69,273 research outputs found
Instability analysis procedure for 3-level multi-bearing rotor-foundation systems
A procedure for the instability analysis of a three-level multispan rotor systems is described. This procedure is based on a distributed mass elastic representation of the rotor system in several eight-coefficient bearings. Each bearing is supported from an elastic foundation on damped, elastic pedestals. The foundation is represented as a general distributed mass elastic structure on discrete supports, which may have different stiffness and damping properties in the horizontal and vertical directions. This system model is suited to studies of instability threshold conditions for multirotor turbomachines on either massive or flexible foundations. The instability conditions is found by obtaining the eigenvalues of the system determinant, which is obtained by the transfer matrix method from the three-level system model. The stability determinant is solved for the lowest rotational speed at which the system damping becomes zero in the complex eigenvalue, and for the whirl frequency corresponding to the natural frequency of the unstable mode. An efficient algorithm for achieving this is described. Application of this procedure to a rigid rotor in two damped-elastic bearings and flexible supports is described. A second example discusses a flexible rotor with four damped-elastic bearings. The third case compares the stability of a six-bearing 300 Mw turbine generator unit, using two different bearing types. These applications validate the computer program and various aspects of the analysis
Temperature-dependent Cross Sections for Charmonium Dissociation in Collisions with Pions and Rhos in Hadronic Matter
Meson-charmonium dissociation reactions governed by the quark interchange are
studied with temperature-dependent quark potentials. Quark-antiquark
relative-motion wave functions and masses of charmonia and charmed mesons are
determined by the central spin-independent part of the potentials or by the
central spin-independent part and a smeared spin-spin interaction. The
prominent temperature dependence of the masses is found. Based on the
potentials, the wave functions, and the meson masses, we obtain
temperature-dependent cross sections for fifteen pion-charmonium and
rho-charmonium dissociation reactions. The numerical cross sections are
parametrized for future applications in hadronic matter. The particular
temperature dependence of the J/psi bound state leads to unusual behavior of
the cross sections for endothermic J/psi dissociation reactions. The quantum
numbers of psi' and chi_c can not make their difference in mass in the
temperature region 0.6T_c < T < T_c, but can make the psi' dissociation
different from the chi_c dissociation.Comment: 52 pages, 23 figures, 6 table
Giant Microwave Absorption in Metallic Grains: Relaxation Mechanism
We show that the low frequency microwave absorption of an ensemble of small
metallic grains at low temperatures is dominated by a mesoscopic relaxation
mechanism. Giant positive magnetoresistance and very strong temperature
dependence of the microwave conductivity is predicted.Comment: 4 pages, REVTeX3+mutlticol+epsf, one EPS figur
A universal relationship between magnetization and local structure changes below the ferromagnetic transition in La_{1-x}Ca_xMnO_3; evidence for magnetic dimers
We present extensive X-ray Absorption Fine Structure (XAFS) measurements on
La_{1-x}Ca_xMnO_3 as a function of B-field (to 11T) and Ca concentration, x
(21-45%). These results reveal local structure changes (associated with polaron
formation) that depend only on the magnetization for a given sample,
irrespective of whether the magnetization is achieved through a decrease in
temperature or an applied magnetic field. Furthermore, the relationship between
local structure and magnetization depends on the hole doping. A model is
proposed in which a filamentary magnetization initially develops via the
aggregation of pairs of Mn atoms involving a hole and an electron site. These
pairs have little distortion and it is likely that they pre-form at
temperatures above T_c.Comment: 5 pages, 5 figures (1 with 2 parts) -- v2. new data added (updated
figures); discussion expande
Determinants of immediate price impacts at the trade level in an emerging order-driven market
The common wisdom argues that, in general, large trades cause large price
changes, while small trades cause small price changes. However, for extremely
large price changes, the trade size and news play a minor role, while the
liquidity (especially price gaps on the limit order book) is a more influencing
factor. Hence, there might be other influencing factors of immediate price
impacts of trades. In this paper, through mechanical analysis of price
variations before and after a trade of arbitrary size, we identify that the
trade size, the bid-ask spread, the price gaps and the outstanding volumes at
the bid and ask sides of the limit order book have impacts on the changes of
prices. We propose two regression models to investigate the influences of these
microscopic factors on the price impact of buyer-initiated partially filled
trades, seller-initiated partially filled trades, buyer-initiated filled
trades, and seller-initiated filled trades. We find that they have
quantitatively similar explanation powers and these factors can account for up
to 44% of the price impacts. Large trade sizes, wide bid-ask spreads, high
liquidity at the same side and low liquidity at the opposite side will cause a
large price impact. We also find that the liquidity at the opposite side has a
more influencing impact than the liquidity at the same side. Our results shed
new lights on the determinants of immediate price impacts.Comment: 21 IOP tex pages including 5 figures and 5 tables. Accepted for
publication in New Journal of Physic
Valence-bond theory of highly disordered quantum antiferromagnets
We present a large-N variational approach to describe the magnetism of
insulating doped semiconductors based on a disorder-generalization of the
resonating-valence-bond theory for quantum antiferromagnets. This method
captures all the qualitative and even quantitative predictions of the
strong-disorder renormalization group approach over the entire experimentally
relevant temperature range. Finally, by mapping the problem on a hard-sphere
fluid, we could provide an essentially exact analytic solution without any
adjustable parameters.Comment: 5 pages, 3 eps figure
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