1,993 research outputs found
Genesis of d'Alembert's paradox and analytical elaboration of the drag problem
We show that the issue of the drag exerted by an incompressible fluid on a
body in uniform motion has played a major role in the early development of
fluid dynamics. In 1745 Euler came close, technically, to proving the vanishing
of the drag for a body of arbitrary shape; for this he exploited and
significantly extended existing ideas on decomposing the flow into thin
fillets; he did not however have a correct picture of the global structure of
the flow around a body. Borda in 1766 showed that the principle of live forces
implied the vanishing of the drag and should thus be inapplicable to the
problem. After having at first refused the possibility of a vanishing drag,
d'Alembert in 1768 established the paradox, but only for bodies with a
head-tail symmetry. A full understanding of the paradox, as due to the neglect
of viscous forces, had to wait until the work of Saint-Venant in 1846.Comment: 10 pages, 4 figures, Physica D, in pres
Nelkin scaling for the Burgers equation and the role of high-precision calculations
Nelkin scaling, the scaling of moments of velocity gradients in terms of the
Reynolds number, is an alternative way of obtaining inertial-range information.
It is shown numerically and theoretically for the Burgers equation that this
procedure works already for Reynolds numbers of the order of 100 (or even lower
when combined with a suitable extended self-similarity technique). At moderate
Reynolds numbers, for the accurate determination of scaling exponents, it is
crucial to use higher than double precision. Similar issues are likely to arise
for three-dimensional Navier--Stokes simulations.Comment: 5 pages, 2 figures, Published in Phys. Rev E (Rapid Communications
Entire solutions of hydrodynamical equations with exponential dissipation
We consider a modification of the three-dimensional Navier--Stokes equations
and other hydrodynamical evolution equations with space-periodic initial
conditions in which the usual Laplacian of the dissipation operator is replaced
by an operator whose Fourier symbol grows exponentially as \ue ^{|k|/\kd} at
high wavenumbers . Using estimates in suitable classes of analytic
functions, we show that the solutions with initially finite energy become
immediately entire in the space variables and that the Fourier coefficients
decay faster than \ue ^{-C(k/\kd) \ln (|k|/\kd)} for any . The
same result holds for the one-dimensional Burgers equation with exponential
dissipation but can be improved: heuristic arguments and very precise
simulations, analyzed by the method of asymptotic extrapolation of van der
Hoeven, indicate that the leading-order asymptotics is precisely of the above
form with . The same behavior with a universal constant
is conjectured for the Navier--Stokes equations with exponential
dissipation in any space dimension. This universality prevents the strong
growth of intermittency in the far dissipation range which is obtained for
ordinary Navier--Stokes turbulence. Possible applications to improved spectral
simulations are briefly discussed.Comment: 29 pages, 3 figures, Comm. Math. Phys., in pres
Hyperviscosity, Galerkin truncation and bottlenecks in turbulence
It is shown that the use of a high power of the Laplacian in the
dissipative term of hydrodynamical equations leads asymptotically to truncated
inviscid \textit{conservative} dynamics with a finite range of spatial Fourier
modes. Those at large wavenumbers thermalize, whereas modes at small
wavenumbers obey ordinary viscous dynamics [C. Cichowlas et al. Phys. Rev.
Lett. 95, 264502 (2005)]. The energy bottleneck observed for finite
may be interpreted as incomplete thermalization. Artifacts arising from models
with are discussed.Comment: 4 pages, 2 figures, Phys. Rev. Lett. in pres
Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data Release
The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained
five-band optical-wavelength imaging near the Galactic plane outside of the
nominal survey boundaries. These additional data were obtained during
commissioning and subsequent testing of the SDSS observing system, and they
provide unique wide-area imaging data in regions of high obscuration and star
formation, including numerous young stellar objects, Herbig-Haro objects and
young star clusters. Because these data are outside the Survey regions in the
Galactic caps, they are not part of the standard SDSS data releases. This paper
presents imaging data for 832 square degrees of sky (including repeats), in the
star-forming regions of Orion, Taurus, and Cygnus. About 470 square degrees are
now released to the public, with the remainder to follow at the time of SDSS
Data Release 4. The public data in Orion include the star-forming region NGC
2068/NGC 2071/HH24 and a large part of Barnard's loop.Comment: 31 pages, 9 figures (3 missing to save space), accepted by AJ, in
press, see http://photo.astro.princeton.edu/oriondatarelease for data and
paper with all figure
The Seventh Data Release of the Sloan Digital Sky Survey
This paper describes the Seventh Data Release of the Sloan Digital Sky Survey
(SDSS), marking the completion of the original goals of the SDSS and the end of
the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most
of the roughly 2000 deg^2 increment over the previous data release lying in
regions of low Galactic latitude. The catalog contains five-band photometry for
357 million distinct objects. The survey also includes repeat photometry over
250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A
coaddition of these data goes roughly two magnitudes fainter than the main
survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2
in the Northern Galactic Cap, closing the gap that was present in previous data
releases. There are over 1.6 million spectra in total, including 930,000
galaxies, 120,000 quasars, and 460,000 stars. The data release includes
improved stellar photometry at low Galactic latitude. The astrometry has all
been recalibrated with the second version of the USNO CCD Astrograph Catalog
(UCAC-2), reducing the rms statistical errors at the bright end to 45
milli-arcseconds per coordinate. A systematic error in bright galaxy photometr
is less severe than previously reported for the majority of galaxies. Finally,
we describe a series of improvements to the spectroscopic reductions, including
better flat-fielding and improved wavelength calibration at the blue end,
better processing of objects with extremely strong narrow emission lines, and
an improved determination of stellar metallicities. (Abridged)Comment: 20 pages, 10 embedded figures. Accepted to ApJS after minor
correction
The Sloan Digital Sky Survey: Technical Summary
The Sloan Digital Sky Survey (SDSS) will provide the data to support detailed
investigations of the distribution of luminous and non- luminous matter in the
Universe: a photometrically and astrometrically calibrated digital imaging
survey of pi steradians above about Galactic latitude 30 degrees in five broad
optical bands to a depth of g' about 23 magnitudes, and a spectroscopic survey
of the approximately one million brightest galaxies and 10^5 brightest quasars
found in the photometric object catalog produced by the imaging survey. This
paper summarizes the observational parameters and data products of the SDSS,
and serves as an introduction to extensive technical on-line documentation.Comment: 9 pages, 7 figures, AAS Latex. To appear in AJ, Sept 200
The Third Data Release of the Sloan Digital Sky Survey
This paper describes the Third Data Release of the Sloan Digital Sky Survey
(SDSS). This release, containing data taken up through June 2003, includes
imaging data in five bands over 5282 deg^2, photometric and astrometric
catalogs of the 141 million objects detected in these imaging data, and spectra
of 528,640 objects selected over 4188 deg^2. The pipelines analyzing both
images and spectroscopy are unchanged from those used in our Second Data
Release.Comment: 14 pages, including 2 postscript figures. Submitted to AJ. Data
available at http://www.sdss.org/dr
Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV
A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe
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