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The phonon drag force acting on a mobile crystal defect: full treatment of discreteness and non-linearity  [PDF]
T. D. Swinburne,S. L. Dudarev
Physics , 2015, DOI: 10.1103/PhysRevB.92.134302
Abstract: Phonon scattering calculations predict the drag force acting on defects and dislocations rises linearly with temperature, in direct contradiction with molecular dynamics simulations that often finds the drag force to be independent of temperature. Using the Mori-Zwanzig projection technique, with no recourse to elasticity or scattering theories, we derive a general Langevin equation for a crystal defect, with full treatment of discreteness and non-linearity in the defect core. We obtain an analytical expression for the drag force that is evaluated in molecular statics and molecular dynamics, extracting the force on a defect directly from the inter-atomic forces. Our results show that a temperature independent drag force arises because vibrations in a discrete crystal are never independent of the defect motion, an implicit assumption in any phonon-based approach. This effect remains even when the Peierls barrier is effectively zero, invalidating qualitative explanations involving the radiation of phonons. We apply our methods to an interstitial defect in tungsten and solitons in the Frenkel-Kontorova model, finding very good agreement with trajectory-based estimations of the thermal drag force.
Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona  [PDF]
Sergio Dolei,Alessandro Bemporad,Daniele Spadaro
Physics , 2014, DOI: 10.1051/0004-6361/201321041
Abstract: In this work we study the kinematics of three small-scale (0.01 R$_\odot$) blobs of chromospheric plasma falling back to the Sun after the huge eruptive event of June 7, 2011. From a study of 3-D trajectories of blobs made with the Solar TErrestrial RElations Observatory (STEREO) data, we demonstrate the existence of a significant drag force acting on the blobs and calculate two drag coefficients, in the radial and tangential directions. The resulting drag coefficients $C_D$ are between 0 and 5, comparable in the two directions, making the drag force only a factor of 0.45-0.75 smaller than the gravitational force. To obtain a correct determination of electron densities in the blobs, we also demonstrate how, by combining measurements of total and polarized brightness, the H$\alpha$ contribution to the white-light emission observed by the COR1 telescopes can be estimated. This component is significant for chromospheric plasma, being between 95 and 98 % of the total white-light emission. Moreover, we demonstrate that the COR1 data can be employed even to estimate the H$\alpha$ polarized component, which turns out to be in the order of a few percent of H$\alpha$ total emission from the blobs. If the drag forces acting on small-scale blobs reported here are similar to those that play a role during the CME propagation, our results suggest that the magnetic drag should be considered even in the CME initiation modelling.
Possibilities for Measurement and Compensation of Stray DC Electric Fields Acting on Drag-Free Test Masses  [PDF]
W. J. Weber,L. Carbone,A. Cavalleri,R. Dolesi,C. D. Hoyle,M. Hueller,S. Vitale
Physics , 2003, DOI: 10.1016/j.asr.2006.03.045
Abstract: DC electric fields can combine with test mass charging and thermal dielectric voltage noise to create significant force noise acting on the drag-free test masses in the LISA (Laser Interferometer Space Antenna) gravitational wave mission. This paper proposes a simple technique to measure and compensate average stray DC potentials at the mV level, yielding substantial reduction in this source of force noise. We discuss the attainable resolution for both flight and ground based experiments.
Theory and Application of Buoyancy in Wells  [cached]
Eirik Kaarstad,Bernt Sigve Aadnoy
Modern Applied Science , 2011, DOI: 10.5539/mas.v5n3p15
Abstract: The paper presents a generalized buoyancy model for application in wells, called the generalized Archimedes’ principle. There exists confusion in the oil industry about buoyancy effects as related to the force-area method (also known as piston force method) or the Archimedes’ principle, the stability force in buckling and axial forces during well intervention operations. This paper will present a general model called the generalized Archimedes’ principle, which is valid for all cases. The paper presents examples of application to ordinary drilling operations, deviated wells and different fluid densities inside and outside the pipe. For well intervention scenarios, the effects of wellhead shut-in pressures on buoyancy are demonstrated. This simple model, not requiring use of a “stability force”, can also analyze buckling. Two field cases are presented to demonstrate the use of the generalized Archimedes’ principle in torque and drag analysis. When the drill string is run in hole having a float valve installed, the buoyancy factor changes depending on how much air is in the drill string. The generalized Archimedes’ principle may simply be applied to the torque and drag model for accurate lowering force calculations. Using the generalized Archimedes’ principle, a correct assessment of the real axial load of the pipe is obtained, a critical factor for tubular work in petroleum wells. The model is also significant as it is simple and can be applied to all well operations.
Computation of energy for diapycnal mixing in the Baltic Sea due to internal wave drag acting on wind-driven barotropic currents  [cached]
Nohr Christian,Gustafsson Bo G.
Oceanologia , 2009,
Abstract: The pathways of energy supply for mixing the deep waters of the Baltic Sea is largely unknown. In this paper, a parameterization of the internal wave drag forces on barotropic motion is developed and implemented into a two-dimensional shallow water model of the Baltic Sea. The model is validated against observed sea levels. The dissipation of barotropic motion by internal wave drag that is quantified from the model results show that breaking internal waves generated by wind forced barotropic motions can contribute significantly to diapycnal mixing in the deep water of the Baltic Sea.
Nonuniform Lowness and Strong Nonuniform Lowness
Li Hongzhou,and Li Guanying,
Li Hongzhou
,Li Guanying

计算机科学技术学报 , 1995,
Abstract: The concepts of the nonuniform and strong nonuniform lownesss are in-troduced. Those notions provide a uniform framework to study connectionsbetween the polynomiaLtime hierarchy and sparse sets.
Analysis of Buoyancy Module Auxiliary Installation Technology Based on Numerical Simulation Analysis of Buoyancy Module Auxiliary Installation Technology Based on Numerical Simulation  [PDF]
XU Songsen,JIAO Chunshuo,NING Meng,DONG Sheng
- , 2018,
Abstract: To reduce the requirement for lifting capacity and decrease the hoist cable force during the descending and laying process of a subsea production system(SPS), a buoyancy module auxiliary installation technology was proposed by loading buoyancy modules on the SPS to reduce the lifting weight. Two models are established, namely, the SPS lowering-down model and the buoyancy module floating-up model. The main study results are the following: 1) When the buoyancy module enters the water under wave condition, the amplitude of tension fluctuation is twice that when SPS enters water; 2) Under current condition, the displacement of SPS becomes three times larger because of the existence of the buoyancy module; 3) After being released, the velocity of the buoyancy module increases to a large speed rapidly and then reaches a balancing speed gradually. The buoyancy module floats up at a balancing speed and rushes out from the water at a pop-up distance; 4) In deep water, the floating-up velocity of the buoyancy module is related to its mass density and shape, and it is not related to water depth; 5) A drag parachute can reduce floating-up velocity and pop-up distance effectively. Good agreement was found between the simulation and experiment results
High Latitude HI in NGC 2613: Buoyant Disk-Halo Outflow  [PDF]
Judith A. Irwin,Tara Chaves
Physics , 2002, DOI: 10.1086/345891
Abstract: We combine new VLA D array HI data of NGC 2613 with previous high resolution data to show new disk-halo features in this galaxy. The global HI distribution is modeled in detail using a technique which can disentangle the effects of inclination from scale height and can also solve for the average volume density distribution in and perpendicular to the disk. The model shows that the galaxy's inclination is on the low end of the range given by Chaves & Irwin (2001) and that the HI disk is thin (z_e = 188 pc), showing no evidence for halo. Numerous discrete disk-halo features are observed, however, achieving z heights up to 28 kpc from mid-plane. One prominent feature in particular, of mass, 8X10^7 Msun and height, 22 kpc, is seen on the advancing side of the galaxy at a projected galactocentric radius of 15.5 kpc. If this feature achieves such high latitudes because of events in the disk alone, then input energies of order ~ 10^{56} ergs are required. We have instead investigated the feasibility of such a large feature being produced via buoyancy (with drag) within a hot, pre-existing X-ray corona. Reasonable plume densities, temperatures, stall height (~ 11 kpc), outflow velocities and ages can indeed be achieved in this way. The advantage of this scenario is that the input energy need only be sufficient to produce blow-out, a condition which requires a reduction of three orders of magnitude in energy. If this is correct, there should be an observable X-ray halo around NGC 2613.
Simple model for reverse buoyancy in a vibrated granular system  [PDF]
G. Gutiérrez,O. Pozo,L. I. Reyes,R. Paredes V.,J. F. Drake,E. Ott
Physics , 2002,
Abstract: Large objects, immersed in a homogeneous granular system, migrate when subjected to vibrations. Under certain conditions large heavy objects rise and similar light ones sink to the bottom. This is called reverse buoyancy. We report an experimental study of this singular behavior, for a large sphere immersed in a deep granular bed. A simple mechanism is proposed to describe the motion of a sphere, inside a vertically vibrated granular system. When reverse buoyancy is observed, the measured vertical velocity of the immersed object, as a function of its density, shows a simple behavior. With a one-dimensional mechanical model that takes into account a buoyancy force and the frictional drag, we obtain the rising velocity for heavy objects and the sinking rate for light ones. The model yields a very good qualitative and quantitative agreement with the experiment.
On deformation spaces of nonuniform hyperbolic lattices  [PDF]
Sungwoon Kim,Inkang Kim
Mathematics , 2013,
Abstract: Let $\Gamma$ be a nonuniform lattice acting on real hyperbolic n-space. We show that in dimension greater than or equal to 4, the volume of a representation is constant on each connected component of the representation variety of $\Gamma$ in SO(n,1). Furthermore, in dimensions 2 and 3, there is a semialgebraic subset of the representation variety such that the volume of a representation is constant on connected components of the semialgebraic subset. Our approach gives a new proof of the local rigidity theorem for nonuniform hyperbolic lattices and the analogue of Soma's theorem, which shows that the number of orientable hyperbolic manifolds dominated by a closed, connected, orientable 3-manifold is finite, for noncompact 3-manifolds.
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