4,587 research outputs found
H\"older Regularity of Geometric Subdivision Schemes
We present a framework for analyzing non-linear -valued
subdivision schemes which are geometric in the sense that they commute with
similarities in . It admits to establish
-regularity for arbitrary schemes of this type, and
-regularity for an important subset thereof, which includes all
real-valued schemes. Our results are constructive in the sense that they can be
verified explicitly for any scheme and any given set of initial data by a
universal procedure. This procedure can be executed automatically and
rigorously by a computer when using interval arithmetics.Comment: 31 pages, 1 figur
Classical light dispersion theory in a regular lattice
We study the dynamics of an infinite regular lattice of classical charged
oscillators. Each individual oscillator is described as a point particle
subject to a harmonic restoring potential, to the retarded electromagnetic
field generated by all the other particles, and to the radiation reaction
expressed according to the Lorentz--Dirac equation. Exact normal mode
solutions, describing the propagation of plane electromagnetic waves through
the lattice, are obtained for the complete linearized system of infinitely many
oscillators. At variance with all the available results, our method is valid
for any values of the frequency, or of the ratio between wavelength and lattice
parameter. A remarkable feature is that the proper inclusion of radiation
reaction in the dynamics of the individual oscillators does not give rise to
any extinction coefficient for the global normal modes of the lattice. The
dispersion relations resulting from our solution are numerically studied for
the case of a simple cubic lattice. New predictions are obtained in this way
about the behavior of the crystal at frequencies near the proper oscillation
frequency of the dipoles.Comment: 15 pages, 1 figure; typos correcte
MEASUREMENT AND EVALUATION OF LOADS ON THE HUMAN BODY DURING SPORTS ACTIVITIES
Introduction: Mechanical loads on the human body are necessary to stimulate bone growth, to maintain bone integrity, and to strengthen the skeletal musculature. However, excessive forces, repetitive shock and high pressures have been identified as contributors to traumatic and overuse injuries. For events of short duration, cinematographic techniques are normally not sufficient to estimate the forces and accelerations experienced by the body's center of mass (CoM) or any one of its parts. Therefore, mechanical sensors are necessary to register forces, accelerations and pressure distributions that occur during sports activities. This methodological overview concentrates on technological aspects and the application of force, pressure and acceleration measurements.
Methods: In the field of biomechanics piezoelectric and strain gage force platforms are commonly used for the determination of ground reaction forces. Properly mounted, these measuring devices provide high accuracy and a good frequency response. Desirable transducer characteristics for biomechanical applications may differ from characteristics advantageous for engineering usage.
Measurement of pressure during sitting or lying on a bed requires a soft and pliable transducer mat that will adapt to the shape of the human body. However, such a transducer will not show good technical specifications. In recent years pressure distribution sensors have been developed using conductive paint as well as capacitive and piezoelectric transducers. Compared to force platforms, pressure distribution sensors allow a much more detailed analysis of the mechanical interaction of the human body with the ground. Pressure devices generally demonstrate reduced accuracy and lower frequency responses as compared to traditional force platforms. Strain gage, inductive, and piezoelectric technologies are also applied for the construction of accelerometers. They are important for the measurements of shock and vibrations at various parts of the body. Skin motion is a major problem for acceleration measurements in biomechanics, and care should be taken to minimize these artifacts.
Applications: The use and the limitations of ground reaction force measurements will be demonstrated for locomotor and other athletic activities. In-shoe pressure distribution techniques have proven valuable in product testing of athletic footwear. Differences between shoes can easily be detected and the aging of material with use can be tested. The substantial influence of skin motion on acceleration measurements will be demonstrated, and methods will be suggested to reduce these skin artifacts. Running and tennis will be chosen as examples to demonstrate the use of acceleration measurements
BIOMECHANICAL EVALUATION OF RUNNING AND SOCCER SHOES: METHODOLOGY AND TESTING PROCEDURES
Running shoes are the footwear, that has been explored the most by scientists in the field of biomechanics. Following the running shoe research peak between 1980 and 1990 other products became the focus of interest. In particular, many shoe studies were performed in the field of basketball and other indoor sports. Only recently, soccer boots have received a lot of attention and were explored by various research groups. Other than in running shoes, soccer boots have additional tasks to perform. These shoes are used for kicking, they should provide sufficient traction for rapid cutting manoeuvres and assist the players in rapid acceleration and stopping movements. Especially, the often conflicting demands of injury prevention and high performance properties remain to be solved. Test methods for athletic footwear as well as research results will be presented here. This will demonstrate how important biomechanics has become in providing the necessary knowledge for the design of functional footwear
Observation of collisions between cold Li atoms and Yb ions
We report on the observation of cold collisions between Li atoms and
Yb ions. This combination of species has recently been proposed as the most
suitable for reaching the quantum limit in hybrid atom-ion systems, due to its
large mass ratio. For atoms and ions prepared in the ground state,
the charge transfer and association rate is found to be at least~10 times
smaller than the Langevin collision rate. These results confirm the excellent
prospects of Li--Yb for sympathetic cooling and quantum information
applications. For ions prepared in the excited electronic states ,
and , we find that the reaction rate is dominated by
charge transfer and does not depend on the ionic isotope nor the collision
energy in the range ~1--120~mK. The low charge transfer rate for ground
state collisions is corroborated by theory, but the shell in the Yb
ion prevents an accurate prediction for the charge transfer rate of the
, and states. Using \textit{ab initio}
methods of quantum chemistry we calculate the atom-ion interaction potentials
up to energies of 30~cm, and use these to give qualitative
explanations of the observed rates.Comment: 8 pages, 7 figures (including appendices
"Mariage des Maillages": A new numerical approach for 3D relativistic core collapse simulations
We present a new 3D general relativistic hydrodynamics code for simulations
of stellar core collapse to a neutron star, as well as pulsations and
instabilities of rotating relativistic stars. It uses spectral methods for
solving the metric equations, assuming the conformal flatness approximation for
the three-metric. The matter equations are solved by high-resolution
shock-capturing schemes. We demonstrate that the combination of a finite
difference grid and a spectral grid can be successfully accomplished. This
"Mariage des Maillages" (French for grid wedding) approach results in high
accuracy of the metric solver and allows for fully 3D applications using
computationally affordable resources, and ensures long term numerical stability
of the evolution. We compare our new approach to two other, finite difference
based, methods to solve the metric equations. A variety of tests in 2D and 3D
is presented, involving highly perturbed neutron star spacetimes and
(axisymmetric) stellar core collapse, demonstrating the ability to handle
spacetimes with and without symmetries in strong gravity. These tests are also
employed to assess gravitational waveform extraction, which is based on the
quadrupole formula.Comment: 29 pages, 16 figures; added more information about convergence tests
and grid setu
Absence of Domain Wall Roughening in a Transverse Field Ising Model with Long-Range Interactions
We investigate roughening transitions in the context of transverse-field
Ising models. As a modification of the transverse Ising model with short range
interactions, which has been shown to exhibit domain wall roughening, we have
looked into the possibility of a roughening transition for the case of
long-range interactions, since such a system is physically realized in the
insulator LiHoF4. The combination of strong Ising anisotropy and long-range
forces lead naturally to the formation of domain walls but we find that the
long-range forces destroy the roughening transition.Comment: 7 pages, 5 figures, revtex
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