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Jet in supersonic crossflow
Based on research into jets in supersonic crossflow carried out by the authors' team over the past 15 years, this book summarizes and presents many cutting-edge findings and analyses on this subject. It tackles the complicated mixing process of gas jets and atomization process of liquid jets in supersonic crossflow, and studies their physical mechanisms. Advanced experimental and numerical techniques are applied to further readers' understanding of atomization, mixing, and combustion of fuel jets in supersonic crossflow, which can promote superior fuel injection design in scramjet engines. The book offers a valuable reference guide for all researchers and engineers working on the design of scramjet engines, and will also benefit graduate students majoring in aeronautical and aerospace engineering.
Book
The merged and superposed sub‐tropical jet and polar‐front jet in the southwest Pacific: A case study
In the southwest Pacific, a meandering jet‐stream in the upper troposphere is sometimes found at ~30° S during austral winters and is usually treated as a sub‐tropical jet (STJ) due to its low latitude. For two contrasting cases, we have conducted analyses from two perspectives to identify the STJ and PFJ: first, using previously published qualitative criteria to identify jet‐cores and second, investigating the jet‐stream axes of STJ and PFJ identified using 2‐PVU curves. The results showed that the chosen meandering jet‐stream case at ~30° S was a merged, and for a time, a superposed STJ and PFJ. Downstream of the jet‐streak, the PFJ split to the south and the STJ to the east. This is in significant contrast to the horizontally well‐separated jet‐stream case chosen in this study. Some processes likely contributing to the superposition of the STJ and PFJ were analyzed and discussed. The movement of PFJ that was closely associated with the movement of the low over the Tasman Sea and the convection in and near the tropical region may have played dominant roles.
Sub‐tropical jet‐streams (STJ) and polar‐front jet‐streams (PFJ) are belts of strong upper‐level winds in the upper troposphere which are generated by different mechanisms. Horizontally well‐separated STJ and PFJ cannot always be detected at the same time in a region. Sometimes, a single meandering jet‐stream is found instead of two separate jets. For example, end of June 2014, a single meandering jet as shown in the diagram (shading) was found in the southwest Pacific at around 30° S and was considered a STJ due to its low latitude. However, this study shows that the single jet is actually a merged and even superposed STJ and PFJ. Correctly identifying a jet‐stream is vital to understand its associated processes and their influence on weather and climate. In addition, jet superpositions have been found to be associated with some high impact weather.
Journal Article
Boosting H → b b ¯ $$ H\\to b\\overline{b} $$ with machine learning
Abstract High-p T Higgs production at hadron colliders provides a direct probe of the internal structure of the gg → H loop with the H → b b ¯ $$ H\\to b\\overline{b} $$ decay offering the most statistics due to the large branching ratio. Despite the overwhelming QCD background, recent advances in jet substructure have put the observation of the gg → H → b b ¯ $$ gg\\to H\\to b\\overline{b} $$ channel at the LHC within the realm of possibility. In order to enhance the sensitivity to this process, we develop a two-stream convolutional neural network, with one stream acting on jet information and one using global event properties. The neural network significantly increases the discovery potential of a Higgs signal, both for high-p T Standard Model production as well for possible beyond the Standard Model contributions. Unlike most studies for boosted hadronically decaying massive particles, the boosted Higgs search is unique because double b-tagging rejects nearly all background processes that do not have two hard prongs. In this context — which goes beyond state-of-the-art two-prong tagging — the network is studied to identify the origin of the additional information leading to the increased significance. The procedures described here are also applicable to related final states where they can be used to identify additional sources of discrimination power that are not being exploited by current techniques.
Journal Article
Importance of the nozzle-exit boundary-layer state in subsonic turbulent jets
To investigate the effects of the nozzle-exit conditions on jet flow and sound fields, large-eddy simulations of an isothermal Mach 0.9 jet issued from a convergent-straight nozzle are performed at a diameter-based Reynolds number of
$1\\times 10^{6}$
. The simulations feature near-wall adaptive mesh refinement, synthetic turbulence and wall modelling inside the nozzle. This leads to fully turbulent nozzle-exit boundary layers and results in significant improvements for the flow field and sound predictions compared with those obtained from the typical approach based on laminar flow in the nozzle. The far-field pressure spectra for the turbulent jet match companion experimental measurements, which use a boundary-layer trip to ensure a turbulent nozzle-exit boundary layer to within 0.5 dB for all relevant angles and frequencies. By contrast, the initially laminar jet results in greater high-frequency noise. For both initially laminar and turbulent jets, decomposition of the radiated noise into azimuthal Fourier modes is performed, and the results show similar azimuthal characteristics for the two jets. The axisymmetric mode is the dominant source of sound at the peak radiation angles and frequencies. The first three azimuthal modes recover more than 97 % of the total acoustic energy at these angles and more than 65 % (i.e. error less than 2 dB) for all angles. For the main azimuthal modes, linear stability analysis of the near-nozzle mean-velocity profiles is conducted in both jets. The analysis suggests that the differences in radiated noise between the initially laminar and turbulent jets are related to the differences in growth rate of the Kelvin–Helmholtz mode in the near-nozzle region.
Journal Article
Measurement of the primary Lund jet plane density in proton-proton collisions at $ \\sqrt{\\textrm{s}} $ = 13 TeV
A measurement is presented of the primary Lund jet plane (LJP) density in inclusive jet production in proton-proton collisions. The analysis uses 138 fb-1 of data collected by the CMS experiment at $\\sqrt{s}$ = 13 TeV. The LJP, a representation of the phase space of emissions inside jets, is constructed using iterative jet declustering. The transverse momentum kT and the splitting angle ΔR of an emission relative to its emitter are measured at each step of the jet declustering process. The average density of emissions as function of ln(kT/GeV) and ln(R/ΔR) is measured for jets with distance parameters R = 0.4 or 0.8, transverse momentum pT > 700 GeV, and rapidity |y| < 1.7. The jet substructure is measured using the charged-particle tracks of the jet. The measured distributions, unfolded to the level of stable charged particles, are compared with theoretical predictions from simulations and with perturbative quantum chromodynamics calculations. Due to the ability of the LJP to factorize physical effects, these measurements can be used to improve different aspects of the physics modeling in event generators.
Journal Article
Search for massive resonances in dijet systems containing jets tagged as W or Z boson decays in pp collisions at ... = 8 TeV
Abstract A search is reported for massive resonances decaying into a quark and a vector boson (W or Z), or two vector bosons (WW, WZ, or ZZ). The analysis is performed on an inclusive sample of multijet events corresponding to an integrated luminosity of 19.7 fb-1, collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. The search uses novel jet-substructure identification techniques that provide sensitivity to the presence of highly boosted vector bosons decaying into a pair of quarks. Exclusion limits are set at a confidence level of 95% on the production of: (i) excited quark resonances q *decaying to qW and qZ for masses less than 3.2 TeV and 2.9 TeV, respectively, (ii) a Randall-Sundrum graviton GRS decaying into WW for masses below 1.2 TeV, and (iii) a heavy partner of the W boson W' decaying into WZ for masses less than 1.7 TeV. For the first time mass limits are set on W' [arrow right] WZ and GRS [arrow right] WW in the all-jets final state. The mass limits on q* [arrow right] qW, q* [arrow right] qZ, W' [arrow right] WZ, GRS [arrow right] WW are the most stringent to date. A model with a \"bulk\" graviton Gbulk that decays into WW or ZZ bosons is also studied. [Figure not available: see fulltext.]
Journal Article
A review on recent advances in machining methods based on abrasive jet polishing (AJP)
Abrasive jet polishing (AJP) is a new non-conventional machining technology for applying to polish the complex surfaces and small areas. Compared with other polishing technologies, AJP has the following advantages: high precision, easy to control, small machining force, good flexibility, without thermal distortion, etc. A review of five main AJP technologies has been conducted to provide an insight into the trends in research of principles, technological method, and impact of polishing quality. Several AJP methods discussed in this work include abrasive water jet polishing, nanoparticle colloid jet polishing, magnetorheological jet polishing, abrasive air jet polishing, and negative pressure cavity jet polishing. The monitoring methods of AJP process are introduced. The jet velocity, material removal, surface roughness, and numerical modeling of jet polishing are also discussed. The effects of some major technological parameters are analyzed. The polish results of metal, glass, and silicon materials are summarized. The probable further research tendency on AJP technology is forecasted. It is a high-potential technology to machine the microstructures and difficult-to-machine materials.
Journal Article
Radio Imaging of the Very-High-Energy γ-Ray Emission Region in the Central Engine of a Radio Galaxy
The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10¹² electron volts and are bright sources of very-high-energy (VHE) γ-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE γ-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.
Journal Article