53,555 research outputs found
Monotone Volume Formulas for Geometric Flows
We consider a closed manifold M with a Riemannian metric g(t) evolving in
direction -2S(t) where S(t) is a symmetric two-tensor on (M,g(t)). We prove
that if S satisfies a certain tensor inequality, then one can construct a
forwards and a backwards reduced volume quantity, the former being
non-increasing, the latter being non-decreasing along the flow. In the case
where S=Ric is the Ricci curvature of M, the result corresponds to Perelman's
well-known reduced volume monotonicity for the Ricci flow. Some other examples
are given in the second section of this article, the main examples and
motivation for this work being List's extended Ricci flow system, the Ricci
flow coupled with harmonic map heat flow and the mean curvature flow in
Lorentzian manifolds with nonnegative sectional curvatures. With our approach,
we find new monotonicity formulas for these flows.Comment: v2: final version (as published
Central American Temnocerus Thunberg, 1815 (Coleoptera: Rhynchitidae)
Twenty eight species of Temnocerus Thunberg, 1815 are recognized from Central America (Mexico to Panama) with eight previously described species and 20 new species as follows: T. abdominalis (Voss), T. chiapensis n. sp., T. chiriquensis (Sharp), T. confertus (Sharp), T. cyaneus n. sp., T. ellus n. sp., T. giganteus n. sp., T. guatemalenus (Sharp), T. guerrerensis n. sp., T. herediensis n. sp., T. mexicanus n. sp., T. michoacensis n. sp., T. minutus n. sp., T. niger n. sp., T. oaxacensis n. sp., T. obrieni, n. sp., T. oculatus (Sharp), T. potosi n. sp., T. pseudaeratus n. sp., T. pueblensis n. sp., T. pusillus (Sharp), T. regularis (Sharp), T. rostralis n. sp., T. rugosus n. sp., T. salvensis n. sp., T. tamaulipensis n. sp., T. thesaurus (Sharp) and T. yucatensis n. sp. Rhynchites debilis Sharp is placed in synonymy with Temnocerus guatemalenus (Sharp) and Pselaphorhynchites lindae Hamilton is placed in synonymy with Temnocerus regularis (Sharp). A key to species based on external characters and male genitalia is provided as well as digital images, aedeagus drawings, and map distributions
Place matters: challenges and opportunities in four rural Americas
A survey of 7,800 rural Americans in 19 counties across the country has led to the Carsey Institute\u27s first major publication that outlines four distinctly different rural Americasâamenity, decline, chronic poverty, and those communities in decline that are also amenity-richâeach has unique challenges in this modern era that will require different policies than their rural neighbors
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Working girls: abuse or choice in street-level sex work? A study of homeless women in Nottingham
This paper aims to explore how abused homeless women understand their choice to sex work. In particular, there is a discussion of the motivations given by women as to why they sex worked, and it is suggested that abused homeless women can experience coercion from abusive partners in deciding to sex work. There is a challenge to the notion of 'victim' within the government's 'Prostitution Strategy' (Home Office, 2006) in specifying responses to the complex needs of sex working women. Recommendations are also made for practice in the context of the 'Strategy' when working with abused and coerced homeless women who choose to sex work. Twenty-six homeless women were interviewed - nine of whom had sex worked - and a structured, qualitative questionnaire was used in a case study design from which information was gathered about the relationship between a woman's experience of abuse and her decision to sex work
Negative Group Velocity
The group velocity for pulses in an optical medium can be negative at
frequencies between those of a pair of laser-pumped spectral lines. The gain
medium then can amplify the leading edge of a pulse resulting in a time advance
of the pulse when it exits the medium, as has been recently demonstrated in the
laboratory. This effect has been called superluminal, but, as a classical
analysis shows, it cannot result in signal propgation at speeds greater than
that of light in vacuum.Comment: v3 adds discussion of "rephasing", and adds a figure. v4 adds
references to the early history of negative group velocity, and adds a
figure; thanks to Alex Grani
Multi-physics simulation of friction stir welding process
Purpose: The Friction Stir Welding (FSW) process comprises of several highly coupled (and non-linear) physical phenomena: large plastic deformation, material flow transportation, mechanical stirring of the tool, tool-workpiece surface interaction, dynamic structural evolution, heat generation from friction and plastic deformation, etc. In this paper, an advanced Finite Element (FE) model encapsulating this complex behavior is presented and various aspects associated with the FE model such as contact modeling, material model and meshing techniques are discussed in detail. Methodology: The numerical model is continuum solid mechanics-based, fully thermomechanically coupled and has successfully simulated the friction stir welding process including plunging, dwelling and welding stages. Findings: The development of several field variables are quantified by the model: temperature, stress, strain, etc. Material movement is visualized by defining tracer particles at the locations of interest. The numerically computed material flow patterns are in very good agreement with the general findings from experiments. Value: The model is, to the best of the authorsâ knowledge, the most advanced simulation of FSW published in the literature
Parametric finite-element studies on the effect of tool shape in friction stir welding
The success of the Friction Stir Welding (FSW) process, and the weld quality produced, depends significantly on the design of the welding tool. In this paper the effect of variation in various tool geometry parameters on FSW process outcomes, during the plunge stage, were investigated. Specifically the tool shoulder surface angle and the ratio of the shoulder radius to pin radius on tool reaction force, tool torque, heat generation, temperature distribution and size of the weld zone were investigated. The studies were carried out numerically using the finite element method. The welding process used AA2024 aluminium alloy plates with a thickness of 3 mm. It was found that, in plunge stage, the larger the pin radius the higher force and torque the tool experiences and the greater heat generated. It is also found that the shoulder angle has very little effect on energy dissipation as well as little effect on temperature distribution
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