1,229 research outputs found
The citric acid cough threshold and the ventilatory response to carbon dioxide on ascent to high altitude
SummaryVentilatory control undergoes profound changes on ascent to high altitude. We hypothesized that the fall in citric acid cough threshold seen on ascent to altitude is mediated by changes in the central control of cough and would parallel changes in central ventilatory control assessed by the hypercapnic ventilatory response (HCVR). Twenty-five healthy volunteers underwent measurements of HCVR and citric acid sensitivity at sea level and during a 9 day sojourn at 5200m. None of the subjects had any evidence of respiratory infection. Citric acid cough threshold fell significantly on ascent to 5200m. The slope, S, of the HCVR increased significantly on ascent to 5200m and during the stay at altitude. There was no correlation between citric acid sensitivity and HCVR. We conclude that the change in citric acid cough threshold seen on exposure to hypobaric hypoxia is unlikely to be mediated by changes in the central control of cough. Sensitivity to citric acid may be due to early subclinical pulmonary edema stimulating airway sensory nerve endings
Particle size segregation and diffusion in saturated granular flows: Implications for grain sorting in debris flows
Sorting of rocks, boulders, and silt/sand-sized particles according to their size is a characteristic feature of debris flow deposits and is an active process during flow which significantly affects the mobility. The degree at which size sorting occurs in debris flows depends on the relative magnitudes of granular processes such as particle size segregation and diffusion. Since debris flows are fluid-saturated phenomena, accurate modelling of size sorting requires the understanding of the influence of fluids on these processes, which have not been systematically studied. Here, we present simulation results and the associated empirical expressions for particle size segregation and diffusion which take into account fluid effects due to buoyant and drag forces. These expressions are developed through scaling analysis of data obtained from coupled granular-fluid simulations of saturated bi-disperse mixtures under simple shear. We further show that using these scaling relationships, an existing segregation-diffusion continuum equation can be extended to model particle sorting in debris flows with various types of fluids
Impact dynamics of debris flow against slit dam: Experimental and numerical investigation
Debris flows are gravity-driven phenomena common in mountainous regions that are hazardous to downstream facilities. To mitigate the impacts of these disastrous processes, structural countermeasures such as slit dams are constructed in gullies and along mountain slopes. Existing studies on the impact dynamics of debris flows against slit dams typically focus only on the flow characteristics but fail to take the geometry of the structure into account. Here we develop an analytical model, derived from the momentum approach, that allows for the estimation of the runup height and impact load of debris flows on slit dams. The model is validated against discrete element simulations and small-scale flume experiments. It is found that the runup height is controlled by both the Froude number and slit size. The proposed analytical model can predict the runup height well within a certain range of Froude numbers. Results from experiments further reveal that the fontal dynamic pressure is sensitive to the flow properties whereas the peak dynamic pressure is strongly affected by the slit size
Cosmic Density Perturbations from Late-Decaying Scalar Condensations
We study the cosmic density perturbations induced from fluctuation of the
amplitude of late-decaying scalar condensations (called \phi) in the scenario
where the scalar field \phi once dominates the universe. In such a scenario,
the cosmic microwave background (CMB) radiation originates to decay products of
the scalar condensation and hence its anisotropy is affected by the fluctuation
of \phi. It is shown that the present cosmic density perturbations can be
dominantly induced from the primordial fluctuation of \phi, not from the
fluctuation of the inflaton field. This scenario may change constraints on the
source of the density perturbations, like inflation. In addition, a correlated
mixture of adiabatic and isocurvature perturbations may arise in such a
scenario; possible signals in the CMB power spectrum are discussed. We also
show that the simplest scenario of generating the cosmic density perturbations
only from the primordial fluctuation of \phi (i.e., so-called ``curvaton''
scenario) is severely constrained by the current measurements of the CMB
angular power spectrum if correlated mixture of the adiabatic and isocurvature
perturbations are generated.Comment: 31pages, 14figure
Thermoelectric effects of an Aharonov-Bohm interferometer with an embedded quantum dot in the Kondo regime
Thermoelectric effects are studied in an Aharonov-Bohm (AB) interferometer
with an embedded quantum dot in the Kondo regime. The AB flux-dependent
transmission probability has an asymmetrical shape arising from the Fano
interference between the direct tunneling path and the Kondo-resonant tunneling
path through a quantum dot. The sign and magnitude of thermopower can be
modulated by the AB flux and the direct tunneling amplitude. In addition, the
thermopower is anomalously enhanced by the Kondo correlation in the quantum dot
near the Kondo temperature (). The Kondo correlation in the quantum dot
also leads to crossover behavior in diagonal transport coefficients as a
function of temperature. The amplitude of an AB oscillation in electric and
thermal conductances is small at temperatures far above , but becomes
enhanced as the system is cooled below . The AB oscillation is strong in
the thermopower and Lorenz number within the crossover region near the Kondo
temperature.Comment: 16 pages, 10 figure
Non-linear response of a Kondo system: Perturbation approach to the time dependent Anderson impurity model
Nonlinear tunneling current through a quantum dot
(an Anderson impurity system) subject to both constant and alternating
electric fields is studied in the Kondo regime. A systematic diagram technique
is developed for perturbation study of the current in physical systems out of
equilibrium governed by time - dependent Hamiltonians of the Anderson and the
Kondo models. The ensuing calculations prove to be too complicated for the
Anderson model, and hence, a mapping on an effective Kondo problem is called
for. This is achieved by constructing a time - dependent version of the
Schrieffer - Wolff transformation. Perturbation expansion of the current is
then carried out up to third order in the Kondo coupling J yielding a set of
remarkably simple analytical expressions for the current. The zero - bias
anomaly of the direct current differential conductance is shown to be
suppressed by the alternating field while side peaks develop at finite source -
drain voltage. Both the direct component and the first harmonics of the time -
dependent response are equally enhanced due to the Kondo effect, while
amplitudes of higher harmonics are shown to be relatively small. A zero
alternating bias anomaly is found in the alternating current differential
conductance, that is, it peaks around zero alternating bias. This peak is
suppressed by the constant bias. No side peaks show up in the differential
alternating - conductance but their counterpart is found in the derivative of
the alternating current with respect to the direct bias. The results pertaining
to nonlinear response are shown to be valid also below the Kondo temperature.Comment: 55 latex pages 11 ps figure
Curvatons in Supersymmetric Models
We study the curvaton scenario in supersymmetric framework paying particular
attention to the fact that scalar fields are inevitably complex in
supersymmetric theories. If there are more than one scalar fields associated
with the curvaton mechanism, isocurvature (entropy) fluctuations between those
fields in general arise, which may significantly affect the properties of the
cosmic density fluctuations. We examine several candidates for the curvaton in
the supersymmetric framework, such as moduli fields, Affleck-Dine field, -
and -flat directions, and right-handed sneutrino. We estimate how the
isocurvature fluctuations generated in each case affect the cosmic microwave
background angular power spectrum. With the use of the recent observational
result of the WMAP, stringent constraints on the models are derived and, in
particular, it is seen that large fraction of the parameter space is excluded
if the Affleck-Dine field plays the role of the curvaton field. Natural and
well-motivated candidates of the curvaton are also listed.Comment: 34 pages, 5 figure
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
Inclusive Hadron Production in Photon-Photon Collisions at Next-to-Leading Order
We study inclusive charged-hadron production in collisions of quasireal
photons at NLO in perturbative QCD, using fragmentation functions recently
extracted from PEP and LEP1 data. We superimpose the direct (DD),
single-resolved (DR), and double-resolved (RR) gamma-gamma channels. First, we
confront existing data taken by TASSO at PETRA and by MARK II at PEP with our
NLO calculations. We also make comparisons with the neutral-kaon to
charged-hadron ratio measured by MARK II. Then, we present NLO predictions for
LEP2, a next-generation e+e- linear collider (NLC) in the TESLA design, and a
Compton collider obtained by converting a NLC. We analyze transverse-momentum
and rapidity spectra with regard to the scale dependence, the interplay of the
DD, DR, and RR components, the sensitivity to the gluon density in the resolved
photon, and the influence of gluon fragmentation. It turns out that the
inclusive measurement of small-p_T hadrons at a Compton collider would greatly
constrain the gluon density of the photon and the gluon fragmentation function.Comment: 16 pages, 12 Postscript figure
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