647 research outputs found
Coexistence and relative abundance in annual plant assemblages: The roles of competition and colonization
Although an interspecific trade-off between competitive and colonizing ability can permit multispecies coexistence, whether this mechanism controls the structure of natural systems remains unresolved. We used models to evaluate the hypothesized importance of this trade-off for explaining coexistence and relative abundance patterns in annual plant assemblages. In a nonspatial model, empirically derived competition-colonization trade-offs related to seed mass were insufficient to generate coexistence. This was unchanged by spatial structure or interspecific variation in the fraction of seeds dispersing globally. These results differ from those of the more generalized competition-colonization models because the latter assume completely asymmetric competition, an assumption that appears unrealistic considering existing data for annual systems. When, for heuristic purposes, completely asymmetric competition was incorporated into our models, unlimited coexistence was possible. However, in the resulting abundance patterns, the best competitors/poorest colonizers were the most abundant, the opposite of that observed in natural systems. By contrast, these natural patterns were produced by competition-colonization models where environmental heterogeneity permitted species coexistence. Thus, despite the failure of the simple competition-colonization trade-off to explain coexistence in annual plant systems, this trade-off may be essential to explaining relative abundance patterns when other processes permit coexistence
Effects of temporal variability on rare plant persistence in annual
Traditional conservation biology regards environmental fluctuations as detrimental to persistence, reducing long-term average growth rates and increasing the probability of extinction. By contrast, coexistence models from community ecology suggest that for species with dormancy, environmental fluctuations may be essential for persistence in competitive communities. We used models based on California grasslands to examine the influence of interannual fluctuations in the environment on the persistence of rare forbs competing with exotic grasses. Despite grasses and forbs independently possessing high fecundity in the same types of years, interspecific differences in germination biology and dormancy caused the rare forb to benefit from variation in the environment. Owing to the buildup of grass competitors, consecutive favorable years proved highly detrimental to forb persistence. Consequently, negative temporal autocorrelation, a low probability of a favorable year, and high variation in year quality all benefited the forb. In addition, the litter produced by grasses in a previously favorable year benefited forb persistence by inhibiting its germination into highly competitive grass environments. We conclude that contrary to conventional predictions of conservation and population biology, yearly fluctuations in climate may be essential for the persistence of rare species in invaded habitats
Viscoelastic Hele-Shaw flow in a cross-slot geometry
In this paper a cross-slot geometry for which the height of the channel is small compared to the
other channel dimensions is considered. The normal components of the viscoelastic stresses are found
analytically for a second order fluid up to numerical inversion. The validity of the theoretical analysis was
corroborated by comparison with numerical simulations based on a stabilized Galerkin least squares finite
element method using an Oldroyd B fluid. Close agreement was found between numerical predictions
and analytical results for Weissenberg numbers up to 0.2. An explicit expression is formulated for
viscoelastic parameters in terms of the variation and strength of the first normal stress difference around
the stagnation point. The analysis is generalized for the case where the inlet channel width is different
from the outlet channel width. For such configurations it was found that uniformity of the elongation rate
was reduced
Dual-plane PIV investigation of acoustically excited jets in a swirl nozzle
Abstract. A novel dual-plane dye laser particle image velocimetry
(PIV) technique used to analyze helicity and energy
dissipation in an unexcited turbulent swirling jet of pressurized
cold air has established that regions within the flow
field of the jet exhibiting high helicity are correlated regions
of high turbulent kinetic energy dissipation. This PIV con-
figuration provides estimates of all components of the velocity
gradient tensor, facilitating calculation of the helicity
from the vorticity components. Application of this novel
dual-plane PIV technique is extended in this study to investigate
helical structures in a turbulent swirling jet where the
underlying shear flow is subjected to external acoustic sinusoidal
forcing in a plane perpendicular to the central axis of
the jet. It was found that acoustic excitation had a significant
effect on the mean velocity profile parallel to the direction of
the jet. The horizontal forcing resulted in the generation of
vorticity that was skewed with a pitch that favored a distribution
of angles around 90â—¦ with respect to the velocity vector.
The distribution of the time-averaged helicity angle indicated
organized helical activity, but such activity is not dominated
by large-scale coherent structures of maximal helicit
Purification of Bioethanol Using Microbubbles Generated by Fluidic Oscillation: A Dynamical Evaporation Model
(Graph Presented) A computational model of a single gas microbubble immersed in a liquid of ethanol-water mixture is developed and solved numerically. This complements earlier binary distillation experiments in which the ethanol-water mixture is stripped by hot air microbubbles achieving around 98% vol. ethanol from the azeotropic mixture. The proposed model has been developed using Galerkin finite element methods to predict the temperature and vapor content of the gas microbubble as a function of its residence time in the liquid phase. This model incorporates a novel rate law that evolves on a time scale related to the internal mixing of microbubbles of 10-3s. The model predictions of a single bubble were shown to be in very good agreement with the existing experimental data, demonstrating that the ratio of ethanol to water in the microbubble regime are higher than the expected ratios that would be consistent with equilibrium theory for all initial bubble temperatures and all liquid ethanol mole fractions considered and within the very short contact times appropriate for thin liquid layers. Our previous experiments showed a decrease in the liquid temperature with decreasing liquid depth in the bubble tank, an increase in the outlet gas temperature with decreasing liquid depth, and an improvement in the stripping efficiency of ethanol upon decreasing the depth of the liquid mixture and increasing the temperature of the air microbubbles, all of which are consistent with the predictions of the computational model
Real-time gauge/gravity duality: Prescription, Renormalization and Examples
We present a comprehensive analysis of the prescription we recently put
forward for the computation of real-time correlation functions using
gauge/gravity duality. The prescription is valid for any holographic
supergravity background and it naturally maps initial and final data in the
bulk to initial and final states or density matrices in the field theory. We
show in detail how the technique of holographic renormalization can be applied
in this setting and we provide numerous illustrative examples, including the
computation of time-ordered, Wightman and retarded 2-point functions in
Poincare and global coordinates, thermal correlators and higher-point
functions.Comment: 85 pages, 13 figures; v2: added comments and reference
Optimal modelling and experimentation for the improved sustainability of microfluidic chemical technology design
Optimization of the dynamics and control of chemical processes holds the promise of improved sustainability for chemical technology by minimizing resource wastage. Anecdotally, chemical plant may be substantially over designed, say by 35-50%, due to designers taking account of uncertainties by providing greater flexibility. Once the plant is commissioned, techniques of nonlinear dynamics analysis can be used by process systems engineers to recoup some of this overdesign by optimization of the plant operation through tighter control. At the design stage, coupling the experimentation with data assimilation into the model, whilst using the partially informed, semi-empirical model to predict from parametric sensitivity studies which experiments to run should optimally improve the model. This approach has been demonstrated for optimal experimentation, but limited to a differential algebraic model of the process. Typically, such models for online monitoring have been limited to low dimensions.
Recently it has been demonstrated that inverse methods such as data assimilation can be applied to PDE systems with algebraic constraints, a substantially more complicated parameter estimation using finite element multiphysics modelling. Parametric sensitivity can be used from such semi-empirical models to predict the optimum placement of sensors to be used to collect data that optimally informs the model for a microfluidic sensor system. This coupled optimum modelling and experiment procedure is ambitious in the scale of the modelling problem, as well as in the scale of the application - a microfluidic device. In general, microfluidic devices are sufficiently easy to fabricate, control, and monitor that they form an ideal platform for developing high dimensional spatio-temporal models for simultaneously coupling with experimentation.
As chemical microreactors already promise low raw materials wastage through tight control of reagent contacting, improved design techniques should be able to augment optimal control systems to achieve very low resource wastage. In this paper, we discuss how the paradigm for optimal modelling and experimentation should be developed and foreshadow the exploitation of this methodology for the development of chemical microreactors and microfluidic sensors for online monitoring of chemical processes. Improvement in both of these areas bodes to improve the sustainability of chemical processes through innovative technology. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved
Modelling spatial and inter-annual variations of nitrous oxide emissions from UK cropland and grasslands using DailyDayCent
This work contributes to the Defra funded projects AC0116: ‘Improving the nitrous oxide inventory’, and AC0114: ‘Data Synthesis, Management and Modelling’. Funding for this work was provided by the UK Department for Environment, Food and Rural Affairs (Defra) AC0116 and AC0114, the Department of Agriculture, Environment and Rural Affairs for Northern Ireland, the Scottish Government and the Welsh Government. Rothamsted Research receives strategic funding from the Biotechnology and Biological Sciences Research Council. This study also contributes to the projects: N-Circle (BB/N013484/1), U-GRASS (NE/M016900/1) and GREENHOUSE (NE/K002589/1).Peer reviewedPublisher PD
Management of incomplete abortions at South African public hospitals
Objective. The objective of this report was to review and describe the management of incomplete abortion by public sector hospitals.Design. A descriptive study in which data were collected prospectively from routine hospital records on all women admitted with incomplete abortion to a stratified random sample of hospitals between 14 and 28 September 1994.Setting. Public sector hospitals in South Africa.Patients. Women with incomplete abortions.Main outcome measures. Length of hospital stay, details of medical management, details of surgical management, determinants of the above.Main results. Data were collected on 803 patients from the 56 participating hospitals. Of these, 767 (95.9%) were in hospital for 1 day or more, and 753 (95.3%) women underwent evacuation of the uterus. Sharp curettage wasthe method employed in 726 (96.9%) and general anaesthesia was used for 601 (88%) of the women requiring uterine evacuation. Antibiotics were prescribed for 396 (49.5%) and blood transfusions were administered to 125 (17%) women. Statistical analysis showed length of stay to be longer in small hospitals (under 500 beds) and when the medical condition was more severe. Antibiotic  usage and blood transfusion were more common with increasing severity and a low haemoglobin level on admission. However, some inappropriate management was identified with regard to both.Main conclusions. It is suggested that uncomplicated incomplete abortion can be more effectively and safely managed using the manual vacuum aspiration technique with sedation/analgesia as an outpatient procedure. Attention should be directed at the introduction of this management routine at all types of hospital and to the ensuring of appropriate management of women with complicated abortion
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