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11.
12.
In the literature one basic work can be found that describes an attempt to verify the applicability of the orthotropic plate wave theory to paper. In the present study we have repeated the key experiment and also modified the original experimental set-up to be able to make new complementary measurements. The performed verification attempt is based on a key experiment in which an ultrasonic resonance method is used. When it is applied to paper, a second resonance can be observed in addition to the one that originates from the asymmetric A0 mode. The result of the verification experiment is conclusive only if the source of this second resonance is the symmetric S0 mode. Our results show that the second resonance is not generated by the S0 mode. Instead, they indicate that the source is a longitudinal wave propagating in the thickness direction of the paper. We therefore conclude that the applicability of the orthotropic plate wave theory to paper cannot be verified by performing the key experiment that is described in the literature. 相似文献
13.
Random waypoint mobility model in cellular networks 总被引:2,自引:0,他引:2
In this paper we study the so-called random waypoint (RWP) mobility model in the context of cellular networks. In the RWP model the nodes, i.e., mobile users, move along a zigzag path consisting of straight legs from one waypoint to the next. Each waypoint is assumed to be drawn from the uniform distribution over the given convex domain. In this paper we characterise the key performance measures, mean handover rate and mean sojourn time from the point of view of an arbitrary cell, as well as the mean handover rate in the network. To this end, we present an exact analytical formula for the mean arrival rate across an arbitrary curve. This result together with the pdf of the node location, allows us to compute all other interesting measures. The results are illustrated by several numerical examples. For instance, as a straightforward application of these results one can easily adjust the model parameters in a simulation so that the scenario matches well with, e.g., the measured sojourn times in a cell. 相似文献
14.
The first third-generation (3G) systems are going to be implemented within the year 2001. Academia and industry are, however, already looking for means to improve the system performance further. Spectral efficiency and higher data rate are the goal. In order to achieve this goal, several means have to be combined. The most important of these are interference cancellation and multiuser detection, optimum space-time processing and adaptive modulation and coding. In the future several wireless systems having different performance characteristics, system structure and parameters will exist. This heterogeneity will make the seamless handover from system to system difficult if not appropriately solved. 4G (fourth generation) is an acronym without any generally accepted concept. We describe some desirable goals and features of the 4G systems. 相似文献
15.
Jorma Heikkinen 《Heat and Mass Transfer》2007,43(4):341-350
The fin efficiency in a heat exchanger element that is a simplification of one row in a tube-and-fin heat exchanger was theoretically
examined within wide ranges of the affecting variables: the conventional fin efficiency and the isothermal effectiveness of
the heat exchanger. These variables are suggested for use also in the further studies. An analytical solution can be found
for the case of a constant heat transfer coefficient. The ambient temperature variation alone decreases the fin efficiency
less than 4%. The local heat transfer coefficient obtained from the numerical fluid flow simulations is strongly affected
by the fin properties because the thermal boundary conditions for the fluid flow changes. On a poorly conducting fin surface
the heat transfer coefficient in front of the fin base is much larger than on an isothermal fin because the heat flux is increasing
in the flow direction. At low fin efficiencies this compensates for the decrease in fin efficiency due to ambient temperature
variation. 相似文献
16.
The random waypoint model (RWP) is one of the most widely used mobility models in performance analysis of mobile wireless networks. In this paper we extend the previous work by deriving an analytical formula for the stationary distribution of a node moving according to a RWP model in n-dimensional space. 相似文献
17.
18.
The kinetics of the reaction between CH3 and HCl was studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (296–495 K) and were fitted to an Arrhenius expression: k1 = 5.0(±0.7) × 10?13 exp{?1.4(±0.3) kcal mol?1/RT} cm3 molecule?1 s?1. This information was combined with known kinetic parameters of the reverse reaction to obtain Second Law determinations of the methyl radical heat of formation {34.7(±0.6) kcal mol?1} and entropy {46(±2) cal mol?1 K?1} at 298 K. Using the known entropy of CH3, a more accurate Third Law determination of the CH3 heat of formation at this temperature was also obtained {34.8(±0.3) kcal mol?1}. The values of k1 obtained in this study are between those reported in prior investigations. The results were also used to test the accuracy of the thermochemical information which can be obtained from kinetic studies of R + HX (X = Cl, Br, I) reactions of the type described here. 相似文献
19.
Solution of linear equations with Hankel and Toeplitz matrices 总被引:1,自引:0,他引:1
Jorma Rissanen 《Numerische Mathematik》1974,22(5):361-366
Summary An algorithm is described for solving a linear equationA x=c, whereA is ann×n Hankel or Toeplitz matrix, inO (n
2) arithmetic operations. As contrasted with earlier such algorithms the present one does not require the principal minors ofA to be non-zero except detA. 相似文献
20.
Thermoluminescence study of persistent luminescence materials: Eu2+- and R3+-doped calcium aluminates, CaAl2O4:Eu2+,R3+ 总被引:3,自引:0,他引:3
Aitasalo T Hölsä J Jungner H Lastusaari M Niittykoski J 《The journal of physical chemistry. B》2006,110(10):4589-4598
Thermoluminescence properties of the Eu2+-, R3+-doped calcium aluminate materials, CaAl2O4:Eu2+,R3+, were studied above room temperature. The trap depths were estimated with the aid of the preheating and initial rise methods. The seemingly simple glow curve of CaAl2O4:Eu2+ peaking at ca. 80 degrees C was found to correspond to several traps. The Nd3+ and Tm3+ ions, which enhance most the intensity of the high-temperature TL peaks, form the most suitable traps for intense and long-lasting persistent luminescence, too. The location of the 4f and 5d ground levels of the R3+ and R2+ ions were deduced in relation to the band structure of CaAl2O4. No clear correlation was found between the trap depths and the R3+ or R2+ level locations. The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R3+ ions. A new persistent luminescence mechanism presented is based on the photoionization of the electrons from Eu2+ to the conduction band followed by the electron trapping to an oxygen vacancy, which is aggregated with a calcium vacancy and a R3+ ion. The migration of the electron from one trap to another and also to the aggregated R3+ ion forming R2+ (or R3+-e-) is then occurring. The reverse process of a release of the electron from traps to Eu2+ will produce the persistent luminescence. The ability of the R3+ ions to trap electrons is probably based on the different reduction potentials and size of the R3+ ions. Hole trapping to a calcium vacancy and/or the R3+ ion may also occur. The mechanism presented can also explain why Na+, Sm3+, and Yb3+ suppress the persistent luminescence. 相似文献