129 research outputs found
Transition in Hypersonic Boundary Layers: Role of Dilatational Waves
Transition and turbulence production in a hypersonic boundary layer is
investigated in a Mach 6 quiet wind tunnel using Rayleigh-scattering
visualization, fast-response pressure measurements, and particle image
velocimetry. It is found that the second instability acoustic mode is the key
modulator of the transition process. The second mode experiences a rapid growth
and a very fast annihilation due to the effect of bulk viscosity. The second
mode interacts strongly with the first vorticity mode to directly promote a
fast growth of the latter and leads to immediate transition to turbulence.Comment: 5 pages, 6 figure
A compact simple HWENO scheme with ADER time discretization for hyperbolic conservation laws I: structured meshes
In this paper, a compact and high order ADER (Arbitrary high order using
DERivatives) scheme using the simple HWENO method (ADER-SHWENO) is proposed for
hyperbolic conservation laws. The newly-developed method employs the
Lax-Wendroff procedure to convert time derivatives to spatial derivatives,
which provides the time evolution of the variables at the cell interfaces. This
information is required for the simple HWENO reconstructions, which take
advantages of the simple WENO and the classic HWENO. Compared with the original
Runge-Kutta HWENO method (RK-HWENO), the new method has two advantages.
Firstly, RK-HWENO method must solve the additional equations for
reconstructions, which is avoided for the new method. Secondly, the SHWENO
reconstruction is performed once with one stencil and is different from the
classic HWENO methods, in which both the function and its derivative values are
reconstructed with two different stencils, respectively. Thus the new method is
more efficient than the RK-HWENO method. Moreover, the new method is more
compact than the existing ADER-WENO method. Besides, the new method makes the
best use of the information in the ADER method. Thus, the time evolution of the
cell averages of the derivatives is simpler than that developed in the work [Li
et. al., 447 (2021), 110661.]. Numerical tests indicate that the new method can
achieve high order for smooth solutions both in space and time, keep
non-oscillatory at discontinuities
Flexible coherent control of plasmonic spin-Hall effect
The surface plasmon polariton is an emerging candidate for miniaturizing optoelectronic circuits. Recent demonstrations of polarization-dependent splitting using metasurfaces, including focal-spot shifting and unidirectional propagation, allow us to exploit the spin degree of freedom in plasmonics. However, further progress has been hampered by the inability to generate more complicated and independent surface plasmon profiles for two incident spins, which work coherently together for more flexible and tunable functionalities. Here by matching the geometric phases of the nano-slots on silver to specific superimpositions of the inward and outward surface plasmon profiles for the two spins, arbitrary spin-dependent orbitals can be generated in a slot-free region. Furthermore, motion pictures with a series of picture frames can be assembled and played by varying the linear polarization angle of incident light. This spin-enabled control of orbitals is potentially useful for tip-free near-field scanning microscopy, holographic data storage, tunable plasmonic tweezers, and integrated optical components
Cure the headache of Transformers via Collinear Constrained Attention
As the rapid progression of practical applications based on Large Language
Models continues, the importance of extrapolating performance has grown
exponentially in the research domain. In our study, we identified an anomalous
behavior in Transformer models that had been previously overlooked, leading to
a chaos around closest tokens which carried the most important information.
We've coined this discovery the "headache of Transformers". To address this at
its core, we introduced a novel self-attention structure named Collinear
Constrained Attention (CoCA). This structure can be seamlessly integrated with
existing extrapolation, interpolation methods, and other optimization
strategies designed for traditional Transformer models. We have achieved
excellent extrapolating performance even for 16 times to 24 times of sequence
lengths during inference without any fine-tuning on our model. We have also
enhanced CoCA's computational and spatial efficiency to ensure its
practicality. We plan to open-source CoCA shortly. In the meantime, we've made
our code available in the appendix for reappearing experiments.Comment: 16 pages, 6 figure
Enlarging Regional Disparities in Energy Intensity within China
As energy saving and emission reduction become a global action, the disparity in energy intensity between different regions is a new rising problem that stems a country's or region's energy-saving potential. Here we collect China's provincial panel data (1995–2017) of primary and final energy consumption to evaluate China's unequal and polarized regional pattern in energy intensity, decompose the inequality index into contributing components, and investigate possible driving factors behind the unequal pattern both regionally and structurally, for the first time. The results show that China's interprovince disparities in energy intensity increase and are exacerbated by the enlarging disparities in energy intensity between the least developed and most developed regions of China. The causes for this phenomenon are as follows: (i) rather loose regulatory measures on mitigating coal consumption; (ii) inferior energy processing technology in areas specializing in energy-intensive industries; (iii) increasing interregional energy fluxes embodied in trade; and (iv) separate jurisdictions at provincial administrative levels. These factors can synthetically result in unintended spillover to areas with inferior green technologies, suggesting an increasingly uneven distribution of energy-intensive and carbon-intensive industries and usage of clean energy. The results reveal the necessities of regional coordination and cooperation to achieve a green economy
Continual Learning in Predictive Autoscaling
Predictive Autoscaling is used to forecast the workloads of servers and
prepare the resources in advance to ensure service level objectives (SLOs) in
dynamic cloud environments. However, in practice, its prediction task often
suffers from performance degradation under abnormal traffics caused by external
events (such as sales promotional activities and applications
re-configurations), for which a common solution is to re-train the model with
data of a long historical period, but at the expense of high computational and
storage costs. To better address this problem, we propose a replay-based
continual learning method, i.e., Density-based Memory Selection and Hint-based
Network Learning Model (DMSHM), using only a small part of the historical log
to achieve accurate predictions. First, we discover the phenomenon of sample
overlap when applying replay-based continual learning in prediction tasks. In
order to surmount this challenge and effectively integrate new sample
distribution, we propose a density-based sample selection strategy that
utilizes kernel density estimation to calculate sample density as a reference
to compute sample weight, and employs weight sampling to construct a new memory
set. Then we implement hint-based network learning based on hint representation
to optimize the parameters. Finally, we conduct experiments on public and
industrial datasets to demonstrate that our proposed method outperforms
state-of-the-art continual learning methods in terms of memory capacity and
prediction accuracy. Furthermore, we demonstrate remarkable practicability of
DMSHM in real industrial applications
Exchange renormalized crystal field excitation in a quantum Ising magnet KTmSe
Rare-earth delafossite compounds, ARCh (A = alkali or monovalent ion, R =
rare earth, Ch = chalcogen), have been proposed for a range of novel quantum
phenomena. Particularly, the Tm series, ATmCh, featuring Tm ions on a
triangular lattice, serves as a representative group of compounds to illustrate
the interplay and competition between spin-orbit coupling, crystal fields, and
exchange couplings in the presence of geometric frustration. Here we report the
thermodynamic and inelastic neutron scattering studies on the newly discovered
triangular-lattice magnet KTmSe. Both heat capacity and neutron diffraction
reveal the absence of long-range magnetic order. Magnetic susceptibility shows
strong Ising-like interactions with antiferromagnetic correlations.
Furthermore, inelastic neutron scattering measurements reveal a branch of
dispersive crystal field excitations. To analyze these observations, we employ
both the transverse field Ising model and the full crystal field scheme, along
with exchange interactions. Our results suggest a strong competition between
spin exchange interactions and crystal field effects. This work is expected to
offer a valuable framework for understanding low-temperature magnetism in
KTmSe and similar materials.Comment: 9 pages, 4 figures. Submitted on the behalf of Shiyi Zhen
Current practice of physical activity counselling within physiotherapy usual care and influences on its use : a cross-sectional survey
Physical activity counselling has demonstrated effectiveness at increasing physical activity when delivered in healthcare, but is not routinely practised. This study aimed to determine (1) current use of physical activity counselling by physiotherapists working within publicly funded hospitals; and (2) influences on this behaviour. A cross‐sectional survey of physiotherapists was conducted across five hospitals within a local health district in Sydney, Australia. The survey investigated physiotherapists’ frequency of incorporating 15 different elements of physical activity counselling into their usual healthcare interactions, and 53 potential influences on their behaviour framed by the COM‐B (Capability, Opportunity, Motivation‐Behaviour) model. The sample comprised 84 physiotherapists (79% female, 48% 90% indicating their patients lacked financial and transport opportunities. These findings confirm that physical activity counselling is not routinely incorporated in physiotherapy practice and help to identify implementation strategies to build clinicians’ opportunities and capabilities to deliver physical activity counselling
Ice-nucleating particles from multiple aerosol sources in the urban environment of Beijing under mixed-phase cloud conditions
Ice crystals occurring in mixed-phase clouds play a vital role in global precipitation and energy balance because of the unstable equilibrium between coexistent liquid droplets and ice crystals, which affects cloud lifetime and radiative properties, as well as precipitation formation. Satellite observations proved that immersion freezing, i.e., ice formation on particles immersed within aqueous droplets, is the dominant ice nucleation (IN) pathway in mixed-phase clouds. However, the impact of anthropogenic emissions on atmospheric IN in the urban environment remains ambiguous. In this study, we present in situ observations of ambient ice-nucleating particle number concentration (NINP) measured at mixed-phase cloud conditions (−30 ∘C, relative humidity with respect to liquid water RHw= 104 %) and the physicochemical properties of ambient aerosol, including chemical composition and size distribution, at an urban site in Beijing during the traditional Chinese Spring Festival. The impact of multiple aerosol sources such as firework emissions, local traffic emissions, mineral dust, and urban secondary aerosols on NINP is investigated. The results show that NINP during the dust event reaches up to 160 # L−1 (where “#” represents number of particles), with an activation fraction (AF) of 0.0036 % ± 0.0011 %. During the rest of the observation, NINP is on the order of 10−1 to 10 # L−1, with an average AF between 0.0001 % and 0.0002 %. No obvious dependence of NINP on the number concentration of particles larger than 500 nm (N500) or black carbon (BC) mass concentration (mBC) is found throughout the field observation. The results indicate a substantial NINP increase during the dust event, although the observation took place at an urban site with high background aerosol concentration. Meanwhile, the presence of atmospheric BC from firework and traffic emissions, along with urban aerosols formed via secondary transformation during heavily polluted periods, does not influence the observed INP concentration. Our study corroborates previous laboratory and field findings that anthropogenic BC emission has a negligible effect on NINP and that NINP is unaffected by heavy pollution in the urban environment under mixed-phase cloud conditions.</p
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