EVALUATION OF TITANIUM ULTRALIGHT MANUAL WHEELCHAIRS USING ANSI/RESNA STANDARDS

A series of commercially available titanium ultralight wheelchairs were tested using ANSI/RESNA testing procedures, and their durability was compared with previously tested aluminum ultralight wheelchairs and light-weight wheelchairs. Three of each of the following titanium wheelchairs were tested: Invacare-TopEnd, Invacare-A4, Quickie-Ti, and TiLite-ZRA. The Quickie-Ti wheelchairs had the most forward and rearward center of gravity adjustability. All of the titanium wheelchairs passed the forward braking effectiveness test, but two chairs of each model tipped backward before the platform inclining to 7 degree in the rearward braking effectiveness test. All titanium wheelchairs passed the impact strength tests, but two failed in the static strength tests: two Invacare-TopEnd wheelchairs and one Invacare-A4 wheelchair failed due to deformation of the armrest mounting plates, and the handgrips of the TiLite-ZRA wheelchairs slid off the push handles. Two Invacare-A4 and one Invacare-TopEnd successfully completed the double drum and curb drop tests, but the remaining 9 wheelchairs failed prematurely. No significant differences were found in the number of the equivalent cycles or the value among the four models. The titanium ultralight wheelchairs had less equivalent cycles and value than the aluminum ultralight wheelchairs that were tested in a previous study. The failure modes in the static strength tests and the fatigue tests were consistent within the model, and revealed important design issues for each model. Our results suggest that manufacturers need to perform more careful analyses before commercializing new products

A heuristic algorithm for the multi-criteria set-covering problems

AbstractA simple greedy heuristic algorithm for the multi-criteria set-covering problem is presented. This result is a multi-criteria generalization of the results established previously by Chvatal

Surface Passivation of Colloidal II-IV Semiconductor Quantum Belts and Quantum Wires: Synthesis, Mechanism and Optical Studies

My research aims to study the formation mechanism of CdSe quantum belts: QBs) and the surface passivation of CdTe quantum wires: QWs). Investigation of QB morphology and QW passivation are fundamentally important to avoid nonradiative recombination and to increase electron-transport efficiency in semiconductor solar-cell devices. The origin of CdSe QBs is a lamellar structure of cadmium-octylamine precursor complex, with an intermediate state of stripe-like assembly of CdSe magic-sized nanoclusters. Transformation of: CdSe)13 nanoclusters to CdSe quantum belts: QBs) is conducted by higher annealing-temperature and confirmed by their optical and structural characterization. The QBs possess a thickness of 1.5-2.0 nm with a width of 7-15 nm and length of \u3e1 micrometer. Most remarkably, the QBs have a superior morphology with noticeably high quantum efficiency: QE, 30 ± 10 %), comparable to quantum rods. High QE is explained by a delocalized exciton recombination with a lower density of defect sites. The colloidal CdTe quantum wires: QWs) are preferred for solar-cell applications due to a tunable diameter range: 5-60 nm), a long length dimensionality: \u3e 5 micrometer), and near-IR band-gap emission energy: 1.5 eV). Successive surface passivation with Lewis acids and bases successfully passivates the QW surface. A significantly enhanced CdTe QW quantum efficiency: QE, 5-8 %) is achieved, which is two orders of magnitude larger than the previous disappointing QE: \u3c 0.01 %). Enhanced photoluminescence: PL) emission reveals intrinsic higher and lower emission bands for CdTe QWs. Enhanced photoluminescence excitation: PLE) features consistently match previous theoretical calculations for electronic transitions. Significantly high QEs of CdSe QBs: 30 ± 10 %) and CdTe QWs: 5-8 %) promise colloidal 1D quantum structures suitable for bio-imaging and solar-cell applications

The Kinetic Structure of Collisionless Slow Shocks and Reconnection Exhausts

A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Simulations are carried out for various ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehose-sense (Pparallel>Pperpendicular) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the downstream region the firehose stability parameter epsilon=1-$mu0(Pparallel-Pperpendicular)/B2 tends to plateau at 0.25. This balance arises from the competition between counterstreaming ions, which drives epsilon down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2-D turbulence also develops in the downstream region. An explanation for the critical value 0.25 is proposed by examining anisotropic fluid theories, in particular the Anisotropic Derivative Nonlinear-Schrodinger-Burgers equations, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes, and corresponds to the lower bound of the transition point in a compound slow shock(SS)/rotational discontinuity(RD) wave. This work implies that it is a pair of compound SS/RD waves that bounds the reconnection outflow, instead of a pair of switch-off slow shocks as in Petschek's model

Particle Acceleration and Plasma Dynamics during Magnetic Reconnection in the Magnetically-dominated Regime

Magnetic reconnection is thought to be the driver for many explosive phenomena in the universe. The energy release and particle acceleration during reconnection have been proposed as a mechanism for producing high-energy emissions and cosmic rays. We carry out two- and three-dimensional kinetic simulations to investigate relativistic magnetic reconnection and the associated particle acceleration. The simulations focus on electron-positron plasmas starting with a magnetically dominated, force-free current sheet ($\sigma \equiv B^2/(4\pi n_e m_e c^2) \gg 1$). For this limit, we demonstrate that relativistic reconnection is highly efficient at accelerating particles through a first-order Fermi process accomplished by the curvature drift of particles along the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra $f \propto (\gamma-1)^{-p}$ and approaches $p = 1$ for sufficiently large $\sigma$ and system size. Eventually most of the available magnetic free energy is converted into nonthermal particle kinetic energy. An analytic model is presented to explain the key results and predict a general condition for the formation of power-law distributions. The development of reconnection in these regimes leads to relativistic inflow and outflow speeds and enhanced reconnection rates relative to non-relativistic regimes. In the three-dimensional simulation, the interplay between secondary kink and tearing instabilities leads to strong magnetic turbulence, but does not significantly change the energy conversion, reconnection rate, or particle acceleration. This study suggests that relativistic reconnection sites are strong sources of nonthermal particles, which may have important implications to a variety of high-energy astrophysical problems.Comment: 18 pages, 13 figures, slightly modified after submitted to Ap

Subsets withoutq-separation and binomial products of fibonacci numbers

The total number of subsets of{1, 2, 3, …, n} withoutq-separation is expressed in terms of binomial products of Fibonacci numbers. Several new combinatorial identities are derived