7,659 research outputs found
The effect of Görtler instability on hypersonic boundary layer transition
AbstractThe evolution of Görtler vortices and its interaction with other instabilities are investigated in this paper. Both the Mack mode and the Görtler mode exist in hypersonic boundary-layer flows over concave surfaces, and their interactions are crucially important in boundary layer transition. We carry out a direct numerical simulation to explore the interaction between the Görtler and the oblique Mack mode. The results indicate that the interaction between the forced Görtler mode and the oblique Mack mode promotes the onset of the transition. The forced oblique Mack mode is susceptible to nonlinear interaction. Because of the development of the Görtler mode, the forced Mack mode and other harmonic modes are excited
Top Quark Rare Decays via Loop-Induced FCNC Interactions in Extended Mirror Fermion Model
Flavor changing neutral current (FCNC) interactions for a top quark
decays into with represents a neutral gauge or Higgs boson, and a
up- or charm-quark are highly suppressed in the Standard Model (SM) due to the
Glashow-Iliopoulos-Miami mechanism. Whilst current limits on the branching
ratios of these processes have been established at the order of from
the Large Hadron Collider experiments, SM predictions are at least nine orders
of magnitude below. In this work, we study some of these FCNC processes in the
context of an extended mirror fermion model, originally proposed to implement
the electroweak scale seesaw mechanism for non-sterile right-handed neutrinos.
We show that one can probe the process for a wide range of parameter
space with branching ratios varying from to , comparable
with various new physics models including the general two Higgs doublet model
with or without flavor violations at tree level, minimal supersymmetric
standard model with or without -parity, and extra dimension model.Comment: 30 pages, 8 figures, 2 tables and 1 appendix. Version to appear in
NP
Safe-by-Construction Autonomous Vehicle Overtaking using Control Barrier Functions and Model Predictive Control
Ensuring safety for vehicle overtaking systems is one of the most fundamental
and challenging tasks in autonomous driving. This task is particularly
intricate when the vehicle must not only overtake its front vehicle safely but
also consider the presence of potential opposing vehicles in the opposite lane
that it will temporarily occupy. In order to tackle the overtaking task in such
challenging scenarios, we introduce a novel integrated framework tailored for
vehicle overtaking maneuvers. Our approach integrates the theories of
varying-level control barrier functions (CBF) and time-optimal model predictive
control (MPC). The main feature of our proposed overtaking strategy is that it
is safe-by-construction, which enables rigorous mathematical proof and
validation of the safety guarantees. We show that the proposed framework is
applicable when the opposing vehicle is either fully autonomous or driven by
human drivers. To demonstrate our framework, we perform a set of simulations
for overtaking scenarios under different settings. The simulation results show
the superiority of our framework in the sense that it ensures collision-free
and achieves better safety performance compared with the standard MPC-based
approach without safety guarantees
Constraining the jet composition of GRB 221009A with the prompt TeV emission limit
Recent LHAASO observations of the prompt emission phase of the
brightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratio
between the TeV and MeV emissions, ,
during the period after the trigger. bf
This period covers the peak of the main MeV burst and is just before the TeV
afterglow emerges. Within the framework of internal shocks, we study the
internal absorption in GRB 221009A by generating a set of
synthetic bursts in a simulation that reproduces the observed feature of GRB
221009A. We find that the absorption does not lead to an
exponential cutoff, but rather a power-law spectrum, consistent with previous
works. We further find that the attenuation due to absorption
alone cannot explain the flux limit ratio of GRB 221009A, suggesting a low
ratio between synchrotron self-Compton (SSC) and synchrotron emission outputs.
This requires the magnetic field energy density to be much larger than the
synchrotron photon energy density so that the SSC flux is greatly suppressed.
This indicates that the jet composition of GRB 221009A is likely
Poynting-flux-dominated.Comment: 11 pages, 5 figures, comments are welcom
Atomically phase-matched second-harmonic generation in a 2D crystal.
Second-harmonic generation (SHG) has found extensive applications from hand-held laser pointers to spectroscopic and microscopic techniques. Recently, some cleavable van der Waals (vdW) crystals have shown SHG arising from a single atomic layer, where the SH light elucidated important information such as the grain boundaries and electronic structure in these ultra-thin materials. However, despite the inversion asymmetry of the single layer, the typical crystal stacking restores inversion symmetry for even numbers of layers leading to an oscillatory SH response, drastically reducing the applicability of vdW crystals such as molybdenum disulfide (MoS2). Here, we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS2. We experimentally observed quadratic dependence of second-harmonic intensity on layer number as a result of atomically phase-matched nonlinear dipoles in layers of the 3R crystal that constructively interfere. By studying the layer evolution of the A and B excitonic transitions in 3R-MoS2 using SHG spectroscopy, we also found distinct electronic structure differences arising from the crystal structure and the dramatic effect of symmetry and layer stacking on the nonlinear properties of these atomic crystals. The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media
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