2,671 research outputs found
Spray automated balancing of rotors: Methods and materials
The work described consists of two parts. In the first part, a survey is performed to assess the state of the art in rotor balancing technology as it applies to Army gas turbine engines and associated power transmission hardware. The second part evaluates thermal spray processes for balancing weight addition in an automated balancing procedure. The industry survey reveals that: (1) computerized balancing equipment is valuable to reduce errors, improve balance quality, and provide documentation; (2) slow-speed balancing is used exclusively, with no forseeable need for production high-speed balancing; (3) automated procedures are desired; and (4) thermal spray balancing is viewed with cautious optimism whereas laser balancing is viewed with concern for flight propulsion hardware. The FARE method (Fuel/Air Repetitive Explosion) was selected for experimental evaluation of bond strength and fatigue strength. Material combinations tested were tungsten carbide on stainless steel (17-4), Inconel 718 on Inconel 718, and Triballoy 800 on Inconel 718. Bond strengths were entirely adequate for use in balancing. Material combinations have been identified for use in hot and cold sections of an engine, with fatigue strengths equivalent to those for hand-ground materials
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
Molecular theory of hydrophobic mismatch between lipids and peptides
Effects of the mismatch between the hydrophobic length, d, of transmembrane
alpha helices of integral proteins and the hydrophobic thickness, D_h, of the
membranes they span are studied theoretically utilizing a microscopic model of
lipids. In particular, we examine the dependence of the period of a lamellar
phase on the hydrophobic length and volume fraction of a rigid, integral,
peptide. We find that the period decreases when a short peptide, such that
d<D_h, is inserted. More surprising, we find that the period increases when a
long peptide, such that d>D_h, is inserted. The effect is due to the
replacement of extensible lipid tails by rigid peptide. As the peptide length
is increased, the lamellar period continues to increase, but at a slower rate,
and can eventually decrease. The amount of peptide which fails to incorporate
and span the membrane increases with the magnitude of the hydrophobic mismatch
|d-D_h|. We explicate these behaviors which are all in accord with experiment.
Predictions are made for the dependence of the tilt of a single trans-membrane
alpha helix on hydrophobic mismatch and helix density.Comment: 14 pages, 5 figure
Lattice gas description of pyrochlore and checkerboard antiferromagnets in a strong magnetic field
Quantum Heisenberg antiferromagnets on pyrochlore and checkerboard lattices
in a strong external magnetic field are mapped onto hard-core lattice gases
with an extended exclusion region. The effective models are studied by the
exchange Monte Carlo simulations and by the transfer matrix method. The
transition point and the critical exponents are obtained numerically for a
square-lattice gas of particles with the second-neighbor exclusion, which
describes a checkerboard antiferromagnet. The exact structure of the magnon
crystal state is determined for a pyrochlore antiferromagnet.Comment: 11 pages, accepted versio
Quantum interface unbinding transitions
We consider interfacial phenomena accompanying bulk quantum phase transitions
in presence of surface fields. On general grounds we argue that the surface
contribution to the system free energy involves a line of singularities
characteristic of an interfacial phase transition, occurring below the bulk
transition temperature T_c down to T=0. This implies the occurrence of an
interfacial quantum critical regime extending into finite temperatures and
located within the portion of the phase diagram where the bulk is ordered. Even
in situations, where the bulk order sets in discontinuously at T=0, the
system's behavior at the boundary may be controlled by a divergent length scale
if the tricritical temperature is sufficiently low. Relying on an effective
interfacial model we compute the surface phase diagram in bulk spatial
dimensionality and extract the values of the exponents describing the
interfacial singularities in
Heterogeneous nucleation near a metastable vapour-liquid transition: the effect of wetting transitions
Phase transformations such as freezing typically start with heterogeneous
nucleation. Heterogeneous nucleation near a wetting transition, of a
crystalline phase is studied. The wetting transition occurs at or near a
vapour-liquid transition which occurs in a metastable fluid. The fluid is
metastable with respect to crystallisation, and it is the crystallisation of
this fluid phase that we are interested in. At a wetting transition a thick
layer of a liquid phase forms at a surface in contact with the vapour phase.
The crystalline nucleus is then immersed in this liquid layer, which reduces
the free energy barrier to nucleation and so dramatically increases the
nucleation rate. The variation in the rate of heterogeneous nucleation close to
wetting transitions is calculated for systems in which the longest-range forces
are dispersion forces.Comment: 11 pages including 3 figure
Liquid drop in a cone - line tension effects
The shape of a liquid drop placed in a cone is analyzed macroscopically.
Depending on the values of the cone opening angle, the Young angle and the line
tension four different interfacial configurations may be realized. The phase
diagram in these variables is constructed and discussed; it contains both the
first- and the second-order transition lines. In particular, the tricritical
point is found and the value of the critical exponent characterizing the
behaviour of the system along the line of the first-order transitions in the
neighbourhood of this point is determined.Comment: 11 pages, 4 figure
Wetting on a spherical wall: influence of liquid-gas interfacial properties
We study the equilibrium of a liquid film on an attractive spherical
substrate for an intermolecular interaction model exhibiting both fluid-fluid
and fluid-wall long-range forces. We first reexamine the wetting properties of
the model in the zero-curvature limit, i.e., for a planar wall, using an
effective interfacial Hamiltonian approach in the framework of the well known
sharp-kink approximation (SKA). We obtain very good agreement with a mean-field
density functional theory (DFT), fully justifying the use of SKA in this limit.
We then turn our attention to substrates of finite curvature and appropriately
modify the so-called soft-interface approximation (SIA) originally formulated
by Napi\'orkowski and Dietrich [Phys. Rev. B 34, 6469 (1986)] for critical
wetting on a planar wall. A detailed asymptotic analysis of SIA confirms the
SKA functional form for the film growth. However, it turns out that the
agreement between SKA and our DFT is only qualitative. We then show that the
quantitative discrepancy between the two is due to the overestimation of the
liquid-gas surface tension within SKA. On the other hand, by relaxing the
assumption of a sharp interface, with, e.g., a simple smoothing of the density
profile there, markedly improves the predictive capability of the theory,
making it quantitative and showing that the liquid-gas surface tension plays a
crucial role when describing wetting on a curved substrate. In addition, we
show that in contrast to SKA, SIA predicts the expected mean-field critical
exponent of the liquid-gas surface tension
3D wedge filling and 2D random-bond wetting
Fluids adsorbed in 3D wedges are shown to exhibit two types of continuous
interfacial unbinding corresponding to critical and tricritical filling
respectively. Analytic solution of an effective interfacial model based on the
transfer-matrix formalism allows us to obtain the asymptotic probability
distribution functions for the interfacial height when criticality and
tricriticality are approached. Generalised random walk arguments show that, for
systems with short-ranged forces, the critical singularities at these
transitions are related to 2D complete and critical wetting with random bond
disorder respectively.Comment: 7 pages, 3 figures, accepted for publication in Europhysics Letter
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