28,155 research outputs found
Discussion on Event Horizon and Quantum Ergosphere of Evaporating Black Holes in a Tunnelling Framework
In this paper, with the Parikh-Wilczek tunnelling framework the positions of
the event horizon of the Vaidya black hole and the Vaidya-Bonner black hole are
calculated respectively. We find that the event horizon and the apparent
horizon of these two black holes correspond respectively to the two turning
points of the Hawking radiation tunnelling barrier. That is, the quantum
ergosphere coincides with the tunnelling barrier. Our calculation also implies
that the Hawking radiation comes from the apparent horizon.Comment: 8 page
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State-of-the-art on research and applications of machine learning in the building life cycle
Fueled by big data, powerful and affordable computing resources, and advanced algorithms, machine learning has been explored and applied to buildings research for the past decades and has demonstrated its potential to enhance building performance. This study systematically surveyed how machine learning has been applied at different stages of building life cycle. By conducting a literature search on the Web of Knowledge platform, we found 9579 papers in this field and selected 153 papers for an in-depth review. The number of published papers is increasing year by year, with a focus on building design, operation, and control. However, no study was found using machine learning in building commissioning. There are successful pilot studies on fault detection and diagnosis of HVAC equipment and systems, load prediction, energy baseline estimate, load shape clustering, occupancy prediction, and learning occupant behaviors and energy use patterns. None of the existing studies were adopted broadly by the building industry, due to common challenges including (1) lack of large scale labeled data to train and validate the model, (2) lack of model transferability, which limits a model trained with one data-rich building to be used in another building with limited data, (3) lack of strong justification of costs and benefits of deploying machine learning, and (4) the performance might not be reliable and robust for the stated goals, as the method might work for some buildings but could not be generalized to others. Findings from the study can inform future machine learning research to improve occupant comfort, energy efficiency, demand flexibility, and resilience of buildings, as well as to inspire young researchers in the field to explore multidisciplinary approaches that integrate building science, computing science, data science, and social science
Shock-induced consolidation and spallation of Cu nanopowders
A useful synthesis technique, shock synthesis of bulk nanomaterials from nanopowders, is explored here with molecular dynamics simulations. We choose nanoporous Cu (∼11 nm in grain size and 6% porosity) as a representative system, and perform consolidation and spallation simulations. The spallation simulations characterize the consolidated nanopowders in terms of spall strength and damage mechanisms. The impactor is full density Cu, and the impact velocity (u_i) ranges from 0.2 to 2 km s^(−1). We present detailed analysis of consolidation and spallation processes, including atomic-level structure and wave propagation features. The critical values of u_i are identified for the onset plasticity at the contact points (0.2 km s^(−1)) and complete void collapse (0.5 km s^(−1)). Void collapse involves dislocations, lattice rotation, shearing/friction, heating, and microkinetic energy. Plasticity initiated at the contact points and its propagation play a key role in void collapse at low u_i, while the pronounced, grain-wise deformation may contribute as well at high u_i. The grain structure gives rise to nonplanar shock response at nanometer scales. Bulk nanomaterials from ultrafine nanopowders (∼10 nm) can be synthesized with shock waves. For spallation, grain boundary (GB) or GB triple junction damage prevails, while we also observe intragranular voids as a result of GB plasticity
Multiple phase transitions in single-crystalline NaFeAs
Specific heat, resistivity, susceptibility and Hall coefficient measurements
were performed on high-quality single crystalline NaFeAs. This
compound is found to undergo three successive phase transitions at around 52,
41, and 23 K, which correspond to structural, magnetic and superconducting
transitions, respectively. The Hall effect result indicates the development of
energy gap at low temperature due to the occurrence of spin-density-wave
instability. Our results provide direct experimental evidence of the magnetic
ordering in the nearly stoichiometric NaFeAs.Comment: 4 pages, 4 figure
Transmutation prospect of long-lived nuclear waste induced by high-charge electron beam from laser plasma accelerator
Photo-transmutation of long-lived nuclear waste induced by high-charge
relativistic electron beam (e-beam) from laser plasma accelerator is
demonstrated. Collimated relativistic e-beam with a high charge of
approximately 100 nC is produced from high-intensity laser interaction with
near-critical-density (NCD) plasma. Such e-beam impinges on a high-Z convertor
and then radiates energetic bremsstrahlung photons with flux approaching
10^{11} per laser shot. Taking long-lived radionuclide ^{126}Sn as an example,
the resulting transmutation reaction yield is the order of 10^{9} per laser
shot, which is two orders of magnitude higher than obtained from previous
studies. It is found that at lower densities, tightly focused laser irradiating
relatively longer NCD plasmas can effectively enhance the transmutation
efficiency. Furthermore, the photo-transmutation is generalized by considering
mixed-nuclide waste samples, which suggests that the laser-accelerated
high-charge e-beam could be an efficient tool to transmute long-lived nuclear
waste.Comment: 13 pages, 8 figures, it has been submitted to Physics of Plasm
Hawking Radiation of Dirac Particles in an Arbitrarily Accelerating Kinnersley Black Hole
Quantum thermal effect of Dirac particles in an arbitrarily accelerating
Kinnersley black hole is investigated by using the method of generalized
tortoise coordinate transformation. Both the location and the temperature of
the event horizon depend on the advanced time and the angles. The Hawking
thermal radiation spectrum of Dirac particles contains a new term which
represents the interaction between particles with spin and black holes with
acceleration. This spin-acceleration coupling effect is absent from the thermal
radiation spectrum of scalar particles.Comment: Revtex, 12pt, 16 pages, no figure, to appear in Gen. Rel. Grav. 34
(2002) N0.
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