539 research outputs found
Viscous Effects on Elliptic Flow and Shock Waves
Fast thermalization and a strong buildup of elliptic flow of QCD matter as
found at RHIC are understood as the consequence of perturbative QCD (pQCD)
interactions within the 3+1 dimensional parton cascade BAMPS. The main
contributions stem from pQCD bremsstrahlung processes.
By comparing to Au+Au data of the flow parameter as a function of
participation number the shear viscosity to entropy ratio is dynamically
extracted, which lies in the range of 0.08 and 0.2, depending on the chosen
coupling constant and freeze out condition. Furthermore, first simulations on
the temporal propagation of dissipative shock waves are given. The cascade can
either simulate true ideal shocks as well as initially diluted, truely viscous
shocks, depending on the employed cross sections or mean free path,
respectively.Comment: 7 pages, 8 figures, to appear in the proceedings of the 2008 Erice
School on Nuclear Physics, Sicil
Optimized Hidden Markov Model based on Constrained Particle Swarm Optimization
International audienceAs one of Bayesian analysis tools, Hidden Markov Model (HMM) has been used to in extensive applications. Most HMMs are solved by Baum-Welch algorithm (BWHMM) to predict the model parameters, which is difficult to find global optimal solutions. This paper proposes an optimized Hidden Markov Model with Particle Swarm Optimization (PSO) algorithm and so is called PSOHMM. In order to overcome the statistical constraints in HMM, the paper develops re-normalization and re-mapping mechanisms to ensure the constraints in HMM. The experiments have shown that PSOHMM can search better solution than BWHMM, and has faster convergence speed
Response to the discussion on “An improved k – ω turbulence model for the simulations of the wind turbine wakes in a neutral atmospheric boundary layer flow” by Y Yang
Investigation of shock waves in the relativistic Riemann problem: A comparison of viscous fluid dynamics to kinetic theory
We solve the relativistic Riemann problem in viscous matter using the
relativistic Boltzmann equation and the relativistic causal dissipative
fluid-dynamical approach of Israel and Stewart. Comparisons between these two
approaches clarify and point out the regime of validity of second-order fluid
dynamics in relativistic shock phenomena. The transition from ideal to viscous
shocks is demonstrated by varying the shear viscosity to entropy density ratio
. We also find that a good agreement between these two approaches
requires a Knudsen number .Comment: Version as published in PRC 82, 024910 (2010); 16 pages, 16 figures,
typos correcte
Relativistic shock waves in viscous gluon matter
We solve the relativistic Riemann problem in viscous gluon matter employing a
microscopic parton cascade. We demonstrate the transition from ideal to viscous
shock waves by varying the shear viscosity to entropy density ratio
from zero to infinity. We show that an ratio larger than 0.2 prevents
the development of well-defined shock waves on timescales typical for
ultrarelativistic heavy-ion collisions. Comparisons with viscous hydrodynamic
calculations confirm our findings.Comment: Version as published in PRL 103, 032301 (2009). 4 pages, 4 figure
Mach Cones in Viscous Matter
Employing a microscopic transport model we investigate the evolution of high
energetic jets moving through a viscous medium. For the scenario of an
unstoppable jet we observe a clearly strong collective behavior for a low
dissipative system , leading to the observation of
cone-like structures. Increasing the dissipation of the system to the Mach Cone structure vanishes. Furthermore, we investigate
jet-associated particle correlations. A double-peak structure, as observed in
experimental data, is even for low-dissipative systems not supported, because
of the large influence of the head shock.Comment: 4 pages, 3 figures, to appear in the conference proceedings of Hot
Quarks 201
Calculation of shear viscosity using Green-Kubo relations within a parton cascade
The shear viscosity of a gluon gas is calculated using the Green-Kubo
relation. Time correlations of the energy-momentum tensor in thermal
equilibrium are extracted from microscopic simulations using a parton cascade
solving various Boltzmann collision processes. We find that the pQCD based
gluon bremsstrahlung described by Gunion-Bertsch processes significantly lowers
the shear viscosity by a factor of 3-8 compared to elastic scatterings. The
shear viscosity scales with the coupling as 1/(alpha_s^2\log(1/alpha_s)). For a
constant coupling constant the shear viscosity to entropy density ratio has no
dependence on temperature. Replacing the pQCD-based collision angle
distribution of binary scatterings by an isotropic form decreases the shear
viscosity by a factor of 3.Comment: 17 pages, 5 figure
Extraction of shear viscosity in stationary states of relativistic particle systems
Starting from a classical picture of shear viscosity we construct a
stationary velocity gradient in a microscopic parton cascade. Employing the
Navier-Stokes ansatz we extract the shear viscosity coefficient . For
elastic isotropic scatterings we find an excellent agreement with the analytic
values. This confirms the applicability of this method. Furthermore for both
elastic and inelastic scatterings with pQCD based cross sections we extract the
shear viscosity coefficient for a pure gluonic system and find a good
agreement with already published calculations.Comment: 17 pages, 7 figure
Web Service Discovery in a Semantically Extended UDDI Registry: the Case of FUSION
Service-oriented computing is being adopted at an unprecedented rate, making the effectiveness of automated service discovery an increasingly important challenge. UDDI has emerged as a de facto industry standard and fundamental building block within SOA infrastructures. Nevertheless, conventional UDDI registries lack means to provide unambiguous, semantically rich representations of Web service capabilities, and the logic inference power required for facilitating automated service discovery. To overcome this important limitation, a number of approaches have been proposed towards augmenting Web service discovery with semantics. This paper discusses the benefits of semantically extending Web service descriptions and UDDI registries, and presents an overview of the approach put forward in project FUSION, towards semantically-enhanced publication and discovery of services based on SAWSDL
An improved k –ω turbulence model for the simulations of the wind turbine wakes in a neutral atmospheric boundary layer flow
Correct prediction of the recovery of wind turbine wakes in terms of the wind velocity and turbulence downstream of the turbine is of paramount importance for the accurate simulations of turbine interactions, overall wind farm energy output and the impact to the facilities downstream of the wind farm. Conventional turbulence models often result in an unrealistic recovery of the wind velocity and turbulence downstream of the turbine. In this paper, a modified k – ω turbulence model has been proposed together with conditions for achieving a zero streamwise gradient for all the fluid flow variables in neutral atmospheric flows. The new model has been implemented in the simulation of the wakes of two different wind turbines and the commonly used actuator disk model has been employed to represent the turbine rotors. The model has been tested for different wind speeds and turbulence levels. The comparison of the computational results shows good agreement with the available experimental data, in both near and far wake regions for all the modeled wind turbines. A zero streamwise gradient has been maintained in the far wake region in terms of both wind speed and turbulence quantities
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