550 research outputs found
GASLESS COMBUSTION FRONTS WITH HEAT LOSS
For a model of gasless combustion with heat loss, we use geometric s
ingular
perturbation theory to show existence of traveling combustion fr
onts. We show that the
fronts are nonlinearly stable in an appropriate sense if an Evans fun
ction criterion, which
can be verified numerically, is satisfied. For a solid reactant and exot
hermicity parameter
that is not too large, we verify numerically that the criterion is satisfi
ed
The effect of phase transitions on the droplet size distribution in homogeneous isotropic turbulence
We investigate the dynamics of an ensemble of discrete aerosol droplets in a homogeneous, isotropic turbulent flow. Our focus is on the stationary distribution of droplet sizes that develops as a result of evaporation and condensation effects. For this purpose we simulate turbulence in a domain with periodic boundary conditions using pseudo-spectral discretization. We solve in addition equations for the temperature and for a scalar field, which represents the background humidity against which the size of the droplets evolves. We apply large-scale forcing of the velocity field to reach a statistically steady state. The droplets are transported by the turbulent field while exchanging heat and mass with the evolving temperature and humidity fields. In this Euler-Lagrange framework, we assume the droplets volume fraction to be sufficiently low to allow one-way coupling of the droplets and turbulence dynamics. The motion of the droplets is time-accurately tracked. The Stokes drag force is included in the equation of motion of the individual droplets. The responsiveness of the droplets to small turbulent scales is directly related to the size of the individual spherical droplets. We perform direct numerical simulation to ultimately obtain the probability density function of the evolving radius of the droplets at different points in time with characteristic heat and mass transfer parameters. We determine the gradual convergence of the distribution function to its statistically stationary state for forced homogeneous, isotropic turbulence
A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy
The existence of pulsations in HgMn stars is still being debated. To provide
the first unambiguous observational detection of pulsations in this class of
chemically peculiar objects, the bright star HD 45975 was monitored for nearly
two months by the CoRoT satellite. Independent analyses of the light curve
provides evidence of monoperiodic variations with a frequency of 0.7572 c/d and
a peak-to-peak amplitude of ~2800 ppm. Multisite, ground-based spectroscopic
observations overlapping the CoRoT observations show the star to be a
long-period, single-lined binary. Furthermore, with the notable exception of
mercury, they reveal the same periodicity as in photometry in the line moments
of chemical species exhibiting strong overabundances (e.g., Mn and Y). In
contrast, lines of other elements do not show significant variations. As found
in other HgMn stars, the pattern of variability consists in an absorption bump
moving redwards across the line profiles. We argue that the photometric and
spectroscopic changes are more consistent with an interpretation in terms of
rotational modulation of spots at the stellar surface. In this framework, the
existence of pulsations producing photometric variations above the ~50 ppm
level is unlikely in HD 45975. This provides strong constraints on the
excitation/damping of pulsation modes in this HgMn star.Comment: Accepted for publication in A&A, 14 pages, 15 colour figures (revised
version after language editing
Search for Doubly-Charged Higgs Boson Production at HERA
A search for the single production of doubly-charged Higgs bosons H^{\pm \pm}
in ep collisions is presented. The signal is searched for via the Higgs decays
into a high mass pair of same charge leptons, one of them being an electron.
The analysis uses up to 118 pb^{-1} of ep data collected by the H1 experiment
at HERA. No evidence for doubly-charged Higgs production is observed and mass
dependent upper limits are derived on the Yukawa couplings h_{el} of the Higgs
boson to an electron-lepton pair. Assuming that the doubly-charged Higgs only
decays into an electron and a muon via a coupling of electromagnetic strength
h_{e \mu} = \sqrt{4 \pi \alpha_{em}} = 0.3, a lower limit of 141 GeV on the
H^{\pm\pm} mass is obtained at the 95% confidence level. For a doubly-charged
Higgs decaying only into an electron and a tau and a coupling h_{e\tau} = 0.3,
masses below 112 GeV are ruled out.Comment: 15 pages, 3 figures, 1 tabl
Forward pi^0 Production and Associated Transverse Energy Flow in Deep-Inelastic Scattering at HERA
Deep-inelastic positron-proton interactions at low values of Bjorken-x down
to x \approx 4.10^-5 which give rise to high transverse momentum pi^0 mesons
are studied with the H1 experiment at HERA. The inclusive cross section for
pi^0 mesons produced at small angles with respect to the proton remnant (the
forward region) is presented as a function of the transverse momentum and
energy of the pi^0 and of the four-momentum transfer Q^2 and Bjorken-x.
Measurements are also presented of the transverse energy flow in events
containing a forward pi^0 meson. Hadronic final state calculations based on QCD
models implementing different parton evolution schemes are confronted with the
data.Comment: 27 pages, 8 figures and 3 table
- …