Suspension-Driven gravity surges on horizontal surfaces: Effect of the initial shape

We present results from highly resolved direct numerical simulations of canonical (axisymmetric and planar) and non-canonical (rectangular) configurations of horizontal suspension-driven gravity surges. We show that the dynamics along the initial minor and major axis of a rectangular release are roughly similar to that of a planar and axisymmetric current, respectively. However, contrary to expectation, we observe under certain conditions the final extent of the deposit from finite releases to surpass that from an equivalent planar current. This is attributed to a converging flow of the particle-laden mixture toward the initial minor axis, a behaviour that was previously reported for scalar-driven currents on uniform slopes [31]. This flow is observed to be correlated with the travelling of a perturbation wave generated at the extremity of the longest side that reaches the front of the shortest side in a finite time. A semi-empirical explicit expression (based on established relations for planar and axisymmetric currents) is proposed to predict the extent of the deposit in the entire x-y plane. Finally, we observe that for the same initial volume of a suspension-driven gravity surge, a release of larger initial horizontal aspect-ratio is able to retain particles in suspension for longer periods of time

Dynamics of non-circular finite-release gravity currents

The present work reports some new aspects of non-axisymmetric gravity currents obtained from laboratory experiments, fully resolved simulations and box models. Following the earlier work of Zgheib et al. (Theor. Comput. Fluid Dyn., vol. 28, 2014, pp. 521–529) which demonstrated that gravity currents initiating from non-axisymmetric cross-sectional geometries do not become axisymmetric, nor do they retain their initial shape during the slumping and inertial phases of spreading, we show that such non-axisymmetric currents eventually reach a self-similar regime during which (i) the local front propagation scales as t1/2 as in circular releases and (ii) the non-axisymmetric front has a self-similar shape that primarily depends on the aspect ratio of the initial release. Complementary experiments of non-Boussinesq currents and top-spreading currents suggest that this self-similar dynamics is independent of the density ratio, vertical aspect ratio, wall friction and Reynolds number Re , provided the last is large, i.e. Re⩾O(104) . The local instantaneous front Froude number obtained from the fully resolved simulations is compared to existing models of Froude functions. The recently reported extended box model is capable of capturing the dynamics of such non-axisymmetric flows. Here we use the extended box model to propose a relation for the self-similar horizontal aspect ratio χ∞ of the propagating front as a function of the initial horizontal aspect ratio χ0 , namely χ∞=1+(lnχ0)/3 . The experimental and numerical results are in good agreement with the proposed relation

Utilization of Small Commercial Grade Nickel Cadmium (NiCd) Cells in Low Earth Orbit (LEO) Applications

The Defense Advanced Research Projects Agency (DARPA) has sponsored the Advanced Space Technology Program (ASTP) to enhance the cost-effectiveness and responsiveness of military space systems. One of the major themes of this program is the development of highly capable small satellites, generally referred to as \u27\u27LightSats, which can perform selected defense missions at relatively low cost. A key element of the programmatic approach is the utilization of commercial grade parts and practices where practical, as opposed to the much more conservative aerospace grade parts. ASTP has incorporated commercial grade batteries into its first generation LightSats; however, an attempt has been made to study the trade-offs and design considerations to optimally employ these batteries on small satellites. For certain applications, particularly for small relatively inexpensive satellites, commercial grade cells may be a viable alternative to aerospace cells. Differences between aerospace and commercial grade cells range from physical construction and technology incorporated, to the level of quality control in manufacturing. These differences are reflected in both greater cost and increased lead time for the aerospace cells. Our research and experience suggest that certain manufacturing technologies are preferable when considering commercial cells for space applications. Once the cell type is chosen, candidate cells must be thoroughly screened to insure survival and acceptable performance in the space environment. To insure optimal performance, cells should be rigorously matched in electrical characteristics when forming batteries. Test procedures should be tailored to fit the application in order to yield the best performance in a specific physical, electrical, and operational environment. An acceptance test plan for screening and matching cells is discussed. The present paper is the first in a series of reports which will document the approach, results, and lessons learned from ASTP\u27s commercial battery studies

Applications of Small Satellites for Defense Space Communication Systems and Technology Development: Pegasus Flight-2 and the Launch of Microsat

DARPA\u27s seven Microsats were placed into orbit on the second flight of the Pegasus. The Microsat program objective is to assess the tactical utility of small, low-cost communications satellites. This paper describes the changes made to the Pegasus since its first flight, provides an overview of the Microsat demonstration program, and outlines the preliminary results of the Pegasus launch. Finally, the near-term Army and Navy demonstration plans for Microsat are discussed

Mind the gap: distributed practice enhances performance in a MOBA game

Understanding how humans master complex skills has the potential for wide-reaching societal benefit. Research has shown that one important aspect of effective skill learning is the temporal distribution of practice episodes (i.e., distributed practice). Using a large observational sample of players (n = 162,417) drawn from a competitive and popular online game (League of Legends), we analysed the relationship between practice distribution and performance through time. We compared groups of players who exhibited different play schedules using data slicing and machine learning techniques, to show that players who cluster gameplay into shorter time frames ultimately achieve lower performance levels than those who space their games across longer time windows. Additionally, we found that the timing of intensive play periods does not affect final performance—it is the overall amount of spacing that matters. These results extend some of the key findings in the literature on practice and learning to an ecologically valid environment with huge n. We discuss our work in relation to recent studies that have examined practice effects using Big Data and suggest solutions for salient confounds

Mixing and recirculation characteristics of gas-liquid Taylor flow in microreactors

The effects of operating parameters (capillary and Reynolds numbers) and microchannel aspect ratio (α = w/h = [1; 2.5; 4]) on the recirculation characteristics of the liquid slug in gas-liquid Taylor flow in microchannels have been investigated using 3-dimensional VOF simulations. The results show a decrease in the recirculation volume in the slug and an increase in recirculation time with increasing capillary number, which is in good agreement with previous results obtained in circular and square geometries (Thulasidas et al., 1997). In addition, increasing the aspect ratio of the channel leads to a slight decrease in recirculating volumes but also a significant increase in recirculation times

On the dispersion of solid particles in a liquid agitated by a bubble swarm

This article deals with the dispersion of solid particles in a liquid agitated by a homogeneous swarm of bubbles. The scale of interest lies between the plant scale (of the order of the tank) and the microscale (less than the bubble diameter). The strategy consists in simulating both the twophase flow of deforming bubbles and the motion of solid particles. The evolution of the spatial distribution of particles together with the encounter and entrainment phenomena is studied as a function of the void fraction and the relative size and mass of particles. The influence of the shape of the bubble and of the model of forces that govern the motion of particles is also considered

Potential-driven Chirality manifestations and impressive enantioselectivity by inherently chiral electroactive films

Molecular materials coupling electroactivity with enantiorecognition capability are an attractive objective in materials research. The usual strategy, hinging on attaching chiral pendants to an electroactive polyconjugated backbone, generally results in modest chirality manifestations. We have thus designed electroactive chiral polyheterocycles, where chirality is not external to the electroactive backbone, but inherent to it, resulting from a tailored torsion produced by the presence of atropisomeric, conjugatively active biheteroaromatic scaffolds. The coincidence of the stereogenic element with the whole electroactive backbone affords by electrooligomerization enantiopure electroactive films of impressive chiroptical activity, which can be finely and reversibly tuned by the electric potential, since injection of positive charges results in decrease of the atropisomeric scaffold angle to favour delocalization, as revealed by CD spectroelectrochemistry, suggesting us the image of a "breathing chirality". To test the enantiorecognition ability of the new inherently chiral conducting films we have developed an efficient protocol in ionic liquid affording preparation of very reproducible electrode surfaces by electrooxidation of the enantiopure monomers on screen printed electrode supports. The resulting specular R and S electrodes have been tested with (R)-(+)- and S-(-)-N,N-dimethyl-1-ferrocenylethylamine specular probes. The response is highly and reproducibly enantioselective (with \uf07e100 mV separation between R and S probes with single enantiomers and even more with the racemate), specular for R vs S surfaces with respect to S and R probes, and reversible in repeated alternating sequences of S and R probe sensing on a single electrode.[1] With the contribution of Fondazione Cariplo, grant no. 2011-0417. [1] F. Sannicol\uf2, S. Arnaboldi, T. Benincori, V. Bonometti, R. Cirilli, L. Dunsch, W. Kutner, G. Longhi, P.R. Mussini, M. Panigati, M. Pierini, S. Rizzo, Angew. Chemie 2014, 53, 2623-2627

Kelvin-Helmholtz instability in the presence of variable viscosity for mudflow resuspension in estuaries

The temporal stability of a parallel shear flow of miscible fluid layers of dif- ferent density and viscosity is investigated through a linear stability analysis and direct numerical simulations. The geometry and rheology of this Newto- nian fluid mixing can be viewed as a simplified model of the behavior of mud- flow at the bottom of estuaries for suspension studies. In this study, focus is on the stability and transition to turbulence of an initially laminar configuration. A parametric analysis is performed by varying the values of three control pa- rameters, namely the viscosity ratio, the Richardson and Reynolds numbers, in the case of initially identical thickness of the velocity, density and viscosity profiles. The range of parameters has been chosen so as to mimic a wide variety of real configurations. This study shows that the Kelvin-Helmholtz instability is controlled by the local Reynolds and Richardson numbers of the inflection point. In addition, at moderate Reynolds number, viscosity strat- ification has a strong influence on the onset of instability, the latter being enhanced at high viscosity ratio, while at high Reynolds number, the influ- ence is less pronounced. In all cases, we show that the thickness of the mixing layer (and thus resuspension) is increased by high viscosity stratification, in particular during the non-linear development of the instability and especially pairing processes. This study suggests that mud viscosity has to be taken into account for resuspension parameterizations because of its impact on the inflec- tion point Reynolds number and the viscosity ratio, which are key parameters for shear instabilities