管内流体交变流动速度的横向分布研究

基于线性理论分析,考虑流体的可压缩性、速度横向分布的相位延迟,对微扰项使用Fourier变换,给出充分发展、层流交变流动管内速度分布解析解.给出的解析解适用于描述各种小振幅振型的振荡,体现出流体速度横向分布的相位延迟,并验证了交变流动状态下的"环形效应".关于流体速度横向分布的解析解给出了流体在谐振状态下的速度分布函数,与肖家华在热声研究中使用常规摄动分析方法假设微扰项以单一频率振荡条件下得到的解析解一致.分析了可压缩流体和不可压缩流体在理论模型上的区别.     

Hydrogen in an oscillating porous Vycor glass

We investigate hydrogen in porous Vycor glass with a torsional oscillator technique. Although our primary purpose is searching for a superfluid transition of hydrogen supercooled in Vycor, we find that hydrogen molecules which are adsorbed and liquefied in Vycor at T > T³ (triple point of bulk H₂) leave the Vycor when decreasing the temperature to below a characteristic value T_c < T₃. We discuss this phenomenon in terms of a free energy balance between solid/liquid hydrogen inside and outside the Vycor.     

Parametric resonance in an oscillating water column

A common type of device for wave-energy extraction is an oscillating water column (OWC) with a compression chamber. Peak performance of most OWC systems occurs at resonance with the driving waves. At resonance, oscillations increase linearly in time until damping inhibits further growth. Parametric resonance is introduced as a means of exciting the oscillations of the water column. In parametric resonance, oscillations increase exponentially in time. The use of this kind of resonance may increase the performance of OWC systems. This type of resonance occurs when one of the parameters in an oscillator varies periodically. Asymptotic methods are used to study the nonlinear dynamics of an OWC with parametric resonance. These results are compared with those of a numerical model of a real experimental laboratory setup.     

Calculation of the Rayleigh-Sommerfeld diffraction integral by exact integration of the fast oscillating factor

We describe a numerical method that can be used to calculate the propagation of light in a medium of constant (possibly complex) index of refraction n. The method integrates the Rayleigh-Sommerfeld diffraction integral numerically. After an appropriate change of integration variables, the integrand of the diffraction integral is split into a slowly varying and an (often fast) oscillating quadratic factor. The slowly varying factor is approximated by a spline fit, and the resulting Fresnel integrals are subsequently integrated exactly. Although the method is not as fast as methods involving a fast Fourier transform, such as plane-wave propagation or Fresnel approximation, it is accurate over a greater range than these methods.     

Radiation from an oscillating dislocation in a tube

The exact solution for the radiation from a screw dislocation oscillating in a long cylindrical tube is obtained. The results show that the fundamental resonant frequencies for thin-walled tubes are considerably lower than in corresponding solid cylinders. Consequently resonance radiation may play an essential role in dynamic fracture and plastic flow of such structures.     

Method for measuring the resonant frequency of an oscillating system

A method for measuring the resonant frequency of an oscillating system providing high accuracy under low Q-factor conditions and a distorted form of amplitude-frequency characteristic of the oscillating system is proposed. A structural diagram of the device is described and time charts are provided explaining the operating principle and advantages of the proposed method.Translated from Izmeritelnaya Tekhnika, No. 9, pp. 49–53, September, 2004.     

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.     

Flow in the Ekman layer on an oscillating porous plate

Summary.  The Laplace transform technique is utilized to obtain the exact solution for thermally-induced wave propagation in a circular piezoelectric plate. The temperature is assumed to satisfy the one-dimensional heat conduction equation that includes a relaxation time. Displacement and electric potential functions are introduced in order to solve the equations of motion and electrodynamics. Numerical results obtained for both a PZT-5A plate and a non-piezoelectric plate illustrate the effects of relaxation time and piezoelasticity on displacement and stress histories. Received October 3, 2002 Published online: February 10, 2003     

Two-scale homogenization of elastic layered composites with interfaces oscillating in two directions

In a variety of situations of practical interest, the interface between two phases in a composite cannot be reasonably assumed to be smooth but has to be taken as being rough at the microscopic scale. How to determine the effective properties of such a composite remains a largely open problem in micromechanics. The present work is concerned with layered composites in which the interface between two neighboring layers oscillates quickly and periodically along two directions in the plane normal to the layering direction. In this case, the classical homogenization theory of layered composites is no longer applicable, since the interfacial oscillations prevent the layered composite in question from being homogeneous in the plane perpendicular to the layering direction. To overcome this difficulty, a two-scale homogenization method is proposed in the present work. First, at the mesoscopic scale, each zone in which an interface oscillates is homogenized as an interphase by an asymptotic analysis. The effective elastic properties of this interphase are determined by using a numerical method based on the fast Fourier transform (FFT) or estimated by applying the generalized self-consistent scheme (GSCS). Then, at the macroscopic scale, the effective elastic moduli of the composite made of the resulting plane layers and interphases are calculated with the help of the classical homogenization theory of layered composites. Finally, numerical examples are provided to illustrate the results for the effective elastic moduli of a layered composite obtained by the two-scale homogenization method proposed and to compare them with the corresponding numerical results given by the finite element method (FEM).     

Stability of a film flowing down along an oscillating surface

The linear approximation for a harmonically oscillating surface is used to obtain the condition of flow stability for a liquid film.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 6, pp. 1006–1012, December, 1982.     

Microscopic analysis of saltation of particles on an obliquely oscillating plate

This paper presents a microscopic analysis of the saltation of particles on an obliquely oscillating plate driven by sine waves with an amplitude on the order of tens of micrometers and a frequency on the order of hundreds of hertz. To examine the effect of the diameter of a particle on its motion, the trajectories and velocities of different-sized particles, from 0.5 to 500 μm in mass median diameter, are analyzed using images captured by a high-speed microscope camera. The results show that larger particles bounce higher, whereas smaller particles easily agglomerate and bounce only slightly, owing to the low restitution caused by their loosely packed structure. In addition, larger particles bounce forward and backward repeatedly, while the agglomerated particles always bounce forward, and consequently have the highest transport velocity among these particles. The particle motion and the transport velocity can be explained by a theoretical probability model.     

Numerical and theoretical study of particle saltation on an obliquely oscillating plate

Particle saltation on an obliquely oscillating plate is simulated using a mass-point model that considers gravity, fluid resistance, restitution, and friction. The calculated results are in good agreement with results obtained experimentally for particles with different diameters and restitutions. A large particle with high restitution bounces forward and backward repeatedly, whereas a particle with low restitution only bounces forward and consequently has a high transport velocity. The mechanism for the difference in the motion of the particles can be explained by taking into account the phase angle of the oscillating plate and the impulse during particle collision.     

Fourier fringe processing by use of an interpolated Fourier-transform technique

Recently a powerful Fourier transform technique was introduced that was able to extract the phase from only one image. However, because the method is based on the two-dimensional Fourier transform, it inherently suffers from leakage effects. A novel procedure is proposed that does not exhibit this distortion. The procedure uses localized information and estimates both the unknown frequencies and the phases of the fringe pattern (using an interpolated fast Fourier transform method). This allows us to demodulate the fringe pattern without any distortion. The proposed technique is validated on both computer simulations and the profile measurements of a tube.     

Detector for liquid chromatography based on acoustic emissions from an oscillating flame

The acoustic flame detector (AFD) is examined as a novel detector for liquid chromatography (LC). It is based upon the acoustic emission frequency of an oscillating hydrogen/oxygen premixed flame and produces a universal response toward organic molecules. A stable frequency near 1000 Hz, which further depends on mobile-phase composition, is achieved for flow rates in the microliter per minute range. The mass flow sensitivity of the AFD demonstrates a linear response over 3 orders of magnitude and a detection limit (S/sigma = 3) of approximately 15 ng of C/s for a series of alcohols. For cyclopentanol, this amounts to an injected mass of approximately 77 ng based on a 0.5-microL injection of a 196 ppm solution in methanol (flow rate 20 microL/min methanol; peak width 30 s). Similar sensitivity is observed using a water mobile phase. Low-frequency (1/f ) noise contributions are dominant with or without mobile phase present. The AFD demonstrates a uniform molar sensitivity toward carbon compounds independent of their optical properties or volatility. Results suggest the device might serve as a simple, inexpensive universal LC detector.     

MHD rotating flow of a third-grade fluid on an oscillating porous plate

Summary The rotating flow of a third-grade fluid on an oscillating porous plate in the presence of a transverse magnetic field is considered. An analytic solution of the governing nonlinear boundary layer equation is obtained. Expressions for the velocity profile are established. It is found that an external magnetic field has the same effect on the flow as the material parameters of the fluid. Further the symmetric and asymmetric nature of the solutions is discussed.     

Waves due to an oscillating and translating disturbance in a two-layer density-stratified fluid

Wave generation due to the steady translational motion of an oscillatory disturbance in a two-layer density-stratified fluid is studied. In the context of linearized two- and three-dimensional potential flow, explicit expressions for the Green functions are derived and limiting cases discussed. Of special interest are the waves in the far field. The number and amplitudes of these waves depend on the characteristics of the disturbance (location, speed and oscillation frequency) and the ocean (stratified layer density and depth ratios). These dependencies are elucidated in concrete examples. An interesting finding, for example, is that, for constant frequency, there are critical speeds (as functions of density and depth ratios) at which the specific amplitudes attain maxima. For the more general problem and as an independent validation, an efficient numerical scheme for the problem based on a spectral method is developed. Direct simulation results compare well with analytical predictions in the near- and far-fields and offer a powerful tool for practical problems with general time-dependent motions of one or more bodies.     

Boundary integral equations as applied to an oscillating bubble near a fluid-fluid interface

A new method is presented to describe the behaviour of an oscillating bubble near a fluid-fluid interface. Such a situation can be found for example in underwater explosions (near muddy bottoms) or in bubbles generated near two (biological) fluids separated by a membrane. The Laplace equation is assumed to be valid in both fluids. The fluids can have different density ratios. A relationship between the two velocity potentials just above and below the fluid-fluid interface can be used to update the co-ordinates of the new interface at the next time step. The boundary integral method is then used for both fluids. With the resulting equations the normal velocities on the interface and the bubble are obtained. Depending on initial distances of the bubble from the fluid-fluid interface and density ratios, the bubbles can develop jets towards or away from this interface. Gravity can be important for bubbles with larger dimensions.