3,176 research outputs found
An Improved Sideband Current Harmonic Model of Interior PMSM Drive by Considering Magnetic Saturation and Cross-Coupling Effects
The sideband current harmonics, as parasitic characteristics in permanent-magnet synchronous machine (PMSM) drives with space vector pulsewidth modulation technique, will increase the corresponding electromagnetic loss, torque ripple, vibration, and acoustic noises. Therefore, fast yet accurate evaluation of the resultant sideband current harmonic components is of particular importance during the design stage of the drive system. However, the inevitable magnetic saturation and cross-coupling effects in interior PMSM drives would have a significant impact on the current components, while the existing analytical sideband current harmonic model neglects those effects. This paper introduces a significant improvement on the analytical model by taking into account these effects with corresponding nonlinear factors. Experimental results are carried out to underpin the accuracy improvements of the predictions from the proposed model over the existing analytical one. The proposed model can offer a very detailed and insightful revelation of impacts of the magnetic saturation and cross-coupling effects on the corresponding sideband current harmonics
Revisiting the anomalous bending elasticity of sharply bent DNA
Several recent experiments suggest that sharply bent DNA has a surprisingly
high bending flexibility, but the cause of this flexibility is poorly
understood. Although excitation of flexible defects can explain these results,
whether such excitation can occur with the level of DNA bending in these
experiments remains unclear. Intriguingly, the DNA contained preexisting nicks
in most of these experiments but whether nicks might play a role in flexibility
has never been considered in the interpretation of experimental results. Here,
using full-atom molecular dynamics simulations, we show that nicks promote DNA
basepair disruption at the nicked sites, which drastically reduces DNA bending
energy. In addition, lower temperatures suppress the nick-dependent basepair
disruption. In the absence of nicks, basepair disruption can also occur but
requires a higher level of DNA bending. Therefore, basepair disruption inside
B-form DNA can be suppressed if the DNA contains preexisting nicks. Overall,
our results suggest that the reported mechanical anomaly of sharply bent DNA is
likely dependent on preexisting nicks, therefore the intrinsic mechanisms of
sharply bent nick-free DNA remain an open question.Comment: 39 pages, 11 figures, 1 supporting materia
Holographic duality from random tensor networks
Tensor networks provide a natural framework for exploring holographic duality
because they obey entanglement area laws. They have been used to construct
explicit toy models realizing many interesting structural features of the
AdS/CFT correspondence, including the non-uniqueness of bulk operator
reconstruction in the boundary theory. In this article, we explore the
holographic properties of networks of random tensors. We find that our models
naturally incorporate many features that are analogous to those of the AdS/CFT
correspondence. When the bond dimension of the tensors is large, we show that
the entanglement entropy of boundary regions, whether connected or not, obey
the Ryu-Takayanagi entropy formula, a fact closely related to known properties
of the multipartite entanglement of assistance. Moreover, we find that each
boundary region faithfully encodes the physics of the entire bulk entanglement
wedge. Our method is to interpret the average over random tensors as the
partition function of a classical ferromagnetic Ising model, so that the
minimal surfaces of Ryu-Takayanagi appear as domain walls. Upon including the
analog of a bulk field, we find that our model reproduces the expected
corrections to the Ryu-Takayanagi formula: the minimal surface is displaced and
the entropy is augmented by the entanglement of the bulk field. Increasing the
entanglement of the bulk field ultimately changes the minimal surface
topologically in a way similar to creation of a black hole. Extrapolating bulk
correlation functions to the boundary permits the calculation of the scaling
dimensions of boundary operators, which exhibit a large gap between a small
number of low-dimension operators and the rest. While we are primarily
motivated by AdS/CFT duality, our main results define a more general form of
bulk-boundary correspondence which could be useful for extending holography to
other spacetimes.Comment: 57 pages, 13 figure
Pedagogical Design for an Online Information Literacy Course: College Students\u27 Learning Experience with Multi-modal Objects
This project is an exploratory study on the use of multi-modal objects in an online information literacy course. This paper reports on the second phase of the project, which focused on students’ learning experience within five course modules employing different multi-modal media objects for instruction. Seven online surveys were conducted at the beginning of the course, immediately after each of the webcast discussion sessions accompanying each course module, and at the end of the course. The findings show significant relationships among computer skills, online teaching materials, use of communication tools, learning experience, and satisfaction with the cours
Linear redshift space distortions for cosmic voids based on galaxies in redshift space
Cosmic voids found in galaxy surveys are defined based on the galaxy
distribution in redshift space. We show that the large scale distribution of
voids in redshift space traces the fluctuations in the dark matter density
field \delta(k) (in Fourier space with \mu being the line of sight projected
k-vector): \delta_v^s(k) = (1 + \beta_v \mu^2) b^s_v \delta(k), with a beta
factor that will be in general different than the one describing the
distribution of galaxies. Only in case voids could be assumed to be quasi-local
transformations of the linear (Gaussian) galaxy redshift space field, one gets
equal beta factors \beta_v=\beta_g=f/b_g with f being the growth rate, and b_g,
b^s_v being the galaxy and void bias on large scales defined in redshift space.
Indeed, in our mock void catalogs we measure void beta factors being in good
agreement with the galaxy one. Further work needs to be done to confirm the
level of accuracy of the beta factor equality between voids and galaxies, but
in general the void beta factor needs to be considered as a free parameter for
RSD studies.Comment: 10 pages, 9 figures; matches the version accepted by PR
Metastable Tight Knots in Semiflexible Chains
Knotted structures can spontaneously occur in polymers such as DNA and proteins, and the formation of knots affects biological functions, mechanical strength and rheological properties. In this work, we calculate the equilibrium size distribution of trefoil knots in linear DNA using off-lattice simulations. We observe metastable knots on DNA, as predicted by Grosberg and Rabin. Furthermore, we extend their theory to incorporate the finite width of chains and show an agreement between our simulations and the modified theory for real chains. Our results suggest localized knots spontaneously occur in long DNA and the contour length in the knot ranges from 600 to 1800 nm.National Science Foundation (U.S.) (NSF Grant No. 1335938)Singapore. National Research FoundationSingapore-MIT Alliance for Research and Technology (SMART
Origin of Metastable Knots in Single Flexible Chains
Recent theoretical progress has explained the physics of knotting of semiflexible polymers, yet knotting of flexible polymers is relatively unexplored. We herein develop a new theory for the size distribution of knots on a flexible polymer and the existence of metastable knots. We show the free energy of a flexible molecule in a tube can be mapped to quantitatively reproduce the free energy distribution of a knot on a flexible chain. The size distribution of knots on flexible chains is expected to be universal and might be observed at a macroscopic scale, such as a string of hard balls.Singapore-MIT Alliance for Research and TechnologyNational Science Foundation (U.S.) (Grant 1335938
Numerical and physical simulation of rapid microstructural evolution of gas atomised Ni superalloy powders
The rapid microstructural evolution of gas atomised Ni superalloy powder compacts over timescales of a few seconds was studied using a Gleeble 3500 thermomechanical simulator, finite element based numerical model and electron microscopy. The study found that the microstructural changes were governed by the characteristic temperatures of the alloy. At a temperature below the Îł' solvus, the powders maintained dendritic structures. Above the Îł' solvus temperature but in the solid-state, rapid grain spheroidisation and coarsening occurred, although the fine-scale microstructures were largely retained. Once the incipient melting temperature of the alloy was exceeded, microstructural change was rapid, and when the temperature was increased into the solid + liquid state, the powder compact partially melted and then re-solidified with no trace of the original structures, despite the fast timescales. The study reveals the relationship between short, severe thermal excursions and microstructural evolution in powder processed components, and gives guidance on the upper limit of temperature and time for powder-based processes if desirable fine-scale features of powders are to be preserved
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