14,071 research outputs found
Multiresolution finite element method based on a new locking-free rectangular Mindlin plate element
A locking-free rectangular Mindlin plate element with a new multi-resolution
analysis (MRA) is proposed and a new finite element method is hence presented.
The MRA framework is formulated out of a mutually nesting displacement subspace
sequence whose basis functions are constructed of scaling and shifting on the
element domain of basic node shape function. The basic node shape function is
constructed by extending the node shape function of a traditional Mindlin plate
element to other three quadrants around the coordinate zero point. As a result,
a new rational MRA concept together with the resolution level (RL) is
constituted for the element. The traditional 4-node rectangular Mindlin plate
element and method is a mono-resolution one and also a special case of the
proposed element and method. The meshing for the monoresolution plate element
model is based on the empiricism while the RL adjusting for the multiresolution
is laid on the rigorous mathematical basis. The analysis clarity of a plate
structure is actually determined by the RL, not by the mesh. Thus, the accuracy
of a plate structural analysis is replaced by the clarity, the irrational MRA
by the rational and the mesh model by the RL that is the discretized model by
the integrated.Comment: 16 pages. arXiv admin note: substantial text overlap with
arXiv:1404.1165, arXiv:1405.677
Impact of Temperature-Dependent Rattling Phonons on Lattice Dynamics and Thermal Transport in AgGeP
Crystalline compounds exhibiting low-frequency rattling phonons constitute an
important class of high-performance thermoelectrics owning to their
intrinsically very low lattice thermal conductivity (). Theoretical
approach that is capable of revealing the physical origin and accurately
predicting is of particular interest, which, however, still
remains an outstanding challenge. In this study, we perform a case study of
lattice dynamics and thermal transport properties of AgGeP,
which has recently been identified as a high-performance thermoelectric
phosphide due to low , arising from rattling vibrations associated
with Ag clusters. Analysis within a first-principles-based
lattice-dynamics framework based on self-consistent phonon theory reveals a
strong temperature dependence of rattling phonons due to high-order anharmonic
interactions. Anharmonic hardening of the rattling optical modes has a strong
effect on the lifetimes of heat-carrying acoustic phonons by decreasing the
rate of three-phonon combination processes. This mechanism results in a
significant increase in and changes its temperature dependence to
Generating Semantically Valid Adversarial Questions for TableQA
Adversarial attack on question answering systems over tabular data (TableQA)
can help evaluate to what extent they can understand natural language questions
and reason with tables. However, generating natural language adversarial
questions is difficult, because even a single character swap could lead to huge
semantic difference in human perception. In this paper, we propose SAGE
(Semantically valid Adversarial GEnerator), a Wasserstein sequence-to-sequence
model for TableQA white-box attack. To preserve meaning of original questions,
we apply minimum risk training with SIMILE and entity delexicalization. We use
Gumbel-Softmax to incorporate adversarial loss for end-to-end training. Our
experiments show that SAGE outperforms existing local attack models on semantic
validity and fluency while achieving a good attack success rate. Finally, we
demonstrate that adversarial training with SAGE augmented data can improve
performance and robustness of TableQA systems.Comment: AAAI 2021 Workshop on Towards Robust, Secure and Efficient Machine
Learnin
Dynamics and flow-coupling in two-layer turbulent thermal convection
We present an experimental investigation of the dynamics and flow-coupling of
convective turbulent flows in a cylindrical Rayleigh-Benard convection cell
with two immiscible fluids, water and fluorinert FC-77 electronic liquid
(FC77). It is found that one large-scale circulation (LSC) roll exists in each
of the fluid layers, and that their circulation planes have two preferred
azimuthal orientations separated by . A surprising finding of the
study is that cessations/reversals of the LSC in FC77 of the two-layer system
occur much more frequently than they do in single-layer turbulent RBC, and that
a cessation is most likely to result in a flow reversal of the LSC, which is in
sharp contrast with the uniform distribution of the orientational angular
change of the LSC before and after cessations in single-layer turbulent RBC.
This implies that the dynamics governing cessations and reversals in the two
systems are very different. Two coupling modes, thermal coupling (flow
directions of the two LSCs are opposite to each other at the fluid-fluid
interface) and viscous coupling (flow directions of the two LSCs are the same
at the fluid-fluid interface), are identified with the former one as the
predominant mode. That most cessations (in the FC77 layer) end up as reversals
can be understood as a symmetry breaking imposed by the orientation of the LSC
in the water layer, which remained unchanged most of the time. Furthermore, the
frequently occurring cessations and reversals are caused by the system
switching between its two metastable states, i.e. thermal and viscous coupling
modes. It is also observed that the strength of the LSC in water becomes weaker
when the LSC in FC77 rotates faster azimuthally and that the flow strength in
FC77 becomes stronger when the LSC in water rotates faster azimuthally, i.e.
the influence of the LSC in one fluid layer on the other is not symmetric.Comment: 13 pages, 8 figure
Microscopic Mechanisms of Glass-Like Lattice Thermal Transport in Cubic CuSbS Tetrahedrites
Materials based on cubic tetrahedrites (CuSbS) are useful
thermoelectrics with unusual thermal and electrical transport properties, such
as very low and nearly temperature-independent lattice thermal conductivity
(). We explain the microscopic origin of the glass-like
in CuSbS by explicitly treating anharmonicity
up to quartic terms for both phonon energies and phonon scattering rates. We
show that the strongly unstable phonon modes associated with trigonally
coordinated Cu atoms are anharmonically stabilized above approximately K
and continue hardening with increasing temperature, in accord with experimental
data. This temperature induced hardening effect reduces scattering of heat
carrying acoustic modes by reducing the available phase space for three-phonon
processes, thereby balancing the conventional increase in
scattering due to phonon population and yielding nearly temperature-independent
. Furthermore, we find that very strong phonon broadening lead to a
qualitative breakdown of the conventional phonon-gas model and modify the
dominant heat transport mechanism from the particle-like phonon wave packet
propagation to incoherent tunneling described by off-diagonal terms in the
heat-flux operator, which are typically prevailing in glasses and disordered
crystals. Our work paves the way to a deeper understanding of glass-like
thermal conductivity in complex crystals with strong anharmonicity
Turbulent thermal convection over rough plates with varying roughness geometries
We present a systematic investigation of the effects of roughness geometry on
turbulent Rayleigh-B\'enard convection (RBC) over rough plates with
pyramid-shaped and periodically distributed roughness elements. Using a
parameter defined as the height of a roughness element over its base
width, the heat transport, the flow dynamics and local temperatures are
measured for the Rayleigh number range , and the Prandtl number from 3.57 to 23.34 at four
values of . It is found that the heat transport scaling, i.e. where is the Nusselt number, may be classified into three
regimes. In Regime I, the system is in a dynamically smooth state. The heat
transport scaling is the same as that in a smooth cell. In Regimes II and III,
the heat transport enhances. When is increased from 0.5 to 4.0,
increases from 0.36 to 0.59 in Regime II, and it increases from 0.30
to 0.50 in Regime III. The experiment demonstrates the heat transport scaling
in turbulent RBC can be manipulated using . Previous studies suggest
that the transition from Regime I to Regime II, occurs when the thermal
boundary layer (BL) thickness becomes smaller than the roughness height .
Direct measurements of the viscous BL in the present study suggest that the
transition from Regime II to Regime III is likely a result of the viscous BL
thickness becoming smaller . The scaling exponent of the Reynolds number
vs. changes from 0.471 to 0.551 when is increased from 0.5
to 4.0. It is also found that increasing increases the clustering of
thermal plumes which effectively increases the plumes lifetime that are
ultimately responsible for the enhanced heat transport.Comment: 27 pages, 19 figure
Anharmonic stabilization and lattice heat transport in rocksalt -GeTe
Peierls-Boltzmann transport equation, coupled with third-order anharmonic
lattice dynamics calculations, has been widely used to model lattice thermal
conductivity () in bulk crystals. However, its application to
materials with structural phase transition at relatively high temperature is
fundamentally challenged by the presence of lattice instabilities (imaginary
phonon modes). Additionally, its accuracy suffers from the absence of
higher-than-third-order phonon scattering processes, which are important
near/above the Debye temperature. In this letter, we present an effective
scheme that combines temperature-induced anharmonic phonon renormalization and
four-phonon scattering to resolve these two theoretical challenges. We apply
this scheme to investigate the lattice dynamics and thermal transport
properties of GeTe, which undergoes a second-order ferroelectric phase
transition from rhombohedral -GeTe to rocksalt -GeTe at about
700~K. Our results on the high-temperature phase -GeTe at 800~K confirm
the stabilization of -GeTe by temperature effects. We find that
considering only three-phonon scattering leads to significantly overestimated
of 3.8~W/mK at 800~K, whereas including four-phonon scattering
reduces to 1.7~W/mK, a value comparable with experiments. To
explore the possibility to further suppress , we show that alloying
-GeTe with heavy cations such as Pb and Bi can effectively reduce
to about 1.0~W/mK, whereas sample size needs to be around 10nm
through nanostructuring to achieve a comparable reduction of
Renormalized Lattice Dynamics and Thermal Transport in VO
Vanadium dioxide (VO) undergoes a first-order metal-insulator
transition (MIT) upon cooling near room temperature, concomitant with
structural change from rutile to monoclinic. Accurate characterization of
lattice vibrations is vital for elucidating the transition mechanism. To
investigate the lattice dynamics and thermal transport properties of VO
across the MIT, we present a phonon renormalization scheme based on
self-consistent phonon theory through iteratively refining vibrational free
energy. Using this technique, we compute temperature-dependent phonon
dispersion and lifetimes, and point out the importance of both magnetic and
vibrational entropy in driving the MIT. The predicted phonon dispersion and
lifetimes show quantitative agreement with experimental measurements. We
demonstrate that lattice thermal conductivity of rutile VO is nearly
temperature independent as a result of strong intrinsic anharmonicity, while
that of monoclinic VO varies according to . Due to phonon softening
and enhanced scattering rates, the lattice thermal conductivity is deduced to
be substantially lower in the rutile phase, suggesting that Wiedemann-Franz law
might not be strongly violated in rutile VO
Distributed Space-Time Coding for Full-Duplex Asynchronous Cooperative Communications
In this paper, we propose two distributed linear convolutional space-time
coding (DLC-STC) schemes for full-duplex (FD) asynchronous cooperative
communications. The DLC-STC Scheme 1 is for the case of the complete loop
channel cancellation, which achieves the full asynchronous cooperative
diversity. The DLC-STC Scheme 2 is for the case of the partial loop channel
cancellation and amplifying, where some loop signals are used as the
self-coding instead of treated as interference to be directly cancelled. We
show this scheme can achieve full asynchronous cooperative diversity. We then
evaluate the performance of the two schemes when loop channel information is
not accurate and present an amplifying factor control method for the DLC-STC
Scheme 2 to improve its performance with inaccurate loop channel information.
Simulation results show that the DLC-STC Scheme 1 outperforms the DLC-STC
Scheme 2 and the delay diversity scheme if perfect or high quality loop channel
information is available at the relay, while the DLC-STC Scheme 2 achieves
better performance if the loop channel information is imperfect.Comment: 9 pages, 7 figure
A Family of Counter Examples to an Approach to Graph Isomorphism
We give a family of counter examples showing that the two sequences of
polytopes and are different. These polytopes were
defined recently by S. Friedland in an attempt at a polynomial time algorithm
for graph isomorphism
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