1,700 research outputs found
The instanton method and its numerical implementation in fluid mechanics
A precise characterization of structures occurring in turbulent fluid flows
at high Reynolds numbers is one of the last open problems of classical physics.
In this review we discuss recent developments related to the application of
instanton methods to turbulence. Instantons are saddle point configurations of
the underlying path integrals. They are equivalent to minimizers of the related
Freidlin-Wentzell action and known to be able to characterize rare events in
such systems. While there is an impressive body of work concerning their
analytical description, this review focuses on the question on how to compute
these minimizers numerically. In a short introduction we present the relevant
mathematical and physical background before we discuss the stochastic Burgers
equation in detail. We present algorithms to compute instantons numerically by
an efficient solution of the corresponding Euler-Lagrange equations. A second
focus is the discussion of a recently developed numerical filtering technique
that allows to extract instantons from direct numerical simulations. In the
following we present modifications of the algorithms to make them efficient
when applied to two- or three-dimensional fluid dynamical problems. We
illustrate these ideas using the two-dimensional Burgers equation and the
three-dimensional Navier-Stokes equations
Stress Tensor and Bulk Viscosity in Relativistic Nuclear Collisions
We discuss the influence of different initial conditions for the stress
tensor and the effect of bulk viscosity on the expansion and cooling of the
fireball created in relativistic heavy-ion collisions. In particular, we
explore the evolution of longitudinal and transverse components of the pressure
and the extent of dissipative entropy production in the one-dimensional,
boost-invariant hydrodynamic model. We find that a bulk viscosity consistent
with recent estimates from lattice QCD further slows the equilibration of the
system, however it does not significantly increase the entropy produced
Lunar Rover with Multiple Science Handling Capability
A rover design study was undertaken for exploration of the Moon. Rovers that have been
launched in the past carried a suite of science payload either onboard its body or on the
robotic arm’s end. No rover has so far been launched and tasked with “carrying and
deploying” a payload on an extraterrestrial surface. This paper describes a lunar rover
designed for deploying payload as well as carrying a suite of instruments onboard for
conventional science tasks. The main consideration during the rover design process was the
usage of existing, in-house technology for development of some rover systems. The
manipulation subsystem design was derived from the technology of Light Weight Robot, a
dexterous arm originally developed for terrestrial applications. Recent efforts have led to
definition of a mission architecture for exploration of the Moon with such a rover. An outline
of its design, the manipulating arm technology and the design decisions that were made has
been presented
Instanton filtering for the stochastic Burgers equation
We address the question whether one can identify instantons in direct
numerical simulations of the stochastically driven Burgers equation. For this
purpose, we first solve the instanton equations using the Chernykh-Stepanov
method [Phys. Rev. E 64, 026306 (2001)]. These results are then compared to
direct numerical simulations by introducing a filtering technique to extract
prescribed rare events from massive data sets of realizations. Using this
approach we can extract the entire time history of the instanton evolution
which allows us to identify the different phases predicted by the direct method
of Chernykh and Stepanov with remarkable agreement
Decoherence and Entropy Production in Relativistic Nuclear Collisions
Short thermalization times of less than 1 fm/c for quark and gluon matter
have been suggested by recent experiments at the Relativistic Heavy Ion
Collider (RHIC). It has been difficult to justify this rapid thermalization in
first-principle calculations based on perturbation theory or the color glass
condensate picture. Here, we address the related question of the decoherence of
the gluon field, which is a necessary component of thermalization. We present a
simplified leading-order computation of the decoherence time of a gluon
ensemble subject to an incoming flux of Weizsacker-Williams gluons. We also
discuss the entropy produced during the decoherence process and its relation to
the entropy in the final state which has been measured experimentally.Comment: 8 pages, 3 figure
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