8,226 research outputs found
Development of a general purpose airborne simulator
Variable stability system development for General Purpose Airborne Simulator /GPAS
Isoprene photooxidation : new insights into the production of acids and organic nitrates
We describe a nearly explicit chemical mechanism for isoprene photooxidation guided by chamber studies that include time-resolved observation of an extensive suite of volatile compounds. We provide new constraints on the chemistry of the poorly-understood isoprene δ-hydroxy channels, which account for more than one third of the total isoprene carbon flux and a larger fraction of the nitrate yields. We show that the cis branch dominates the chemistry of the δ-hydroxy channel with less than 5% of the carbon following the trans branch. The modelled yield of isoprene nitrates is 12±3% with a large difference between the δ and β branches. The oxidation of these nitrates releases about 50% of the NOx. Methacrolein nitrates (modelled yield ≃15±3% from methacrolein) and methylvinylketone nitrates (modelled yield ≃11±3% yield from methylvinylketone) are also observed. Propanone nitrate, produced with a yield of 1% from isoprene, appears to be the longest-lived nitrate formed in the total oxidation of isoprene. We find a large molar yield of formic acid and suggest a novel mechanism leading to its formation from the organic nitrates. Finally, the most important features of this mechanism are summarized in a condensed scheme appropriate for use in global chemical transport models
Multi-particle-collision dynamics: Flow around a circular and a square cylinder
A particle-based model for mesoscopic fluid dynamics is used to simulate
steady and unsteady flows around a circular and a square cylinder in a
two-dimensional channel for a range of Reynolds number between 10 and 130.
Numerical results for the recirculation length, the drag coefficient, and the
Strouhal number are reported and compared with previous experimental
measurements and computational fluid dynamics data. The good agreement
demonstrates the potential of this method for the investigation of complex
flows.Comment: 6 pages, separated figures in .jpg format, to be published in
Europhysics Letter
Infinite Momentum Frame Calculation of Semileptonic Heavy \Lambda_b\to\Lambda_c Transitions Including HQET Improvements
We calculate the transition form factors that occur in heavy -type
baryon semileptonic decays as e.g. in . We use Bauer-Stech-Wirbel type infinite momentum frame wave
functions for the heavy -type baryons which we assume to consist of a
heavy quark and a light spin-isospin zero diquark system. The form factors at are calculated from the overlap integrals of the initial and final
-type baryon states. To leading order in the heavy mass scale the
structure of the form factors agrees with the HQET predictions including the
normalization at zero recoil. The leading order -dependence of the form
factors is extracted by scaling arguments. By comparing the model form factors
with the HQET predictions at we obtain a consistent set of
model form factors up to . With our preferred choice of
parameter values we find that the contribution of the non-leading form factor
is practically negligible. We use our form factor predictions to compute rates,
spectra and various asymmetry parameters for the semi-leptonic decay
.Comment: 24 pages, LaTeX, 6 figures are included in PostScript format. Final
version of paper to appear in Phys.Rev.
Hygroscopicity of secondary organic aerosols formed by oxidation of cycloalkenes, monoterpenes, sesquiterpenes, and related compounds
A series of experiments has been conducted in the Caltech indoor smog chamber facility to investigate the water uptake properties of aerosol formed by oxidation of various organic precursors. Secondary organic aerosol (SOA) from simple and substituted cycloalkenes (C5-C8) is produced in dark ozonolysis experiments in a dry chamber (RH~5%). Biogenic SOA from monoterpenes, sesquiterpenes, and oxygenated terpenes is formed by photooxidation in a humid chamber (~50% RH). Using the hygroscopicity tandem differential mobility analyzer (HTDMA), we measure the diameter-based hygroscopic growth factor (GF) of the SOA as a function of time and relative humidity. All SOA studied is found to be slightly hygroscopic, with smaller water uptake than that of typical inorganic aerosol substances. The aerosol water uptake increases with time early in the experiments for the cycloalkene SOA, but decreases with time for the biogenic SOA. This behavior could indicate competing effects between the formation of more highly oxidized polar compounds (more hygroscopic), and formation of longer-chained oligomers (less hygroscopic). All SOA also exhibit a smooth water uptake with RH with no deliquescence or efflorescence. The water uptake curves are found to be fitted well with an empirical three-parameter functional form. The measured pure organic GF values at 85% RH are between 1.09–1.16 for SOA from ozonolysis of cycloalkenes, 1.01–1.04 for sesquiterpene photooxidation SOA, and 1.06–1.11 for the monoterpene and oxygenated terpene SOA. The GF of pure SOA (GForg) in experiments in which inorganic seed aerosol is used is determined by assuming volume-weighted water uptake (Zdanovskii-Stokes-Robinson or ''ZSR'' approach) and using the size-resolved organic mass fraction measured by the Aerodyne Aerosol Mass Spectrometer. Knowing the water content associated with the inorganic fraction yields GForg values. However, for each precursor, the GForg values computed from different HTDMA-classified diameters agree with each other to varying degrees. Lack of complete agreement may be a result of the non-idealities of the solutions that are not captured by the ZSR method. Comparing growth factors from different precursors, we find that GForg is inversely proportional to the precursor molecular weight and SOA yield, which is likely a result of the fact that higher-molecular weight precursors tend to produce larger and less hygroscopic oxidation products
Exclusive Photoproduction of Large Momentum-Transfer K and K* Mesons
The reactions gamma p -> K+ Lambda and gamma p -> K* Lambda are analyzed
within perturbative QCD, allowing for diquarks as quasi-elementary constituents
of baryons. The diquark-model parameters and the quark-diquark distribution
amplitudes of proton and Lambda are taken from previous investigations of
electromagnetic baryon form factors and Compton-scattering off protons.
Unpolarized differential cross sections and polarization observables are
computed for different choices of the K and K* distribution amplitudes. The
asymptotic form of the K distribution amplitude (proportional to x1 x2) is
found to provide a satisfactory description of the K photoproduction data.Comment: 32 pages, 7 figures available as tared, compressed and uuencoded
PS-file
Scaling of Selfavoiding Tethered Membranes: 2-Loop Renormalization Group Results
The scaling properties of selfavoiding polymerized membranes are studied
using renormalization group methods. The scaling exponent \nu is calculated for
the first time at two loop order. \nu is found to agree with the Gaussian
variational estimate for large space dimension d and to be close to the Flory
estimate for d=3.Comment: 4 pages, RevTeX + 20 .eps file
Hard exclusive photoproduction of mesons
We calculate the differential cross section and single-polarization
observables for the reaction within perturbative QCD,
treating the proton as a quark-diquark system. The phenomenological couplings
of gauge bosons to (spatially extended) diquarks and the quark-diquark
distribution amplitude of the proton are adopted from previous investigations
of baryon form factors and two-photon processes. Going beyond leading order, we
take into account hadron-mass effects by means of a systematic expansion in the
small parameter (hadron mass/ photon energy). With the -meson
distribution amplitude taken from the literature our predictions for the
differential cross section at | t | \agt 4 \text{GeV}^2 seem to provide a
reasonable extrapolation of the low-t data and are also comparable in magnitude
with the results of a two-gluon exchange model in which the gluons are
considered as a remnant of the pomeron. For momentum transfers of a few GeV
hadron-mass effects appear still to be sizeable.Comment: 37 pages, 7 figures, uses RevTeX styl
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