10,649 research outputs found
Stationary shocks in periodic highly nonlinear granular chains
We study the existence of stationary shock waves in uniform and periodic heterogeneous highly nonlinear granular chains governed by a power-law contact interaction, comparing discrete and continuum approaches, as well as experiments. We report the presence of quasisteady shock fronts without the need for dissipative effects. When viscous effects are neglected, the structure of the leading front appears to be solely the result of dispersive effects related to the lattice wave dispersion and, for heterogeneous bead chains, to the impedance mismatch between material domains. We report analytically and numerically the shock-width scaling with the variation in the particles periodicity (cell size) and compare the obtained results with experiments. We check the state (â) behind the shock front via quasistatic compression analysis and report a very good agreement between theory and numerical data
Plastic Response of a 2D Amorphous Solid to Quasi-Static Shear : II - Dynamical Noise and Avalanches in a Mean Field Model
We build a minimal, mean-field, model of plasticity of amorphous solids,
based upon a phenomenology of dissipative events derived, in a preceding paper
[A. Lemaitre, C. Caroli, arXiv:0705.0823] from extensive molecular simulations.
It reduces to the dynamics of an ensemble of identical shear transformation
zones interacting via the dynamic noise due to the long ranged elastic fields
induced by zone flips themselves. We find that these ingredients are sufficient
to generate flip avalanches with a power-law scaling with system size,
analogous to that observed in molecular simulations. We further show that the
scaling properties of avalanches sensitively depend on the detailed shape of
the noise spectrum. This points out the importance of developing a realistic
coarse-grained description of elasticity in these systems
Separation of timescales in a two-layered network
We investigate a computer network consisting of two layers occurring in, for
example, application servers. The first layer incorporates the arrival of jobs
at a network of multi-server nodes, which we model as a many-server Jackson
network. At the second layer, active servers at these nodes act now as
customers who are served by a common CPU. Our main result shows a separation of
time scales in heavy traffic: the main source of randomness occurs at the
(aggregate) CPU layer; the interactions between different types of nodes at the
other layer is shown to converge to a fixed point at a faster time scale; this
also yields a state-space collapse property. Apart from these fundamental
insights, we also obtain an explicit approximation for the joint law of the
number of jobs in the system, which is provably accurate for heavily loaded
systems and performs numerically well for moderately loaded systems. The
obtained results for the model under consideration can be applied to
thread-pool dimensioning in application servers, while the technique seems
applicable to other layered systems too.Comment: 8 pages, 2 figures, 1 table, ITC 24 (2012
EUROPEAN CONFERENCE ON QUEUEING THEORY 2016
International audienceThis booklet contains the proceedings of the second European Conference in Queueing Theory (ECQT) that was held from the 18th to the 20th of July 2016 at the engineering school ENSEEIHT, Toulouse, France. ECQT is a biannual event where scientists and technicians in queueing theory and related areas get together to promote research, encourage interaction and exchange ideas. The spirit of the conference is to be a queueing event organized from within Europe, but open to participants from all over the world. The technical program of the 2016 edition consisted of 112 presentations organized in 29 sessions covering all trends in queueing theory, including the development of the theory, methodology advances, computational aspects and applications. Another exciting feature of ECQT2016 was the institution of the TakĂĄcs Award for outstanding PhD thesis on "Queueing Theory and its Applications"
Experiments on the dynamic behavior of cavitating pumps
This paper describes experiments performed to measure the dynamic transfer matrices for cavitating (and noncavitating) pumps. These transfer matrices describe the relationship between small linear oscillatory perturbations in the pressures and mass flow rates at inlet and discharge from the hydraulic machine. The matrices were deduced from direct measurements of these fluctuating quantities for different modes of excitation of the machine. Results for a cavitating inducer are presented as functions of frequency and mean operating state. Though some of the trends in the data are consistent with existing theoretical models of inducer dynamics, others are not, indicating a need for further theoretical investigation of the dynamic characteristics of such flows. The results exhibit increasingly complex dynamics with increasing cavitation; it appears that the hydraulic machine deviates from an essentially passive response without cavitation to an increasingly active response as the cavitation number is reduced
Computations of unsteady transonic flow governed by the conservative full potential equation using an alternating direction implicit algorithm
A development was the time linearization of the density function. This linearization reduces the solution process from solving just a single equation. Two sample cases were computed. First, a one dimensional traveling shock wave was computed and compared with the analytic solution. Second, a two dimensional case was calculated for a flow field which resulted from a thickening and subsequently, thinning airfoil. The resulting flow field, which included a traveling shock wave, was compared to the flow field obtained from the low frequency, small disturbance, transonic equation
DSMC-LBM mapping scheme for rarefied and non-rarefied gas flows
We present the formulation of a kinetic mapping scheme between the Direct
Simulation Monte Carlo (DSMC) and the Lattice Boltzmann Method (LBM) which is
at the basis of the hybrid model used to couple the two methods in view of
efficiently and accurately simulate isothermal flows characterized by variable
rarefaction effects. Owing to the kinetic nature of the LBM, the procedure we
propose ensures to accurately couple DSMC and LBM at a larger Kn number than
usually done in traditional hybrid DSMC-Navier-Stokes equation models. We show
the main steps of the mapping algorithm and illustrate details of the
implementation. Good agreement is found between the moments of the single
particle distribution function as obtained from the mapping scheme and from
independent LBM or DSMC simulations at the grid nodes where the coupling is
imposed. We also show results on the application of the hybrid scheme based on
a simpler mapping scheme for plane Poiseuille flow at finite Kn number.
Potential gains in the computational efficiency assured by the application of
the coupling scheme are estimated for the same flow.Comment: Submitted to Journal of Computational Scienc
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