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This study investigates, by means of numerical simulations, extreme mechanical force exerted by a turbulent flow impinging on a bluff body, and examines the relevance of two distinct rare-event algorithms to efficiently sample these events. The drag experienced by a square obstacle placed in a turbulent channel flow (in two dimensions) is taken as a representative case study. Direct sampling shows that extreme fluctuations are closely related to the presence of a strong vortex blocked in the near wake of the obstacle. This vortex is responsible for a significant pressure drop between the forebody and the base of the obstacle, thus yielding a very high value of the drag. Two algorithms are then considered to speed up the sampling of such flow scenarios, namely the adaptive multilevel splitting (AMS) and the Giardina–Kurchan–Tailleur–Lecomte (GKTL) algorithms. The general idea behind these algorithms is to replace a long simulation by a set of much shorter ones, running in parallel, with dynamics that is replicated or pruned, according to some specific rules designed to sample large-amplitude events more frequently. These algorithms have been shown to be relevant for a wide range of problems in statistical physics, computer science, biochemistry. The present study is the first application to a fluid–structure interaction problem. Practical evidence is given that the fast sweeping time of turbulent fluid structures past the obstacle has a strong influence on the efficiency of the rare-event algorithm. While the AMS algorithm does not yield significant run-time savings as compared to direct sampling, the GKTL algorithm appears to be effective in sampling very efficiently extreme fluctuations of the time-averaged drag and estimating related statistics such as return times. Software used for simulations and data processing is available at https://github.com/tlestang/paper_extreme_drag_fluctuations.
On 3 October 2015, the Médecins Sans Frontières (MSF) Trauma Centre in Kunduz, Afghanistan was bombed during a US–Afghan joint military operation to retake the city. Even before that night, attacks on health-care facilities in war zones were already a worrying trend and a major concern for humanitarian organizations. Such attacks have led both MSF and the International Committee of the Red Cross (ICRC) to launch campaigns1 addressing the need for greater protection of the medical mission in situations of armed conflict. Nonetheless, the scale and specific context of the attack on the Kunduz Trauma Centre have given rise to various specific investigations2 and provoked many more questions that this article will explore. The article will delve into the “many mistakes” scenario that has been presented by the US investigation in order to critically analyze whether these mistakes may originate from either incorrect or biased interpretations or implementation of international humanitarian law.
We model the dynamics of Jupiter’s jets by the stochastic barotropic
-plane model. In this simple framework, by analytic computation of the averaged effect of eddies, we obtain three new explicit results about the equilibrium structure of jets. First we obtain a very simple explicit relation between the Reynolds stresses, the energy injection rate and the averaged velocity shear. This predicts the averaged velocity profile far from the jet edges (extrema of zonal velocity). Our approach takes advantage of a time-scale separation between the inertial dynamics on one hand, and the spin-up (or spin-down) time on the other. Second, a specific asymptotic expansion close to the eastward jet extremum explains the formation of a cusp at the scale of energy injection, characterized by a curvature that is independent of the forcing spectrum. Finally, we derive equations that describe the evolution of the westward tip of the jets. The analysis of these equations is consistent with the previously discussed picture of barotropic adjustment, explaining the relation between the westward jet curvature and the
-effect. Our results give a consistent overall theory of the stationary velocity profile of inertial barotropic zonal jets, in the limit of small-scale forcing.
The taxonomy of Mediterranean populations of Diodora is assessed based on new molecular (COI and 28S) data. The recently described Diodora demartiniorum Buzzurro & Russo, 2005, is found to be a valid species restricted to the Gulf of Gabès (Tunisia) but possibly occurring also on the coast of Libya. However, specimens from the Aegean Sea previously identified as D. demartiniorum are molecularly (and morphologically pseudocryptically) distinct and represent a previously unrecognized species here described as D. giannispadai n. sp. It is hypothesized that the current distribution of these two species corresponds to glacial refuges during Pleistocene climate changes.
This paper uses a quantitative overlapping generation model to suggest a pension reform able to sustain a retirement system, in the face of deep demographic changes. We derive the reform design from an optimization program that selects one or more policy instruments – and their values – among a predefined set, to minimize the welfare loss of the median voter while keeping sound public finances, sustaining gross domestic product growth and considering the welfare of the newborn generation. We calibrate the model to the Luxembourg economy. The European Commission (2012) forecasts that, among all euro area countries, Luxembourg will experience the largest increase in pension costs between now and 2060. Our simulations show that a single instrument reform would imply severe backlashes on the rest of the economy. The suggested pension reform instead consists of a policy mix including taxation, benefits and the effective retirement age. We stress the need to design pension reforms based on optimization programs that lead to the achievement of desired targets. Indeed, the reform implemented by the Luxembourg government in 2013, which does not result from an optimization program, will not keep public finances sound over the medium term.
We take up the old problem of Calvert (J. Fluid Mech., vol. 29, 1967, pp. 691–703) concerning the wake of a cylinder inclined with respect to the flow direction, and consider it from the viewpoint of transition to turbulence. For cylinders placed perpendicular to the flow direction, we address the disagreement between numerical simulation of the ideal axisymmetric configuration and experimental observations. We demonstrate that for a disk (a cylinder of aspect ratio infinity) and a flat cylinder of aspect ratio
(ratio of diameter to height), the numerically predicted transition scenario is limited to very small inclination angles and is thus difficult to test experimentally. For inclination angles of about
and more, a joint numerical and experimental study shows that the experimentally observed scenario agrees qualitatively well with the results of numerical simulations. For the flat cylinder
, we obtain satisfactory agreement with regard to dependence of the critical Reynolds number (
) of the onset of vortex shedding on the inclination angle. Both for infinitely flat disks and cylinders of aspect ratio
, a small inclination tends to promote vortex shedding, that is, to lower the instability threshold, whereas for inclination angles exceeding
the opposite effect is exhibited. The Strouhal number of oscillations is found to be only very weakly dependent on the Reynolds number, and very good agreement is obtained between values reported by Calvert (J. Fluid Mech., vol. 29, 1967, pp. 691–703) at high Reynolds numbers and our simulations at
. In contrast, we observe relatively poor agreement in Strouhal numbers when comparing the results of our numerical simulations and the data acquired from the experimental set-up described in this paper. Closer analysis shows that confidence can be placed in the numerical results because the discrepancy can be attributed to the influence of the support system of the flat cylinder. Suggestions for improvement of the experimental set-up are provided.
The debate about the legality of cross-border relief operations has been revived in the wake of the failures experienced by international humanitarian organizations in their response to humanitarian needs in Syria.
Buccinidae—like other canivorous and predatory molluscs—are generally considered to be occasional visitors or rare colonizers in deep-sea biogenic habitats. However, casual observations during tropical deep-sea cruises suggest that associations between buccinids and sunken wood, in particular, are not fortuitous. Enigmatocolus monnieri has been found to co-occur in Madagascar with bathymodiolines, vesicomyids and solemyids, indicating the presence of seeps, and species of Thermosipho gen. nov. have been sampled by submersibles and remotely operated vehicles, exclusively from hydrothermal vents. A molecular phylogeny (based on CO1, 12S and 28S genes) reveals that buccinid genera potentially associated with sunken wood (Eosipho, Gaillea gen. nov., Calagrassor gen. nov., and Manaria) are closely related to taxa from vents (Thermosipho gen. nov.) and seeps (Enigmaticolus). The anatomy of several dissected species did not reveal any special trait that could be interpreted as a special adaptation to biogenic substrates. Buccinids from sunken wood are most diverse in the Indo-Pacific centre of marine biodiversity, the ‘Coral Triangle’, at depths between 100 and 1000 m, with numerous species still undescribed.
The effects of simulant groundwater composition, pH and temperature on the dissolution and alteration of Na- and Ca-montmorillonite have been studied. Prior to the experiments, Wyoming type Na-montmorillonite, Swy-2, was purified to decrease the amount of accessory minerals. For Ca-montmorillonite experiments, the interlayer cation Na+ of purified Swy-2 was exchanged with Ca2+. The batch experiments were conducted with the purified montmorillonites in simulated fresh and saline waters at 25°C and 60°C under anaerobic conditions in an Ar atmosphere. The concentrations of Si, Al, Fe and Mg were analysed from ultra-filtered solution samples with High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS) as a function of dissolution time. The pH evolution was also measured. The solid smectite phases were analysed with X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD analyses indicated that the nature of the smectite mineral did not change over 140 days. However, the experimental conditions, more or less, modified the structure (e.g. the layer stacking of montmorilllonite; the partial dissolution of the smectite), which cannot be detected by XRD but was evidenced by chemical data, and can be considered as a possible contributor to the stacking faults of the montmorillonite. The log rates (mol g–1 s–1), based on the dissolved amount of Si, varied between –10.64 and –12.13 depending on the experimental conditions.
Refractory castables containing alumina-magnesia/self-forming spinel
(MgAl2O4) are used in impact pads of steel ladles in steelmaking
processes. In order to understand the wear mechanisms of refractory materials, several
recipes were tested from a corrosion, slag resistance and thermal shock point of view. The
results show that the corrosion is extremely limited at the slag/refractory interface for
all cases. Nevertheless, for higher cement alumina content castables, the formation of
cracks is observed in refractory castables into which slag can penetrate. The slag reacts
with the alumina to form a new phase such as hibonite (CA6) and calcium dialuminate (CA2).
The volumetric change of these reactions involving CA2 and CA6 lead to the apparition of
macro-cracks. Thus, the penetration of slag and steel are increased, causing hot
mechanical properties to degrade. For lower cement alumina castables, the formation of
micro-cracks is avoided by controlling volume expansion. Thus, the slag deposit reacts
with alumina grains and the matrix at the slag/refractory interface to produce a
monomineral layer of hibonite. In this way, the monomineral layer acts as a barrier and
limits the penetration of slag and steel into the refractory lining. Thus, to increase the
lifetime of refractory castables containing alumina-magnesia/self-forming spinel, it is
advised to control volume expansion in order to avoid the formation of cracks and limit
the penetration of secondary metallurgy steel ladle slag.
Saint-Augustin Lake is an urban lake in Québec, Canada that has been subjected to long periods of direct human impact, mainly due to agricultural and urban activities, with great changes in trophic status and chemistry occurring within the last few decades. In 2009, during an examination of the lake bottom substrate, the presence of the invasive species Cipangopaludina chinensis (Reeve, 1863) was found on floor bottom sediments. The gastropods soft tissues were mineralized and analyzed by ICP-OES. The purpose of this study was to estimate concentrations of heavy metals in C. chinensis, describing the relations of these values with the sediment metal. In gastropod soft tissues the overall common trend in the heavy metal concentrations was revealed in the following order: Fe>Mn>Zn>Cu>As>Ni>Pb>Cd>Cr. Biota-sediment accumulation factors (BSAFs) have shown that C. chinensis cannot be used as a bioindicator of heavy metal pollution and exposure in the Canadian lakes where it is present. In fact, while the sediments of Saint-Augustin Lake are characterized by high metal concentrations, C. chinensis does not have high bioaccumulation factors (BSAFs <1.0). By literature comparison with other aquatic organisms in polluted ecosystems at different latitudes it was possible to affirm that the concentrations of Fe, Mn and Zn in C. chinensis tissues are considerable if compared with these sites.
The one-in-a-life-time event Supernova SN 1987A, the brightest supernova seen since Kepler's in 1604, has given us a unique opportunity to study the mechanics of a supernova explosion and now to witness the birth of a supernova remnant. A violent encounter is underway between the fastest-moving debris and the circumstellar ring: shocks excite “hotspots”. ATCA/ANTF, Gemini, VLT, HST, Spitzer, Chandra, and recently ALMA observations have been so far organized to help understanding the several emission mechanisms at work. In the mid-infrared SN 1987A has transformed from a SN with the bulk of its radiation from the ejecta to a SNR whose emission is dominated by the interaction of the blast wave with the surrounding interstellar medium, a process in which kinetic energy is converted into radiative energy. Currently this remnant emission is dominated by material in or near the inner equatorial ring (ER). We give here a brief history of our mid-infrared observations, and present our last data obtained with the SPITZER infrared satellite and the ESO VLT and Gemini telescopes: we show how together with Chandra observations, they contribute to the understanding of this fascinating object. We argue also that our imaging observations suggest that warm dust is still present in the ejecta, and we dispute the presence of huge amount of very cold dust in it, as it has been claimed on the basis of data obtained with the HERSCHELl satellite.
This study aims at gaining a better understanding of the behaviour of montmorillonite in contact with different ground waters; alteration of montmorillonite and possible formation of secondary minerals. Batch experiments were conducted with purified Swy-2 montmorillonite in simulated fresh (I=0.05 M, pH 8) and saline (I=0.1 M, pH 11) waters at 25 and 60ºC in anaerobic (Ar(g)) conditions. The concentrations of Al, Fe; Mg and Si were analysed from ultra-filtered solution samples with HR-ICP-MS (High Resolution Inductively Coupled Plasma Mass Spectrometry). The amount of released Si depended strongly on the experimental conditions. The Si concentrations at 60oC in the saline and fresh waters showed a difference greater than an order of magnitude. The initial purified montmorillonite and the solid materials from experiments were analysed with XRD. The analysis indicated that the nature of smectite did not change, but the experimental conditions, more or less, modified the structure of montmorillonite, e.g., in fresh waters the XRD spectra showed peaks typical of mixed layer minerals, which can refer to the presence of either randomly ordered illite/smectite or randomly ordered collapsed smectite/ hydrated smectite layers. The dissolution of montmorillonite was studied also by modelling with TOUGHREACT. The experimental and modelled results were compared revealing a need to develop the model e.g. in respect of the evolution of pH.
An exhaustive parametric study of the transition scenario in the wake of oblate spheroids and flat cylinders placed with their rotation axis parallel to the flow is presented. The flatness of the investigated objects is classified by the aspect ratio χ defined as χ = d/a for spheroids (with d the diameter and a the length of the polar axis) and as χ = d/h) for cylinders (with h the cylinder height). We find a significant qualitative similarity between both configurations. At large aspect ratios (χ > 2.3 for spheroids and χ ≥ 4 for cylinders), the secondary bifurcation giving rise to a periodic state without planar symmetry is subcritical with a hysteresis interval of about two Reynolds number units. For spheroids, the sphere-like scenario is recovered only at aspect ratios very close to one (χ ≥ 1 are considered), while for cylindrical bodies the same holds for χ ≤ 1.7. For intermediate aspect ratios, a domain of states with non-zero net helicity separates states typical for the sphere wake from those of an infinitely flat disk.