5,752 research outputs found
The 3D structure of the Lagrangian acceleration in turbulent flows
We report experimental results on the three dimensional Lagrangian
acceleration in highly turbulent flows. Tracer particles are tracked optically
using four silicon strip detectors from high energy physics that provide high
temporal and spatial resolution. The components of the acceleration are shown
to be statistically dependent. The probability density function (PDF) of the
acceleration magnitude is comparable to a log-normal distribution. Assuming
isotropy, a log-normal distribution of the magnitude can account for the
observed dependency of the components. The time dynamics of the acceleration
components is found to be typical of the dissipation scales whereas the
magnitude evolves over longer times, possibly close to the integral time scale.Comment: accepted for publication in Physical Review Letter
Virtual Topology Design for Minimizing Network Diameter and Average Hop Count in WDM Networks
We design virtual topologies in wavelength division multiplexing (WDM) networks to minimize the network diameter and average hop count, where network diameter refers to the number of hops of the longest shortest path and average hop count is the average number of hops among the shortest paths of all node pairs. Such objectives are important to WDM networks, especially to those with statistical multiplexing mechanisms such as optical burst switching (OBS) and optical packet switching (OPS). By minimizing the network diameter and average hop count, optical packets or bursts will experience less contention loss and smaller delay due to a reduced number of intermediate nodes en route. In this paper, we first formulate an integer linear program (ILP) for optimal design of virtual topologies with minimized network diameter and average hop count. Then, a novel heuristic least weight minimum diameter (LWMD) is proposed to find good solutions efficiently. Based on the virtual topology obtained, we further design two traffic accommodation schemes to provision wavelengths under a given traffic matrix, with guaranteed network diameter and minimized network resource consumption.published_or_final_versio
Derivative moments in turbulent shear flows
We propose a generalized perspective on the behavior of high-order derivative
moments in turbulent shear flows by taking account of the roles of small-scale
intermittency and mean shear, in addition to the Reynolds number. Two
asymptotic regimes are discussed with respect to shear effects. By these means,
some existing disagreements on the Reynolds number dependence of derivative
moments can be explained. That odd-order moments of transverse velocity
derivatives tend not vanish as expected from elementary scaling considerations
does not necessarily imply that small-scale anisotropy persists at all Reynolds
numbers.Comment: 11 pages, 7 Postscript figure
Measurement of Lagrangian velocity in fully developed turbulence
We have developed a new experimental technique to measure the Lagrangian
velocity of tracer particles in a turbulent flow, based on ultrasonic Doppler
tracking. This method yields a direct access to the velocity of a single
particule at a turbulent Reynolds number . Its dynamics is
analyzed with two decades of time resolution, below the Lagrangian correlation
time. We observe that the Lagrangian velocity spectrum has a Lorentzian form
, in agreement
with a Kolmogorov-like scaling in the inertial range. The probability density
function (PDF) of the velocity time increments displays a change of shape from
quasi-Gaussian a integral time scale to stretched exponential tails at the
smallest time increments. This intermittency, when measured from relative
scaling exponents of structure functions, is more pronounced than in the
Eulerian framework.Comment: 4 pages, 5 figures. to appear in PR
Optimal Location of Sources in Transportation Networks
We consider the problem of optimizing the locations of source nodes in
transportation networks. A reduction of the fraction of surplus nodes induces a
glassy transition. In contrast to most constraint satisfaction problems
involving discrete variables, our problem involves continuous variables which
lead to cavity fields in the form of functions. The one-step replica symmetry
breaking (1RSB) solution involves solving a stable distribution of functionals,
which is in general infeasible. In this paper, we obtain small closed sets of
functional cavity fields and demonstrate how functional recursions are
converted to simple recursions of probabilities, which make the 1RSB solution
feasible. The physical results in the replica symmetric (RS) and the 1RSB
frameworks are thus derived and the stability of the RS and 1RSB solutions are
examined.Comment: 38 pages, 18 figure
Fast Domain Growth through Density-Dependent Diffusion in a Driven Lattice Gas
We study electromigration in a driven diffusive lattice gas (DDLG) whose
continuous Monte Carlo dynamics generate higher particle mobility in areas with
lower particle density. At low vacancy concentrations and low temperatures,
vacancy domains tend to be faceted: the external driving force causes large
domains to move much more quickly than small ones, producing exponential domain
growth. At higher vacancy concentrations and temperatures, even small domains
have rough boundaries: velocity differences between domains are smaller, and
modest simulation times produce an average domain length scale which roughly
follows , where varies from near .55 at 50% filling
to near .75 at 70% filling. This growth is faster than the behavior
of a standard conserved order parameter Ising model. Some runs may be
approaching a scaling regime. At low fields and early times, fast growth is
delayed until the characteristic domain size reaches a crossover length which
follows . Rough numerical estimates give and simple theoretical arguments give . Our conclusion that
small driving forces can significantly enhance coarsening may be relevant to
the YBCuO electromigration experiments of Moeckly {\it et
al.}(Appl. Phys. Let., {\bf 64}, 1427 (1994)).Comment: 18 pages, RevTex3.
Improved Lagrangian mixing models for passive scalars in isotropic turbulence
Lagrangian data for velocity, scalars, and energy and scalar dissipation from direct numerical simulations are used to validate Lagrangian mixing models for inert passive scalars in stationary isotropic turbulence. The scalar fluctuations are nearly Gaussian, and, as a result of production by uniform mean gradients, statistically stationary. Comparisons are made for Taylor-scale Reynolds numbers in the range 38 to about 240 and Schmidt numbers in the range 1/8 to 1. Model predictions for one-point, one-time Eulerian statistics ~Eulerian correspondence! and one-particle, two-time Lagrangian statistics ~Lagrangian correspondence! are examined. Two scalar mixing models, namely the Lagrangian Fokker–Planck model and the Lagrangian colored-noise ~LCN! model, are proposed and written in terms of stochastic differential equations ~SDE! with specified drift and diffusion terms. Both of these models rely on statistics of the scalar field conditioned upon the energy dissipation, as provided by the Lagrangian spectral relaxation ~LSR! model. With the exception of the scalar dissipation, the models are shown to capture the Reynolds and Schmidt-number dependence of the Lagrangian integral time scales. However, the LCN model provides a more realistic description of the Lagrangian scalar fluctuations as differentiable time series having the correct form of the scalar autocorrelation function. Further extensions of the new mixing models to non-Gaussian scalars are conceptually straightforward, but require a closure for the scalar-conditioned scalar dissipation rate matrix. Likewise, accurate prediction of joint statistics for differential diffusion between different scalars with unequal molecular diffusivities will require the formulation of a multiscale SDE similar to the LSR model
Evaluation of Noise Generated by Train Movement at Rail Near to Residential Areas – Case Study at Kluang, Johor.
Rail transportation is one of the largest transportations that is widely used in Malaysia. Therefore, many railways have been built in urban and rural areas to improve interstate connectivity in Malaysia. However, train noise generated from train movement can disturb the comfort of nearby residents that live near railways. This study was conducted nearby the residential area between the Kluang – Renggam rail line to investigate the noise level and compare the existing noise level with the guideline from the Department of Environment (DOE) and World Health Organization (WHO) limits. The measurement data was taken by using the Sound Level Meter (SLM) type of SLM-25 at three different locations with distance of 20 m, 30 m, and 55 m away from the train track during morning, noon, and evening. The result shows the maximum value is 102.6 dBA during the evening at point 1 with a distance of 20 m. The residential area at point 1 which is 20 m distance between rail track and point 2 with the distance of 30 m from the rail track was indicated exposed to train noise disturbance whereby the measurement data for each location was exceeded the permissible noise for DOE and WHO which is 60 dBA and 54 dBA. The result can show that the noise level was dependent on the distance of the receiver to the train track. A better method such as developed noise mapping and the questionnaire of annoyance level for the residential area can improve the findings on the effect of train noise in the future
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