Density gradient centrifugation of sperm is a common assisted-reproduction procedure in humans used to improve semen quality. The technique allows sperm separation based on their isopycnic points. Sperm with morphologic abnormalities are often more buoyant, leading to their retention above centrifuged density gradients, with structurally normal sperm passing through the gradient. Three experiments were conducted to evaluate the effects of tube size, sperm number following centrifugation, and density gradient volume (height) on stallion sperm quality and recovery rate in sperm pellets following centrifugation. In all three experiments, equine semen was initially centrifuged to increase sperm concentration. In Experiment 1, one-mL aliquots were layered over EquiPure? Bottom Layer (1-Layer) or over-tiered EquiPure? Top and Bottom Layers (2-Layer). For Experiment 2, one-mL aliquots were layered over three different heights of EquiPure? Bottom Layer in 15-mL or 50-mL conical-bottom tubes. For Experiment 3, four different aliquots containing a sperm load of 1-4x were layered over a constant volume of EquiPure? Bottom Layer in 15-mL or 50-mL conical bottom tubes. The tubes were then centrifuged. Resulting sperm pellets were evaluated for morphologic quality, DNA integrity, motility and recovery rate. Sperm-EquiPure? centrifugation yielded improvements in motility, morphology and DNA integrity parameters (P<0.05), as compared to controls. The 1-Layer method resulted in a higher recovery rate than the 2-Layer method (P<0.05). Sperm processed in the 15-mL tubes yielded higher velocity and higher recovery rates than sperm processed in the 50-mL tubes (P<0.05). Within tube type, gradient volume did not impact parameters of semen quality or recovery rate. An increase in sperm number for density gradient centrifugation resulted in a decreased recovery rate (P<0.05) when 15-mL tubes were used.

High-speed high Reynolds number boundary layer flows with mechanical non-equilibrium effects have numerous practical applications; examples include access-to-space ascent, re-entry and descent, and military hypersonic systems. However, many of the basic turbulent flow processes in this regime are poorly understood and are beyond the realm of modern direct numerical simulations Previous studies have shown that curvature driven pressure gradients significantly alter the state of the turbulence in high-speed boundary layers; the turbulence levels have been shown to decrease by large amounts (up to 100 percent) and the Reynolds shear stress has been shown to change sign. However, most of our understanding is based on point measurement techniques such as hot-wire and Laser Doppler anemometry acquired at low to moderate supersonic Mach numbers (i.e., M = 2-3). After reviewing the available literature, the following scientific questions remain unanswered pertaining to the effect of favorable pressure gradients: (1) How is state of the mean flow and turbulence statistics altered? (2) How is the structure of wall turbulence; break-up, stretch or a combination? (3) How are the Reynolds stress component production mechanisms altered? (4) What is the effect of Mach number on the above processes? To answer these questions and to enhance the current database, an experimental analysis was performed to provide high fidelity documentation of the mean and turbulent flow properties using two-dimensional particle image velocimetry (PIV) along with flow visualizations of a high speed (M4.88=), high Reynolds number (Re36,000θ≈) supersonic turbulent boundary layer with curvature-driven favorable pressure gradients (a nominally zero, a weak, and a strong favorable pressure gradient). From these data, detailed turbulence analyses were performed including calculating classical mean flow and turbulence statistics, examining turbulent stress production, and performing quadrant decomposition of the Reynolds stress for each pressure gradient case. It was shown that the effect of curvature-driven favorable pressure gradients on the turbulent structure of a supersonic boundary layer was significant. For the strong pressure gradient model, the turbulent shear stress changed sign throughout the entire boundary layer; a phenomena was not observed to this magnitude in previous studies. Additionally, significant changes were seen in the turbulent structure of the boundary layer. It is believed that hairpin vortices organized within the boundary layer are stretched and then broken up over the favorable pressure gradient. Energy from these hairpin structures is transferred to smaller turbulent eddies as well as back into the mean flow creating a fuller mean velocity profile. It was determined that the effects of favorable pressure gradients on the basic structure of a turbulent Mach 5.0 boundary layer were significant, therefore increasing the complexity of computational modeling.

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We consider the constrained convex minimization problem min x2C f(x) we will present gradient projection method which generates a sequence fxkg according to the formula xk+1 = PC(xk

Reproduction de : Thèse de doctorat : Informatique et mathématiques appliquées : Toulouse, INPT : 2005. / Titre provenant de l'écran-titre. Bibliogr. 91 réf.

Thèse de doctorat : PHYSIQUE/SCIENCES APPLIQUEES : EP : 1998. / 1998EPXX0046. 63 REF.

No description available.

Global warming will affect every level of biological organization from the metabolism of individuals to the functioning of ecosystems. I explored the effects of warming on three rock-pool meioinvertebrate communities along a latitudinal gradient (temperate, tropical, and sub-Arctic regions) to determine effects on community and food-web structure and functioning. Warming affected regions differently, having a positive effect on sub-Arctic communities, a negative effect on temperate communities, and intermediate responses in tropical communities in terms of abundance, stability and extinction frequency. Differences in structural properties of the food webs supported the insurance hypothesis: that greater redundancy in webs results in greater stability, and helped to explain why the tropical community was more stable than the temperate community in warmed treatments. My study highlights the importance of considering differential response of species and communities from different latitudes and the importance of food web structure in predicting species response to global climate change.

No description available.

Human activities have dramatically altered the functioning of ecosystems through the ages. Urbanization illustrates the effect of anthropogenic activity by the transformation of natural areas to ecologically disturbed regions (development of towns, cities and settlements). The growing need for urban employment in South Africa has led to an increase in the number of informal settlements on the periphery of urban areas. These settlements result in fragmentation and sprawling of cities, which intensifies strain on the natural environment. Fragmentation in urban regions then leads to the formation of 'patches' of land which exhibit different disturbance levels and are generally typified as either urban, suburban or rural areas. These land use types may be ecologically studied along an urbanization gradient, with the intention of obtaining meaningful comparisons. An urbanization gradient contains an urban landscape which consists of a densely built and developed core surrounded by an area of decreasing development and increasing 'naturalness'. The use of urbanization gradients has been proven world-wide as a useful tool for the study of changes in ecological patterns and processes across landscapes. This approach has been used to examine many different impacts of urbanization, namely on invertebrate communities, bird community composition and plant community composition. Using biological indicators to determine the degree of anthropogenic impact on the environment has proven effective in past studies. These indicators can be used to monitor ecological change following habitat disturbance, identify changing trends over time, provide early warning systems of degradation and diagnose the cause of existing problems. Several authors have supported the use of arthropods as suitable indicators of disturbance. The aim of this study was to determine what impact disturbance, due to urbanization, may have had on the diversity and abundances of epigeal (surface roaming) arthropods (focussing on ants, beetles and spiders) following an urbanization gradient approach. In addition, plant and soil data were combined with the arthropod analysis for each site studied, in order to obtain a better picture of how arthropod community composition would change in relation to these factors. The ant group were the numerically dominant group of the arthropods studied, although the beetles did have the highest number of species captured. Spiders were caught in low abundances, but were also represented by a high number of species. Dramatic decreasing trends were observed with respect to the ant abundances and diversity from rural to the more urbanized sites. Quite the opposite, seemed to occur with the beetles and spiders, who were dominant in species and numbers in the urbanized areas. This trend may be explained on account of the occurrence of generalists and opportunistic beetle and spider species, which seem to thrive in these heterogenous urban habitats. When considering environmental components, percentage bare-ground and sand concentration seemed to be the determining factors in the rural sites, around which the ant group aggregated. Sandy habitats with patches of bare-ground provide more favourable micro-habitats for the ant species to roam and scavenge in, and are advantageous for nest building. Clay concentration and abundance of fruit seemed to assist in providing favourable habitats for the opportunistic and generalist beetle species, in the urbanized areas. High clay concentrations in the urban areas provided ideal conditions for abundant organic covering which would favour saprophagous (feed on decaying organic matter) beetle species and support diverse prey for the predatory beetle and spider species to feed on. Abundance of fruit may have attracted numerous herbivorous beetles (frugivorous beetles). Urbanization seemed to have a more pronounced effect on ant diversity and abundances in comparison to the beetles and spiders, and therefore recommended for future utilization as a suitable "Bio-indicator" group for further local disturbance studies. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.