Coherent helical structures in swirl flows

A nonstationary flow with rotating helical filaments formed in the region of swirl flow breakdown has been studied using the method of phase averaging of the output signal of a laser Doppler velocimeter. Vortices in the measured flow field were separated using the λ₂ technique based on an analysis of the velocity gradient tensor. It is shown for the first time that precession of the central monopolar vortex is accompanied by the formation of a pair of secondary vortices with the opposite directions of circulation, which form a helical dipole structure.     

Damping of the acoustic vibrations of individual gold nanoparticles

In this letter, the ultrafast vibrational dynamics of individual gold nanorings has been investigated by femtosecond transient absorption spectroscopy. Two acoustic vibration modes have been detected and identified. The influence of the mechanical coupling at the nanoparticle/substrate interface on the acoustic vibrations of the nano-objects is discussed. Moreover, by changing the environment of the nanoring, we provide a clear evidence of the impact of the surrounding medium on the damping of the acoustic vibrations. Such results are reported here for the first time on individual nanoparticles. This work points out a new sensing method based on the sensitivity of the acoustic vibration damping to the surrounding medium.     

Experimental investigation on the avoidance of self-excited vibrations

Self-excited vibrations are observed in many technical applications. Frictional contacts are often involved in the mechanism which generates vibrations. Reasons for the excitation mechanisms are decreasing friction characteristics depending on the sliding velocity, fluctuating normal loads or different geometrical effects. First, the mechanisms are explained using simple examples. The practical relevance of self-excited, friction-induced vibrations is exemplified with three technical systems: a system with an axial seal; a tread block of a tyre; and a disc brake. The knowledge of the excitation mechanism is necessary to introduce successfully design countermeasures. These measures to avoid self-excited vibrations are important to solve practical problems. They are the main focus of this work. Further, additional passive and active subsystems are described and validated experimentally. Therefore, a large range of design, active and passive solutions are given.     

Structure-dependent coherent acoustic vibrations of hollow gold nanospheres

Hollow gold nanospheres (HGNs) were excited with ultrashort laser pulses, and the coherent vibrational response was examined using femtosecond time-resolved transient absorption. The results indicated that HGNs support an isotropic mode, resulting in periodic modulation of the surface plasmon differential absorption. Two different categories of coherent acoustic vibrations, which depend on particle dimensions, were observed for HGNs. Further, the vibration launching mechanism was dependent upon the dimensions of the HGN. Coherent vibrations in HGNs characterized by small outer radii (<10 nm) and low cavity-radius-to-outer-shell radius aspect ratios (<0.5) were excited by a direct mechanism, whereas the vibrations observed for the larger particles (>25 nm OR) with higher aspect ratios (>0.5) resulted from an indirect mechanism. These findings may be significant for developing a predictive understanding of nanostructure optical and mechanical properties.     

Modal vibrations of an infinite cylinder in an acoustic halfspace

Acoustic radiation from an infinite cylindrical surface vibrating with an arbitrary, time-harmonic surface velocity distribution while positioned near the rigid/compliant boundary of a semi-infinite ideal compressible fluid medium is determined in an exact fashion using the classical method of separation of variables. The formulation utilizes the appropriate wave-field expansions and the method of images along with the pertinent translational addition theorem to develop a closed-form solution in form of infinite series. The analytical results are illustrated with numerical examples in which the cylindrical source, vibrating in the monopole and dipole-like modes, is positioned near the rigid/compliant boundary of a water-filled acoustic halfspace. Subsequently, the basic acoustic field quantities such as the modal acoustic radiation impedance load, radiated far-field pressure and the radiation intensity distribution are evaluated for representative values of the parameters characterizing the system.     

Excitation of the opposite acoustic flows in thermodynamically nonequilibrium gaseous media

It is shown that the acoustic wind direction may change in an acoustically active gaseous medium.     

An investigation of the kinetics of structure transformations and phase transition in nickel using acoustic methods

Results are given of experimental investigation of the temperature dependence of the acoustic and acoustic-emission properties of nickel in the vicinity of magnetic phase transition (Curie point), as well as of their behavior in the process of annealing and quenching. The information content of these properties is demonstrated when investigating the evolution of nonequilibrium structures and phase transitions of metal materials.     

An investigation of the parameters of heat-exchanging devices using the vapor-lift mechanism

An analysis has been made of the conditions of heat transfer which makes it possible to determine the optimum range of working temperatures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 1, pp. 44–48, January, 1989.     

An experimental investigation of the hydrodynamics of two-phase flows (mixtures and jets) in pebble beds

An experimental investigation is performed of two-phase flows through pebble beds. A flow presents either a premixed gas-water mixture or a jet of gas in water. The dependences of pressure loss on the main characteristics of flow are obtained. A formula is suggested for the estimation of overall pressure loss of a two-phase medium in beds. The opening angle of a jet of gas as a function of the ratio of phase velocities is investigated for the case of jet flow of a two-phase medium. Some estimates are given of the physical pattern of the process of phase mixing.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 954–960.Original Russian Text Copyright © 2004 by G. A. Filippov, L. E. Melamed, V. P. Mastyukin, M. V. Konditerov, and A. I. Tropkina.     

Computational-theoretical investigation of buoyant jet flows

The results of mathematical simulation of fully developed plane buoyant jet flows are presented. The solution is found within the framework of the model of a laminar boundary layer in the Boussinesq approximation using the method of matched asymptotic expansions. Analytical dependences of the basic characteristics of a jet flow on the Prandtl number and density parameter have been constructed.     

Damping of acoustic vibrations of single gold nanoparticles optically trapped in water

We combine ultrafast pump-probe spectroscopy with optical trapping to study homogeneous damping of the acoustic vibrations of single gold nanospheres (80 nm diameter) and nanorods (25 nm diameter by 60 nm length) in water. We find a significant particle-to-particle variation in damping times. Our results indicate that vibrational damping occurs not only by dissipation into the liquid, but also by damping mechanisms intrinsic to the particle. Our experiment opens the study of mechanisms of intrinsic mechanical dissipation in metals at frequencies 1-1000 GHz, a range that has been difficult to access thus far.     

Radioisotope investigation of impurity transport in air flows

The use of the radioactive tracer method of studying impurity transport in air is investigated. An expression relating the counting rate and the tracer concentration in the gas mixture is obtained. Certain experimental results are presented.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 1, pp. 136–144, July, 1971.     

Ring vibrations in an acoustic medium as a source of ultrasonic radiation

In this paper, the vibrational characteristics of ultrasonic transducers consisting of a ceramic ring filled with a plastic disk are considered. For axially symmetric resonances, the vibrational amplitude on the surface of the plastic core is approximately described by a truncated Bessel function of zero order and first kind J(0), leading to an ultrasonic beam which is nondiffracting near the transducer. The near- and far-field pressure distributions of such transducers are measured and compared with model predictions. In the near zone, the width of the measured main lobe is narrow, which is in accordance with the theoretical results for circular transducers with a truncated Bessel function amplitude distribution. By changing the inner diameter of the ring, it is possible to control the resonance frequency of the plastic core. On the other hand, the resonance frequency of the ring may be regulated by varying its width. The existing coupling of resonant vibrations of the ceramic ring and plastic core enables variations of the bandwidths of the considered circular transducers.     

An investigation of the effects of material anisotropy and heterogeneity on pulsed, laser-generated acoustic signals

Point-source and line-source models for the laser ultrasonic source in materials exhibiting transverse isotropy are applied to the specific problem of laser generation and ultrasonic propagation in unidirectional, polymer matrix composite materials. Comparing experiment and theory, it is shown that these composite materials exhibit homogeneous behavior, at the frequencies investigated, for ultrasonic wave propagation perpendicular to the fiber direction. For ultrasonic propagation in the fiber direction, ultrasonic dispersion, resulting from the inhomogeneous nature of the composite, affects the laser ultrasonic signal.