1,978 research outputs found
Mesoscopic two-phase model for describing apparent slip in micro-channel flows
The phenomenon of apparent slip in micro-channel flows is analyzed by means
of a two-phase mesoscopic lattice Boltzmann model including non-ideal
fluid-fluid and fluid-wall interactins. The weakly-inhomogeneous limit of this
model is solved analytically.
The present mesoscopic approach permits to access much larger scales than
molecular dynamics, and comparable with those attained by continuum methods.
However, at variance with the continuum approach, the existence of a gas layer
near the wall does not need to be postulated a priori, but emerges naturally
from the underlying non-ideal mesoscopic dynamics. It is therefore argued that
a mesoscopic Lattice Boltzmann approach with non-ideal fluid-fluid and
fluid-wall interactions might achieve an optimal compromise between physical
realism and computational efficiency for the study of channel micro-flows.Comment: 5 pages, 3 figure
High cycle fatigue of ARMCO iron severely deformed by ECAP
The high-cycle fatigue behavior of ARMCO iron severely deformed by Equal Channel Angular Pressing (ECAP) at room temperature through route Bc until 8 passes, with an average grain size of ~365 nm, was studied and compared with the same material in the annealed state with an average grain size of ~72 µm. The fatigue limit of the 8 passes ECAPed sample increased with respect to the annealed material by more than 250% rising from 274 MPa to 717 MPa. Striations and dimpled relief were observed on the fracture surfaces of the fatigued ultrafine and coarse grain fatigue samples. The microstructure was characterized by Electron Backscattered Diffraction (EBSD) before and after the fatigue tests and it was observed in both samples an increment in the fraction of Low Angle Grain Boundaries (LAGB) at high number of cycles to failure. A texture analysis for the materials after the fatigue failure was done. This study shown a preferential orientation towards the ¿ fiber for both conditions.Peer ReviewedPostprint (author's final draft
Optimal streaks in a Falkner-Skan boundary layer
This paper deals with the optimal streaky perturbations (which maximize the
perturbed energy growth) in a wedge flow boundary layer. These three
dimensional perturbations are governed by a system of linearized boundary layer
equations around the Falkner-Skan base flow. Based on an asymptotic analysis of
this system near the free stream and the leading edge singularity, we show that
for acute wedge semi-angle, all solutions converge after a streamwise transient
to a single streamwise-growing solution of the linearized equations, whose
initial condition near the leading edge is given by an eigenvalue problem first
formulated in this context by Tumin (2001). Such a solution may be regarded as
a streamwise evolving most unstable streaky mode, in analogy with the usual
eigenmodes in strictly parallel flows, and shows an approximate
self-similarity, which was partially known and is completed in this paper. An
important consequence of this result is that the optimization procedure based
on the adjoint equations heretofore used to define optimal streaks is not
necessary. Instead, a simple low-dimensional optimization process is proposed
and used to obtain optimal streaks. Comparison with previous results by Tumin
and Ashpis (2003) shows an excellent agreement. The unstable streaky mode
exhibits transient growth if the wedge semi-angle is smaller than a critical
value that is slightly larger than , and decays otherwise. Thus the
cases of right and obtuse wedge semi-angles exhibit less practical interest,
but they show a qualitatively different behavior, which is briefly described to
complete the analysis
Matching pursuit-based compressive sensing in a wearable biomedical accelerometer fall diagnosis device
There is a significant high fall risk population, where individuals are susceptible to frequent falls and obtaining significant injury, where quick medical response and fall information are critical to providing efficient aid. This article presents an evaluation of compressive sensing techniques in an accelerometer-based intelligent fall detection system modelled on a wearable Shimmer biomedical embedded computing device with Matlab. The presented fall detection system utilises a database of fall and activities of daily living signals evaluated with discrete wavelet transforms and principal component analysis to obtain binary tree classifiers for fall evaluation. 14 test subjects undertook various fall and activities of daily living experiments with a Shimmer device to generate data for principal component analysis-based fall classifiers and evaluate the proposed fall analysis system. The presented system obtains highly accurate fall detection results, demonstrating significant advantages in comparison with the thresholding method presented. Additionally, the presented approach offers advantageous fall diagnostic information. Furthermore, transmitted data accounts for over 80% battery current usage of the Shimmer device, hence it is critical the acceleration data is reduced to increase transmission efficiency and in-turn improve battery usage performance. Various Matching pursuit-based compressive sensing techniques have been utilised to significantly reduce acceleration information required for transmission.Scopu
A note on the lattice Boltzmann method beyond the Chapman Enskog limits
A non-perturbative analysis of the Bhatnagar-Gross-Krook (BGK) model kinetic
equation for finite values of the Knudsen number is presented. This analysis
indicates why discrete kinetic versions of the BGK equation, and notably the
Lattice Boltzmann method, can provide semi-quantitative results also in the
non-hydrodynamic, finite-Knudsen regime, up to . This may
help the interpretation of recent Lattice Boltzmann simulations of microflows,
which show satisfactory agreement with continuum kinetic theory in the
moderate-Knudsen regime.Comment: 7 PAGES, 1 FIGUR
Multi-particle-collision dynamics: Flow around a circular and a square cylinder
A particle-based model for mesoscopic fluid dynamics is used to simulate
steady and unsteady flows around a circular and a square cylinder in a
two-dimensional channel for a range of Reynolds number between 10 and 130.
Numerical results for the recirculation length, the drag coefficient, and the
Strouhal number are reported and compared with previous experimental
measurements and computational fluid dynamics data. The good agreement
demonstrates the potential of this method for the investigation of complex
flows.Comment: 6 pages, separated figures in .jpg format, to be published in
Europhysics Letter
Investigation of a lattice Boltzmann model with a variable speed of sound
A lattice Boltzmann model is considered in which the speed of sound can be
varied independently of the other parameters. The range over which the speed of
sound can be varied is investigated and good agreement is found between
simulations and theory. The onset of nonlinear effects due to variations in the
speed of sound is also investigated and good agreement is again found with
theory. It is also shown that the fluid viscosity is not altered by changing
the speed of sound
High speed single photon detection in the near-infrared
InGaAs avalanche photodiodes (APDs) are convenient for single photon
detection in the near-infrared (NIR) including the fibre communication bands
(1.31/1.55 m). However, to suppress afterpulse noise due to trapped
avalanche charge, they must be gated with MHz repetition frequencies, thereby
severely limiting the count rate in NIR applications. Here we show gating
frequencies for InGaAs-APDs well beyond 1 GHz. Using a self-differencing
technique to sense much weaker avalanches, we reduce drastically afterpulse
noise. At 1.25 GHz, we obtain a detection efficiency of 10.8% with an
afterpulse probability of 6.16%. In addition, the detector features low jitter
(55 ps) and a count rate of 100 MHz
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