472 research outputs found
Incipient Separation in Shock Wave Boundary Layer Interactions as Induced by Sharp Fin
The incipient separation induced by the shock wave turbulent boundary layer
interaction at the sharp fin is the subject of present study. Existing theories
for the prediction of incipient separation, such as those put forward by McCabe
(1966) and Dou and Deng (1992), can have thus far only predicting the direction
of surface streamline and tend to over-predict the incipient separation
condition based on the Stanbrook's criterion. In this paper, the incipient
separation is firstly predicted with Dou and Deng (1992)'s theory and then
compared with Lu and Settles (1990)' experimental data. The physical mechanism
of the incipient separation as induced by the shock wave/turbulent boundary
layer interactions at sharp fin is explained via the surface flow pattern
analysis. Furthermore, the reason for the observed discrepancy between the
predicted and experimental incipient separation conditions is clarified. It is
found that when the wall limiting streamlines behind the shock wave becomes\
aligning with one ray from the virtual origin as the strength of shock wave
increases, the incipient separation line is formed at which the wall limiting
streamline becomes perpendicular to the local pressure gradient. The formation
of this incipient separation line is the beginning of the separation process.
The effects of Reynolds number and the Mach number on incipient separation are
also discussed. Finally, a correlation for the correction of the incipient
separation angle as predicted by the theory is also given.Comment: 34 pages; 9 figure
Second harmonic generation and birefringence of some ternary pnictide semiconductors
A first-principles study of the birefringence and the frequency dependent
second harmonic generation (SHG) coefficients of the ternary pnictide
semiconductors with formula ABC (A = Zn, Cd; B = Si, Ge; C = As, P) with
the chalcopyrite structures was carried out. We show that a simple empirical
observation that a smaller value of the gap is correlated with larger value of
SHG is qualitatively true. However, simple inverse power scaling laws between
gaps and SHG were not found. Instead, the real value of the nonlinear response
is a result of a very delicate balance between different intraband and
interband terms.Comment: 13 pages, 12 figure
Anisotropic splitting of intersubband spin plasmons in quantum wells with bulk and structural inversion asymmetry
In semiconductor heterostructures, bulk and structural inversion asymmetry
and spin-orbit coupling induce a k-dependent spin splitting of valence and
conduction subbands, which can be viewed as being caused by momentum-dependent
crystal magnetic fields. This paper studies the influence of these effective
magnetic fields on the intersubband spin dynamics in an asymmetric n-type
GaAs/AlGaAs quantum well. We calculate the dispersions of intersubband spin
plasmons using linear response theory. The so-called D'yakonov-Perel'
decoherence mechanism is inactive for collective intersubband excitations,
i.e., crystal magnetic fields do not lead to decoherence of spin plasmons.
Instead, we predict that the main signature of bulk and structural inversion
asymmetry in intersubband spin dynamics is a three-fold, anisotropic splitting
of the spin plasmon dispersion. The importance of many-body effects is pointed
out, and conditions for experimental observation with inelastic light
scattering are discussed.Comment: 8 pages, 6 figure
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
Barcodes of marine invertebrates from north Iberian ports: Native diversity and resistance to biological invasions
Ports are gateways for many marine organisms transported by ships worldwide, especially non-indigenous species (NIS). In this study carried out in North Iberian ports (Cantabrian Sea, Bay of Biscay) we have observed 38% of exotic macroinvertebrates. Four species, namely the barnacle Austrominius modestus, the tubeworm Ficopomatus enigmaticus, the Pacific oyster Crassostrea gigas and the pygmy mussel Xenostrobus securis, exhibited clear signs of invasiveness. A total of 671 barcode (cytochrome oxidase subunit I or 18S rRNA) genes were obtained and confirmed the species status of some cryptic NIS. Negative and significant correlation between diversity estimators of native biota and proportion of NIS suggests biotic resistance in ports. This could be applied to management of port biota for contributing to prevent the settlement of biopollutants in these areas which are very sensitive to biological invasions.Versión del editor2,359
Recommended operating room practice during the COVID-19 pandemic: systematic review
Background
The COVID-19 pandemic poses a critical global public health crisis. Operating room (OR) best practice in this crisis is poorly defined. This systematic review was performed to identify contemporary evidence relating to OR practice in the context of COVID-19.
Methods
MEDLINE was searched systematically using PubMed (search date 19 March 2020) for relevant studies in accordance with PRISMA guidelines. Documented practices and guidance were assessed to determine Oxford Centre for Evidence-Based Medicine (OCEBM) levels of evidence, and recommendations for practice within five domains were extracted: physical OR, personnel, patient, procedure, and other factors.
Results
Thirty-five articles were identified, of which 11 met eligibility criteria. Nine articles constituted expert opinion and two were retrospective studies. All articles originated from the Far East (China, 9; Singapore, 2); eight of the articles concerned general surgery. Common themes were identified within each domain, but all recommendations were based on low levels of evidence (median OCEBM level 5 (range 4–5)). The highest number of overlapping recommendations related to physical OR (8 articles) and procedural factors (13). Although few recommendations related to personnel factors, consensus was high in this domain, with all studies mandating the use of personal protective equipment.
Conclusion
There was little evidence to inform this systematic review, but there was consensus regarding many aspects of OR practice. Within the context of a rapidly evolving pandemic, timely amalgamation of global practice and experiences is needed to inform best practice
Astronomical Distance Determination in the Space Age: Secondary Distance Indicators
The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations. © 2018, The Author(s)
The emission of X-rays by fission fragments
Approximately 5% of the γ-ray transitions in fission fragments are internally converted, giving rise to X-rays. X-ray energies are characteristic of Z, but yields depend strongly and not smoothly on Z and A of the fragments and on time. If these biases can be accepted, X-rays can be used to identify the atomic number of fission fragments. More generally, internal conversion and X-ray emission must be interpreted in the context of detailed knowledge of γ-ray transitions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43110/1/10967_2005_Article_BF02039468.pd
Norms, Networks, Power, and Control: Understanding Informal Payments and Brokerage in Cross-Border Trade in Sierra Leone
Recent research has cast light on the variety of informal payments and practices that govern the day-to-day interactions between traders and customs agents at border posts in low-income countries. Building on this literature, this paper draws on survey and qualitative evidence in an effort to explore which groups are most advantaged and disadvantaged by the largely informal processes and norms governing cross-border trade. We find that understanding variation in strategies and outcomes across traders can only be effectively understood with reference to the importance of norms, networks, power, and the logic of control.Department for International DevelopmentBill and Melinda Gates Foundatio
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