15,067 research outputs found
WPU-Net: Boundary Learning by Using Weighted Propagation in Convolution Network
Deep learning has driven a great progress in natural and biological image
processing. However, in material science and engineering, there are often some
flaws and indistinctions in material microscopic images induced from complex
sample preparation, even due to the material itself, hindering the detection of
target objects. In this work, we propose WPU-net that redesigns the
architecture and weighted loss of U-Net, which forces the network to integrate
information from adjacent slices and pays more attention to the topology in
boundary detection task. Then, the WPU-net is applied into a typical material
example, i.e., the grain boundary detection of polycrystalline material.
Experiments demonstrate that the proposed method achieves promising performance
and outperforms state-of-the-art methods. Besides, we propose a new method for
object tracking between adjacent slices, which can effectively reconstruct 3D
structure of the whole material. Finally, we present a material microscopic
image dataset with the goal of advancing the state-of-the-art in image
processing for material science.Comment: technical repor
High-frequency ultrasonic speckle velocimetry in sheared complex fluids
High-frequency ultrasonic pulses at 36 MHz are used to measure velocity
profiles in a complex fluid sheared in the Couette geometry. Our technique is
based on time-domain cross-correlation of ultrasonic speckle signals
backscattered by the moving medium. Post-processing of acoustic data allows us
to record a velocity profile in 0.02--2 s with a spatial resolution of 40
m over 1 mm. After a careful calibration using a Newtonian suspension, the
technique is applied to a sheared lyotropic lamellar phase seeded with
polystyrene spheres of diameter 3--10 m. Time-averaged velocity profiles
reveal the existence of inhomogeneous flows, with both wall slip and shear
bands, in the vicinity of a shear-induced ``layering'' transition. Slow
transient regimes and/or temporal fluctuations can also be resolved and exhibit
complex spatio-temporal flow behaviors with sometimes more than two shear
bands.Comment: 15 pages, 18 figures, submitted to Eur. Phys. J. A
The evolution of energy in flow driven by rising bubbles
We investigate by direct numerical simulations the flow that rising bubbles
cause in an originally quiescent fluid. We employ the Eulerian-Lagrangian
method with two-way coupling and periodic boundary conditions. In order to be
able to treat up to 288000 bubbles, the following approximations and
simplifications had to be introduced: (i) The bubbles were treated as
point-particles, thus (ii) disregarding the near-field interactions among them,
and (iii) effective force models for the lift and the drag forces were used. In
particular, the lift coefficient was assumed to be 1/2, independent of the
bubble Reynolds number and the local flow field. The results suggest that large
scale motions are generated, owing to an inverse energy cascade from the small
to the large scales. However, as the Taylor-Reynolds number is only in the
range of 1, the corresponding scaling of the energy spectrum with an exponent
of -5/3 cannot develop over a pronounced range. In the long term, the property
of local energy transfer, characteristic of real turbulence, is lost and the
input of energy equals the viscous dissipation at all scales. Due to the lack
of strong vortices the bubbles spread rather uniformly in the flow. The
mechanism for uniform spreading is as follows: Rising bubbles induce a velocity
field behind them that acts on the following bubbles. Owing to the shear, those
bubbles experience a lift force which make them spread to the left or right,
thus preventing the formation of vertical bubble clusters and therefore of
efficient forcing. Indeed, when the lift is artifically put to zero in the
simulations, the flow is forced much more efficiently and a more pronounced
energy accumulates at large scales is achieved.Comment: 9 pages, 7 figure
Probing white-matter microstructure with higher-order diffusion tensors and susceptibility tensor MRI.
Diffusion MRI has become an invaluable tool for studying white matter microstructure and brain connectivity. The emergence of quantitative susceptibility mapping and susceptibility tensor imaging (STI) has provided another unique tool for assessing the structure of white matter. In the highly ordered white matter structure, diffusion MRI measures hindered water mobility induced by various tissue and cell membranes, while susceptibility sensitizes to the molecular composition and axonal arrangement. Integrating these two methods may produce new insights into the complex physiology of white matter. In this study, we investigated the relationship between diffusion and magnetic susceptibility in the white matter. Experiments were conducted on phantoms and human brains in vivo. Diffusion properties were quantified with the diffusion tensor model and also with the higher order tensor model based on the cumulant expansion. Frequency shift and susceptibility tensor were measured with quantitative susceptibility mapping and susceptibility tensor imaging. These diffusion and susceptibility quantities were compared and correlated in regions of single fiber bundles and regions of multiple fiber orientations. Relationships were established with similarities and differences identified. It is believed that diffusion MRI and susceptibility MRI provide complementary information of the microstructure of white matter. Together, they allow a more complete assessment of healthy and diseased brains
Fully coupled simulations of non-colloidal monodisperse sheared suspensions
In this work we investigate numerically the dynamics of sheared suspensions in the limit of vanishingly small fluid and particle inertia. The numerical model we used is able to handle the multi-body hydrodynamic interactions between thousands of particles embedded in a linear shear flow. The presence of the particles is modeled by momentum source terms spread out on a spherical envelop forcing the Stokes equations of the creeping flow. Therefore all the velocity perturbations induced by the moving particles are simultaneously accounted for.
The statistical properties of the sheared suspensions are related to the velocity fluctuation of the particles. We formed averages for the resulting velocity fluctuation and rotation rate tensors. We found that the latter are highly anisotropic and that all the velocity fluctuation terms grow linearly with particle volume fraction. Only one off-diagonal term is found to be non zero (clearly related to trajectory symmetry breaking induced by the non-hydrodynamic repulsion force). We also found a strong correlation of positive/negative velocities in the shear plane, on a time scale controlled by the shear rate (direct interaction of two particles). The time scale required to restore uncorrelated velocity fluctuations decreases continuously as the concentration increases. We calculated the shear induced self-diffusion coefficients using two different methods and the resulting diffusion tensor appears to be anisotropic too.
The microstructure of the suspension is found to be drastically modified by particle interactions. First the probability density function of velocity fluctuations showed a transition from exponential to Gaussian behavior as particle concentration varies. Second the probability of finding close pairs while the particles move under shear flow is strongly enhanced by hydrodynamic interactions when the concentration increases
A Neural Model of How the Brain Represents and Compares Multi-Digit Numbers: Spatial and Categorical Processes
Both animals and humans are capable of representing and comparing numerical quantities, but only humans seem to have evolved multi-digit place-value number systems. This article develops a neural model, called the Spatial Number Network, or SpaN model, which predicts how these shared numerical capabilities are computed using a spatial representation of number quantities in the Where cortical processing stream, notably the Inferior Parietal Cortex. Multi-digit numerical representations that obey a place-value principle are proposed to arise through learned interactions between categorical language representations in the What cortical processing stream and the Where spatial representation. It is proposed that learned semantic categories that symbolize separate digits, as well as place markers like "tens," "hundreds," "thousands," etc., are associated through learning with the corresponding spatial locations of the Where representation, leading to a place-value number system as an emergent property of What-Where information fusion. The model quantitatively simulates error rates in quantification and numerical comparison tasks, and reaction times for number priming and numerical assessment and comparison tasks. In the Where cortical process, it is proposed that transient responses to inputs are integrated before they activate an ordered spatial map that selectively responds to the number of events in a sequence. Neural mechanisms are defined which give rise to an ordered spatial numerical map ordering and Weber law characteristics as emergent properties. The dynamics of numerical comparison are encoded in activity pattern changes within this spatial map. Such changes cause a "directional comparison wave" whose properties mimic data about numerical comparison. These model mechanisms are variants of neural mechanisms that have elsewhere been used to explain data about motion perception, attention shifts, and target tracking. Thus, the present model suggests how numerical representations may have emerged as specializations of more primitive mechanisms in the cortical Where processing stream. The model's What-Where interactions can explain human psychophysical data, such as error rates and reaction times, about multi-digit (base 10) numerical stimuli, and describe how such a competence can develop through learning. The SpaN model and its explanatory range arc compared with other models of numerical representation.Defense Advanced Research Projects Agency and the Office of Naval Research (N00014-95-1-0409); National Science Foundation (IRI-97-20333
A Neural Model of How The Brain Represents and Compares Numbers
Many psychophysical experiments have shown that the representation of numbers and numerical quantities in humans and animals is related to number magnitude. A neural network model is proposed to quantitatively simulate error rates in quantification and numerical comparison tasks, and reaction times for number priming and numerical assessment and comparison tasks. Transient responses to inputs arc integrated before they activate an ordered spatial map that selectively responds to the number of events in a sequence. The dynamics of numerical comparison are encoded in activity pattern changes within this spatial map. Such changes cause a "directional comparison wave" whose properties mimic data about numerical comparison. These model mechanisms are variants of neural mechanisms that have elsewhere been used to explain data about motion perception, attention shifts, and target tracking. Thus, the present model suggests how numerical representations may have emerged as specializations of more primitive mechanisms in the cortical Where processing stream.National Science Foundation (IRI-97-20333); Defense Advanced research Projects Agency and the Office of Naval Research (N00014-95-1-0409); National Institute of Health (1-R29-DC02952-01
Spherically symmetric solutions to a model for phase transitions driven by configurational forces
We prove the global in time existence of spherically symmetric solutions to
an initial-boundary value problem for a system of partial differential
equations, which consists of the equations of linear elasticity and a
nonlinear, non-uniformly parabolic equation of second order. The problem models
the behavior in time of materials in which martensitic phase transitions,
driven by configurational forces, take place, and can be considered to be a
regularization of the corresponding sharp interface model. By assuming that the
solutions are spherically symmetric, we reduce the original multidimensional
problem to the one in one space dimension, then prove the existence of
spherically symmetric solutions. Our proof is valid due to the essential
feature that the reduced problem is one space dimensional.Comment: 25 page
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