8,830 research outputs found
The Impacts of Spatially Variable Demand Patterns on Water Distribution System Design and Operation
Open Access articleResilient water distribution systems (WDSs) need to minimize the level of service failure in terms of magnitude and duration over its design life when subject to exceptional conditions. This requires WDS design to consider scenarios as close as possible to real conditions of the WDS to avoid any unexpected level of service failure in future operation (e.g., insufficient pressure, much higher operational cost, water quality issues, etc.). Thus, this research aims at exploring the impacts of design flow scenarios (i.e., spatial-variant demand patterns) on water distribution system design and operation. WDSs are traditionally designed by using a uniform demand pattern for the whole system. Nevertheless, in reality, the patterns are highly related to the number of consumers, service areas, and the duration of peak flows. Thus, water distribution systems are comprised of distribution blocks (communities) organized in a hierarchical structure. As each community may be significantly different from the others in scale and water use, the WDSs have spatially variable demand patterns. Hence, there might be considerable variability of real flow patterns for different parts of the system. Consequently, the system operation might not reach the expected performance determined during the design stage, since all corresponding facilities are commonly tailor-made to serve the design flow scenario instead of the real situation. To quantify the impacts, WDSs’ performances under both uniform and spatial distributed patterns are compared based on case studies. The corresponding impacts on system performances are then quantified based on three major metrics; i.e., capital cost, energy cost, and water quality. This study exemplifies that designing a WDS using spatial distributed demand patterns might result in decreased life-cycle cost (i.e., lower capital cost and nearly the same pump operating cost) and longer water ages. The outcomes of this study provide valuable information regarding design and operation of water supply infrastructures; e.g., assisting the optimal design
Strain Hardening in Polymer Glasses: Limitations of Network Models
Simulations are used to examine the microscopic origins of strain hardening
in polymer glasses. While traditional entropic network models can be fit to the
total stress, their underlying assumptions are inconsistent with simulation
results. There is a substantial energetic contribution to the stress that rises
rapidly as segments between entanglements are pulled taut. The thermal
component of stress is less sensitive to entanglements, mostly irreversible,
and directly related to the rate of local plastic arrangements. Entangled and
unentangled chains show the same strain hardening when plotted against the
microscopic chain orientation rather than the macroscopic strain.Comment: 4 pages, 3 figure
The rotational velocity of the sdOB primary of the eclipsing binary system LB 3459 (AA Dor)
We present an analysis of the rotational velocity of the primary of LB 3459
based on 107 new high-resolution and high-S/N ESO VLT UVES spectra. 105 of them
cover a complete orbital period (0.26 d) of this binary system. We have
determined an orbital period of P = 22600.702 +/- 0.005 sec, a radial velocity
amplitude of A_1 = 39.19 +/- 0.05 km/sec, and T_0 = 2451917.152690 +/-
0.000005. From simulations of the He II 4686A line profile (based on NLTE model
atmosphere calculations), we derive v_rot = 47 +/- 5 km/sec.
We present an animation which shows the orbital movement of the binary
system, its synthetic lightcurve, and compares the phase-dependent variation of
the predicted with the observed He II 4686A line profile.
The radius of the cool component is almost the same size like Jupiter but its
mass is about 70 times higher than Jupiter's mass. Thus, from its present mass
(M_2 = 0.066 M_sun), the secondary of LB 3459 lies formally within the
brown-dwarf mass range (0.013 - 0.08 M_sun). It might be a former planet which
has survived the previous common-envelope phase and even has gained mass.Comment: 7 pages, 11 Postscript figures, to appear in A&
Metal abundances in hot white dwarfs with signatures of a superionized wind
About a dozen hot white dwarfs with effective temperatures Teff =
65,000-120,000 K exhibit unusual absorption features in their optical spectra.
These objects were tentatively identified as Rydberg lines of ultra-high
excited metals in ionization stages V-X, indicating line formation in a dense
environment with temperatures near one million Kelvin. Since some features show
blueward extensions, it was argued that they stem from a superionized wind. A
unique assignment of the lines to particular elements is not possible, although
they probably stem from C, N, O, and Ne. To further investigate this
phenomenon, we analyzed the ultraviolet spectra available from only three stars
of this group; that is, two helium-rich white dwarfs, HE 0504-2408 and HS
0713+3958 with spectral type DO, and a hydrogen-rich white dwarf, HS 2115+1148
with spectral type DAO. We identified light metals (C, N, O, Si, P, and S) with
generally subsolar abundances and heavy elements from the iron group (Cr, Mn,
Fe, Co, Ni) with solar or oversolar abundance. The abundance patterns are not
unusual for hot WDs and can be interpreted as the result of gravitational
settling and radiative levitation of elements. As to the origin of the
ultra-high ionized metals lines, we discuss the possible presence of a
multicomponent radiatively driven wind that is frictionally heated.Comment: Accepted for publication in A&
The hot white dwarf in the peculiar binary nucleus of the planetary nebula EGB6
EGB6 is an extended, faint old planetary nebula (PN) with an enigmatic
nucleus. The central star (PG0950+139) is a hot DAOZ-type white dwarf (WD). An
unresolved, compact emission knot was discovered to be located 0.166" away from
the WD and it was shown to be centered around a dust-enshrouded low-luminosity
star. It was argued that the dust disk and evaporated gas (photoionized by the
hot WD) around the companion are remnants of a disk formed by wind material
captured from the WD progenitor when it was an asymptotic giant branch (AGB)
star. In this paper, we assess the hot WD to determine its atmospheric and
stellar parameters. We performed a model-atmosphere analysis of ultraviolet
(UV) and optical spectra. We found Teff = 105,000 +/- 5000 K, log g = 7.4 +/-
0.4, and a solar helium abundance (He = 0.25 +/- 0.1, mass fraction). We
measured the abundances of ten more species (C, N, O, F, Si, P, S, Ar, Fe, Ni)
and found essentially solar abundance values, indicating that radiation-driven
wind mass-loss, with a theoretical rate of log(dot-M/M_sun/yr) = -11.0
(+1.1)(-0.8) prevents the gravitational separation of elements in the
photosphere. The WD has a mass of M/M_sun = 0.58 (+0.12)(-0.04) and its
post-AGB age (log(t_evol/yr) = 3.60 (+1.26)(-0.09)) is compatible with the PN
kinematical age of log(t_PN}/yr) = 4.2. In addition, we examined the UV
spectrum of the hot nucleus of a similar object with a compact emission region,
TOL26 (PN G298.0+34.8), and found that it is a slightly cooler DAOZ WD (Teff
about 85,000 K), but this WD shows signatures of gravitational settling of
heavy elements.Comment: A&A accepte
The far-ultraviolet spectra of two hot PG1159 stars
PG1159 stars are hot, hydrogen-deficient (pre-) white dwarfs with atmospheres
mainly composed of helium, carbon, and oxygen. The unusual surface chemistry is
the result of a late helium-shell flash. Observed element abundances enable us
to test stellar evolution models quantitatively with respect to their
nucleosynthesis products formed near the helium-burning shell of the progenitor
asymptotic giant branch stars. Because of the high effective temperatures
(Teff), abundance determinations require ultraviolet spectroscopy and non-local
thermodynamic equilibrium model atmosphere analyses. Up to now, we have
presented results for the prototype of this spectral class and two cooler
members (Teff in the range 85,000-140,000 K). Here we report on the results for
two even hotter stars (PG1520+525 and PG1144+005, both with Teff = 150,000 K)
which are the only two objects in this temperature-gravity region for which
useful far-ultraviolet spectra are available, and revisit the prototype star.
Previous results on the abundances of some species are confirmed, while results
on others (Si, P, S) are revised. In particular, a solar abundance of sulphur
is measured in contrast to earlier claims of a strong S deficiency that
contradicted stellar evolution models. For the first time, we assess the
abundances of Na, Al, and Cl with newly constructed non-LTE model atoms.
Besides the main constituents (He, C, O), we determine the abundances (or upper
limits) of N, F, Ne, Na, Al, Si, P, S, Cl, Ar, and Fe. Generally, good
agreement with stellar models is found.Comment: Accepted for publication in A&
Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)
We present a spectral analysis of the sdO central star K 648 based on
medium-resolution optical and high-resolution UV spectra. The photospheric
parameters are determined by means of state-of-the-art NLTE model atmosphere
techniques.
We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08)
and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a
factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a
sub-solar value. However, these metal abundances are much higher than the
cluster's metallicity M 15: [Fe/H] = -2.25).
The surface composition appears to be a mixture of the original hydrogen-rich
material and products of helium burning (3 alpha process) which have been mixed
up to the surface. The abundances of He, C, and N are consistent with the
nebular abundance, while O is considerably more abundant in the photosphere
than in the nebula.
From a comparison of its position in the log Teff - log g plane with
evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity
of 3810 +/- 1200 Lsun are deduced.
Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the
distance of M 15 as determined by Alves et al. (2000).
From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.Comment: 8 pages, 13 figure
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