Stochastic modeling of tire–snow interaction using a polynomial chaos approach

Developing accurate models to simulate the interaction between pneumatic tires and unprepared terrain is a demanding task. Such tire–terrain contact models are often used to analyze the mobility of a wheeled vehicle on a given type of soil, or to predict the vehicle performance under specified operational conditions (as related to the vehicle and tires, as well as to the running support). Due to the complex nature of the interaction between a tire and off-road environment, one usually needs to make simplifying assumptions when modeling such an interaction. It is often assumed that the tire–terrain interaction can be captured using a deterministic approach, which means that one assumes fixed values for several vehicle or tire parameters, and expects exact responses from the system. While this is rarely the case in real life, it is nevertheless a necessary step in the modeling process of a deterministic framework. In reality, the external excitations affecting the system, as well as the values of the vehicle and terrain parameters, do not have fixed values, but vary in time or space. Thus, although a deterministic model may capture the response of the system given one set of deterministic values for the system parameters, inputs, etc., this is in fact only one possible realization of the multitude of responses that could occur in reality. The goal of our study is to develop a mathematically sound methodology to improve the prediction of the tire–snow interaction by considering the variability of snow depth and snow density, which will lead to a significantly better understanding and a more realistic representation of tire–snow interaction. We constructed stochastic snow models using a polynomial chaos approach developed at Virginia Tech, to account for the variability of snow depth and of snow density. The stochastic tire–snow models developed are based on the extension of two representative deterministic tire–snow interaction models developed at the University of Alaska, including the pressure–stress deterministic model and the hybrid (on-road extended for off-road) deterministic model. Case studies of a select combination of uncertainties were conducted to quantify the uncertainties of the interfacial forces, sinkage, entry angle, and the friction ellipses as a function of wheel load, longitudinal slip, and slip angle. The simulation results of the stochastic pressure–stress model and the stochastic hybrid model are compared and analyzed to identify the most convenient tire design stage for which they are more suitable. The computational efficiency of the two models is also discussed.     

Analytical performance characteristics of thin and thick film amperometric microcells

The analytical performance of amperometric microcells with different electrode geometries is compared for enzyme activity measurements. The microcells were fabricated with thin film photolithography or thick film screen-printing in four different designs. The cells made with the thin film process used flexible substrate with microelectrode array or a circular, disk-shaped working electrode. The screen-printed working electrodes had semicircle or disk shape on ceramic chips. Putrescine oxidase (PUO) activity measurement was used as a model. The determination of PUO activity is important in the clinical diagnosis of premature rupture of the amniotic membrane. An electropolymerized m-phenylenediamine size-exclusion layer was used to eliminate common interferences. The size exclusion layer revealed also to be advantageous in protecting the electrodes from fouling by putrescine (enzyme substrate). The electrode fouling of bare electrodes was insignificant for screen-printed electrodes, but very severe for electroplated platinum working electrodes. The microelectrode array electrodes demonstrated smaller RSD and higher normalized sensitivities for hydrogen peroxide and PUO activity. All the other electrodes were demonstrating comparable analytical performances.     

Effect of vertical grid variability on a free surface flow model

A previously developed numerical model that solves the incompressible, non‐hydrostatic, Navier–Stokes equations for free surface flow is analysed on a non‐uniform vertical grid. The equations are vertically transformed to the σ‐coordinate system and solved in a fractional step manner in which the pressure is computed implicitly by correcting the hydrostatic flow field to be divergence free. Numerical consistency, accuracy and efficiency are assessed with analytical methods and numerical experiments for a varying vertical grid discretization. Specific discretizations are proposed that attain greater accuracy and minimize computational effort when compared to a uniform vertical discretization. Published in 2007 by John Wiley & Sons, Ltd.     

A novel bicyclic ketal fragmentation reaction

Bicyclic ketals of the 6,8-dioxabicyclo[3.2.1]octane series are specifically cleaved to give δ, ε-unsaturated ketones by treatment of the ketal with acetyl iodide.     

Molecular structures of neopentane and di-tert-butylmethane by vapor-phase electron diffraction

Although the present molecules are much less strained than the tri-t-butyl member of the series CH_n(t-Bu)_4–n studied previously, di-t-butylmethane nevertheless exhibits striking steric deformations due to its pair of inescapable GG' conformations. The two adjacent t-butyl groups respond to the steric stress by undergoing torsional displacements of 15 ± 6° (3σ), by tilting away from each other by 3–5°, and by opening up the central CCC bond angle to 125–128° (parameter value sensitive to assumptions in analysis). Carbon-carbon bonds, with mean lengths of 1.545 ± 0.005Å, are stretched on the average by about 0.008Åfrom the neopentane reference value. Derived molecular parameters are in substantial agreement with values calculated by molecular mechanics using model fields MUB-1 and MUB-2. The methylene¹³C-H nmr coupling constant was found to be 125 Hz, a value indistinguishable from those reported for unstrained alkanes but not in accord with predictions from the formulas of Foote or Mislow for severely distorted methylene groups.Molecular parameters for neopentane included r_g(C-C) = 1.534 ± 0.003Å, r_g(C-H) = 1.114 ± 0.008Åand ∠CCH = 112 ± 3°. The new value for the C-C bond satisfactorily resolves a discrepancy between previously reported bond lengths. These had disagreed significantly with each other but, to within their uncertainties, they are consistent with the new, intermediate value. Amplitudes of vibration were determined for both neopentane and di-t-butylmethane.     

Pulsed-field ionization electron spectroscopy and binding energies of alkali metal-dimethyl ether and -dimethoxyethane complexes

Lithium and sodium complexes of dimethyl ether (DME) and dimethoxyethane (DXE) were produced by reactions of laser-vaporized metal atoms with organic vapors in a pulsed nozzle cluster source. The mono-ligand complexes were studied by photoionization and pulsed field ionization zero electron kinetic energy (ZEKE) spectroscopy. Vibrationally resolved ZEKE spectra were obtained for Li(DME), Na(DME) and Li(DXE) and a photoionization efficiency spectrum for Na(DXE). The ZEKE spectra were analyzed by comparing with the spectra of other metal-ether complexes and with electronic structure calculations and spectral simulations. Major vibrations measured for the M(DME) (M=Li,Na) ions were M-O and C-O stretches and M-O-C and C-O-C bends. These vibrations and additional O-Li-O and O-C-C-O bends were observed for the Li(DXE) ion. The M(DME) complexes were in C2v symmetry with the metal atom binding to oxygen, whereas Li(DXE) was in a C2 ring configuration with the Li atom attaching to both oxygen atoms. Moreover, the ionization energies of these complexes were measured from the ZEKE or photoionization spectra and bond dissociation energies were derived from a thermodynamic cycle.     

Absorption und Adsorption

Ohne Zusammenfassung     

A Simple Propellane Synthesis

A dianion-mediated dialkylation reaction provides ready access to a heterocyclic propellane. The methodology should provide a general entry into a variety of similar systems.     

Tabak und Tabakrauch

Ohne Zusammenfassung