Semi-active hydropneumatic spring and damper system

This paper is concerned with the development of a semi-active hydropneumatic spring and damper system, comprising of a two state hydropneumatic spring and a two state hydraulic damper. The system was specifically developed to improve the ride comfort and handling of large off-road vehicles. The suspension requirements for good ride comfort and handling for heavy off-road vehicles are discussed with special reference to the advantages of semi-active hydropneumatic springs and semi-active dampers. The layout and functioning of an experimental spring and damper unit used for laboratory tests are discussed. Spring and damper characteristics, as well as valve response times for both the semi-active spring and semi-active damper were determined. A single degree of freedom test rig with a sprung mass of 3 tons was used to perform first order ride comfort tests. Tests include step response and random input response. The test rig was also used to evaluate semi-active control strategies for both spring and damper as well as a control strategy for implementing ride height adjustment without using an external hydraulic pump.     

The influence of pressure on the lower critical solution temperature miscibility behavior of aqueous solutions of poly(vinyl methyl ether) and the relation to the compositional curvature of the volume of mixing

In mixtures of PVME and water, the influence of pressure on the LCST miscibility gap is determined covering the whole composition range and pressures from atmospheric pressure up to 900 MPa. The cloud point curve at atmospheric pressure has the characteristic bimodal shape in agreement with literature data. Upon increasing pressure the cloud point curve at the low concentration side decreases with pressure, whereas at the high concentrations the cloud point curve increases with pressure. The overall influence of pressure results in a less pronounced bimodality and ultimately the bimodal shape disappears. In addition to the pressure dependence of the miscibility behavior, the density of mixtures of water and PVME are determined at atmospheric pressure. The experimental excess specific volumes are negative for all measured compositions, but the compositional curvature varies with composition. The curvature of the excess specific volume is positive for the higher concentrations but it is negative in the lower composition range. The density measurements are linked to the pressure dependence of the LCST miscibility behavior using exact thermodynamic relationships. The excess specific volume and miscibility results are shown to be in good agreement. Moreover, it is shown that the Clapeyron equation, which is exact for pure components and also frequently assumed to apply to mixtures, is not valid in the system PVME/water. The system PVME/water is an example where the usual approximation of one-to-one correspondence between curvature and excess volume does not apply. Finally, the molecular origins for the observed excess volume and miscibility behavior are briefly discussed from theoretical and molecular simulation points of view.     

Simulation and experimental verification of the diffusion in an anisotropic fiber phantom

Diffusion weighted magnetic resonance imaging enables the visualization of fibrous tissues such as brain white matter. The validation of this non-invasive technique requires phantoms with a well-known structure and diffusion behavior. This paper presents anisotropic diffusion phantoms consisting of parallel fibers. The diffusion properties of the fiber phantoms are measured using diffusion weighted magnetic resonance imaging and bulk NMR measurements. To enable quantitative evaluation of the measurements, the diffusion in the interstitial space between fibers is modeled using Monte Carlo simulations of random walkers. The time-dependent apparent diffusion coefficient and kurtosis, quantifying the deviation from a Gaussian diffusion profile, are simulated in 3D geometries of parallel fibers with varying packing geometries and packing densities. The simulated diffusion coefficients are compared to the theory of diffusion in porous media, showing a good agreement. Based on the correspondence between simulations and experimental measurements, the fiber phantoms are shown to be useful for the quantitative validation of diffusion imaging on clinical MRI-scanners.     

Time-dependence of the isotope effects in the unimolecular dissociation of tertiary amine molecular ions

The intramolecular secondary isotope effects on the α-cleavage of deuterium-labelled N-methyldipentylamine radical cations have been studied as a function of ion lifetime by field ionization kinetics. The isotope effects observed are all normal and increase in magnitude with increasing ion lifetime, with the exception of the δ-labelled compound which shows an inverse effect (predominant loss of the labelled radical) at times shorter than 10^?9 s, and a normal effect at longer times. The isotope effects reflect differences in zero-point energies of the transition states as well as the influence of slight reductions of isotope-dependent frequencies on the state sums–a statistical weight effect. The latter is particularly important at high ion energies and is the primary reason for the occurrence of the inverse isotope effect. The time dependence of the normal and inverse isotope effect is reproduced by QET/RRKM calculations.