Acoustic characteristics of interacting supersonic jets

This study was intended primarily to reveal more information about the noise producing mechanisms of supersonic jets. Two identical, small, cold air, supersonic, overexpanded jets were tested at selected angles, varying from parallel to 90 degrees intersecting, and at various distances apart. Schlieren photographs of the jet structure and far field sound data were obtained. Close spacing of the parallel jets caused acoustic attenuation, which reached a maximum at one diameter centerline spacing, where the sound of two jets nearly equals that of a single jet. In every case the intersecting jets merged into a single supersonic jet.The overall sound power level of intersecting jets is generally higher than that of two independent jets, because of the turbulent mixing of the two jet flows. A maximum level is reached when the jets intersect at a point near the middle of the flow region containing repetitive shocks. For the parallel jets and intersecting jets at large separation, the sound levels are lower in the plane containing the jet centerlines. For intersecting jets at small separation, however, this shielding effect is reversed.     

Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m _λ ). At low fluence, small permanent increases of E(m _λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.     

Optical properties of metals with inhomogeneously heated electrons

We propose a new approach for describing the optical properties of metals with inhomogeneously heated electrons in the skin layer. The absorption coefficient and phase shift of the reflected wave, which are not described by the usual Fresnel formulas, were found. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

Acoustic properties of unconsolidated granular mixes

Simple empirical relations have been proposed to relate a limited number of directly measurable non-acoustic properties of an unconsolidated granular mix to its characteristic acoustic impedance and propagation constant. These properties are: characteristic particle dimension, porosity, tortuosity and the density of the grain base. It is believed that the model accounts heuristically for the mechanical friction between the elements of the rigid frame, the absorption in the frame micro-pores, and the degree of compaction. These effects can be important and are linked to the value of material density. This work presents practical applications of the proposed model for the prediction of the acoustic characteristics of hard-backed layers of loose granular mixes which can be used for acoustic absorption and insulation. It is shown that the predictions are in excellent agreement with the measured data for a representative range of loose granular mixes. A comparison of the results of the 4-parameter Attenborough model for the acoustic properties of porous media and the experimental data is made also. This model is used extensively to predict the acoustic properties of porous ground and granular media. The accuracy of this model with respect to loose granular materials is discussed.     

Acoustic properties of a human chest

A cross-spectral method for determining the longitudinal velocity of sound in the tissues of a human chest in vivo is proposed and substantiated. The method is based on the detection of a percussion stroke by two acoustic sensors positioned over opposite parts of a lung. Statistical estimates are obtained for the longitudinal velocity of sound in chest tissues (the middle part of the right lung) from a group of three men (40–47 years old) without any evident lung disorders in the frequency ranges of 80–130, 170–290, and 350–500 Hz. The adequacy of the double-resonance acoustic model of the human respiratory tract, which combines the resonance of the air volume in the human chest and the wave resonances of the bronchial tree as a narrow pipe, is experimentally verified.     

Acoustic properties of humpback whale songs

A vertical array of five hydrophones was used to measure the acoustic field in the vertical plane of singing humpback whales. Once a singer was located, two swimmers with snorkel gear were deployed to determine the orientation of the whale and position the boat so that the array could be deployed in front of the whale at a minimum standoff distance of at least 10 m. The spacing of the hydrophones was 7 m with the deepest hydrophone deployed at a depth of 35 m. An eight-channel TASCAM recorder with a bandwidth of 24 kHz was used to record the hydrophone signals. The location (distance and depth) of the singer was determined by computing the time of arrival differences between the hydrophone signals. The maximum source level varied between individual units in a song, with values between 151 and 173 dB re 1 microPa. One of the purposes of this study was to estimate potential sound exposure of nearby conspecifics. The acoustic field determined by considering the relative intensity of higher frequency harmonics in the signals indicated that the sounds are projected in the horizontal direction despite the singer being canted head downward anywhere from about 25 degrees to 90 degrees. High-frequency harmonics extended beyond 24 kHz, suggesting that humpback whales may have an upper frequency limit of hearing as high as 24 kHz.     

Quantitative properties of colour singlet clusters in QCD jets

Using a Monte Carlo technique we have made detailed calculations of the properties of colour singlet clusters in QCD jets, first discussed by Amati and Veneziano. Although the results confirm a mass damping for colour singlets, we find that, quantitatively, it is not enough to justify a phenomenological use of colour singlets as an intermediate step in the materialization of QCD quanta into hadrons.     

Acoustic streaming related to minor loss phenomenon in differentially heated elements of thermoacoustic devices

It is demonstrated that the differentially heated stack, the heart of all thermoacoustic devices, provides a source of streaming additional to those associated with Reynolds stresses in quasi-unidirectional gas flow. This source of streaming is related to temperature-induced asymmetry in the generation of vortices and turbulence near the stack ends. The asymmetry of the hydrodynamic effects in an otherwise geometrically symmetric stack is due to the temperature difference between stack ends. The proposed mechanism of streaming excitation in annular thermoacoustic devices operates even in the absence of thermo-viscous interaction of sound waves with resonator walls.     

Acoustic tomography of temperature distribution in a medium heated by a focused ultrasound beam

A new method is proposed for measuring temperature in the focal region of a high-intensity ultrasonic emitter on the assumption of axially symmetric temperature field. This method is based on solving the integral equation relating the temperature of the medium to the delay times of the probe ultrasound pulse intersecting the heated region in the transverse direction at different distances from the beam axis. The accuracy of the algorithm for calculating temperature with allowance for the finite experimental data set is analyzed. The calculation results are compared with the experimental data. Original Russian Text ? S.A. Tsysar’, O.A. Sapozhnikov, V.G. Andreev, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 4, pp. 558–561.     

二硼化钛陶瓷的声学性能

用超声相比较方法测量了TiB₂陶瓷样品不同方向的声速,以此计算了样品的弹性模量,并与别的作者的结果进行了比较,讨论了引起各种结果差异的原因。     

超硬材料的声学特性

利用超声相干技术的几种不同测速方法,测量金刚石聚晶及其它一些超硬材料在室温或960℃以下温区的纵波和横波速度,以此研究这类材料的声学特性和弹性性能。     

径向声子晶体隔声特性*

该文构造了由两种匀质材料交替分布的径向声子晶体柱壳模型。首先,针对声波在其中的轴对称传播情况进行了理论分析,建立了声波由内向外传播的传递矩阵,进而导出了声压透射系数和隔声量表达式。采用数值分析的方法系统地讨论了径向声子晶体柱壳的隔声特性,并与单一材质柱壳的传播规律进行对比分析,其次,借助有限元仿真分析的手段对数值结果进行了验证。最后,详细分析了内外流体的特性阻抗对径向声子晶体柱壳隔声特性的影响,得到了相应的参数影响规律。研究表明,径向声子晶体柱壳存在声波带隙,导致其在带隙范围内的隔声效果远远优于单材质柱壳,并且该结构的固有特性突破了质量定律的限制;声波带隙内表面局域态现象出现与否由内外声场和结构场共同决定。     

Structure and electrical properties of p-type twin ZnTe nanowires

Resonant tunneling is firstly found in twin p-type ZnTe nanowire field-effect transistors. The twin ZnTe nanowires are synthesized via the thermal evaporation process. X-ray diffraction and high-resolution transmission electron microscopy characterization indicate that the as-grown twin nanowire has a zinc-blende crystal structure with an integrated growth direction of [11-1]. The twin plane is (11-1) and the angle between the mirror symmetrical planes is 141°. The formation of twins is attributed to the surface tension from the eutectic liquid droplet. Field-effect transistors based on single ZnTe twin nanowire are constructed, the corresponding electrical measurements demonstrate that the twin nanowires have a p-type conductivity with a mobility (μ _h ) of 0.11 cm² V⁻¹ S⁻¹, and a carrier concentration (n _h ) of 1.1×10¹⁷ cm⁻³. Significantly, the negative differential resistance with a peak-to-valley current ratio of about 1.3 is observed in p-type twin ZnTe nanowire field-effect transistors at room temperature. As the periodic barriers produced in the periodic twin interfaces can form multi-barrier and multi-well along one-dimensional direction. The multibarrier can be modulated under external electrical field. When the resonant condition is met, the space charge will be enhanced with the inherent feedback mechanism, and the resonant tunneling will occur.     

Experimental properties of gluon and quark jets from a point source

Gluon jets are identified in hadronic Z decays as all the particles in a hemisphere opposite to a hemisphere containing two tagged quark jets. Gluon jets defined in this manner are equivalent to gluon jets produced from a color singlet point source and thus correspond to the definition employed for most theoretical calculations. In a separate stage of the analysis, we select quark jets in a manner to correspond to calculations, as the particles in hemispheres of flavor tagged light quark (uds) events. We present the distributions of rapidity, scaled energy, the logarithm of the momentum, and transverse momentum with respect to the jet axes, for charged particles in these gluon and quark jets. We also examine the charged particle multiplicity distributions of the jets in restricted intervals of rapidity. For soft particles at large , we observe the charged particle multiplicity ratio of gluon to quark jets to be , in agreement with the prediction that this ratio should approximately equal the ratio of QCD color factors, . The intervals used to define soft particles and large for this result, GeV/ and GeV/, are motivated by the predictions of the Herwig Monte Carlo multihadronic event generator. Additionally, our gluon jet data allow a sensitive test of the phenomenon of non-leading QCD terms known as color reconnection. We test the model of color reconnection implemented in the Ariadne Monte Carlo multihadronic event generator and find it to be disfavored by our data. Received: 24 February 1999 / Published online: 8 September 1999