20,491 research outputs found
Viscous three-dimensional analyses for nozzles for hypersonic propulsion
A Navier-Stokes computer code was validated using a number of two- and three-dimensional configurations for both laminar and turbulent flows. The validation data covers a range of freestream Mach numbers from 3 to 14, includes wall pressures, velocity profiles, and skin friction. Nozzle flow fields computed for a generic scramjet nozzle from Mach 3 to 20, wall pressures, wall skin friction values, heat transfer values, and overall performance are presented. In addition, three-dimensional solutions obtained for two asymmetric, single expansion ramp nozzles at a pressure ratio of 10 consists of the internal expansion region in the converging/diverging sections and the external supersonic exhaust in a quiescent ambient environment. The fundamental characteristics that were captured successfully include expansion fans; Mach wave reflections; mixing layers; and nonsymmetrical, multiple inviscid cell, supersonic exhausts. Comparison with experimental data for wall pressure distributions at the center planes shows good agreement
Using bijective maps to improve free energy estimates
We derive a fluctuation theorem for generalized work distributions, related
to bijective mappings of the phase spaces of two physical systems, and use it
to derive a two-sided constraint maximum likelihood estimator of their free
energy difference which uses samples from the equilibrium configurations of
both systems. As an application, we evaluate the chemical potential of a dense
Lennard-Jones fluid and study the construction and performance of suitable
maps.Comment: 17 pages, 11 figure
NONLINEAR POLARIZATION SPECTROSCOPY (FREQUENCY DOMAIN) STUDIES OF EXCITED STATE PROCESSES: THE B800–850ANTENNA OF RHODOBACTER SPHAEROIDES
Nonlinear polarization spectroscopy in the frequency domain allows rate constant determinations of fast electronic energy and phase relaxations together with characterization of the type of line broadening. Application of this method to the B850 component of the isolated B800–850antenna ofRhodobacter sphaeroides at room temperature shows that B850 is inhomogeneously broadened, with homogeneous widths between 30 and 200 cm−1, depending on the spectral position of the subforms. The corresponding phase relaxation times are clearly in the subpicosecond range. There is also indication of an up-to-now unspecified1–5 ps energy relaxation channel per subunit
Bulk viscosity of strange quark matter: Urca versus non-leptonic processes
A general formalism for calculating the bulk viscosity of strange quark
matter is developed. Contrary to the common belief that the non-leptonic
processes alone give the dominant contribution to the bulk viscosity, the
inclusion of the Urca processes is shown to play an important role at
intermediate densities when the characteristic r-mode oscillation frequencies
are not too high. The interplay of non-leptonic and Urca processes is analyzed
in detailComment: 9 pages, 4 figures, v2: revised figures, no change in result
BCS BEC crossover and phase structure of relativistic system: a variational approach
We investigate here the BCS BEC crossover in relativistic systems using a
variational construct for the ground state and the minimization of the
thermodynamic potential. This is first studied in a four fermion point
interaction model and with a BCS type ansatz for the ground state with fermion
pairs. It is shown that the antiparticle degrees of freedom play an important
role in the BCS BEC crossover physics, even when the ratio of fermi momentum to
the mass of the fermion is small. We also consider the phase structure for the
case of fermion pairing with imbalanced populations. Within the ansatz,
thermodynamically stable gapless modes for both fermions and anti fermions are
seen for strong coupling in the BEC regime. We further investigate the effect
of fluctuations of the condensate field by treating it as a dynamical field and
generalize the BCS ansatz to include quanta of the condensate field also in a
boson fermion model with quartic self interaction of the condensate field. It
is seen that the critical temperature decreases with inclusion of fluctuations.Comment: 18 pages, 13 figures, one more section added, title modified, version
to appear in Phys Rev
Bulk viscosity in hyperonic star and r-mode instability
We consider a rotating neutron star with the presence of hyperons in its
core, using an equation of state in an effective chiral model within the
relativistic mean field approximation. We calculate the hyperonic bulk
viscosity coefficient due to nonleptonic weak interactions. By estimating the
damping timescales of the dissipative processes, we investigate its role in the
suppression of gravitationally driven instabilities in the -mode. We observe
that -mode instability remains very much significant for hyperon core
temperature of around K, resulting in a comparatively larger instability
window. We find that such instability can reduce the angular velocity of the
rapidly rotating star considerably upto , with as
the Keplerian angular velocity.Comment: 10 pages including 7 figure
Non-Compositional Term Dependence for Information Retrieval
Modelling term dependence in IR aims to identify co-occurring terms that are
too heavily dependent on each other to be treated as a bag of words, and to
adapt the indexing and ranking accordingly. Dependent terms are predominantly
identified using lexical frequency statistics, assuming that (a) if terms
co-occur often enough in some corpus, they are semantically dependent; (b) the
more often they co-occur, the more semantically dependent they are. This
assumption is not always correct: the frequency of co-occurring terms can be
separate from the strength of their semantic dependence. E.g. "red tape" might
be overall less frequent than "tape measure" in some corpus, but this does not
mean that "red"+"tape" are less dependent than "tape"+"measure". This is
especially the case for non-compositional phrases, i.e. phrases whose meaning
cannot be composed from the individual meanings of their terms (such as the
phrase "red tape" meaning bureaucracy). Motivated by this lack of distinction
between the frequency and strength of term dependence in IR, we present a
principled approach for handling term dependence in queries, using both lexical
frequency and semantic evidence. We focus on non-compositional phrases,
extending a recent unsupervised model for their detection [21] to IR. Our
approach, integrated into ranking using Markov Random Fields [31], yields
effectiveness gains over competitive TREC baselines, showing that there is
still room for improvement in the very well-studied area of term dependence in
IR
Experimental Evidence of Time Delay Induced Death in Coupled Limit Cycle Oscillators
Experimental observations of time delay induced amplitude death in a pair of
coupled nonlinear electronic circuits that are individually capable of
exhibiting limit cycle oscillations are described. In particular, the existence
of multiply connected death islands in the parameter space of the coupling
strength and the time delay parameter for coupled identical oscillators is
established. The existence of such regions was predicted earlier on theoretical
grounds in [Phys. Rev. Lett. 80, 5109 (1998); Physica 129D, 15 (1999)]. The
experiments also reveal the occurrence of multiple frequency states, frequency
suppression of oscillations with increased time delay and the onset of both
in-phase and anti-phase collective oscillations.Comment: 4 aps formatted RevTeX pages; 6 figures; to appear in Phys. Rev. Let
Neutrino Propagation In Color Superconducting Quark Matter
We calculate the neutrino mean free path in color superconducting quark
matter, and employ it to study the cooling of matter via neutrino diffusion in
the superconducting phase as compared to a free quark phase. The cooling
process slows when quark matter undergoes a second order phase transition to a
superconducting phase at the critical temperature . Cooling subsequently
accelerates as the temperature decreases below . This will directly impact
the early evolution of a newly born neutron star should its core contain quark
matter. Consequently, there may be observable changes in the early neutrino
emission which would provide evidence for superconductivity in hot and dense
matter.Comment: 12 pages, 5 figure
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