtyid polychaete Nephtys cirrosa, the cirolanid isopod Eurydice pulchra, and the haustorid amphipods Bathyporeia spp. Based on species composition, specific densities, and biomass, two species associations were defined: a relatively species-poor, high intertidal species association, dominated by S. squamata and with an average density of 1,413 ind m-2 and biomass of 808 mg AFDW m-2 (summer); and a relatively species-rich, low intertidal species association, dominated by N. cirrosa, and with an average density of 104 ind m-2 and biomass of 162 mg AFDW m-2 in summer. For both seasons, the high intertidal species association was restricted in its intertidal distribution between the mean tidal and the mean high-water spring level, whereas the low intertidal species association was found from the mean tidal level to the subtidal. The latter showed good affinities with the subtidal N. cirrosa species association, occurring just offshore of De Panne beach, confirming the existence of a relationship between the low intertidal and subtidal macrobenthic species associations. Summer-winter comparison revealed a strong decrease in densities and biomass in the high intertidal zone during winter. Habitat continuity of the low intertidal zone with the subtidal allows subtidal organisms to repopulate the low intertidal zone
Effect of Sn on generalized stacking fault energy surfaces in zirconium and its hydrides
Hydrogen embrittlement in Zr alloy fuel cladding is a primary safety concern
for water based nuclear reactors. Here we investigated the stabilisation of
planar defects within the forming hydrides by Sn, the primary alloying element
of Zircaloy-4 used in the cladding. In order to explain formation of hydrides
and planar defects observed in our experiments, we performed atomic-scale ab
initio calculations focusing on the solute interactions with generalized
stacking faults in hcp -Zr and fcc zirconium hydrides. Our calculations
showed that an increase in Sn concentration leads to a stabilisation of
stacking faults in both -Zr and hydride phases. However, the solution
enthalpy of Sn is lower in the -Zr as compared to the other hydride
phases indicative of two competing processes of Sn depletion/enrichment at the
Zr hydride/matrix interface. This is corroborated by experimental findings,
where Sn is repelled by hydrides and is mostly found trapped at interfaces and
planar defects indicative of stacking faults inside the hydride phases. Our
systematic investigation enables us to understand the presence and distribution
of solutes in the hydride phases, which provides a deeper insight into the
microstructural evolution of such alloy's properties during its service
lifetime.Comment: 17 pages, 8 figure
Atomic-scale investigation of hydrogen distribution in a Ti–Mo alloy
Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully β and α + β Ti–Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully β sample but segregates at some α/β phase boundaries with a composition exceeding 20 at.% in the α + β sample. No stable hydrides were observed in either sample. The hydrogen concentration in β phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition
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