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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
Modification of jet substructure in heavy ion collisions as a probe of the resolution length of quark-gluon plasma
A
bstract
We present an analysis of the role that the quark-gluon plasma (QGP) resolution length, the minimal distance by which two nearby colored charges in a jet must be separated such that they engage with the plasma independently, plays in understanding the modification of jet substructure due to interaction with QGP. The shorter the resolution length of QGP, the better its resolving power. We identify a set of observables that are sen- sitive to whether jets are quenched as if they are single energetic colored objects or whether the medium that quenches them has the ability to resolve the internal structure of the jet. Using the hybrid strong/weak coupling model, we find that although the ungroomed jet mass is not suitable for this purpose (because it is more sensitive to effects coming from particles reconstructed as a part of a jet that originate from the wake that the jet leaves in the plasma), groomed observables such as the number of Soft Drop splittings
n
SD
, the momentum sharing fraction
z
g
, or the
groomed
jet mass are particularly well-suited to discriminate the degree to which the QGP medium resolves substructure within a jet. In order to find the optimal grooming strategy, we explore different cuts in the Lund plane that allow for a clear identification of the regions of Soft Drop phase space that enhance the differences in the jet substructure between jets in vacuum and quenched jets. Comparison with present data seems to disfavor an “infinite resolution length”, which is to say the hypothesis that the medium interacts with the jet as if it were a single energetic colored object. Our analysis indicates that as the precision of experimental measurements of jet substructure observables and the control over uncertainties in their calculation improves, it will become possible to use comparisons like this to constrain the value of the resolution length of QGP, in addition to seeing how the substructure of jets is modified via their passage through it.
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
Experimental investigation of the structure of plane turbulent wall jets. Part 1. Spectral analysis
Plane turbulent wall jets are traditionally considered to be composed of a turbulent boundary layer (TBL) topped by a half-free jet. However, certain peculiar features, such as counter-gradient momentum flux occurring below velocity maximum in experiments and numerical simulations, suggest a different structure of turbulence therein. Here, we hypothesize that turbulence in wall jets has two distinct structural modes, wall mode scaling on wall variables and free-jet mode scaling on jet variables. To investigate this hypothesis, experimental data from our wall jet facility are acquired using single hot-wire anemometry and two-dimensional particle image velocimetry at three nozzle Reynolds numbers 10 244, 15 742 and 21 228. Particle image velocimetry measurements with four side-by-side cameras capture the longest field of view studied so far in wall jets. Direct spatial spectra of these fields reveal modal spectral contributions to variances of velocity fluctuations, Reynolds shear stress, shear force, turbulence production, velocity fluctuation triple products and turbulent transport. The free-jet mode has wavelengths scaling on the jet length scale ${z_{T}}$, and contains two dominant submodes with wavelengths $5{z_{T}}$ and $2.5{z_{T}}$. The region of flow above the velocity maximum shows the presence of the outer jet mode whereas the region below it shows robust bimodal behaviour attributed to both wall and inner jet modes. Counter-gradient momentum flux is effected by the outer jet mode intruding into the region below velocity maximum. These findings support the hypothesis of wall and free-jet structural modes, and indicate that the region below velocity maximum could be much complex than a conventional TBL.
Journal Article
Flowfield and Noise Dynamics of Supersonic Rectangular Impinging Jets: Major versus Minor Axis Orientations
The current study explores the flowfield and noise characteristics of an ideally expanded supersonic (Mach 1.44) rectangular jet impinging on a flat surface. The existing literature is primarily concentrated on axisymmetric jets, known for their resonance dominance, pronounced unsteadiness, and acoustic signatures. In contrast, non-axisymmetric jets remain relatively less understood, particularly those impinging on a ground surface. By employing Schlieren imaging, high-frequency pressure measurements using high-bandwidth transducers, and particle image velocimetry (PIV), this research comprehensively examines the flow-acoustic phenomena. Schlieren imaging revealed distinct, coherent structures and strong acoustic waves, while pressure measurements at the impingement surface exhibited high-amplitude fluctuations, peaking at approximately 186 dB. Acoustic analysis identified multiple high-amplitude tones with unique directional characteristics, suggesting the potential for multiple simultaneous modes in rectangular jets. Furthermore, the PIV data elucidated differences in the jet shear layer and wall jet development attributed to the nozzle orientation. These findings contribute to a deeper understanding of non-axisymmetric jet behavior, offering insights relevant to fundamental flow physics and practical applications such as vertical takeoff and landing aircraft.
Journal Article
Flying Camelot
Winner of the Gardner-Lasser Aerospace History Literature Award
Flying Camelot brings us back to the post-Vietnam era, when the US Air Force launched two new, state-of-the art fighter aircraft: the F-15 Eagle and the F-16 Fighting Falcon.
It was an era when debates about aircraft superiority went public-and these were not uncontested discussions. Michael W. Hankins delves deep into the fighter pilot culture that gave rise to both designs, showing how a small but vocal group of pilots, engineers, and analysts in the Department of Defense weaponized their own culture to affect technological development and larger political change.
The design and advancement of the F-15 and F-16 reflected this group's nostalgic desire to recapture the best of World War I air combat. Known as the \"Fighter Mafia,\" and later growing into the media savvy political powerhouse \"Reform Movement,\" it believed that American weapons systems were too complicated and expensive, and thus vulnerable. The group's leader was Colonel John Boyd, a contentious former fighter pilot heralded as a messianic figure by many in its ranks. He and his group advocated for a shift in focus from the multi-role interceptors the Air Force had designed in the early Cold War towards specialized air-to-air combat dogfighters. Their influence stretched beyond design and into larger politicized debates about US national security, debates that still resonate today.
A biography of fighter pilot culture and the nostalgia that drove decision-making, Flying Camelot deftly engages both popular culture and archives to animate the movement that shook the foundations of the Pentagon and Congress.
eBook
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