7,079 research outputs found
Universal Scaling Laws for Dense Particle Suspensions in Turbulent Wall-Bounded Flows
The macroscopic behavior of dense suspensions of neutrally-buoyant spheres in
turbulent plane channel flow is examined. We show that particles larger than
the smallest turbulence scales cause the suspension to deviate from the
continuum limit in which its dynamics is well described by an effective
suspension viscosity. This deviation is caused by the formation of a particle
layer close to the wall with significant slip velocity. By assuming two
distinct transport mechanisms in the near-wall layer and the turbulence in the
bulk, we define an effective wall location such that the flow in the bulk can
still be accurately described by an effective suspension viscosity. We thus
propose scaling laws for the mean velocity profile of the suspension flow,
together with a master equation able to predict the increase in drag as
function of the particle size and volume fraction.Comment: Accepted for publication in PRL. Supplemental material include
Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions
We use interface-resolved simulations to study finite-size effects in
turbulent channel flow of neutrally-buoyant spheres. Two cases with particle
sizes differing by a factor of 2, at the same solid volume fraction of 20% and
bulk Reynolds number are considered. These are complemented with two reference
single-phase flows: the unladen case, and the flow of a Newtonian fluid with
the effective suspension viscosity of the same mixture in the laminar regime.
As recently highlighted in Costa et al. (PRL 117, 134501), a particle-wall
layer is responsible for deviations of the statistics from what is observed in
the continuum limit where the suspension is modeled as a Newtonian fluid with
an effective viscosity. Here we investigate the fluid and particle dynamics in
this layer and in the bulk. In the particle-wall layer, the near wall
inhomogeneity has an influence on the suspension micro-structure over a
distance proportional to the particle size. In this layer, particles have a
significant (apparent) slip velocity that is reflected in the distribution of
wall shear stresses. This is characterized by extreme events (both much higher
and much lower than the mean). Based on these observations we provide a scaling
for the particle-to-fluid apparent slip velocity as a function of the flow
parameters. We also extend the flow scaling laws in to second-order Eulerian
statistics in the homogeneous suspension region away from the wall. Finite-size
effects in the bulk of the channel become important for larger particles, while
negligible for lower-order statistics and smaller particles. Finally, we study
the particle dynamics along the wall-normal direction. Our results suggest that
1-point dispersion is dominated by particle-turbulence (and not
particle-particle) interactions, while differences in 2-point dispersion and
collisional dynamics are consistent with a picture of shear-driven
interactions
Understanding Mobile Search Task Relevance and User Behaviour in Context
Improvements in mobile technologies have led to a dramatic change in how and
when people access and use information, and is having a profound impact on how
users address their daily information needs. Smart phones are rapidly becoming
our main method of accessing information and are frequently used to perform
`on-the-go' search tasks. As research into information retrieval continues to
evolve, evaluating search behaviour in context is relatively new. Previous
research has studied the effects of context through either self-reported diary
studies or quantitative log analysis; however, neither approach is able to
accurately capture context of use at the time of searching. In this study, we
aim to gain a better understanding of task relevance and search behaviour via a
task-based user study (n=31) employing a bespoke Android app. The app allowed
us to accurately capture the user's context when completing tasks at different
times of the day over the period of a week. Through analysis of the collected
data, we gain a better understanding of how using smart phones on the go
impacts search behaviour, search performance and task relevance and whether or
not the actual context is an important factor.Comment: To appear in CHIIR 2019 in Glasgow, U
Imaging material properties of biological samples with a Force Feedback Microscope
Mechanical properties of biological samples have been imaged with a
\textit{Force Feedback Microscope}. Force, force gradient and dissipation are
measured simultaneously and quantitatively, merely knowing the AFM cantilever
spring constant. Our first results demonstrate that this robust method provides
quantitative high resolution force measurements of the interaction The little
oscillation imposed to the cantilever and the small value of its stiffness
result in a vibrational energy much smaller than the thermal energy, reducing
the interaction with the sample to a minimum. We show that the observed
mechanical properties of the sample depend on the force applied by the tip and
consequently on the sample indentation.
Moreover, the frequency of the excitation imposed to the cantilever can be
chosen arbitrarily, opening the way to frequency-dependent studies in
biomechanics, sort of spectroscopic AFM investigations
Model selection in hidden Markov models : a simulation study
A review of model selection procedures in hidden Markov models reveals contrasting evidence about the reliability and the precision of the most commonly used methods. In order to evaluate and compare existing proposals, we develop a Monte Carlo experiment which allows a powerful insight on the behaviour of the most widespread model selection methods. We find that the number of observations, the conditional state-dependent probabilities, and the latent transition matrix are the main factors influencing information criteria and likelihood ratio test results. We also find evidence that, for shorter univariate time series, AIC strongly outperforms BIC
Spectroscopic investigation of local mechanical impedance of living cells
The mechanical properties of PC12 living cells have been studied at the
nanoscale with a Force Feedback Microscope using two experimental approaches.
Firstly, the local mechanical impedance of the cell membrane has been mapped
simultaneously to the cell morphology at constant force. As the force of the
interaction is gradually increased, we observed the appearance of the
sub-membrane cytoskeleton. We shall compare the results obtained with this
method with the measurement of other existing techniques. Secondly, a
spectroscopic investigation has been performed varying the indentation of the
tip in the cell membrane and consequently the force applied on it. In contrast
with conventional dynamic atomic force microscopy techniques, here the small
oscillation amplitude of the tip is not necessarily imposed at the cantilever
first eigenmode. This allows the user to arbitrarily choose the excitation
frequency in developing spectroscopic AFM techniques. The mechanical response
of the PC12 cell membrane is found to be frequency dependent in the 1 kHz - 10
kHz range. The damping coefficient is reproducibly observed to decrease when
the excitation frequency is increased.Comment: 8 pages, 8 figure
Out of equilibrium anomalous elastic response of a water nano-meniscus
We report the observation of a transition in the dynamical properties of
water nano-menicus which dramatically change when probed at different time
scales. Using a AFM mode that we name Force Feedback Microscopy, we observe
this change in the simultaneous measurements, at different frequencies, of the
stiffness G'(N/m), the dissipative coefficient G''(kg/sec) together with the
static force. At low frequency we observe a negative stiffness as expected for
capillary forces. As the measuring time approaches the microsecond, the dynamic
response exhibits a transition toward a very large positive stiffness. When
evaporation and condensation gradually lose efficiency, the contact line
progressively becomes immobile. This transition is essentially controlled by
variations of Laplace pressure
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