540 research outputs found
Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer
In this study, a detailed model of an urban landscape has been re-constructed inthe wind tunnel and the flow structure inside and above the urban canopy has beeninvestigated. Vertical profiles of all three velocity components have been measuredwith a Laser-Doppler velocimeter, and an extensive analysis of the measured meanflow and turbulence profiles carried out. With respect to the flow structure inside thecanopy, two types of velocity profiles can be distinguished. Within street canyons,the mean wind velocities are almost zero or negative below roof level, while closeto intersections or open squares, significantly higher mean velocities are observed.In the latter case, the turbulent velocities inside the canopy also tend to be higherthan at street-canyon locations. For both types, turbulence kinetic energy and shearstress profiles show pronounced maxima in the flow region immediately above rooflevel. Based on the experimental data, a shear-stress parameterization is proposed, inwhich the velocity scale, us, and length scale, zs, are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, ds, is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature. The length scales derived from the shear-stress parameterization also allowdetermination of appropriate scales for the mean wind profile. The roughnesslength, z0, and displacement height, d0, can both be described as fractions of the distance, zs - ds, between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth zs - ds as the roughness layer. With respect to the lower part of the canopy (z < ds) the flow behaves as a skimming flow. Correlations between the length scales zs and ds and morphometric parameters are discussed. The mean wind profiles above the urban structure follow a logarithmic windlaw. A combination of morphometric estimation methods for d0 and z0 with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u*, appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughnesssublayer, a local scaling approach results in good agreement between measuredand predicted mean wind profile
Electronic transport in films of colloidal CdSe nanocrystals
We present results for electronic transport measurements on large
three-dimensional arrays of CdSe nanocrystals. In response to a step in the
applied voltage, we observe a power-law decay of the current over five orders
of magnitude in time. Furthermore, we observe no steady-state dark current for
fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the
power-law form of the decay is quite general, there are quantitative variations
with temperature, applied field, sample history, and the material parameters of
the array. Despite evidence that the charge injected into the film during the
measurement causes the decay of current, we find field-scaling of the current
at all times. The observation of extremely long-lived current transients
suggests the importance of long-range Coulomb interactions between charges on
different nanocrystals.Comment: 11 pages, 10 figure
Compact Lossy Trapdoor Functions and Selective Opening Security From LWE
Selective opening (SO) security is a security notion for public-key
encryption schemes that captures security against adaptive corruptions
of senders. SO security comes in chosen-plaintext (SO-CPA) and
chosen-ciphertext (SO-CCA) variants, neither of which is implied by
standard security notions like IND-CPA or IND-CCA security.
In this paper, we present the first SO-CCA secure encryption scheme that
combines the following two properties: (1) it has a constant ciphertext
expansion (i.e., ciphertexts are only larger than plaintexts by a constant
factor), and (2) its security can be proven from a standard assumption.
Previously, the only known SO-CCA secure encryption scheme achieving
(1) was built from an ad-hoc assumption in the RSA regime.
Our construction builds upon LWE, and in particular on a new and surprisingly
simple construction of compact lossy trapdoor functions (LTFs).
Our LTF can be converted into an “all-but-many LTF” (or ABM-LTF),
which is known to be sufficient to obtain SO-CCA security. Along the
way, we fix a technical problem in that previous ABM-LTF-based construction
of SO-CCA security
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In-street wind direction variability in the vicinity of a busy intersection in central London
We present results from fast-response wind measurements within and above a busy intersection between two street canyons (Marylebone Road and Gloucester Place) in Westminster, London taken as part of the DAPPLE (Dispersion of Air Pollution and Penetration into the Local Environment; www.dapple.org.uk) 2007 field campaign. The data reported here were collected using ultrasonic anemometers on the roof-top of a building adjacent to the intersection and at two heights on a pair of lamp-posts on opposite sides of the intersection. Site characteristics, data analysis and the variation of intersection flow with the above-roof wind direction (θref) are discussed. Evidence of both flow channelling and recirculation was identified within the canyon, only a few metres from the intersection for along-street and across-street roof-top winds respectively. Results also indicate that for oblique rooftop flows, the intersection flow is a complex combination of bifurcated channelled flows, recirculation and corner vortices. Asymmetries in local building geometry around the intersection and small changes in the background wind direction (changes in 15-min mean θref of 5–10 degrees) were also observed to have profound influences on the behaviour of intersection flow patterns. Consequently, short time-scale variability in the background flow direction can lead to highly scattered in-street mean flow angles masking the true multi-modal features of the flow and thus further complicating modelling challenges
Herschel PACS and SPIRE Observations of TWA brown dwarf discs
We present Herschel SPIRE observations for the TW Hydrae association (TWA)
brown dwarf discs SSSPM J1102-3431 (SS1102) and 2MASSW J1207334-393254
(2M1207). Both discs are undetected in the SPIRE 200-500mu bands. We have also
analyzed the archival PACS data and find no detection for either source in the
160mu band. Based on radiative transfer modeling, we estimate an upper limit to
the disc mass for both sources of 0.1 M_Jup. The lack of detection in the SPIRE
bands could be due to a paucity of millimeter sized dust grains in the 2M1207
and SS1102 discs. We also report a non-detection for the brown dwarf 2MASS
J1139511-315921 (2M1139) in the PACS 70 and 160mu bands. We have argued for the
presence of a warm debris disc around 2M1139, based on an excess emission
observed at 24mu. The mid-infrared colors for 2M1139 are similar to the
transition discs in the Taurus and Ophuichus regions. A comparison of the brown
dwarf disc masses over a ~1-10 Myr age interval suggests a decline in the disc
mass with the age of the system.Comment: Accepted in A&
Magnetic Field Dependence of the Level Spacing of a Small Electron Droplet
The temperature dependence of conductance resonances is used to measure the
evolution with the magnetic field of the average level spacing
of a droplet containing electrons created by lateral confinement of a
two-dimensional electron gas in GaAs. becomes very small (eV) near two critical magnetic fields at which the symmetry of the
droplet changes and these decreases of are predicted by
Hartree-Fock (HF) for charge excitations. Between the two critical fields,
however, the largest measured eV is an order of
magnitude smaller than predicted by HF but comparable to the Zeeman splitting
at this field, which suggests that the spin degrees of freedom are important.
PACS: 73.20.Dx, 73.20.MfComment: 11 pages of text in RevTeX, 4 figures in Postscript (files in the
form of uuencoded compressed tar file
Spin Exciton in quantum dot with spin orbit coupling in high magnetic field
Coulomb interactions of few () electrons confined in a disk shaped
quantum dot, with a large magnetic field applied in the z-direction
(orthogonal to the dot), produce a fully spin polarized ground state. We
numerically study the splitting of the levels corresponding to the multiplet of
total spin (each labeled by a different total angular momentum )
in presence of an electric field parallel to , coupled to by a
Rashba term. We find that the first excited state is a spin exciton with a
reversed spin at the origin. This is reminiscent of the Quantum Hall
Ferromagnet at filling one which has the skyrmion-like state as its first
excited state. The spin exciton level can be tuned with the electric field and
infrared radiation can provide energy and angular momentum to excite it.Comment: 9 pages, 9 figures. submitted to Phys.Rev.
Artificial Atoms
Contains research goals and objectives, reports on six research projects and a list of publications.Joint Services Electronics Program Contract DAAL03-92-C-0001Joint Services Electronics Program Grant DAAH04-95-1-0038National Science Foundation Grant ECS 92-0342
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