1,000 research outputs found
Inertial effects on two-particle relative dispersion in turbulent flows
We report experimental results on the relative motion of pairs of solid
spheric particles with initial separations in the inertial range of fully
developed turbulence in water. The particle densities were in the range of , \textit{i.e.}, from neutrally
buoyant to highly inertial; and their sizes were of the Kolmogorov scale. For
all particles, we observed a Batchelor like regime, in which particles
separated ballistically. Similar to the Batchelor regime for tracers, this
regime was observed in the early stages of the relative separation for times with determined by the turbulence energy dissipation
rate and the initial separation between particle pairs. In this time interval
heavier particles separated faster than fluid tracers. The second order
Eulerian velocity structure functions was found to increase with density. In
other words, both observations show that the relative velocity between inertial
particles was larger than that between tracers. Based on the widely used,
simplified equation of motion for inertial point-particles, we derived a model
that shows an increase in relative velocity between inertial particles. In its
scale dependence, however, it disagrees quantitatively with the experimental
results. This we attribute to the preferential sampling of the flow field by
inertial particles, which is not captured by the model.Comment: 6 pages, 5 figures, 2 tables, epl2.cls, submitted to EP
Turbulence attenuation by large neutrally buoyant particles
Turbulence modulation by inertial-range-size, neutrally-buoyant particles is
investigated experimentally in a von K\'arm\'an flow. Increasing the particle
volume fraction , maintaining constant impellers Reynolds
number attenuates the fluid turbulence. The inertial-range energy transfer rate
decreases as , suggesting that only particles
located on a surface affect the flow. Small-scale turbulent properties, such as
structure functions or acceleration distribution, are unchanged. Finally,
measurements hint at the existence of a transition between two different
regimes occurring when the average distance between large particles is of the
order of the thickness of their boundary layers.Comment: 7 pages, 4 figure
River inflow and salinity changes in the Caspian Sea during the last 5500 years
Pollen, spores and dinoflagellate cysts have been analysed on three sediment cores (1.8–1.4 m-long) taken from the south and middle basins of the Caspian Sea. A chronology available for one of the cores is based on calibrated radiocarbon dates (ca 5.5–0.8 cal. ka BP). The pollen and spores assemblages indicate fluctuations between steppe and desert. In addition there are some outstanding zones with a bias introduced by strong river inflow. The dinocyst assemblages change between slightly brackish (abundance of Pyxidinopsis psilata and Spiniferites cruciformis) and more brackish (dominance of Impagidinium caspienense) conditions. During the second part of the Holocene, important flow modifications of the Uzboy River and the Volga River as well as salinity changes of the Caspian Sea, causing sea-level fluctuations, have been reconstructed. A major change is suggested at ca 4 cal. ka BP with the end of a high level phase in the south basin. Amongst other hypotheses, this could be caused by the end of a late and abundant flow of the Uzboy River (now defunct), carrying to the Caspian Sea either meltwater from higher latitudes or water from the Amu-Daria. A similar, later clear phase of water inflow has also been observed from 2.1 to 1.7 cal. ka BP in the south basin and probably also in the north of the middle basin
Simultaneous 3D measurement of the translation and rotation of finite size particles and the flow field in a fully developed turbulent water flow
We report a novel experimental technique that measures simultaneously in
three dimensions the trajectories, the translation, and the rotation of finite
size inertial particles together with the turbulent flow. The flow field is
analyzed by tracking the temporal evolution of small fluorescent tracer
particles. The inertial particles consist of a super-absorbent polymer that
renders them index and density matched with water and thus invisible. The
particles are marked by inserting at various locations tracer particles into
the polymer. Translation and rotation, as well as the flow field around the
particle are recovered dynamically from the analysis of the marker and tracer
particle trajectories. We apply this technique to study the dynamics of
inertial particles much larger in size (Rp/{\eta} \approx 100) than the
Kolmogorov length scale {\eta} in a von K\'arm\'an swirling water flow
(R{\lambda} \approx 400). We show, using the mixed (particle/fluid) Eulerian
second order velocity structure function, that the interaction zone between the
particle and the flow develops in a spherical shell of width 2Rp around the
particle of radius Rp. This we interpret as an indication of a wake induced by
the particle. This measurement technique has many additional advantages that
will make it useful to address other problems such as particle collisions,
dynamics of non-spherical solid objects, or even of wet granular matter.Comment: 18 pages, 7 figures, submitted to "Measurement Science and
Technology" special issue on "Advances in 3D velocimetry
Optically switched magnetism in photovoltaic perovskite CHNH(Mn:Pb)I
The demand for ever-increasing density of information storage and speed of
manipulation boosts an intense search for new magnetic materials and novel ways
of controlling the magnetic bit. Here, we report the synthesis of a
ferromagnetic photovoltaic CHNH(Mn:Pb)I material in which the
photo-excited electrons rapidly melt the local magnetic order through the
Ruderman-Kittel-Kasuya-Yosida interactions without heating up the spin system.
Our finding offers an alternative, very simple and efficient way of optical
spin control, and opens an avenue for applications in low power, light
controlling magnetic devices
Optical spectroscopy and the nature of the insulating state of rare-earth nickelates
Using a combination of spectroscopic ellipsometry and DC transport
measurements, we determine the temperature dependence of the optical
conductivity of NdNiO and SmNiO films. The optical spectra show the
appearance of a characteristic two-peak structure in the near-infrared when the
material passes from the metal to the insulator phase. Dynamical mean-field
theory calculations confirm this two-peak structure, and allow to identify
these spectral changes and the associated changes in the electronic structure.
We demonstrate that the insulating phase in these compounds and the associated
characteristic two-peak structure are due to the combined effect of
bond-disproportionation and Mott physics associated with half of the
disproportionated sites. We also provide insights into the structure of excited
states above the gap.Comment: 12 pages, 13 figure
Ergodic and non-ergodic clustering of inertial particles
We compute the fractal dimension of clusters of inertial particles in mixing
flows at finite values of Kubo (Ku) and Stokes (St) numbers, by a new series
expansion in Ku. At small St, the theory includes clustering by Maxey's
non-ergodic 'centrifuge' effect. In the limit of St to infinity and Ku to zero
(so that Ku^2 St remains finite) it explains clustering in terms of ergodic
'multiplicative amplification'. In this limit, the theory is consistent with
the asymptotic perturbation series in [Duncan et al., Phys. Rev. Lett. 95
(2005) 240602]. The new theory allows to analyse how the two clustering
mechanisms compete at finite values of St and Ku. For particles suspended in
two-dimensional random Gaussian incompressible flows, the theory yields
excellent results for Ku < 0.2 for arbitrary values of St; the ergodic
mechanism is found to contribute significantly unless St is very small. For
higher values of Ku the new series is likely to require resummation. But
numerical simulations show that for Ku ~ St ~ 1 too, ergodic 'multiplicative
amplification' makes a substantial contribution to the observed clustering.Comment: 4 pages, 2 figure
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Examination of Build Height in Ultrasonic Consolidation for Foil Width Specimens Using Supports
Ultrasonic consolidation (UC) is a novel, solid-state, additive manufacturing
fabrication process. It consists of ultrasonic joining of thin metal foils and contour milling
to directly produce functional components in a variety of geometries. The bond between
layers forms when an ultrasonic horn creates a local oscillating stress field at the mating
surfaces. It is commonly theorized that the high frequency vibration under pressure
produces a metallurgical bond without melting the base material. The mechanism behind
the bond is believed to be due to interfacial motion and friction that disrupts surface
contaminants, arguably allowing direct metal to metal contact, and producing sufficient
stress to induce plastic flow and promote the growth of grains across the mating surfaces.
Ignored in this explanation is the role of substrate dimensions on the quality and strength
of the joining process. Researchers have previously examined the effective height
limitations of the build process, i.e., the limiting height to width ratio of one of the
component features being fabricated. This paper extends the experimental work on using
support materials to extend build height on specimens using two different candidate materials, tin bismuth, and a mixture of sugar, corn syrup, and water, referred to as
“candy”. Tin bismuth and candy the represent the extremes of a tradeoff between
convenience and stiffness that a support material must possess.Mechanical Engineerin
Impacts of changes in groundwater recharge on the isotopic composition and geochemistry of seasonally ice-covered lakes: insights for sustainable management
Lakes are under increasing pressure due to widespread anthropogenic impacts
related to rapid development and population growth. Accordingly, many lakes
are currently undergoing a systematic decline in water quality. Recent
studies have highlighted that global warming and the subsequent changes in
water use may further exacerbate eutrophication in lakes. Lake evolution
depends strongly on hydrologic balance, and therefore on groundwater
connectivity. Groundwater also influences the sensitivity of lacustrine
ecosystems to climate and environmental changes, and governs their
resilience. Improved characterization of groundwater exchange with lakes is
needed today for lake preservation, lake restoration, and sustainable
management of lake water quality into the future. In this context, the aim of
the present paper is to determine if the future evolution of the climate, the
population, and the recharge could modify the geochemistry of lakes (mainly
isotopic signature and quality via phosphorous load) and if the isotopic
monitoring of lakes could be an efficient tool to highlight the variability
of the water budget and quality.
Small groundwater-connected lakes were chosen to simulate changes in water
balance and water quality expected under future climate change scenarios,
namely representative concentration pathways (RCPs) 4.5 and 8.5. Contemporary
baseline conditions, including isotope mass balance and geochemical
characteristics, were determined through an intensive field-based research
program prior to the simulations. Results highlight that future lake
geochemistry and isotopic composition trends will depend on four main
parameters: location (and therefore climate conditions), lake catchment size
(which impacts the intensity of the flux change), lake volume (which impacts
the range of variation), and lake G index (i.e., the percentage of
groundwater that makes up total lake inflows), the latter being the dominant
control on water balance conditions, as revealed by the sensitivity of lake
isotopic composition. Based on these model simulations, stable isotopes
appear to be especially useful for detecting changes in recharge to lakes
with a G index of between 50 and 80 %, but response is non-linear.
Simulated monthly trends reveal that evolution of annual lake isotopic
composition can be dampened by opposing monthly recharge fluctuations. It is
also shown that changes in water quality in groundwater-connected lakes
depend significantly on lake location and on the intensity of recharge
change
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