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291.
Spyros Beltaos 《Canadian Metallurgical Quarterly》2001,127(8):650-656
Lack of field data on the thickness and roughness of river ice jams inhibits reliable calibration of ice jam models and can result in erroneous evaluation of important physical parameters. Detailed thickness data that have been obtained on several ice jams by means of a remote profiling technique reveal a highly irregular underside. Statistical analysis points to a normal probability distribution of the irregularities and allows direct assessment of the absolute roughness of each jam. The latter varies linearly with the average jam thickness, at least to a thickness value of 3 m, and is shown to be consistent with previously reported roughness values deduced from purely hydraulic considerations. Using the present findings in conjunction with past data and modelling experience, simple equations are developed to compute composite-flow resistance characteristics in terms of jam thickness and flow depth. 相似文献
292.
Martha Ouzounidou Dimitris Ipsakis Spyros Voutetakis Simira Papadopoulou Panos Seferlis 《Energy》2009,34(10):1733
An integrated system for hydrogen production via autothermal steam reforming of methanol and consequent power generation in a polymer electrolyte membrane (PEM) fuel cell has been developed and operated at C.P.E.R.I. The pilot plant comprises an autothermal reforming reactor to produce hydrogen, a preferential oxidation reactor (PROX) to reduce CO concentration below 50 ppm and a PEM fuel cell for power generation.The present paper deals with the study of this system, both from an experimental and a theoretical point of view. The experimental work aims to: (a) examine the effect of the reforming temperature on methanol conversion and on the effluent stream concentration, (b) investigate the effect of reaction temperature and O2/CO ratio on the performance of the PROX reactor, and (c) evaluate the operation of a 10-cell PEM fuel cell, using pure hydrogen and air at three temperature levels. The experimental data are subsequently utilized for the validation of one-dimensional pseudo-homogeneous models that have been developed for the two reactors and also for the identification of the voltage–current characteristic curve of the PEM fuel cell. The validated models are then used to investigate the behavior and explore the interactions, both static and dynamic, among the various process subsystems. 相似文献
293.
Tas DO Thomson IN Löffler FE Pavlostathis SG 《Environmental science & technology》2006,40(14):4467-4472
The microbial reductive dechlorination kinetics of pentachloroaniline (PCA) and less chlorinated anilines (CAs) were investigated with a mixed, fermentative/ methanogenic culture. Batch dechlorination assays were performed with all available CAs at an initial concentration of 3 microM, and an incubation temperature of 22 degrees C. Dechlorination of PCA, two tetrachloroanilines (2,3,4,5- and 2,3,5,6-TeCA), five trichloroanilines (2,3,4-, 2,3,5-, 2,4,5-, 2,4,6-, and 3,4,5-TrCA), and one dichloroaniline (3,5-DCA; low extent) was observed but none of the five remaining dichloroanilines and three monochloroanilines were dechlorinated by the enrichment culture during batch assays. The dechlorination rates (k') and half-saturation coefficients (Kc) were measured using nonlinear regression based on the integrated Michaelis-Menten equation under conditions of electron donor saturation and assuming constant biomass concentration over the relatively short batch incubation period. At an initial concentration of CAs of about 3 microM, the values of k' and Kc ranged from 0.25 to 1.19 microM/day and from 0.11 to 1.72 microM, respectively, corresponding to half-lives in the range of 1.5-8.5 days. Model simulations of the sequential dechlorination reactions based on a branched-chain Michaelis-Menten model and using independently measured k' and Kc values matched the experimental data very well. 相似文献
294.
The onset of stall cells (SCs) is experimentally investigated on a flattop loaded 18% thick airfoil optimized for use on wind turbine blades, exhibiting trailing edge separation. SCs are dynamic coherent vortical structures that appear on wings under separated flow conditions. Although SCs have been known for long, neither are their characteristics completely documented nor their generating mechanisms fully understood. The present investigation aims at providing additional information on the geometric characteristics in terms of width, length and occupied area. The relevant data are presented as functions of Reynolds (Re) number, angle of attack and aspect ratio (AR) of the model. In the tests reported, the dynamic character of SCs is suppressed by imposing a localized flow disturbance. For the specific airfoil and for the Re and AR range tested, it is found that: the angle of attack at which SCs are initially formed decreases linearly with Re number and independently of the AR; unlike two‐dimensional separation, their chordwise length increases with Re; the SC area relative to the wing planform area (defined as the relative SC area) grows asymptotically with angle of attack and Re number reaching an upper bound, which is independent of the AR; at intermediate angles of attack, the SC relative area is higher for the lower AR wing; for a fixed increment in Re number, the growth of the SC relative area is independent of the initial Re number; at lower angles of attack, the actual SC area is independent of the wing span. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
295.
296.
Nuclear magnetic resonance spectroscopy is proposed as an efficient analytical tool in the study of painted artworks. The binding medium from two original oil paintings, dated from the early 20th and the late 17th century, was studied via high-resolution 1D and 2D NMR, establishing the advanced state of hydrolysis and oxidation of the oil paint. Studies of the solvent-extractable component from model samples of various drying oils, raw oil paints, and aged oil paints allowed the definition of several markers based on the integral ratios of various chemical species present in the 1H and 13C NMR spectra. These markers are sensitive to hydrolytic and oxidative processes that reflect the extent of aging in oil paintings. The rapidity, simplicity, and nondestructive nature of the proposed analytical NMR methodology represents a great advantage, since the usually minute sample quantities available from original artwork can be subsequently analyzed further by other analytical techniques, if necessary. 相似文献
297.
298.
Keigo Watanabe Yamato Shiraishi Spyros G. Tzafestas Jun Tang Toshio Fukuda 《Journal of Intelligent and Robotic Systems》1998,22(3-4):315-330
This paper proposes a feedback control scheme for an omnidirectional holonomic autonomous platform, which is equipped with three lateral orthogonal-wheel assemblies. Firstly, the dynamic properties of the platform are studied, and a dynamic model suitable for the application of control is derived. The control scheme constructed is of the resolved-acceleration type, with PI and PD feedback. The control scheme was experimentally applied to an actual mobile robotic platform. The results obtained show that full omnidirectionality can be achieved with decoupled rotational and translational motions. Omnidirectionality is one of the principal requirements for mobile robots designed for health-care and other general-hospital services. 相似文献
299.
Robust sliding-mode control applied to a 5-link biped robot 总被引:2,自引:0,他引:2
Spyros Tzafestas Mark Raibert Costas Tzafestas 《Journal of Intelligent and Robotic Systems》1996,15(1):67-133
In this paper the application of robust control to a 5-link biped robotic model is investigated through the sliding mode approach, and compared to pure computed torque control. The biped consists of five links, namely the torso and two links in each leg. These links are connected via four (two hip and two knee) rotating joints which are considered to be friction-free and driven by independent d.c. motors. The locomotion of the biped is assumed to be constrained on the sagittal plane. The paper provides a full derivation of the biped dynamic model (single-leg support phase, biped-in-the-air phase) and an outline of the computed torque and sliding mode control algorithms. The simulation results were derived with two sets of parameters (one of which corresponds to a human-sized biped) and several degrees of parametric uncertainty (from 10% to 200%). In all cases the results obtained through the sliding mode control were much better than those obtained with the computed torque control. This superiority was shown to become stronger as the degree of uncertainty and the size of the biped increases. 相似文献
300.
Spyros A. Svoronos Wilbur W. Woo Stuart J. C. Irvine Haluk O. Sankur Jagmohan Bajaj 《Journal of Electronic Materials》1996,25(9):1561-1569
The interdiffused multilayer process (IMP) is a novel approach to growing Hg1−xCdxTe. In this process, alternating thin films of HgTe and CdTe are grown and allowed to interdiffuse resulting in a bulk material
of constant composition. A model of the IMP must include the effects of both the deposition of new material and the interdiffusion
of the material. It must also be able handle the flush phases of the IMP where the growth rate decays to zero. Existing approaches
to modeling epitaxial growth of Hg1−xCdxTe treat growth and interdiffusion as separate, sequential steps resulting in numerical stability problems, pseudodiffusion
effects, or flush phase modeling problems. The model presented here, however, is based on an incremental balance where growth
and diffusion occur simultaneously, resulting in a model exhibiting none of the difficulties mentioned above. The IMP growth
model is integrated with a model for calculating reflectance from a laser directed at near normal incidence angle. The predicted
reflectance is compared to experimental measurements and showed a good preliminary fit when the model employed default parameters.
The agreement is greatly improved after parameter fitting. 相似文献