220 research outputs found
Temperature development in the leakage flow of screw extruders
Temperature distribution at the exit of the leakage gap is of interest for a number of problems. For the calculation of temperatures, the leakage flow may be considered to be a pure drag flow to a good approximation. In the Newtonian case, thermal development length may be expressed in terms of gap height as L ≈ 3/8Pe ·δ ;usually this is less than the available gap length. Pe is the Peclet number and δ the height of leakage gap. Therefore the existing flow may be considered fully developed. For power law fluids, numerical calculations lead to results of the same order. Martin's results therefore may be applied to the flow at the exit of the leakage gap
AN EVALUATION OF THE NON-LINEAR VISCOELASTIC PROPERTIES OF THE HEALING MEDIAL COLLATERAL LIGAMENT
Injuries to knee ligaments are frequent, demanding an increased understanding of the healing process. Clinically, the injured medial collateral ligament (MCL) has been found to heal without surgical intervention. However, laboratory studies have shown that, even one year after injury, the biomechanical properties, biochemical composition, and histomorphological appearance of the healing MCL remains suboptimal. While research has focused on the changes in mechanical properties (i.e. stress-strain behavior) of the healed MCL, studies on its viscoelastic properties are limited. Yet, this knowledge is critical to determine the overall kinetic response of the knee joint.The quasi-linear viscoelastic (QLV) theory proposed by Professor Y.C. Fung has been frequently used to model the viscoelastic properties of the MCL. This theory was developed based on an idealized step-elongation during a stress relaxation test. As this is experimentally impossible, the constants of the theory may not be representative when they are determine based on experiments that utilize finite strain rates. Thus, the overall objectives of this dissertation were to 1) develop and validate a novel experimental and analytical approach that accounts for finite strain rates and provides an accurate determination of the viscoelastic properties of the normal MCL, 2) apply this new approach to describe the viscoelastic behavior of the healing MCL, and 3) to determine whether the new approach can describe the response of the MCL to harmonic oscillations.This work demonstrated that a newly developed approach could be utilized to determine the constants of the quasi-linear viscoelastic theory and successfully describe the viscoelastic behavior of both the normal and healing MCLs. Interestingly, the healing ligaments display a lower initial slope of the stress-strain curve and a greater propensity to dissipate energy, suggesting other structures within the knee would have to play a compensatory role in knee function. It was also found that the mechanisms governing the viscoelastic response of the MCL to harmonic oscillations may not be the same as that which governs stress relaxation behavior. Thus, a more general theory may be necessary to describe both phenomena
Viscoelastic spectrum analysis and the identification of a fung viscoelastic material
Despite its many limitations, the Fung “quasi-linear viscoelastic” constitutive model continues to serve as a workhorse of the biomechanics community. A central challenge in applying the model is that it requires a specific form for the relaxation spectrum that is difficult to relate to easily obtained experimental spectra such as a generalized Maxwell relaxation spectrum. Here, we present a simple and general technique for obtaining a from relaxation data a viscoelastic spectrum appropriate to the Fung model. We apply the model to identify several biomaterials that are modeled reasonably by a Fung model, and many more that are not
Spectrum of the non-commutative spherical well
We give precise meaning to piecewise constant potentials in non-commutative
quantum mechanics. In particular we discuss the infinite and finite
non-commutative spherical well in two dimensions. Using this, bound-states and
scattering can be discussed unambiguously. Here we focus on the infinite well
and solve for the eigenvalues and eigenfunctions. We find that time reversal
symmetry is broken by the non-commutativity. We show that in the commutative
and thermodynamic limits the eigenstates and eigenfunctions of the commutative
spherical well are recovered and time reversal symmetry is restored
Microscopic mechanism for experimentally observed anomalous elasticity of DNA in 2D
By exploring a recent model [Palmeri, J., M. Manghi, and N. Destainville.
2007. Phys. Rev. Lett. 99:088103] where DNA bending elasticity, described by
the wormlike chain model, is coupled to base-pair denaturation, we demonstrate
that small denaturation bubbles lead to anomalies in the flexibility of DNA at
the nanometric scale, when confined in two dimensions (2D), as reported in
atomic force microscopy (AFM) experiments [Wiggins, P. A., et al. 2006. Nature
Nanotech. 1:137-141]. Our model yields very good fits to experimental data and
quantitative predictions that can be tested experimentally. Although such
anomalies exist when DNA fluctuates freely in three dimensions (3D), they are
too weak to be detected. Interactions between bases in the helical
double-stranded DNA are modified by electrostatic adsorption on a 2D substrate,
which facilitates local denaturation. This work reconciles the apparent
discrepancy between observed 2D and 3D DNA elastic properties and points out
that conclusions about the 3D properties of DNA (and its companion proteins and
enzymes) do not directly follow from 2D experiments by AFM.Comment: To appear in Biophys. J. 8 pages, supplementary information included
(7 pages
Spectroscopic properties of a two-level atom interacting with a complex spherical nanoshell
Frequency shifts, radiative decay rates, the Ohmic loss contribution to the
nonradiative decay rates, fluorescence yields, and photobleaching of a
two-level atom radiating anywhere inside or outside a complex spherical
nanoshell, i.e. a stratified sphere consisting of alternating silica and gold
concentric spherical shells, are studied. The changes in the spectroscopic
properties of an atom interacting with complex nanoshells are significantly
enhanced, often more than two orders of magnitude, compared to the same atom
interacting with a homogeneous dielectric sphere. The detected fluorescence
intensity can be enhanced by 5 or more orders of magnitude. The changes
strongly depend on the nanoshell parameters and the atom position. When an atom
approaches a metal shell, decay rates are strongly enhanced yet fluorescence
exhibits a well-known quenching. Rather contra-intuitively, the Ohmic loss
contribution to the nonradiative decay rates for an atomic dipole within the
silica core of larger nanoshells may be decreasing when the silica core - inner
gold shell interface is approached. The quasistatic result that the radial
frequency shift in a close proximity of a spherical shell interface is
approximately twice as large as the tangential frequency shift appears to apply
also for complex nanoshells. Significantly modified spectroscopic properties
(see computer program (pending publication of this manuscript) freely available
at http://www.wave-scattering.com) can be observed in a broad band comprising
all (nonresonant) optical and near-infrared wavelengths.Comment: 20 pages plus 63 references and 11 figures, plain LaTex, for more
information see http://www.wave-scattering.com (color of D sphere in figures
2-6 altered, minor typos corrected.
Feasibility study of using the dispersion of surface acoustic wave impulse for viscoelasticity characterization in tissue mimicking phantoms
The Single-Particle density of States, Bound States, Phase-Shift Flip, and a Resonance in the Presence of an Aharonov-Bohm Potential
Both the nonrelativistic scattering and the spectrum in the presence of the
Aharonov-Bohm potential are analyzed. The single-particle density of states
(DOS) for different self-adjoint extensions is calculated. The DOS provides a
link between different physical quantities and is a natural starting point for
their calculation. The consequences of an asymmetry of the S matrix for the
generic self-adjoint extension are examined.
I. Introduction
II. Impenetrable flux tube and the density of states
III. Penetrable flux tube and self-adjoint extensions
IV. The S matrix and scattering cross sections
V. The Krein-Friedel formula and the resonance
VI. Regularization
VII. The R --> 0 limit and the interpretation of self-adjoint extensions
VIII. Energy calculations
IX. The Hall effect in the dilute vortex limit
X. Persistent current of free electrons in the plane pierced by a flux tube
XI. The 2nd virial coefficient of nonrelativistic interacting anyons
XII. Discussion of the results and open questionsComment: 68 pages, plain latex, 7 figures, 3 references and one figure added
plus a few minor text correction
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