1,686 research outputs found
Motion and wake structure of spherical particles
This paper presents results from a flow visualization study of the wake
structures behind solid spheres rising or falling freely in liquids under the
action of gravity. These show remarkable differences to the wake structures
observed behind spheres held fixed. The two parameters controlling the rise or
fall velocity (i.e., the Reynolds number) are the density ratio between sphere
and liquid and the Galileo number.Comment: 9 pages, 8 figures. Higher resolution on demand. To appear in
Nonlinearity January 200
A brief summary of L. van Wijngaarden's work up till his retirement
This paper attempts to provide an overview of Professor Leen van Wijngaarden's scientific work by briefly summarizing a number of his papers. The review is organized by topic and covers his work on pressure waves in bubbly liquids, bubble dynamics, two-phase flow, standing waves in resonant systems, and flow cavitation noise. A list of publications up till his retirement in March 1997 is provided in the Appendix
On the characteristics of the equations of motion for a bubbly flow and the related problem of critical flow
For the study of transients in gas-liquid flows, the equations of the so-called separated flow model are inadequate, because they possess, in the general case where gas and liquid move at different velocities, complex characteristics. This paper is concerned with the equations of motion for bubbly flow. The equations are discussed with emphasis on the aspects of relative motion and the characteristics are calculated. It is found that all characteristics are real. The results are used to establish a relation between gas velocity, liquid velocity, void fraction and sound velocity at critical flow. This relation agrees very well with experimental data for these quantities as measured by Muir and Eichhorn in the throat of a converging-diverging nozzle
Logarithmic two-loop corrections to the Lamb shift in hydrogen
Higher order logarithmic corrections to the
hydrogen Lamb shift are calculated. The results obtained show the two-loop
contribution has a very peculiar behavior, and significantly alter the
theoretical predictions for low lying S-states.Comment: 14 pages, including 2 figures, submitted to Phys. Rev. A, updated
with minor change
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
Oscillations of a gas pocket on a liquid-covered solid surface
The dynamic response of a gas bubble entrapped in a cavity on the surface of
a submerged solid subject to an acoustic field is investigated in the linear
approximation. We derive semi-analytical expressions for the resonance
frequency, damping and interface shape of the bubble. For the liquid phase, we
consider two limit cases: potential flow and unsteady Stokes flow. The
oscillation frequency and interface shape are found to depend on two
dimensionless parameters: the ratio of the gas stiffness to the surface tension
stiffness, and the Ohnesorge number, representing the relative importance of
viscous forces. We perform a parametric study and show, among others, that an
increase in the gas pressure or a decrease in the surface tension leads to an
increase in the resonance frequency until an asymptotic value is reached
Instantaneous Clear Sky Radiative Forcings of Halogenated Gases
The clear sky instantaneous radiative forcings of the 14 halogenated gases
previously shown to have the largest contribution to global warming, were
found. The calculation used the absorption cross sections for the halogenated
gases which are assumed to be independent of temperature as well as over 1/3
million line strengths for the 5 naturally occurring greenhouse gases: HO,
CO, O, CH and NO, from the Hitran database. The total radiative
forcing of the halogenated gases at their 2020 concentrations is 0.52 (0.67)
W/m at the tropopause (mesopause). Over half of this forcing is due to
CFC11 and CFC12 whose concentrations are declining as a result of the Montreal
Protocol. The rate of total forcing change for all 14 halogenated gases is 1.5
(2.2) mW/m/year at the tropopause (mesopause). The calculations assumed a
constant altitude concentration for all halogenated gases except CFC11, CFC12
and SF. Using the observed altitude dependence for those 3 molecules
reduced their radiative forcings by about 10%. The global warming potential
values were comparable to those given by the Intergovernmental Panel on Climate
Change. The contribution of a gas to global warming was estimated using the
forcing power per molecule defined as the derivative of its radiative forcing
with respect to its column density. For the present atmosphere, the
per-molecule forcing powers of halogenated gases are orders of magnitude larger
than those for the 5 naturally occuring greenhouse gases because the latter
have much higher concentrations and are strongly saturated. But, the rates of
concentration increase of the 5 main greenhouse gases are orders of magnitude
greater than that of any halogenated gas. Assuming the temperature increase
caused by each gas is proportional to its radiative forcing increase, the 14
halogenated gases are responsible for only 2% of the total global warming.Comment: arXiv admin note: text overlap with arXiv:2103.16465,
arXiv:2006.0309
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