3,069 research outputs found
Active dry granular flows: rheology and rigidity transitions
The constitutive relations of a dense granular flow composed of
self-propelling frictional hard particles are investigated by means of DEM
numerical simulations. We show that the rheology, which relates the dynamical
friction and the volume fraction to the inertial number ,
depends on a dimensionless number , which compares the active
force to the confining pressure. Two liquid/solid transitions -- in the Maxwell
rigidity sense -- are observed. As soon as the activity is turned on, the
packing becomes an `active solid' with a mean number of particle contacts
larger than the isostatic value. The quasi-static values of and
decrease with . At a finite value of the activity ,
corresponding to the isostatic condition, a second `active rigidity transition'
is observed beyond which the quasi-static values of the friction vanishes and
the rheology becomes Newtonian. For , we provide
evidence for a highly intermittent dynamics of this 'active fluid'.Comment: 7 pages, 7 figures, final version, accepted for publication in
Europhys. Let
Creep and fluidity of a real granular packing near jamming
We study the internal dynamical processes taking place in a granular packing
below yield stress. At all packing fractions and down to vanishingly low
applied shear, a logarithmic creep is evidenced. The experiments are analyzed
under the scope of a visco-elastic model introducing an internal "fluidity"
variable. For all experiments, the creep dynamics can be rescaled onto a unique
curve which displays jamming at the random-close-packing limit. At each packing
fraction, a stress value is evidenced, corresponding to the onset of internal
granular reorganisation leading to a slowing down the creep dynamics before the
final yield
Stripe formation in horizontally oscillating granular suspensions
We present the results of an experimental study of pattern formation in
horizontally oscillating granular suspensions. Starting from a homogeneous
state, the suspension turns into a striped pattern within a specific range of
frequencies and amplitudes of oscillation. We observe an initial development of
layered structures perpendicular to the vibration direction and a gradual
coarsening of the stripes. However, both processes gradually slow down and
eventually saturate. The probability distribution of the stripe width
approaches a nonmonotonic steady-state form which can be approximated by a
Poisson distribution. We observe similar structures in MD simulations of soft
spherical particles coupled to the motion of the surrounding fluid.Comment: 7 pages, 8 figures, to appear in Europhys. Lett. (2014
Reduction of acquisition time using partition of the signal decay in spectroscopic imaging technique (RAPID-SI)
To overcome long acquisition times of Chemical Shift Imaging (CSI), a new Magnetic Resonance Spectroscopic Imaging (MRSI) technique called Reduction of Acquisition time by Partition of the signal Decay in Spectroscopic Imaging (RAPID-SI) using blipped phase encoding gradients inserted during signal acquisition was developed. To validate the results using RAPID-SI and to demonstrate its usefulness in terms of acquisition time and data quantification; simulations, phantom and in vivo studies were conducted, and the results were compared to standard CSI. The method was based upon the partition of a magnetic resonance spectroscopy (MRS) signal into sequential sub-signals encoded using blipped phase encoding gradients inserted during signal acquisition at a constant time interval. The RAPID-SI technique was implemented on a clinical 3 T Siemens scanner to demonstrate its clinical utility. Acceleration of data collection was performed by inserting R (R= acceleration factor) blipped gradients along a given spatial direction during data acquisition. Compared to CSI, RAPID-SI reduced acquisition time by the acceleration factor R. For example, a 2D 16x16 data set acquired in about 17 min with CSI, was reduced to approximately 2 min with the RAPID-SI (R= 8). While the SNR of the acquired RAPID-SI signal was lower compared to CSI by approximately the factor root R, it can be improved after data pre-processing and reconstruction. Compared to CSI, RAPID-SI reduces acquisition time, while preserving metabolites information. Furthermore, the method is flexible and could be combined with other acceleration methods such as Parallel Imaging
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Rapidly expanding lake heatwaves under climate change
Lake heatwaves – prolonged periods of hot surface water temperature in lakes – have recently been shown to increase in intensity and duration, with numerous potential implications for aquatic ecosystems. However, an important physical attribute of lake heatwaves that has not yet been investigated is their spatial extent, and how it varies within a warming world. Here, we show that the spatial extent of lake heatwaves, defined as contiguous regions within a lake that simultaneously experience extreme warm conditions, is increasing in the largest group of freshwater lakes on Earth, The Laurentian Great Lakes. We show that the maximum spatial extent of lake heatwaves is sensitive to inter-annual variations in winter ice cover and the timing of stratification onset in spring. Notably, we find that a lengthening of the warm summer season and, in turn, an overall increase in surface water temperature, stimulates the development of larger lake heatwaves. On average, our results suggest that the mean spatial extent of lake heatwaves has increased two-fold since 1995. We anticipate this rapid expansion of lake heatwaves to have widespread implications for heat-related impacts on aquatic species
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