Movement of a finite body in channel flow

A body of finite size is moving freely inside, and interacting with, a channel flow. The description of this unsteady interaction for a comparatively dense thin body moving slowly relative to flow at medium-to-high Reynolds number shows that an inviscid core problem with vorticity determines much, but not all, of the dominant response. It is found that the lift induced on a body of length comparable to the channel width leads to differences in flow direction upstream and downstream on the body scale which are smoothed out axially over a longer viscous length scale; the latter directly affects the change in flow directions. The change is such that in any symmetric incident flow the ratio of slopes is found to be [Formula: see text], i.e. approximately 0.900969, independently of Reynolds number, wall shear stresses and velocity profile. The two axial scales determine the evolution of the body and the flow, always yielding instability. This unusual evolution and linear or nonlinear instability mechanism arise outside the conventional range of flow instability and are influenced substantially by the lateral positioning, length and axial velocity of the body

On the slow motion of a spheroid in a rotating stratified fluid

We consider the slow motion generated when a body is set into motion relative to an incompressible, inviscid, non-diffusive rotating stratified fluid, showing that there is generated in general a topographic Rossby wave which leads to non-decaying fluctuations in the lift on the obstacle and a fluctuating non-zero drag. The problem is relevant to the flow patterns and forces excited when slow oceanic flows cross bottom topography and suggests a mechanism for slow fluctuations observed in laboratory experiments

Towards a classification framework for social machines

The state of the art in human interaction with computational systems blurs the line between computations performed by machine logic and algorithms, and those that result from input by humans, arising from their own psychological processes and life experience. Current socio-technical systems, known as ‘social machines’ exploit the large-scale interaction of humans with machines. Interactions that are motivated by numerous goals and purposes including financial gain, charitable aid, and simply for fun. In this paper we explore the landscape of social machines, both past and present, with the aim of defining an initial classificatory framework. Through a number of knowledge elicitation and refinement exercises we have identified the polyarchical relationship between infrastructure, social machines, and large-scale social initiatives. Our initial framework describes classification constructs in the areas of contributions, participants, and motivation. We present an initial characterization of some of the most popular social machines, as demonstration of the use of the identified constructs. We believe that it is important to undertake an analysis of the behaviour and phenomenology of social machines, and of their growth and evolution over time. Our future work will seek to elicit additional opinions, classifications and validation from a wider audience, to produce a comprehensive framework for the description, analysis and comparison of social machines

Synthesis and pinning properties of the infinite-layer superconductor Sr0.9La0.1CuO

We report the high-pressure synthesis of the electron-doped infinite-layer superconductor Sr0.9La0.1CuO2 and its superconducting properties. A Rietveld analysis of X-ray powder diffraction data showed that, within the resolution of the measurement, the sample had purely an infinite-layer structure without any discernible impurities. The superconducting volume fraction and the transition width were greatly improved compared to those in previous reports. The irreversibility field line and the intragranular critical current density were much higher than those of La1.85Sr0.15CuO4 and Nd1.85Ce0.15CuO4. The stronger pinning behaviors are consistent with the strong interlayer coupling due to the short distance between CuO2 planes.Comment: Physica C (in press) 5 pages, 4 figur

The Importance of Reference Frame for Pressure at the Liquid-Vapour Interface

Copyright © 2021 The Author(s). The local pressure tensor is non-unique, a fact which has generated confusion and debate in the 70 years since the seminal work by Irving Kirkwood. This non-uniqueness is normally attributed to the interaction path between molecules, especially in the interfacial-science community. In this work, we reframe this discussion of non-uniqueness in terms of the location, or reference frame, used to measure the pressure. By using a general mathematical description of the liquid–vapour interface, we obtain a reference frame that moves with the interface through time, providing new insight into the pressure. We compare this instantaneous moving reference frame with the fixed Eulerian one. Through this process, we show the requirement that normal pressure balance at the moving surface is satisfied by surface fluxes; however, an additional corrective term based on surface curvature is required for the average pressure in a volume. We make the case that a focus on the path of integration is the cause of confusion in the literature. Using an explicit reference frame with a more general derivation of pressure clarifies some of the issues of uniqueness, providing a pressure tensor which is defined at any instant in time and valid away from thermodynamic equilibrium.Engineering and Physical Sciences Research Council [EP/T022213/1] (partially funded UK Materials and Molecular Modelling Hub for computational resources).UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by Engineering and Physical Sciences Research Council [EP/T022213/1]

The TREC2001 video track: information retrieval on digital video information

The development of techniques to support content-based access to archives of digital video information has recently started to receive much attention from the research community. During 2001, the annual TREC activity, which has been benchmarking the performance of information retrieval techniques on a range of media for 10 years, included a ”track“ or activity which allowed investigation into approaches to support searching through a video library. This paper is not intended to provide a comprehensive picture of the different approaches taken by the TREC2001 video track participants but instead we give an overview of the TREC video search task and a thumbnail sketch of the approaches taken by different groups. The reason for writing this paper is to highlight the message from the TREC video track that there are now a variety of approaches available for searching and browsing through digital video archives, that these approaches do work, are scalable to larger archives and can yield useful retrieval performance for users. This has important implications in making digital libraries of video information attainable

Multiscale Simulation of Fluids: Coupling Molecular and Continuum

This is an arXiv preprint which has not been certified by peer review. 21 pages, 8 figures perspective paper submitted to Physical Chemistry Chemical Physics (PCCP) published by Royal Society of Chemistry (RSC), (Print ISSN: 1463-9076, Electronic ISSN: 1463-9084).Computer simulation is an important tool for scientific progress, especially when lab experiments are either extremely costly and difficult or lack the required resolution. However, all of the simulation methods come with limitations. In molecular dynamics (MD) simulation, the length and time scales that can be captured are limited, while computational fluid dynamics (CFD) methods are built on a range of assumptions, from the continuum hypothesis itself, to a variety of closure assumptions. To address these issues, the coupling of different methodologies provides a way to retain the best of both methods. Here, we provide a perspective on multiscale simulation based on the coupling of MD and CFD with each a distinct part of the simulation domain. This style of coupling allows molecular detail to be present only where it is needed, so CFD can model larger scales than possible with MD alone. We present a unified perspective of the literature, showing the links between state and flux coupling and discuss the various assumptions required for both. A unique challenge in such coupled simulation is obtaining averages and constraining local parts of a molecular simulation. We highlight that incorrect localisation has resulted in an error in the literature for both pressure tensor and coupling constraints. We then finish with some applications, focused on the simulation of fluids. Thus, we hope to motivate further research in this exciting area with applications across the spectrum of scientific disciplines...

A new mass-loss rate prescription for red supergiants

Evolutionary models have shown the substantial effect that strong mass-loss rates (⁠M˙s) can have on the fate of massive stars. Red supergiant (RSG) mass-loss is poorly understood theoretically, and so stellar models rely on purely empirical M˙–luminosity relations to calculate evolution. Empirical prescriptions usually scale with luminosity and effective temperature, but M˙ should also depend on the current mass and hence the surface gravity of the star, yielding more than one possible M˙ for the same position on the Hertzsprung–Russell diagram. One can solve this degeneracy by measuring M˙ for RSGs that reside in clusters, where age and initial mass (Minit) are known. In this paper we derive M˙ values and luminosities for RSGs in two clusters, NGC 2004 and RSGC1. Using newly derived Minit measurements, we combine the results with those of clusters with a range of ages and derive an Minit-dependent M˙ prescription. When comparing this new prescription to the treatment of mass-loss currently implemented in evolutionary models, we find models drastically overpredict the total mass-loss, by up to a factor of 20. Importantly, the most massive RSGs experience the largest downward revision in their mass-loss rates, drastically changing the impact of wind mass-loss on their evolution. Our results suggest that for most initial masses of RSG progenitors, quiescent mass-loss during the RSG phase is not effective at removing a significant fraction of the H-envelope prior to core-collapse, and we discuss the implications of this for stellar evolution and observations of SNe and SN progenitors