The stability of immiscible viscous fingering in Hele-Shaw cells with spatially varying permeability

In this paper, we investigate the stability of immiscible viscous fingering in Hele-Shaw cells with spatially varying permeability, across a range of capillary numbers. We utilise a coupled boundary element - radial basis function (BE –RBF) numerical method that adapts and moves with the growing interface, providing an efficient, high accuracy scheme to track the interfacial displacement of immiscible fluids. By comparing the interfacial evolution and growth rate in varying permeability cells to that in uniform cells, we can assess the relative stability of the perturbations as a consequence of the variable permeability. Numerical experiments in Hele-Shaw cells with gradually varying permeability highlight 3 aperture effects that control the interfacial stability: (1) Gradients in the capillary pressure (2) Local changes in fluid mobility (3) Variation in the viscous pressure gradient. In low capillary number regimes, we find that aperture effect 1 and 2 dominate, which (relatively) stabilise interfacial perturbations in converging geometries and destabilise perturbations in diverging geometries. In high capillary number regimes, aperture effect 3 dominates meaning the relative stability transitions; the interface is destabilised in converging cells and stabilised in diverging cells. We find an upper bound critical capillary number Cagt at which the relative stability transitions in our gradually varying cell as 1000<Cagt<1250, which is independent of both α and ϵ0. This result is much lower than the value of Cagt=9139 predicted by linear stability theory, due to significant non-linear perturbation growth. This transition links the results found in previous works performed at low and high capillary numbers, providing new insight into the viscous fingering instability in variable permeability cells. To conclude, we present simulations in Hele-Shaw cells with large geometric heterogeneities and anisotropy, in order to demonstrate the significant fluid re-distribution that can occur due to localised variations in cell permeability. Using periodic permeability distributions, we show the significant re-distribution of fluid that can occur due to large capillary pressure gradients in the capillary limit, and the channelling of flow that can occur in the viscous limit along anisotropic features

Visual Interest Prediction with Attentive Multi-Task Transfer Learning

Visual interest & affect prediction is a very interesting area of research in the area of computer vision. In this paper, we propose a transfer learning and attention mechanism based neural network model to predict visual interest & affective dimensions in digital photos. Learning the multi-dimensional affects is addressed through a multi-task learning framework. With various experiments we show the effectiveness of the proposed approach. Evaluation of our model on the benchmark dataset shows large improvement over current state-of-the-art systems

Local field topology behind light localization and metamaterial topological transitions

We revisit the mechanisms governing the sub-wavelength spatial localization of light in surface plasmon polariton (SPP) modes by investigating both local and global features in optical powerflow at SPP frequencies. Close inspection of the instantaneous Poynting vector reveals formation of optical vortices - localized areas of cyclic powerflow - at the metal-dielectric interface. As a result, optical energy circulates through a subwavelength-thick 'conveyor belt' between the metal and dielectric where it creates a high density of optical states (DOS), tight optical energy localization, and low group velocity associated with SPP waves. The formation of bonding and anti-bonding SPP modes in metal-dielectric-metal waveguides can also be conveniently explained in terms of different spatial arrangements of localized powerflow vortices between two metal interfaces. Finally, we investigate the underlying mechanisms of global topological transitions in metamaterials composed of multiple metal and dielectric films, i.e., transitions of their iso-frequency surfaces from ellipsoids to hyperboloids, which are not accompanied by the breaking of lattice symmetry. Our analysis reveals that such global topological transitions are governed by the dynamic local re-arrangement of local topological features of the optical interference field, such as vortices and saddle points, which reconfigures global optical powerflow within the metamaterial. These new insights into plasmonic light localization and DOS manipulation not only help to explain the well-known properties of SPP waves but also provide useful guidelines for the design of plasmonic components and materials for a variety of practical applications.Comment: 25 pages, 9 figures, Ch. 8 of Singular and Chiral Nanoplasmonics (S.V. Boriskina and N.I. Zheludev Eds.) Pan Stanford, Singapore, 201

Individual isotopic specializations predict subsequent inter-individual variation in movement in a freshwater fish

Despite many similarities and intuitive links between individual dietary specialization and behavioral inter-individual variation, these phenomena have been studied in isolation, and empirical data confirming relationships between these intraspecific variance sources are lacking. Here we use stable isotope analysis and acoustic telemetry to test the hypothesis that individual specialization in trophic (d15N) and littoral/pelagic prey reliance (d13C) covary with inter-individual variation in movement in a group of 34 free-swimming burbot (Lota lota). By performing stable isotope analysis on tissues with differing isotopic turnover rates (anal fin and dorsal muscle), in 24 lethally sampled burbot, we demonstrate that individual specialization in trophic niche (d15N) and li

N-body simulations of gravitational dynamics

We describe the astrophysical and numerical basis of N-body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and large-scale structure). We explain and discuss the state-of-the-art algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N-body simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu

Immiscible thermo-viscous fingering in Hele-Shaw cells

We investigate immiscible radial displacement in a Hele-Shaw cell with a temperature dependent viscosity using two coupled high resolution numerical methods. Thermal gradients created in the domain through the injection of a low viscosity fluid at a different temperature to the resident high viscosity fluid can lead to the formation of unstable thermo-viscous fingers, which we explore in the context of immiscible flows. The transient, multi-zone heat transfer is evaluated using a newly developed auxiliary radial basis function-finite collocation (RBF-FC) method, which locally captures variation in flux and field variable over the moving interface, without the need for ghost node extrapolation. The viscosity couples the transient heat transfer to the Darcy pressure/velocity field, which is solved using a boundary element - RBF-FC method, providing an accurate and robust interface tracking scheme for the full thermo-viscous problem. We explore the thermo-viscous problem space using systematic numerical experiments, revealing that the early stage finger growth is controlled by the pressure gradient induced by the varying temperature and mobility field. In hot injection regimes, negative temperature gradients normal to the interface act to accelerate the interface, promoting finger bifurcation and enhancing the viscous fingering instability. Correspondingly, cold injection regimes stabilise the flow compared to isothermal cases, hindering finger formation. The interfacial mobility distribution controls the late stage bifurcation mode, with non-uniformities induced by the thermal diffusivity creating alternate bifurcation modes. Further numerical experiments reveal the neutral stability of the thermal effects on the fingering evolution, with classical viscous fingering dynamics eventually dominating the evolution. We conclude the paper with a mechanistic summary of the immiscible thermo-viscous fingering regime, providing the first detailed analysis of the thermal problem in immiscible flows

'Education, education, education' : legal, moral and clinical

This article brings together Professor Donald Nicolson's intellectual interest in professional legal ethics and his long-standing involvement with law clinics both as an advisor at the University of Cape Town and Director of the University of Bristol Law Clinic and the University of Strathclyde Law Clinic. In this article he looks at how legal education may help start this process of character development, arguing that the best means is through student involvement in voluntary law clinics. And here he builds upon his recent article which argues for voluntary, community service oriented law clinics over those which emphasise the education of students

A rapid staining technique for the detection of the initiation of germination of bacterial spores

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75645/1/j.1472-765x.2002.01047.x.pd

Using screen video capture software to aide and inform cognitive interviewing

Web-based surveys are a salient tool in the repertoire of social and behavioral scientists. The increase in web-based surveys is understandable considering the distinct advantages offered, including: (a) lower costs and reduced labor time, (b) ability to directly transfer data into statistical packages (reducing coding errors), (c) customization options enabling more attractive presentation, (d) ability to reduce respondent burden by embedding skip patterns, and (e) access to larger sample sizes in different geographic regions. It is important to note, however, that administering web-based surveys also introduces distinct sources of error (e.g., coverage, sampling and non-response). Regardless of format (e.g., paper-and-pencil or web-based), specific, prescribed steps must be followed when constructing an instrument in order to reduce survey error and lend credence to the data collected before subsequent analysis is performed. One of those crucial stages integral to the pretesting process is cognitive interviewing. Cognitive interviewing is a qualitative process, encompassing two main techniques: think aloud interviewing and verbal probing. Collectively, these two methods seek to (a) produce information on what the respondent is thinking while answering the questions, (b) the cognitive processes used to answer the questions, and (c) how the respondent answers the questions. The purpose of this article is to provide a practical guide outlining how Camtasia, a screen video capture software, can aide and inform the cognitive interview process