1,642 research outputs found
Strong wave-mean-flow coupling in baroclinic acoustic streaming
The interaction of an acoustic wave with a stratified fluid can drive strong
streaming flows owing to the baroclinic production of fluctuating vorticity, as
recently demonstrated by Chini et al. (J. Fluid Mech., 744, 2014, pp. 329). In
the present investigation, a set of wave/mean-flow interaction equations is
derived that governs the coupled dynamics of a standing acoustic wave mode of
characteristic (small) amplitude {\epsilon} and the streaming flow it drives in
a thin channel with walls maintained at differing temperatures. Unlike
classical Rayleigh streaming, the resulting mean flow arises at O({\epsilon})
rather than at O({\epsilon^2}). Consequently, fully two-way coupling between
the waves and the mean flow is possible: the streaming is sufficiently strong
to induce O(1) rearrangements of the imposed background temperature and density
fields, which modifies the spatial structure and frequency of the acoustic mode
on the streaming time scale. A novel Wentzel-Kramers-Brillouin-Jeffreys
analysis is developed to average over the fast wave dynamics, enabling the
coupled system to be integrated strictly on the slow time scale of the
streaming flow. Analytical solutions of the reduced system are derived for weak
wave forcing and are shown to reproduce results from prior direct numerical
simulations (DNS) of the compressible Navier Stokes and heat equations with
remarkable accuracy. Moreover, numerical simulations of the reduced system are
performed in the regime of strong wave mean flow coupling for a fraction of the
computational cost of the corresponding DNS. These simulations shed light on
the potential for baroclinic acoustic streaming to be used as an effective
means to enhance heat transfer.Comment: 29 pages, 7 figure
Contribution of Extragalactic Infrared Sources to CMB Foreground Anisotropy
We estimate the level of confusion to Cosmic Microwave Background anisotropy
measurements caused by extragalactic infrared sources. CMB anisotropy
observations at high resolution and high frequencies are especially sensitive
to this foreground. We use data from the COBE satellite to generate a Galactic
emission spectrum covering mm and sub-mm wavelengths. Using this spectrum as a
template, we predict the microwave emission of the 5319 brightest infrared
galaxies seen by IRAS. We simulate skymaps over the relevant range of
frequencies (30-900 GHz) and instrument resolutions (10'-10 degrees Full Width
Half Max). Analysis of the temperature anisotropy of these skymaps shows that a
reasonable observational window is available for CMB anisotropy measurements.Comment: 14 pages (LaTex source), 3 PostScript figures. Final version, to
appear in ApJLetters May 1. Expanded discussion of systematic error
Cyclic ADP-ribose metabolism in rat kidney: High capacity for synthesis in glomeruli
Cyclic ADP-ribose metabolism in rat kidney: High capacity for synthesis in glomeruli. Recent discovery of cyclic ADP-ribose (cADPR) as an agent that triggers Ca2+ release from intracellular stores, through ryanodine receptor channel, is an important new development in the investigation of intracellular signaling mechanisms. We determined the capacity of kidney and its components for synthesis of cADPR from β-NAD, that is catalyzed by enzyme ADP-ribosyl cyclase, and enzymatic inactivation that is catalyzed by cADPR-glycohydrolase. Little or no activity of ADP-ribosyl cyclase was found in extracts from the whole rat kidney, renal cortex, outer and inner medulla. On the other hand, incubation of β-NAD with similar extracts from rat liver, spleen, heart, and brain resulted in biosynthesis of cADPR. In addition, extracts from suspension of proximal tubules or microdissected proximal convoluted tubules virtually lacked ADP-ribosyl cyclase activity. In sharp contrast to proximal tubules and cortex, extracts from glomeruli had high ADP-ribosyl cyclase activity, similar to that found in non-renal tissues. Authenticity of cADPR biosynthesized in glomeruli was documented by several criteria such as HPLC analysis, effect of inhibitors and homologous desensitization of Ca2+-release bioassay. On the other hand, the activity of cADPR-glycohydrolase was similar in extracts from glomeruli and in extracts from kidney cortex. Mesangial cells and vascular smooth muscle cells grown in primary culture displayed considerable ADPR-ribose cyclase activity. Our results show that extracts from glomeruli, unlike extracts from renal tissue zones and proximal tubules, have a singularly high capacity for synthesis of cADPR. We surmise that cADPR-triggered Ca2+-releasing system can serve as an intracellular signaling pathway that may be operant in regulations of glomerular cell functions
HD 152246 - a new high-mass triple system and its basic properties
Analyses of multi-epoch, high-resolution (R ~ 50.000) optical spectra of the
O-type star HD 152246 (O9 IV according to the most recent classification),
complemented by a limited number of earlier published radial velocities, led to
the finding that the object is a hierarchical triple system, where a close
inner pair (Ba-Bb) with a slightly eccentric orbit (e = 0.11) and a period of
6.0049 days revolves in a 470-day highly eccentric orbit (e = 0.865) with
another massive and brighter component A. The mass ratio of the inner system
must be low since we were unable to find any traces of the secondary spectrum.
The mass ratio A/(Ba+Bb) is 0.89. The outer system has recently been resolved
using long-baseline interferometry on three occasions. The interferometry
confirms the spectroscopic results and specifies elements of the system. Our
orbital solutions, including the combined radial-velocity and interferometric
solution indicate an orbital inclination of the outer orbit of 112{\deg} and
stellar masses of 20.4 and 22.8 solar masses. We also disentangled the spectra
of components A and Ba and compare them to synthetic spectra from two
independent programmes, TLUSTY and FASTWIND. In either case, the fit was not
satisfactory and we postpone a better determination of the system properties
for a future study, after obtaining observations during the periastron passage
of the outer orbit (the nearest chance being March 2015). For the moment, we
can only conclude that component A is an O9 IV star with v*sin(i) = 210 +\- 10
km/s and effective temperature of 33000 +\- 500 K, while component Ba is an O9
V object with v*sin(i) = 65 +/- 3 km/s and T_eff = 33600 +\- 600 K.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Time-stepping approach for solving upper-bound problems: Application to two-dimensional Rayleigh-Benard convection
An alternative computational procedure for numerically solving a class of variational problems arising from rigorous upper-bound analysis of forced-dissipative infinite-dimensional nonlinear dynamical systems, including the Navier-Stokes and Oberbeck-Boussinesq equations, is analyzed and applied to Rayleigh-Benard convection. A proof that the only steady state to which this numerical algorithm can converge is the required global optimal of the relevant variational problem is given for three canonical flow configurations. In contrast with most other numerical schemes for computing the optimal bounds on transported quantities (e.g., heat or momentum) within the "background field" variational framework, which employ variants of Newton's method and hence require very accurate initial iterates, the new computational method is easy to implement and, crucially, does not require numerical continuation. The algorithm is used to determine the optimal background-method bound on the heat transport enhancement factor, i.e., the Nusselt number (Nu), as a function of the Rayleigh number (Ra), Prandtl number (Pr), and domain aspect ratio L in two-dimensional Rayleigh-Benard convection between stress-free isothermal boundaries (Rayleigh's original 1916 model of convection). The result of the computation is significant because analyses, laboratory experiments, and numerical simulations have suggested a range of exponents alpha and beta in the presumed Nu similar to (PrRa beta)-Ra-alpha scaling relation. The computations clearly show that for Ra <= 10(10) at fixed L = 2 root 2, Nu <= 0.106Pr(0)Ra(5/12), which indicates that molecular transport cannot generally be neglected in the "ultimate" high-Ra regime.NSF DMS-0928098 DMS-1515161 DMS-0927587 PHY-1205219Simons FoundationNSFONRInstitute for Computational Engineering and Sciences (ICES
Monitoramento da eficiência de remoção de nitrogênio no tratamento.
Projeto/Plano de Ação: 04.10.00.011
Regimes of stratified turbulence at low Prandtl number
Quantifying transport by strongly stratified turbulence in low Prandtl number
() fluids is critically important for the development of better models for
the structure and evolution of stellar interiors. Motivated by recent numerical
simulations showing strongly anisotropic flows suggestive of scale-separated
dynamics, we perform a multiscale asymptotic analysis of the governing
equations. We find that, in all cases, the resulting slow-fast system naturally
takes a quasilinear form. Our analysis also reveals the existence of several
distinct dynamical regimes depending on the emergent buoyancy Reynolds and
P\'eclet numbers, and , respectively,
where is the aspect ratio of the large-scale turbulent flow
structures, and is the outer scale Reynolds number. Scaling relationships
relating the aspect ratio, the characteristic vertical velocity, and the
strength of the stratification (measured by the Froude number ) naturally
emerge from the analysis. When , the dynamics at all scales is
dominated by buoyancy diffusion, and our results recover the scaling laws
empirically obtained from direct numerical simulations by Cope et al. (2020).
For , diffusion is negligible (or at least subdominant) at all
scales and our results are consistent with those of Chini et al. (2022) for
strongly stratified geophysical turbulence at .Finally, we have
identified a new regime for , in which slow, large
scales are diffusive while fast, small scales are not. We conclude by
presenting a map of parameter space that clearly indicates the transitions
between isotropic turbulence, non-diffusive stratified turbulence, diffusive
stratified turbulence and viscously-dominated flows.Comment: 25 pages, 1 figur
Variability survey in the CoRoT SRa01 field: Implications of eclipsing binary distribution on cluster formation in NGC 2264
Time-series photometry of the CoRoT field SRa01 was carried out with the
Berlin Exoplanet Search Telescope II (BEST II) in 2008/2009. A total of 1,161
variable stars were detected, of which 241 were previously known and 920 are
newly found. Several new, variable young stellar objects have been discovered.
The study of the spatial distribution of eclipsing binaries revealed the higher
relative frequency of Algols toward the center of the young open cluster NGC
2264. In general Algol frequency obeys an isotropic distribution of their
angular momentum vectors, except inside the cluster, where a specific
orientation of the inclinations is the case. We suggest that we see the orbital
plane of the binaries almost edge-on.Comment: 18 pages, 8 figures, accepted for publication in Ap
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