5,588 research outputs found
Critical Torsional Oscillations of a Rotating Accelerated Shaft
We assume that the forces applied to the shaft have a variable part which is a moment of constant amplitude M(x) per unit length, distributed along the shaft, and varying with a frequency proportional to the angular velocity.
From the solution corresponding to the harmonic steady state vibration, we deduce, by using Heaviside's expansion, the motion due to a sudden application of the moment M(x). This enables us to compute the effect of the moment when applied with a linearly increasing frequency.
In this analysis the damping will be neglected. In case of a viscous damping the linear character of the equations is not affected and the same method might be used. We did not carry this calculation for two reasons:
1. The exact result will be in general complicated, and involve viscous friction coefficients which will not be very accurately known. Besides, the effect of friction might be roughly taken into account by considering the steady state solution.
2. In most cases the damping is not viscous but due to the hysteresis or internal friction of the material. This is proved by the experimental fact that the energy absorbed in the vibration of elastic bodies is proportional to the frequency and not to its square. This effect might be taken roughly into account by energetic considerations
PERFORMANCE MANAGEMENT IN THE PUBLIC SECTOR:
Education supply in universities of most European countries has for the last ten years become a strategic matter. At present, French universities consider education supply as an investment. But they do not utilize all incentive mechanisms in order to drive their strategies. At the beginning of the year 2006, the public sector reform will tend to impose performance measurements of research and educational activities, in order to improve organizational efficiency. The aim of this reform in the French context is to provide driving elements to increase internal efficiency, social and economic impact of higher education system and to reinforce international attractiveness of public education institutions. The substitution of resources management by result management involves an agent's performance responsibility measurement. Evaluation becomes a central factor and is articulated with incentives system. The weakening of the property right system drives project bearers to maximize their utility instead of their incomes. In such a context, the understanding of individual strategies permits to understand constraints of management within universities, and to take into account the impact of stakeholders who take part in the value generation process. The major risk is to constraint the utility function of projects bearers by increasing their burden and their motivation. The result could be the limitation of the number of projects, and as well, the decreasing of university investments.performance, public sector, universities, efficiency, value generation
Nanoscale buckling deformation in layered copolymer materials
In layered materials, a common mode of deformation involves buckling of the
layers under tensile deformation in the direction perpendicular to the layers.
The instability mechanism, which operates in elastic materials from geological
to nanometer scales, involves the elastic contrast between different layers. In
a regular stacking of "hard" and "soft" layers, the tensile stress is first
accommodated by a large deformation of the soft layers. The inhibited Poisson
contraction results in a compressive stress in the direction transverse to the
tensile deformation axis. The hard layers sustain this transverse compression
until buckling takes place and results in an undulated structure. Using
molecular simulations, we demonstrate this scenario for a material made of
triblock copolymers. The buckling deformation is observed to take place at the
nanoscale, at a wavelength that depends on strain rate. In contrast to what is
commonly assumed, the wavelength of the undulation is not determined by defects
in the microstructure. Rather, it results from kinetic effects, with a
competition between the rate of strain and the growth rate of the instability.
http://www.pnas.org/content/early/2011/12/23/1111367109.abstrac
Can one hear the shape of a saturation patch?
The theory of the acoustics of patchy-saturation in porous media is used to
analyze experimental data on wave velocity and attenuation in partially water
saturated limestones. It is demonstrated that the theory can be used to deduce
the value of V/A, the ratio of the volume to area of the water patch, and l_f,
the Poisson size of the water patch. One can ``hear'' the shape of a patch if
the properties of the rock and the measurement frequencies are such as to
satisfy the specific requirements for the validity of the theory
Scattering of a longitudinal wave by a circular crack in a fluid-saturated porous medium
Physical properties of many natural and man-made materials can be modelled using the concept of poroelasticity. Some porous materials, in addition to the network of pores, contain larger inhomogeneities such as inclusions, cavities, fractures or cracks. A common method of detecting such inhomogeneities is based on the use of elastic wave scattering. We consider interaction of a normally incident time-harmonic longitudinal plane wave with a circular crack imbedded in a porous medium governed by Biot's equations of dynamic poroelasticity. The problem is formulated in cylindrical co-ordinates as a system of dual integral equations for the Hankel transform of the wave field, which is then reduced to a single Fredholm integral equation of the second kind. It is found that the scattering that takes place is predominantly due to wave inducedfluid flow between the pores and the crack. The scattering magnitude depends on the size of the crack relative to the slow wave wavelength and has it's maximum value when they are of the same order
Dynamic stability of bombs and projectiles
The four chapters comprising pages 1 to 123, inclusive,
report were previously released in limited distribution with
desigtions and dates as follows:
Chapter I, CIT/JPC 4, January 2, 1943
Chapter II, CIT/JPC 5, January 2, 1943
Chapter III, CIT/JPC 6, January 2, 1943
Chapter IV, CIT/JPC 11, May 26, 1943
Chapters I and II were prepared as of July 1, 1942, Chapter III as
of, September 1, 1942, and Chapter IV as of the publication date. The text
is herewith reproduced from the original vellum copy by the
photo-offset process, with mathematical notation in the author's hand. The references
listed at the end of Chapter IV were originally contained in CIT/JPC 11
and since this list includes all references cited in JPC 4, 5, and 6, the
lists originally contained in these reports are here omitted.
The study as planned was intended to cover all aspects of the
dynamic stability problems involved in solids moving through fluids. The
chapters contained herein only partially cover the general subject. The
results presented are immediately applicable to the ballistics of aerial
bombs or the air trajectory of torpedoes. They also apply to the underwater
ballistics of depth bombs and the underwater behavior of torpedoes
in that phase of the trajectory where cavitation is absent
Unfolding the Sulcus
Sulci are localized furrows on the surface of soft materials that form by a
compression-induced instability. We unfold this instability by breaking its
natural scale and translation invariance, and compute a limiting bifurcation
diagram for sulcfication showing that it is a scale-free, sub-critical {\em
nonlinear} instability. In contrast with classical nucleation, sulcification is
{\em continuous}, occurs in purely elastic continua and is structurally stable
in the limit of vanishing surface energy. During loading, a sulcus nucleates at
a point with an upper critical strain and an essential singularity in the
linearized spectrum. On unloading, it quasi-statically shrinks to a point with
a lower critical strain, explained by breaking of scale symmetry. At
intermediate strains the system is linearly stable but nonlinearly unstable
with {\em no} energy barrier. Simple experiments confirm the existence of these
two critical strains.Comment: Main text with supporting appendix. Revised to agree with published
version. New result in the Supplementary Informatio
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