Search results for "gradient"
showing 10 items of 725 documents
Global properties of generalized Ornstein–Uhlenbeck operators on Lp(RN,RN) with more than linearly growing coefficients
2009
AbstractWe show that the realization Ap of the elliptic operator Au=div(Q∇u)+F⋅∇u+Vu in Lp(RN,RN), p∈[1,+∞[, generates a strongly continuous semigroup, and we determine its domain D(Ap)={u∈W2,p(RN,RN):F⋅∇u+Vu∈Lp(RN,RN)} if 1<p<+∞. The diffusion coefficients Q=(qij) are uniformly elliptic and bounded together with their first-order derivatives, the drift coefficients F can grow as |x|log|x|, and V can grow logarithmically. Our approach relies on the Monniaux–Prüss theorem on the sum of noncommuting operators. We also prove Lp–Lq estimates and, under somewhat stronger assumptions, we establish pointwise gradient estimates and smoothing of the semigroup in the spaces Wα,p(RN,RN), α∈[0,1], wher…
Soil Bacterial Community Along an Altitudinal Gradient in the Sumaco, a Stratovolcano in the Amazon Region
2022
Our study is a pioneering exploration of the microbiome in the soil of the Sumaco stratovolcano and an assessment of the effects of an elevational gradient and related physicochemical soil parameters on richness and community structure. The Sumaco, as an isolated Amazonian stratovolcano, may be among one of the least studied ecosystems in Ecuador and perhaps the Amazon region. Universal patterns remain unresolved or available information inconclusive to establish a supported consensus on general governing processes by which elevation and its associated environmental gradients may determine the microbial richness and community structure. We tested a recent proposal on how microbial diversity…
An Innovative Method to Produce Scaffolds with a Pore Size Gradient for Tissue Engineering Applications
2014
Thermally Induced Phase Separation (TIPS) is a technique for the production of porous scaffold for Tissue Engineering applications. A wide range of microporous morphologies, in terms of pore size and distribution, can be obtained by tuning TIPS processing parameters, especially thermal history. The production of scaffolds for bone tissue regeneration is a challenging target: as a matter of fact, scaffolds must mimic the bone morphology, thus requiring a gradient of pore dimension and morphology along one dimension. To attain this goal, an experimental apparatus capable to impose different thermal histories on the two sides of a sample was designed, set up and tested. The sample (35x35 mm su…
Phase separation in thin films: Effect of temperature gradients
2013
We study the phase-separation kinetics of a binary (AB) mixture confined in a thin film of thickness D with a temperature gradient. Starting from a Kawasaki-exchange kinetic Ising model, we use a master-equation approach to systematically derive an extension of the Cahn-Hilliard model for this system. We study the effect of temperature gradients perpendicular to the film with "neutral" (no preference for either A or B) surfaces. We highlight the rich phenomenology and pattern dynamics which arises from the interplay of phase separation and the temperature gradient.
A model of oil burnout from glass fabric
1997
A mathematical model is proposed for the process of the removal (by burning) of oil contained in a glass fibre insulation fabric manufactured in Latvia. The small aspect ratio of the fabric allows simplifications to the modelling which reduce the problem to a single nonlinear ordinary differential equation. When the effects of reflected radiation are also included, the differential equation is supplemented by two integral equations. Predictions of the position of the ‘burning zone’ accord well with observations made at the factory. The effect of the inclusion of extra heating chambers is also examined, and it is found that the temperature gradient in the fabric may be greatly decreased in t…
CFD simulation of a membrane distillation module channel
2009
The interest towards the use of membrane distillation (MD) processes for seawater desalination has been rising recently due to the ease of coupling MD with waste and/or solar thermal energy. Notwithstanding the flexibility of the process and its potential for further developments in membrane performances, one of the main drawbacks is the thermal efficiency reduction caused by temperature polarization. Because of such phenomenon, only a small amount of the driving force potentially available for the separation process, i.e. the temperature difference between evaporating and condensing fluids, is actually used for the separation. In order to reduce temperature polarization a study on the effe…
A Unified Perspective on the Dynamics of Axisymmetric Hurricanes and Monsoons
2006
Abstract This paper provides a unified perspective on the dynamics of hurricane- and monsoonlike vortices by identifying them as specific limiting cases of a more general flow system. This more general system is defined as stationary axisymmetric balanced flow of a stably stratified non-Boussinesq atmosphere on the f plane. The model is based on the primitive equations assuming gradient wind balance in the radial momentum equation. The flow is forced by heating in the vortex center, which is implemented as relaxation toward a specified equilibrium temperature Te. The flow is dissipated through surface friction, and it is assumed to be almost inviscid in the interior. The heating is assumed …
Abdominal and Thoracic Magnetic Resonance Angiography: Challenges, Technical Approaches, and First Clinical Applications
1993
The main goal of magnetic resonance angiography (MRA) is to enrich the huge potential of magnetic resonance imaging (MRI) by additional morphological and functional information about the vasculature. In spin echo sequences fast flowing blood leads to signal extinction, providing high contrast between tissue and vascular structures without the need for contrast media. However, various artifacts are frequently observed which alter the assessment of the vasculature. They are due to alterations of flow (slow velocity or turbulences) or induced by the acquisition technique (Mills et al. 1983; Bradley and Waluch 1985). By using gradient echo sequences, high-quality angiograms can be obtained from…
Global retention models and their application to the prediction of chromatographic fingerprints
2020
Abstract The resolution of samples containing unknown compounds of different nature, or without standards available, as is the case of chromatographic fingerprints, is still a challenge. Possibly, the most problematic aspect that prevents systematic method development is finding models that describe without bias the retention behaviour of the compounds in the samples. In this work, the use of global models (able to describe the whole sample) is proposed as an alternative to the use of individual models for each solute. Global models contain parameters that are specific for each solute, while other parameters ‒related to the column and solvent‒ are common for all solutes. A special regressio…
Estimation of annual energy output of a wind turbine using wind speed probability distribution
2016
In this work, annual energy output of a variable speed wind turbine is analyzed using annual Weibull wind speed probability distribution function. The power coefficient variation with tip speed ratio in torque control region as well as pitch angle variation for maximum power output from wind turbine are analyzed for identifying control system parameters. The wind turbine power output and variation of power coefficient with tip speed ratio as well as pitch angle are examined / reported using annual Weibull distribution function. Finally the variation of the estimated annual energy output of the given wind turbine with the mean wind speed is presented.