8,013 research outputs found
Large-Eddy Simulation of Axisymmetric Compression Corner Flow
The Wall-Modeled Large Eddy Simulation (WMLES) approach is used to study the interaction of a shock wave with a high Reynolds number turbulent boundary layer. Since the near wall region is modeled, high Reynolds number turbulent flows can be simulated at a moderate computational cost. The case considered is that of an axisymmetric Mach 2.85 turbulent boundary layer over a 30 compression corner. The Reynolds number of the boundary layer upstream of the interaction based on momentum thickness (Re theta = u sub infinity theta/v sub infinity) is ~12,000. The geometry and flow conditions match the experiments of Dunagan et al. (NASA TM 88227, 1986). The simulations were performed using equilibrium and non-equilibrium wall models. The agreement with experiment is encouraging for the finest grid with respect to the separation bubble length, unsteady shock structure and wall pressure distribution. Sensitivity ofWMLES results to grid, wall model, and blockage effects in the tunnel are reported
Improved filters for gravitational waves from inspiraling compact binaries
The order of the post-Newtonian expansion needed to extract in a reliable and accurate manner the fully general relativistic gravitational wave signal from inspiraling compact binaries is explored. A class of approximate wave forms, called P-approximants, is constructed based on the following two inputs: (a) the introduction of two new energy-type and flux-type functions e(v) and f(v), respectively, (b) the systematic use of the Padé approximation for constructing successive approximants of e(v) and f(v). The new P-approximants are not only more effectual (larger overlaps) and more faithful (smaller biases) than the standard Taylor approximants, but also converge faster and monotonically. The presently available (v/c)^5-accurate post-Newtonian results can be used to construct P-approximate wave forms that provide overlaps with the exact wave form larger than 96.5%, implying that more than 90% of potential events can be detected with the aid of P-approximants as opposed to a mere 10–15 % that would be detectable using standard post-Newtonian approximants
Centrifugal Force and Ellipticity behaviour of a slowly rotating ultra compact object
Using the optical reference geometry approach, we have derived in the
following, a general expression for the ellipticity of a slowly rotating fluid
configuration using Newtonian force balance equation in the conformally
projected absolute 3-space, in the realm of general relativity. Further with
the help of Hartle-Thorne (H-T) metric for a slowly rotating compact object, we
have evaluated the centrifugal force acting on a fluid element and also
evaluated the ellipticity and found that the centrifugal reversal occurs at
around , and the ellipticity maximum at around . The result has been compared with that of Chandrasekhar and
Miller which was obtained in the full 4-spacetime formalism
Lagrangian perfect fluids and black hole mechanics
The first law of black hole mechanics (in the form derived by Wald), is
expressed in terms of integrals over surfaces, at the horizon and spatial
infinity, of a stationary, axisymmetric black hole, in a diffeomorphism
invariant Lagrangian theory of gravity. The original statement of the first law
given by Bardeen, Carter and Hawking for an Einstein-perfect fluid system
contained, in addition, volume integrals of the fluid fields, over a spacelike
slice stretching between these two surfaces. When applied to the
Einstein-perfect fluid system, however, Wald's methods yield restricted
results. The reason is that the fluid fields in the Lagrangian of a gravitating
perfect fluid are typically nonstationary. We therefore first derive a first
law-like relation for an arbitrary Lagrangian metric theory of gravity coupled
to arbitrary Lagrangian matter fields, requiring only that the metric field be
stationary. This relation includes a volume integral of matter fields over a
spacelike slice between the black hole horizon and spatial infinity, and
reduces to the first law originally derived by Bardeen, Carter and Hawking when
the theory is general relativity coupled to a perfect fluid. We also consider a
specific Lagrangian formulation for an isentropic perfect fluid given by
Carter, and directly apply Wald's analysis. The resulting first law contains
only surface integrals at the black hole horizon and spatial infinity, but this
relation is much more restrictive in its allowed fluid configurations and
perturbations than that given by Bardeen, Carter and Hawking. In the Appendix,
we use the symplectic structure of the Einstein-perfect fluid system to derive
a conserved current for perturbations of this system: this current reduces to
one derived ab initio for this system by Chandrasekhar and Ferrari.Comment: 26 pages LaTeX-2
Tracing very high energy neutrinos from cosmological distances in ice
Astrophysical sources of ultrahigh energy neutrinos yield tau neutrino fluxes
due to neutrino oscillations. We study in detail the contribution of tau
neutrinos with energies above PeV relative to the contribution of the other
flavors. We consider several different initial neutrino fluxes and include tau
neutrino regeneration in transit through the Earth and energy loss of charged
leptons. We discuss signals of tau neutrinos in detectors such as IceCube, RICE
and ANITA.Comment: 27 pages, 19 figure
CO2 lidar system for atmospheric studies
A lidar facility using a TEA CO2 laser source is being developed at the ENEA Laboratories for Atmospheric Studies. The different subsystems and the proposed experimental activities are described
Probing the non-linear structure of general relativity with black hole binaries
Observations of the inspiral of massive binary black holes (BBH) in the Laser
Interferometer Space Antenna (LISA) and stellar mass binary black holes in the
European Gravitational-Wave Observatory (EGO) offer an unique opportunity to
test the non-linear structure of general relativity. For a binary composed of
two non-spinning black holes, the non-linear general relativistic effects
depend only on the masses of the constituents. In a recent letter, we explored
the possibility of a test to determine all the post-Newtonian coefficients in
the gravitational wave-phasing.
However, mutual covariances dilute the effectiveness of such a test. In this
paper, we propose a more powerful test in which the various post-Newtonian
coefficients in the gravitational wave phasing are systematically measured by
treating three of them as independent parameters and demanding their mutual
consistency. LISA (EGO) will observe BBH inspirals with a signal-to-noise ratio
of more than 1000 (100) and thereby test the self-consistency of each of the
nine post-Newtonian coefficients that have so-far been computed, by measuring
the lower order coefficients to a relative accuracy of
(respectively, ) and the higher order coefficients to a relative
accuracy in the range -0.1 (respectively, -1).Comment: 5 pages, 4 figures. Revised version, accepted for publication in
Phys. Rev
On the Noether charge form of the first law of black hole mechanics
The first law of black hole mechanics was derived by Wald in a general
covariant theory of gravity for stationary variations around a stationary black
hole. It is formulated in terms of Noether charges, and has many advantages. In
this paper several issues are discussed to strengthen the validity of the
Noether charge form of the first law. In particular, a gauge condition used in
the derivation is justified. After that, we justify the generalization to
non-stationary variations done by Iyer-Wald.Comment: Latex, 16 pages, arguments on gauge conditions and near-stationary
entropy are added, accepted for publication in Physical Review
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