Experimental investigation of the outer wave field at the open end of a pipe

The results of investigating the external wave field radiated from the open end of a tube on the linear and nonlinear resonance intervals are presented. The presence of a velocity amplitude maximum in the wall zone near the open end is detected. The limits of vortex motion of the gas within the tube are determined. The flow core in the wave field is investigated.Translated from Inzhenerno-fizicheskie Zhurnal, Vol. 61, No. 5, pp. 714–716, November, 1991.     

Kopplung von Gasdruckschwingungen in Radialverdichtern

Unsteady flow processes occur in centrifugal compressors, especially noticeable in passing region between impeller flow and reposing diffusers like return bend or radial diffuser. This unsteady flow causes gas pressure oscillations of considerable extent. If the unsteady flow generates two or more gas pressure oscillations, which result of different arousals with different frequencies, interconnection of gas pressure oscillations occur, leading into new vibration occurrences and side bands at base frequency in frequency spectrum. If frequencies and magnitude of pressure oscillation amplitudes of single oscillations with sinusoidal oscillation progression are known, the resulting gas pressure oscillation progression of interconnected gas pressure oscillations can be calculated and also experimental proven.     

Dynamics of the conductivity solid bodies in a high-frequency alternating magnetic field

The conductivity bodies and systems of pendulum types situated in alternating magnetic field are very interesting objects from physical and mathematical point of view. In dependence of correlation between eigen frequency and frequency of external alternating magnetic field, many motions with very different characters are possible. So in high-frequency magnetic field is experimentally observed and mathematically described the stability of overturn pendulum state and also self-excited oscillations. If the magnetic field with average frequency like to eigen frequency, many others kinds of motions are possible. Particular motions of rare attractors type and others. The technical sense of these effects may be used for support stability of beforehand unstable positions conductivity bodies and so the excitement of oscillations with necessary characters, particular oscillations with necessary frequency.     

Linear and nonlinear forced oscillations of a flow of a bubbly liquid in a deformable pipe (survey)

Data reported in numerous publications is used as a basis for analyzing the possibility of the generation of shock waves in a flow moving inside a pipe. A piston vibrating at one end of a tube having a nozzle attached to its other end was chosen as the source of the oscillations. The equations of flow of a bubbly liquid are given for the case when long waves propagate in the liquid and a transverse velocity exists in it. Different formulations of the boundary conditions on the nozzle are given, and methods are described for solving the boundary-value problem far from resonance frequencies. The theoretical results obtained by passing to the gas-flow limit agree with the information obtained by other authors and experimental data. It is established that there are frequencies at which periodic hydraulic shocks are generated in the pipes that are examined. Their amplitude is much greater than the amplitude of the shock waves generated during resonance in a column of gas. By varying the initial content of gas in the mixture, the acoustic properties of the nozzle, and the elastic properties of the pipe, it is possible to control oscillations in the flow, alter the resonance frequencies, and eliminate or generate hydraulic shocks. When the frequencies of oscillation associated with the shocks are the same as the natural frequency of the pipe, the latter may undergo large displacements that can ultimately lead to fatigue failure of the pipe or its supports.Translated from Problemy Prochnosti, No. 9, pp. 3–29, September, 1994.     

声弹性法测量铝合金预拉伸板中的应力

研究温度和应力对7050铝合金预拉伸板中的超声纵波、偏振横波和临界折射纵波传播速度的影响,并分析不同频率临界折射纵波在梯度应力场中的传播规律。结果表明:温度对声速的影响大于声弹效应的影响;单轴拉伸时,沿轴向传播的临界折射纵波、平行轴向偏振的横波的速度降低,垂直应力方向偏振横波的速度升高,垂直轴向传播的纵波速度变化不大;临界折射纵波的频率越高,其所反映的应力越接近表面;声弹性法测得的应力是声传播路径上各点应力在超声波造成的质点振动方向上分量的综合反映。     

声场对流场影响的研究

借助于传声电容、传声放大器等声学测量仪器和热线风速仪等流场测量仪器及计算机数据采集系统，本文实验观察了有无强声场叠加及不同声场强度时流场瞬时速度波形的变化，并对声场作用下的平均速度、脉动速度、速度频谱、速度自相关、脉动速度和脉动声强互相关等流动特征参数进行了计算与分析。发现了强声对流场的调制作用，流动速度会随声音信号同步脉动，且声强越大，速度脉动幅度越大。当声场强度超过约155dB时，声场进入非线性区，波形从正弦波变为锯齿波。声波诱发的大幅度周期脉动及非线性效应，提高了流动的湍流特性。这些发现，将有助于对声凝聚机理的进一步研究。     

Experimental study of aerosol oscillations in an open tube in the shock-free wave mode

Forced longitudinal oscillations of a finely dispersed aerosol in an open tube in the shock-free wave mode near the first fundamental frequency have been studied experimentally. The time dependences of the numerical concentration of the aerosol droplets have been obtained. The effect of the frequency and amplitudes of the excitation on the aerosol clearing, which consists of coagulation, sedimentation on the tube walls, and partial ejection of the aerosol droplets from the open end of the tube, has been studied. It has been shown that the process of aerosol clearing occurs 6–12 times more efficiently than natural sedimentation.     

声流对单管道周围换热特性的影响

通过仿真与实验的方式,研究声波影响管道外部的流动和换热特性。在仿真中,对输入声压级相同时,不同频率的声流速度进行研究,结果表明频率对管壁处平均声压级的影响较小,但对管壁周围切向速度的影响较大。基于声波对流场分布的影响,在实验中研究不同声压级和频率对单根钢管换热的影响,结果表明声压级不变,频率越低,钢管换热效果越好;频率不变,声压级越大,钢管换热效果越好。仿真与实验结果表明：管道周围的换热效果与声流速度的大小成正比,而声流大小与频率和激励速度直接相关,激励速度越大,频率越低,钢管附近声流运动越剧烈,采用低频高声压级的声波能够加速钢管的换热过程,该研究为声波影响管道换热特性提供了理论依据。     

Velocity Fluctuations in a Particle-Laden Turbulent Flow over a Backward-Facing Step

Dilute gas-particle turbulent flow over a backward-facing step is numerically simulated. Large Eddy Simulation (LES) is used for the continuous phase and a Lagrangian trajectory method is adopted for the particle phase. Four typical locations in the flow field are chosen to investigate the two-phase velocity fluctuations. Time-series velocities of the gas phase with particles of different sizes are obtained. Velocity of the small particles is found to be similar to that of the gas phase, while high frequency noise exists in the velocity of the large particles. While the mean and rms velocities of the gas phase and small particles are correlated, the rms velocities of large particles have no correlation with the gas phase. The frequency spectrum of the velocity of the gas phase and the small particle phase show the -5/3 decay for higher wave number, as expected in a turbulent flow. However, there is a "rising tail' in the high frequency end of the spectrum for larger particles. It is shown that large particles behave differently in the flow field, while small particles behave similarly and dominated by the local gas phase flow.     

Gas flow influences on bubbling and flow characteristics of fluidized bed with immersed tube under electrostatic effects

Industrial bubbling fluidized beds are used to fluidize particles. When particles are fluidized, electrostatic effects will cause the particles to form obvious agglomerates, thus reducing fluidization performance. For better fluidization performance, internal component immersed tubes are usually placed in fluidized bed to limit the bubble size and reduce particle agglomerates. Meanwhile, pulsed gas flow can increase particle disturbance, which is also an effective method to reduce particle agglomerates. In this paper, the CFD-DEM model under electrostatic effects is constructed to research the bubbling and flow characteristics in fluidized beds. Firstly, particle mixing qualities with and without the immersed tube are compared. Then, the effects of different superficial gas velocities are investigated with an immersed tube. Finally, different frequencies are applied to study the energy loss and flow characteristics around the immersed tube. The results show that the addition of the immersed tube can reduce bubble size to facilitate particle mixing. Due to the obstruction of the immersed tube, the bubbles are generated near the wall. As the superficial gas velocity increases, the larger bubbles are generated. Moreover, the electrostatic force applied to the particles varies periodically with the frequency of incoming pulsed gas flow, with fluctuations maximal at 2.5 Hz.     

Periodic shock waves in spherical resonators (survey)

This article surveys the literature on the problem of shock waves in spherical resonators. The published data is used to examine the feasibility of exciting shock waves in such resonators by means of a source of low-amplitude harmonic oscillations. A nonlinear wave equation is obtained to describe the propagation of unidimensional spherical waves in solids, liquids, and gases, as well as in bubbly liquids. A solution to the equation is constructed by the small-parameter method with the use of traveling-wave functions. Then, in solving boundary-value problems, linearized equations are integrated and the resonance frequencies at which the amplitudes of the oscillation increase without limit according to the linear solution are determined. Near these frequencies, the linear analysis is then refined by allowing for the nonlinear terms in the boundary-value problems. It is shown that an increase in the amplitude of the oscillations at resonance frequencies may lead to the formation of spherical periodic shock waves in the given resonators. An analogy is made between these waves and resonance shock waves excited in long unidimensional resonators.Translated from Problemy Prochnosti, No. 10, pp. 49–73, October, 1995.     

Vibrations and stability of smooth and ribbed spherical bands

Vibrations of preloaded smooth and ribbed spherical bands are studied. A solution of linearized movement equations in displacements is presented in the form of single trigonometric series for a circumferential coordinate. Critical loads and natural vibration frequencies are determined with a linear and geometrically linear stress-strained state of static equilibrium. The effect of a longitudinal compressive load on the magnitude of minimum natural frequencies for oscillations of smooth and ribbed spherical bands is considered as a specific example.Translated from Problemy Prochnosti, No. 2, pp. 86–91, February, 1996.     

Automodulation onset in a gyro-backward-wave oscillator with external feedback

The onset of multimode oscillation regimes (automodulation) in a helical electron beam-backward electromagnetic wave system (gyro-backward wave oscillator, gyro-BWO) with external feedback has been studied. Thresholds for the self-excitation and the onset of multimode oscillations in the gyro-BWO significantly depend on the feedback parameters and are determined by the quality and the natural frequencies of a resonator formed by an electrodynamic system of the gyro-BWO and the delay-feedback chain.     

Enhanced fluidization of solid particles in an oscillating acoustic field

Compared with an ordinary fluidized bed, the fluidization quality of solid particles can be effectively improved by vibration induced by appropriate acoustic fields. The effects of sound on the hydrodynamic behavior of fluidized bed have been investigated under the application of acoustic fields of different intensities (110–130 dB) and frequencies (50–500 Hz). The obtained results show that the perturbation effect of the sound field on fixed-bed pressure drop becomes more significant with increasing sound pressure level, exerting a larger pressure than present under ordinary conditions, due to the change in particle arrangement induced by the acoustic field. Except for a particular frequency, the minimum fluidization velocity in the bed decreases gradually with the increase in the ratio of bed height to bed diameter. The rising velocity of the bubble and the average overflow velocity of residual gas in collapse tests are reduced by the acoustic field.     

Electron Transport and Velocity Oscillations in a Carbon Nanotube

We report Monte Carlo (MC) simulation results showing applied bias dependent spatial velocity oscillations for semiconducting single-walled zig-zag carbon nanotubes (CNTs). MC simulations show velocity oscillations in CNTs at different lengths due to high scattering rates associated with optical phonon emissions in the one-dimensional CNT system. The frequencies of these oscillations are as high as 30 THz. In addition, we investigate average ensemble electron velocities as a function of tube length.     

Numerical simulation of the interaction between reflected shock wave and near-wall boundary layer

The interaction of a shock wave, reflected from the flat end of a cylindrical shock tube, with a cocurrent gas flow at the side wall, is considered in the axisymmetric case. The Navier-Stokes system of equations has been numerically integrated in the thin layer approximation using a predictor-corrector scheme. The appearance of a recirculation gas flow structure behind the reflected shock front and the evolution of this structure in the reflected wave going away from the tube end are analyzed. The corresponding patterns of constant density lines and the velocity vector field are presented.     

Influence of boundary condition on the sound velocity in granular assembly: Spiral tube versus cylinder

The properties of elastic wave propagation in granular assemblies have become a subject of immense interest in recent years, however, the influence of different confinements on the sound velocity is seldom investigated. This study provides a method to determine the contact point between spherical, super-ellipsoidal particles and complex boundaries, in order to investigate how the anisotropy induced by particle shape or boundary affects velocity. Taking cylinder and spiral tube confinements as examples, the falling process of spherical and super-ellipsoidal assemblies are simulated to verify the validation by the discrete element method (DEM). The convergence of the kinetic energy during the falling process and the equilibrium state with zero residual kinetic energy guarantees the stability and correctness. On the basis, elastic wave propagation of spherical and super-ellipsoidal systems in spiral tube and cylindrical confinements under different pressures are modelled, and sound velocities are calculated. The effective medium theory (EMT), granular solid hydrodynamics (GSH), and elastic stiffness are used to interpret the relationship between velocity and stress in cylindrical confinement. However, the results in the spiral tube deviate from EMT and GSH, which means the boundary affects velocity significantly. The difference of velocity between spiral tube and cylinder is qualitatively explained from the perspective of anisotropy of contact force distribution in the system. The simulation results show that anisotropy introduced by the curved surface affects the acoustic properties greatly. The method used for spiral boundary is also suitable for other complicated confinements.     

Characteristics of a single jet injected into an incipiently fluidized bed: A magnetic resonance imaging study

Rapid magnetic resonance imaging (MRI) was used to characterize properties of a single central gas jet injected into a 3D gas fluidized bed under incipient fluidization conditions. Snapshots of both particle concentration and particle velocity are provided. The average jet height, oscillations in jet height and the size of bubbles breaking off from the jet increased with increasing jet velocity. The frequency of bubble breakoff from the jet decreased with increasing jet velocity. The jet height measurements are compared with various correlations in the literature, and the quantitative data provided here can be compared directly with that from numerical simulations and theoretical predictions for validation purposes.     

Thermal acoustic oscillations in triple point liquid hydrogen systems

Thermal acoustic oscillations are often observed in tubes which penetrate a cryogenic system and are closed at the warm end and open at the cold end. Such tubes are genrally used for filling or vetning the tank, providing relief pressure or inserting instruments taps. Large amounts of heat (of the order of ten to a thousand times more than by normal heat conduction) can be transferred into a cryogenic system when such thermaloscillations occur. A number of studies examining thermal acoustic oscillations in liquid helium systems have been performed by Rott et al. However, only minimal consideration has been given to such oscillationsin liquid and sluch hydrogen systems. This study extends Rott's theory to the stability aspects of thermal acoustic oscillations for a straight tube closed at the warm end and inserted into a Dewar flask filled with triple point liquid hydrogen when the cold open end is located above the liquid surface. These results can also be applied to a slush hydrogen when the pressure in the Dewar flask is reduced to the triple point pressure of hydrogen. Numerical results have been obtained in this study for developing stability curves, establishing oscillation frequency characteristics and identifying critical configurations for initiating such oscillation. The mechanisms associated with the two branches of the stability curves for thermal acoustic oscillations have also been investigated.