4,468 research outputs found
Optimal bounds with semidefinite programming: an application to stress driven shear flows
We introduce an innovative numerical technique based on convex optimization
to solve a range of infinite dimensional variational problems arising from the
application of the background method to fluid flows. In contrast to most
existing schemes, we do not consider the Euler--Lagrange equations for the
minimizer. Instead, we use series expansions to formulate a finite dimensional
semidefinite program (SDP) whose solution converges to that of the original
variational problem. Our formulation accounts for the influence of all modes in
the expansion, and the feasible set of the SDP corresponds to a subset of the
feasible set of the original problem. Moreover, SDPs can be easily formulated
when the fluid is subject to imposed boundary fluxes, which pose a challenge
for the traditional methods. We apply this technique to compute rigorous and
near-optimal upper bounds on the dissipation coefficient for flows driven by a
surface stress. We improve previous analytical bounds by more than 10 times,
and show that the bounds become independent of the domain aspect ratio in the
limit of vanishing viscosity. We also confirm that the dissipation properties
of stress driven flows are similar to those of flows subject to a body force
localized in a narrow layer near the surface. Finally, we show that SDP
relaxations are an efficient method to investigate the energy stability of
laminar flows driven by a surface stress.Comment: 17 pages; typos removed; extended discussion of linear matrix
inequalities in Section III; revised argument in Section IVC, results
unchanged; extended discussion of computational setup and limitations in
Sectios IVE-IVF. Submitted to Phys. Rev.
Mortgage valuation report forms and the identification of subsidence
This paper examines whether surveyors engaged in mortgage valuation inspections using questionnaire style report forms supplied by lending institutions, are subject to an increased risk of liability in respect of identifying the present and future threat of subsidence to domestic properties. Analysis of the mortgage valuation report forms used by 34 different lending institutions, showed that 20% failed to ask any subsidence related questions, only 6% asked about the geology or soil type of the site, and only 9% asked about the location of trees relative to the building. Evaluation of the report forms showed that the type, quality and quantity of questioning were such that 24 out of the 34 were inadequate and unreliable, leaving the surveyor at an increased risk of litigation
Star formation in normal galaxies
The ways in which recent infrared observations, particularly by the Infrared Astronomy Satellite (IRAS), have influenced ideas about star formation in normal galaxies, are discussed
Atmospheres and Spectra of Strongly Magnetized Neutron Stars II: Effect of Vacuum Polarization
We study the effect of vacuum polarization on the atmosphere structure and
radiation spectra of neutron stars with surface magnetic fields B=10^14-10^15
G, as appropriate for magnetars. Vacuum polarization modifies the dielectric
property of the medium and gives rise to a resonance feature in the opacity;
this feature is narrow and occurs at a photon energy that depends on the plasma
density. Vacuum polarization can also induce resonant conversion of photon
modes via a mechanism analogous to the MSW mechanism for neutrino oscillation.
We construct atmosphere models in radiative equilibrium with an effective
temperature of a few \times 10^6 K by solving the full radiative transfer
equations for both polarization modes in a fully ionized hydrogen plasma. We
discuss the subtleties in treating the vacuum polarization effects in the
atmosphere models and present approximate solutions to the radiative transfer
problem which bracket the true answer. We show from both analytic
considerations and numerical calculations that vacuum polarization produces a
broad depression in the X-ray flux at high energies (a few keV \la E \la a few
tens of keV) as compared to models without vacuum polarization; this arises
from the density dependence of the vacuum resonance feature and the large
density gradient present in the atmosphere. Thus the vacuum polarization effect
softens the high energy tail of the thermal spectrum, although the atmospheric
emission is still harder than the blackbody spectrum because of the non-grey
opacities. We also show that the depression of continuum flux strongly
suppresses the equivalent width of the ion cyclotron line and therefore makes
the line more difficult to observe.Comment: 21 pages, 21 figures; MNRAS; corrected minor typo
The Spin Period of EX Hydrae
We show that the spin period of the white dwarf in the magnetic CV EX Hydrae
represents an equilibrium state in which the corotation radius is comparable
with the distance from the white dwarf to the inner Lagrange point. We also
show that a continuum of spin equilibria exists at which Pspin is significantly
longer than \sim 0.1 Porb. Most systems occupying these equilibrium states
should have orbital periods below the CV period gap, as observed.Comment: MNRAS, accepte
Atmospheres and Spectra of Strongly Magnetized Neutron Stars
We construct atmosphere models for strongly magnetized neutron stars with
surface fields G and effective temperatures K. The atmospheres directly determine the characteristics
of thermal emission from isolated neutron stars, including radio pulsars, soft
gamma-ray repeaters, and anomalous X-ray pulsars. In our models, the atmosphere
is composed of pure hydrogen or helium and is assumed to be fully ionized. The
radiative opacities include free-free absorption and scattering by both
electrons and ions computed for the two photon polarization modes in the
magnetized electron-ion plasma. Since the radiation emerges from deep layers in
the atmosphere with \rho\ga 10^2 g/cm, plasma effects can significantly
modify the photon opacities by changing the properties of the polarization
modes. In the case where the magnetic field and the surface normal are
parallel, we solve the full, angle-dependent, coupled radiative transfer
equations for both polarization modes. We also construct atmosphere models for
general field orientations based on the diffusion approximation of the
transport equations and compare the results with models based on full radiative
transport. In general, the emergent thermal radiation exhibits significant
deviation from blackbody, with harder spectra at high energies. The spectra
also show a broad feature (\Delta E/\Ebi\sim 1) around the ion cyclotron
resonance \Ebi=0.63 (Z/A)(B/10^{14}{G}) keV, where and are the atomic
charge and atomic mass of the ion, respectively; this feature is particularly
pronounced when \Ebi\ga 3k\Teff. Detection of the resonance feature would
provide a direct measurement of the surface magnetic fields on magnetars.Comment: 29 pages, 11 figures; corrected factor of 2 in He models: minor
changes to figs 4 and 9 as a result; other very minor change
The Accretion Flows and Evolution of Magnetic Cataclysmic Variables
We have used a model of magnetic accretion to investigate the accretion flows
of magnetic cataclysmic variables. Numerical simulations demonstrate that four
types of flow are possible: discs, streams, rings and propellers. The
fundamental observable determining the accretion flow, for a given mass ratio,
is the spin-to-orbital period ratio of the system. If IPs are accreting at
their equilibrium spin rates, then for a mass ratio of 0.5, those with
Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be
stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. The spin to
orbital period ratio at which the systems transition between these flow types
increases as the mass ratio of the stellar components decreases.
For the first time we present evolutionary tracks of mCVs which allow
investigation of how their accretion flow changes with time. As systems evolve
to shorter orbital periods and smaller mass ratios, in order to maintain spin
equilibrium, their spin-to-orbital period ratio will generally increase. As a
result, the relative occurrence of ring-like flows will increase, and the
occurrence of disc-like flows will decrease, at short orbital periods. The
growing number of systems observed at high spin-to-orbital period ratios with
orbital periods below 2h, and the observational evidence for ring-like
accretion in EX Hya, are fully consistent with this picture.Comment: Accepted for publication in ApJ. 6 figures - included here at low
resolutio
- âŠ