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Recent work has described screech noise from a supersonic jet as being due to leakage of a wave that is otherwise trapped in the jet's interior. In that work, the simplest of many techniques used is ray tracing for a single shear-...
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Recent work has described screech noise from a supersonic jet as being due to leakage of a wave that is otherwise trapped in the jet's interior. In that work, the simplest of many techniques used is ray tracing for a single shear-layer modeled as a row of Stuart vortices. In the present work, a lower row of vortices is added to form a plane jet. Instead of plotting ray paths, a technique of visualization analogous to streaklines is used that better corresponds to instantaneous density fields as observed, for instance, by the Schlieren method. This produces striking images that show leakage of waves at each internal reflection resulting in a row of acoustic sources as envisioned since the 1950s. However, the sources are not isotropic and each has a zone of silence in the downstream direction. Leakage creates a fold in the wave pattern internal to the jet which leads to fine scale features. Reported experiments have also observed fine scale features (described as splitting) in the shock-cell pattern; they may be related to those observed here. Internally reflected rays also undergo a diffusive process as they propagate down the jet. In particular, each successive internal reflection at an unsteady shear-layer scatters rays along a wider range of wave angle and makes them more susceptible to leakage at the next reflection. It also causes more downstream directivity for the more downstream sources. An important result is that as the Mach number Mj is varied, maxima in leakage rate and mean acoustic amplitude occur at (near) resonances between the Mach-wave and shear-layer periods. Maxima in sound pressure level versus Mj have also been reported for laboratory round jets. Finally, as the shear-layer thickness is increased, a minimum in the rate of leakage (correlated with a minimum in radiation amplitude) occurs due to the competing effects of increased shear-layer penetration versus reduced eddy passage frequency.
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This paper presents model numerical simulations of photoassociation and ionization of cold Rb+Cs atoms steered by broadband optical pulses operating within the weak-field limit over a picosecond time scale. The primary focus of th...
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This paper presents model numerical simulations of photoassociation and ionization of cold Rb+Cs atoms steered by broadband optical pulses operating within the weak-field limit over a picosecond time scale. The primary focus of the work is on generation of RbCs molecules in bound levels of the (1)0~+ (X~1Σ~+) electronic ground state through a sequence of pump-dump transitions between (1)0~+ and (4)0~+ states driven by a field centered at 811 nm with different phase modulations. It is found that chirped fields generate substantially bound oscillator levels of the X~1Σ~+ state by avoiding promotion of amplitude to vibrational levels in the upper state whose Franck-Condon factors for stimulated emission to levels in the X Σ~+ state are detrimentally low, but which otherwise come into play when the driving field is transform limited. Optimal generation of molecules in the electronic ground state irrespective of vibrational level selectivity is calculated to occur when the temporal phase of the driving field is modulated by the classical energy difference for promotion of (4)0~+← (1)0~+ photon absorption. Conversely, generation of deeply bound vibrational eigenlevels of the X~1Σ~+state is optimally promoted by a field whose temporal phase enhances (4)0~+ → (1)0~+ stimulated emission. Driving fields phase modulated by the classical difference potential or linearly down-chirped enhance molecule formation vis-a-vis the thermal collision; fields whose temporal phase is modulated according to the shape of the (1)0~+ potential energy curve or which are linearly up-chirped suppress molecule formation relative to the thermal probability, but generate deeply bound X~1Σ~+ vibrational levels. Application of a bichromatic field comprising temporally overlapped components centered 811 and 622 nm also improves the probability of generating deeply bound vibrational levels, but at the expense of increased ionization. Model calculations of transition probabilities between neutral and ionic states of the collision lead to the conclusions that: transitions to individual vibrational levels of the X~1Σ~+ state occur with a higher probability than resonant three-photon ionization prompted by a single-color field at 811 nm; and application of bichromatic light at 811 and 622 nm preferentially ionizes the incipient molecule via a two-color two-photon absorption, the likelihood of which masks the occupation of deeply bound X~1Σ~+ oscillator levels.
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A dually activated ketene enolate, generated from an acid chloride, the unusual chelating nucleophile (1,8- naphthyridine), and a Lewis acid, reacts to afford a host of α,α-difluorinated products in the presence of a benchtop-st...
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A dually activated ketene enolate, generated from an acid chloride, the unusual chelating nucleophile (1,8- naphthyridine), and a Lewis acid, reacts to afford a host of α,α-difluorinated products in the presence of a benchtop-stable fluorinating agent (Selectfluor). The use of this method to synthesize otherwise difficult to make products is highlighted along with computational and spectroscopic support for the proposed chelate.
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Because can be a really useful word in an A-level unit test. Add it to a response becomes worth a mark. Use it sloppily though, perhaps when 'and' might have been more appropriate, and you may fail to gain credit. With these point...
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Because can be a really useful word in an A-level unit test. Add it to a response becomes worth a mark. Use it sloppily though, perhaps when 'and' might have been more appropriate, and you may fail to gain credit. With these points in mind, this Upgrade concentrates on just one word -- because.
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Allergic reactions to otherwise innocuous substances involve a complex interplay of molecular interactions-some strong, some weak. This study reveals a key role for low-affinity antibodies and thus a possible point of weakness tha...
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Allergic reactions to otherwise innocuous substances involve a complex interplay of molecular interactions-some strong, some weak. This study reveals a key role for low-affinity antibodies and thus a possible point of weakness that may be exploited for therapeutic intervention.
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Entries for this diary should be sent to the Journal Editor [editor@britastro.org] as soon as dates and locations are known. Details of all astronomical meetings of regional or national interest are welcome. The Editor's decision ...
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Entries for this diary should be sent to the Journal Editor [editor@britastro.org] as soon as dates and locations are known. Details of all astronomical meetings of regional or national interest are welcome. The Editor's decision on inclusion or otherwise of any meeting in this listing is final.
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Pseudouridine is the most abundant RNA modification, yet except for a few well-studied cases, little is known about the modified positions and their function(s). Here, we develop Ψ-seq for transcriptomewide quantitative mapping o...
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Pseudouridine is the most abundant RNA modification, yet except for a few well-studied cases, little is known about the modified positions and their function(s). Here, we develop Ψ-seq for transcriptomewide quantitative mapping of pseudouridine. We validate Ψ-seq with spike-ins and de novo identification of previously reported positions and discover hundreds of unique sites in human and yeast mRNAs and snoRNAs. Perturbing pseudouridine synthases (PUS) uncovers which pseudouridine synthase modifies each site and their target sequence features. mRNA pseudouridinylation depends on both sitespecific and snoRNA-guided pseudouridine synthases. Upon heat shock in yeast, Pus7p-mediated pseudouridylation is induced at >200 sites, and PUS7 deletion decreases the levels of otherwise pseudouridylated mRNA, suggesting a role in enhancing transcript stability. rRNA pseudouridine stoichiometries are conserved but reduced in cells from dyskeratosis congenita patients, where the PUS DKC1 is mutated. Our work identifies an enhanced, transcriptome-wide scope for pseudouridine and methods to dissect its underlying mechanisms and function.
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Records in Around the Region are published for interest only; their inclusion does not imply acceptance by the records committee of the relevant country. All records refer to 2017 unless stated otherwise.
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IN MY ADULT-EDUCATION COURSE Constellations of Spring, I always try toshowcase — outside, in the dark — someof the very sights we talk about in class.We observe from a spot just 6 miles (asthe crow flies) from the capitol dome i...
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IN MY ADULT-EDUCATION COURSE Constellations of Spring, I always try toshowcase — outside, in the dark — someof the very sights we talk about in class.We observe from a spot just 6 miles (asthe crow flies) from the capitol dome inAugusta, Maine. There's a new megamall5 miles away, hurting skies toward thesoutheast. Otherwise it's a decent spot.
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The term "aurora borealis", or "northern dawn" dates back centuries and refers to emissions of light from the otherwise-dark nighttime atmosphere, usually occurring in the polar regions except during highly disturbed periods. Auro...
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The term "aurora borealis", or "northern dawn" dates back centuries and refers to emissions of light from the otherwise-dark nighttime atmosphere, usually occurring in the polar regions except during highly disturbed periods. Aurora is distinct from airglow, which is a weak and relatively unstructured emission in the thermosphere caused by chemical reactions and ionization driven primarily by solar UV illumination during the day. In contrast, auroral emissions are the result of excitation of neutral atoms and molecules in the upper atmosphere by collisions with charged particles, typically which originate in the magnetosphere and precipitate along geomagnetic field lines with energies of hundreds of eV to tens of keV.
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