In this paper we present the experimental results of a detailed investigation of the flow and acoustic properties of a turbulent
jet with Mach number 0·75 and Reynolds number 3·5 103. We describe the methods and experimental procedures followed during the measurements, and subsequently present the flow
field and acoustic field. The experiment presented here is designed to provide accurate and reliable data for validation of
Direct Numerical Simulations of the same flow. Mean Mach number surveys provide detailed information on the centreline mean
Mach number distribution, radial development of the mean Mach number and the evolution of the jet mixing layer thickness both
downstream and in the early stages of jet development. Exit conditions are documented by measuring the mean Mach number profile
immediately above the nozzle exit. The fluctuating flow field is characterised by means of a hot-wire, which produced radial
profiles of axial turbulence at several stations along the jet axis and the development of flow fluctuations through the jet
mixing layer. The axial growth rate of the jet instabilities are determined as function of Strouhal number, and the axial
development of several spectral components is documented. The directivity of the overall sound pressure level and several
spectral components were investigated. The spectral content of the acoustic far field is shown to be compatible with findings
of hot-wire experiments in the mixing layer of the jet. In addition, the measured acoustic spectra agree with Tam’s large-scale
similarity and fine-scale similarity spectra (Tam et al., AIAA Pap 96, 1996). 相似文献
We report a singly resonant optical parametric oscillator (SRO) based on a ZnGeP(2) crystal directly pumped by a lamp-pumped Q-switched CrTmHo:YAG laser. The IR was tunable from 4.7 to 7.8 microm via crystal angle tuning. A maximum optical to optical efficiency of 56% was obtained from the pump (2.09 microm) to total IR at a pump energy of 6.5 mJ. The corresponding idler energy was 1.45 mJ. The SRO was measured to have a slope efficiency of 64% and a threshold of 1 mJ. The spatial beam quality of the idler, characterized by the M(2) parameter, was 1.38 when the SRO was pumped at 2.5 times threshold. These results show that ZnGeP(2) optical parametric oscillators directly pumped by a CrTmHo:YAG laser can be operated efficiently, while maintaining good IR beam quality. 相似文献
The activation of dioxygen for selective oxidation of organic molecules is a major catalytic challenge. Inspired by the activity of nitrogen‐doped carbons in electrocatalytic oxygen reduction, we combined such a carbon with metal‐oxide catalysts to yield cooperative catalysts. These simple materials boost the catalytic oxidation of several alcohols, using molecular oxygen at atmospheric pressure and low temperature (80 °C). Cobalt and copper oxide demonstrate the highest activities. The high activity and selectivity of these catalysts arises from the cooperative action of their components, as proven by various control experiments and spectroscopic techniques. We propose that the reaction should not be viewed as occurring at an ‘active site’, but rather at an ‘active doughnut’–the volume surrounding the base of a carbon‐supported metal‐oxide particle. 相似文献
A Monte Carlo simulation procedure has been set up and applied to generate representative ensembles of randomly branched step‐growth polymers based on their reaction recipe. The molecular distributions thus obtained are consistent with those from statistical/analytical approaches. However, because the current method gives access to the complete ensemble of simulated molecules, a very detailed structural analysis is possible. Our procedures are applicable to any ‘AfBg’ system with f + g ≥ 1. We apply this approach to randomly branched polyamides in order to gain insight into their molecular structure and understand the effect of the reaction recipe on the final product.
In this paper we investigate in a systematic way the influence of polydispersity in the block lengths on the phase behavior
of AB-multiblock copolymer melts. As model system we take a polydisperse multiblock copolymer for which both the A-blocks and the B-blocks satisfy a Schultz-Zimm distribution. In the limit of low polydispersity the expressions for the vertex functions are
clarified by using simple physical arguments. For various values of the polydispersity the phase diagram is presented, which
shows that the region of stability of the bcc phase increases considerably with increasing polydispersity. The strong dependence
of the periodicity of the microstructure on the polydispersity and on the interaction strength is presented.
Received 2 July 1998 相似文献
This study considers step‐growth polymerizing systems of general type “AfiBgi” whereby one or more of the reacting monomer species bear at least three reactive groups. The random polymerization process will lead to a population of polymer molecules in which the individual molecules may differ widely with respect to degree of polymerization and number of branch points. This study presents an algorithmic method to calculate the statistical distribution of weight over these two molecular properties. The method uses bivariate generating functions, recurrences, and integer arithmetic.
In the present paper we obtain sufficient conditions for the existence of equivalent local martingale measures for Lévy-driven moving averages and other non-Markovian jump processes. The conditions that we obtain are, under mild assumptions, also necessary. For instance, this is the case for moving averages driven by an -stable Lévy process with .Our proofs rely on various techniques for showing the martingale property of stochastic exponentials. 相似文献
We report a systematic study of the transport properties of coupled one-dimensional metallic chains as a function of the number of parallel chains. When the number of parallel chains is less than 2000, the transport properties show power-law behavior on temperature and voltage, characteristic for one-dimensional systems. 相似文献