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排序方式: 共有182条查询结果,搜索用时 15 毫秒
1.
Prof. Dr. J. H. Ahrens 《Computing》1989,41(1-2):163-166
In many algorithms for sampling from non-uniform distributions the logarithm of a uniform deviate must be compared with some test quantity. It is shown that all these comparisons can be done efficiently without calling a logarithm subprogram: two procedures for this task are presented and compared. 相似文献
2.
This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency. 相似文献
3.
Equilibrium potentials of the iron(II)—iron electrode vs TlTlCl; 0.1 M NH4Cl in liquid ammonia are determined from intersections of Tafel lines for iron deposition and dissolution. The hydrogen electrode is shown to behave reversibly at palladium hydride, but not at platinum with respect to the dependence of hydrogen pressure. The standard potential of iron at 293 K is estimated at E° = ?0,356 (±0,010)V in 0.1 M NH4NO3. Standard potentials of other electrodes are reevaluated and shown to be consistent with earlier measurements except for the standard potential of lead. Steady state and transient polarization curves indicate in simple mechanism of the iron electrode involving transfer of iron(II) in one step. The temperature-independent anodic transfer coefficient is α+ = 0.41 (±0.02), the cathodic transfer coefficient α? = 0.61 (±0.03). The standard exchange current density at E° is j°° = 7.10?7 Acm?2 for 293 K. Exchange current densities of the hydrogen electrode are j° = 1.6·10?9 Acm?2 at platinized platinum in 0.1 M NH4Cl both for 293 K. 相似文献
4.
An accurate acceptance-rejection algorithm is devised and tested. The procedure requires an average of less than 3 uniform deviates whenever the standard deviation of the distribution is at least 4, and this number decreases monotonically to 2.63 as . Variable parameters are permitted, and no subroutines for sampling from other statistical distributions are needed.This research was supported by the Austrian Research Council (Fonds zur Förderung der wissenschaftlichen Forschung). 相似文献
5.
This study used cluster analysis to identify three patterns of sexual health risk behaviors in a sample of adult rape survivors (N=102). Women in the 1st cluster (high risk) reported substantial increases from pre- to postrape in their frequency of sexual activity, number of sexual partners, infrequency of condom use, and frequency of using alcohol and/or drugs during sex. The 2nd cluster (moderate risk) reported increases in frequency of sexual activity and number of partners but mitigated that risk with increased condom use. Survivors in the 3rd cluster (low risk) indicated that their sexual health behaviors had become much less risky postrape. An ecological model predicting cluster membership revealed that individual-level and contextual factors predict patterns of risk behaviors. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
6.
G. Gregoris F. Bouton C. De Keukeleire P. Siliprandi F. Baio L. De Schepper W. De Ceuninck L. Tielemans T. Ahrens M. Krumm 《Quality and Reliability Engineering International》1996,12(4):247-252
We present the results of SHORTEST, a BRITE EURAM II project for the introduction in industry of an advanced approach to the reliability evaluation of electronic assemblies. The achievements so far are the development and validation of seven in-situ test techniques. Five of the test techniques demonstrate the capability of detecting failure mechanisms at 48 hours with moderate stress conditions, and half of the in-situ test results obtained at 48 hours on 10 subtechnologies correlate with conventional tests. All correlation results are presented. The application of the method for quality assurance and building-in reliability is discussed. The final aim of SHORTEST is to look for the exploitation of the developed in-situ techniques. The marketing study that will define the industrial needs of European companies is presented. In parallel, the insertion in the CECC specification of recommendations based on the SHORTEST principle is foreseen. 相似文献
7.
Ahrens O Buhrdorf A Hohlfeld D Tebje L Binder J 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2002,49(9):1321-1329
A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer. All previous approaches either require an additional metallic electrode or do not possess a structured sacrificial layer and, consequently, may make exact adjustment of the membrane dimensions difficult. The present set ups are especially suited for the fabrication of cMUT with gap heights ranging between 50 nm and 2 /spl mu/m between the electrodes. Large gaps are a prerequisite to enabling sufficient deflections of the membrane and, therewith, to generating high pressure gradients. On the other hand, small gap sizes are desirable for detecting weak ultrasonic sources. This paper focuses on the fabrication process of cMUT to realize electrode separation above 500 nm and, in addition, on the manufacturing of cMUT with gaps below 500 nm. The successful realization has been proven by some basic experimental investigations. Finally, the fundamental equations of a frequently chosen simulation model are documented, as a number of ambiguities exist in the common literature. 相似文献
8.
Haberer P du Toit M Dicks LM Ahrens F Holzapfel WH 《International journal of food microbiology》2003,87(3):287-291
Minipigs were fed a "Western-style", high-cholesterol diet for a baseline period, followed by the diet containing a mixture of three Lactobacillus strains with potential probiotic features, after which a normal pig diet was followed. The faecal enzyme activity for beta-glucuronidase and azoreductase, which are commonly considered as markers for procarcinogenic activity, was significantly reduced during the 5 weeks of "probiotic" supplementation. During the period of Lactobacillus administration, the cell counts for total anaerobes increased, whereas the total number of aerobes showed no change. 相似文献
9.
10.
Vuong T.H.H. Gibson W.C. Ahrens R.E. Parsey J.M. Jr. 《Electron Devices, IEEE Transactions on》1990,37(1):51-57
The sidegating effect in a MBE-grown HFET GaAs structure is considerable and persists to large distances. On the other hand, the leakage current is very low. A model that takes into account the unusual features of sidegating in this structure and is consistent with other studies of similar structures is proposed. On the basis of the model, a change in the isolation process was implemented. This resulted in a large reduction of the sidegating effect and confirmed the main features of the model 相似文献