Wake-Driven Dynamics of Finite-Sized Buoyant Spheres in Turbulence

Particles suspended in turbulent flows are affected by the turbulence and at the same time act back on the flow. The resulting coupling can give rise to rich variability in their dynamics. Here we report experimental results from an investigation of finite-sized buoyant spheres in turbulence. We find that even a marginal reduction in the particle's density from that of the fluid can result in strong modification of its dynamics. In contrast to classical spatial filtering arguments and predictions of particle models, we find that the particle acceleration variance increases with size. We trace this reversed trend back to the growing contribution from wake-induced forces, unaccounted for in current particle models in turbulence. Our findings highlight the need for improved multi-physics based models that account for particle wake effects for a faithful representation of buoyant-sphere dynamics in turbulence.Comment: 5 pages, 4 figures, http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.12450

Mouse Natural Killer (NK) Cells Express the Nerve Growth Factor Receptor TrkA, which Is Dynamically Regulated

Background: Nerve growth factor (NGF) is a neurotrophin crucial for the development and survival of neurons. It also acts on cells of the immune system which express the NGF receptors TrkA and p75 NTR and can be produced by them. However, mouse NK cells have not yet been studied in this context. Methodology/Principal Findings: We used cell culture, flow cytometry, confocal microscopy and ELISA assays to investigate the expression of NGF receptors by NK cells and their secretion of NGF. We show that resting NK cells express TrkA and that the expression is different on NK cell subpopulations defined by the relative presence of CD27 and CD11b. Expression of TrkA is dramatically increased in IL-2-activated NK cells. The p75 NTR is expressed only on a very low percentage of NK cells. Functionally, NGF moderately inhibits NK cell degranulation, but does not influence proliferation or cytokine production. NK cells do not produce NGF. Conclusions/Significance: We demonstrate for the first time that mouse NK cells express the NGF receptor TrkA and tha

Turbulence-induced melting of a nonequilibrium vortex crystal in a forced thin fluid film

To develop an understanding of recent experiments on the turbulence-induced melting of a periodic array of vortices in a thin fluid film, we perform a direct numerical simulation of the two-dimensional Navier-Stokes equations forced such that, at low Reynolds numbers, the steady state of the film is a square lattice of vortices. We find that, as we increase the Reynolds number, this lattice undergoes a series of nonequilibrium phase transitions, first to a crystal with a different reciprocal lattice and then to a sequence of crystals that oscillate in time. Initially the temporal oscillations are periodic; this periodic behaviour becomes more and more complicated, with increasing Reynolds number, until the film enters a spatially disordered nonequilibrium statistical steady that is turbulent. We study this sequence of transitions by using fluid-dynamics measures, such as the Okubo-Weiss parameter that distinguishes between vortical and extensional regions in the flow, ideas from nonlinear dynamics, e.g., \Poincare maps, and theoretical methods that have been developed to study the melting of an equilibrium crystal or the freezing of a liquid and which lead to a natural set of order parameters for the crystalline phases and spatial autocorrelation functions that characterise short- and long-range order in the turbulent and crystalline phases, respectively.Comment: 31 pages, 56 figures, movie files not include

Parental socioeconomic status and the timing of first marriage: What is the role of unmarried cohabitation? Results from a cross-national comparison

Background: Previous research has shown that individuals from high-status families enter marriage later than those from low-status families. However, in many Western societies, it has become common to cohabit prior to marriage. Does this change the link between parental socioeconomic status (SES) and marriage timing? Objective: This study examines to what extent the impact of parental SES on the timing of first marriage weakens after young adults start a cohabiting union. It also examines cross-national variation in the link between parental SES and marriage timing before and after young adults cohabit and whether this variation depends on countries' position in the cohabitation transition. Methods: We apply discrete-time hazard models and meta-analytical tools using data from 20 Western countries. To examine whether the cohabitation stage of countries explains country differences, we construct a four-stage cohabitation typology. Results: In most countries, higher parental SES results in later entry into marriage. The impact of parental SES on marriage timing weakens considerably after young adults entered a cohabiting union. Substantial cross-national variation is found in the strength of the link between parental SES and marriage timing. However, this variation cannot be explained by the cohabitation stage countries are in. Contribution: First, this study provides fresh evidence of the influence of parental SES on family formation in Western countries. Second, it shows the importance of a life-course perspective, as parental SES matters less after young adults start a cohabiting union. Third, it presents a theory-based and empirically-tested typology of stages in the cohabitation transition

Mesoscopic structure and social aspects of human mobility

The individual movements of large numbers of people are important in many contexts, from urban planning to disease spreading. Datasets that capture human mobility are now available and many interesting features have been discovered, including the ultra-slow spatial growth of individual mobility. However, the detailed substructures and spatiotemporal flows of mobility - the sets and sequences of visited locations - have not been well studied. We show that individual mobility is dominated by small groups of frequently visited, dynamically close locations, forming primary "habitats" capturing typical daily activity, along with subsidiary habitats representing additional travel. These habitats do not correspond to typical contexts such as home or work. The temporal evolution of mobility within habitats, which constitutes most motion, is universal across habitats and exhibits scaling patterns both distinct from all previous observations and unpredicted by current models. The delay to enter subsidiary habitats is a primary factor in the spatiotemporal growth of human travel. Interestingly, habitats correlate with non-mobility dynamics such as communication activity, implying that habitats may influence processes such as information spreading and revealing new connections between human mobility and social networks.Comment: 7 pages, 5 figures (main text); 11 pages, 9 figures, 1 table (supporting information

Proton Interaction Vertex Imaging With Silicon-Pixel CMOS Telescope For Carbon Therapy Quality control

International audienceMonitoring of the dose deposition during carbon ion therapy is a crucial issue for the quality control of such treatments. Recent studies have demonstrated that an ion-range control with millimeter resolution is feasible on a pencil-beam basis in homogeneous targets with prompt gamma detection for proton beams [1] and with Proton Interaction Vertex Imaging (PIVI) for carbon beams [2]. The present communication aims at describing our experimental and Monte Carlo simulation results. [1] J. Smeets et al., Phys. Med. Biol. 57 (2012) 3371-3405 [2] P. Henriquet et al., Phys. Med. Biol. 57 (2012) 4655-466

Development of Cryogenic Current Comparators with DC Squid Readout for the Calibration of Electrical Standards

For the realization of the electrical quantum metrology triangle (V-A-Ω) a device to amplify very small currents with high precision is needed. The cryogenic current comparator (CCC) is by far the best instrument to do this. In order to make a very current sensitive CCC for calibration of electrical standards, we have developed optimum dc Superconducting QUantum Interference Devices (SQUIDs). The design, fabrication and characterisation of these devices is presented. The measurements concern the flux-to-voltage transfer and the noise properties, especially the input current noise. The optimisation of the flux transformer circuit that links the CCC with the SQUID will be treated. In addition, typical fabrication aspects of the CCC as the wires and tube assembly, the shields and the support system will be addressed

Search for the exotic Ξ−−(1860)\Xi^{--}(1860) Resonance in 340GeV/c Σ−\Sigma^--Nucleus Interactions

We report on a high statistics search for the $\Xi^{--}(1860)$ resonance in $\Sigma^-$-nucleus collisions at 340GeV/c. No evidence for this resonance is found in our data sample which contains 676000 $\Xi^-$ candidates above background. For the decay channel $\Xi^{--}(1860) \to \Xi^-\pi^-$ and the kinematic range 0.15$<x_F<$0.9 we find a 3$\sigma$ upper limit for the production cross section of 3.1 and 3.5 $\mu$b per nucleon for reactions with carbon and copper, respectively.Comment: 5 pages, 4 figures, modification of ref. 43 and 4