807 research outputs found
Transport of Brownian particles confined to a weakly corrugated channel
We investigate the average velocity of Brownian particles driven by a
constant external force when constrained to move in two-dimensional,
weakly-corrugated channels. We consider both the geometric confinement of the
particles between solid walls as well as the soft confinement induced by a
periodic potential. Using perturbation methods we show that the leading order
correction to the marginal probability distribution of particles in the case of
soft confinement is equal to that obtained in the case of geometric
confinement, provided that the (configuration) integral over the cross-section
of the confining potential is equal to the width of the solid channel. We then
calculate the probability distribution and average velocity in the case of a
sinusoidal variation in the width of the channels. The reduction on the average
velocity is larger in the case of soft channels at small P\'eclet numbers and
for relatively narrow channels and the opposite is true at large P\'eclet
numbers and for wide channels. In the limit of large P\'eclet numbers the
convergence to bulk velocity is faster in the case of soft channels. The
leading order correction to the average velocity and marginal probability
distribution agree well with Brownian Dynamics simulations for the two types of
confinement and over a wide range of P\'eclet numbers
On the rolling motion of viscous fluid on a rigid surface
This thesis considers two closely related problems. First, the influence of insoluble surfactant at a moving contact line is considered. This work is mostly motivated by the air entrainment during the coating process where there is a three-phase contact point (e.g., air, liquid and solid). For moving contact line problems, when the fluid is assumed to be an incompressible Newtonian fluid and a no-slip boundary condition is enforced at the solid boundary, the non-integrable stress singularity arises at the contact line, which is physically unrealistic. The contact angle of 180° is considered as a special case in which the singularity is absent. The previous work showed that there exists a non-singular local solution in the vicinity of the contact line for any capillary number. A simplified asymptotic model is used here to find a global solution with a 180° contact angle. Also the effects of insoluble surfactant are checked and numerical results show there exists a critical capillary number above which there is no steady state solution.
The second problem is viscous droplets rolling down a non-wettable inclined plane. The recent experiments show that the contact angle is very large (close to 180°) when a droplet rolls on a super-hydrophobic surface. The biharmonic boundary element method (BBEM) is used to implement numerical simulations of rolling motion. The numerical results agree well with the experimental results and theoretical prediction. Numerical evidence is also found that the stress singularity at the contact line is alleviated with a 180° contact angle. For droplets with insoluble surfactant on the surface, the finite volume method is used to track the evolution of surfactant. It shows that the rolling motion is retarded because of Marangoni force due to nonuniform concentration distribution of surfactant
Transport properties of Brownian particles confined to a narrow channel by a periodic potential
We investigate the transport of Brownian particles in a two-dimensional
potential under the action of a uniform external force. The potential is
periodic in one direction and confines the particle to a narrow channel of
varying cross-section in the other direction. We apply the standard long-wave
asymptotic analysis in the narrow dimension and show that the leading order
term is equivalent to that obtained previously from a direct extension of the
Fick-Jacobs approximation. We also show that the confining potential has
similar effects on the transport of Brownian particles to those induced by a
solid channel. Finally, we compare the analytical results with Brownian
dynamics simulations in the case of a sinusoidal variation of the width of the
parabolic potential in the cross-section. We obtain excellent agreement for the
marginal probability distribution, the average velocity of the Brownian
particles and the asymptotic dispersion coefficient, over a wide range of
Peclet numbers
语言分类系统、真值间隔和不可通约性 — 对库恩关于不可通约性的分类学解释之重建
摘要:通常所接受对库恩有关不可通约性的分类学解释的理解建立在不可译性概念之上,因而是相当不完善的。为了弥补此不足,本文在分类之逻辑语义理论 (a logical-semantic theory of taxonomy)、真值语义理论 (a semantic theory of truth-value),以及跨语言交流的真值条件理论 (a truth-value conditional theory of cross-language communication) 之基础上,对库恩有关不可通约性的分类学解释进行了重建。根据这种重建,如果两个科学语言具有不相匹配的分类结构,则当一科学语言的核心语句在其所属语境中具有真值、而在另一科学语言之语境中考虑缺乏真值时,这两种科学语言乃不可通约。通过这种重建,库恩对于不可通约性的成熟解释并非基于保真 (truth-preserving) 可译性或不可译性的概念,而是基于保留真值状态 (truth-value-preserving) 的跨语言交流概念之上。这种重建使得库恩有关不可通约性的解释更具坚实的理论基础、也在概念上更加完整。
Active snort rules and the needs for computing resources: Computing resources needed to activate different numbers of snort rules
This project was designed to discover the relationship between the number of enabled rules maintained by Snort and the amount of computing resources necessary to operate this intrusion detection system (IDS) as a sensor. A physical environment was set up to loosely simulate a network and an IDS sensor monitoring it.
The experiment was conducted in five trials. A different number of Snort rules was enabled in each trial and the corresponding utilization of computing resources was measured. Remarkable variation and a clear trend of CPU usage were observed in the experiment
Heegaard Floer correction terms and Dedekind-Rademacher sums
We derive a closed formula for the Heegaard Floer correction terms of lens
spaces in terms of the classical Dedekind sum and its generalization, the
Dedekind-Rademacher sum. Our proof relies on a reciprocity formula for the
correction terms established by Ozsvath and Szabo. A consequence of our result
is that the Casson-Walker invariant of a lens space equals the average of its
Heegaard-Floer correction terms. Additionally, we find an obstruction for the
equality and equality with opposite sign, of two correction terms of the same
lens space. Using this obstruction we are able to derive an optimal upper bound
on the number of vanishing correction terms of lens spaces with square order
second cohomology.Comment: 11 pages, new reference adde
Bis(2,2′-bipyridyl)bromidocopper(II) bromide bromoacetic acid hemihydrate
In the title compound, [CuBr(C10H8N2)2]Br·BrCH2COOH·0.5H2O, the CuII ion is coordinated by four N atoms [Cu—N = 1.985 (6)–2.125 (7) Å] from two 2,2′-bipyridine ligand molecules and a bromide anion [Cu—Br = 2.471 (2) Å] in a distorted trigonal-bipyramidal geometry. Short centroid–centroid distances [3.762 (5) and 3.867 (5) Å] between the aromatic rings of neighbouring cations suggest the existence of π–π interactions. Intermolecular O—H⋯Br hydrogen bonds and weak C—H⋯O and C—H⋯Br interactions consolidate the crystal packing
Trade-Offs between Economic Benefits and Ecosystem Services Value under Three Cropland Protection Scenarios for Wuhan City in China
Over the past few decades urbanization and population growth have been the main trend all over the world, which brings the increase of economic benefits (EB) and the decrease of cropland. Cropland protection policies play an important role in the urbanization progress. In this study, we assess the trade-offs between EB and ecosystem services value (ESV) under three cropland protection policy scenarios using the LAND System Cellular Automata for Potential Effects (LANDSCAPE) model. The empirical results reveal that trade-offs between EB and ESV in urbanizing areas are dynamic, and that they considerably vary under different cropland protection policy scenarios. Especially, the results identify certain "turning points" for each policy scenario at which a small to moderate growth in EB would result in greater ESV losses. Among the three scenarios, we found that the cropland protection policy has the most adverse effect on trade-offs between EB and ESV and the results in the business as usual scenario have the least effect on the trade-offs. Furthermore, the results show that a strict balance between requisition and compensation of cropland is an inappropriate policy option in areas where built-up areas are increasing rapidly from the perspective of mitigating conflict between EB and ESV and the numbers of cropland protection that restrained by land use planning policy of Wuhan is a better choice
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