Mathematical simulation of a gas flow in the vicinity of the open end of a tube for harmonic oscillations of a piston at the resonance frequency at the other end of the tube

The structure of the gas flow in the vicinity of the open end of a tube for the oscillating gas flow caused by piston oscillations at the first resonance frequency at the other end of the tube has been determined by numerical integration of the Navier–Stokes equations using the ANSYS FLUENT program package. For the variant of the tube with an infinitely long flange and a sharp edge, the influence of the piston displacement amplitude on the gas flow rate in the tube is investigated, and the phases of gas inflow and outflow during the period of oscillation have been determined.     

Mathematical model for non-linear standing waves in a tube

A mathematical model is developed to describe the behavior of one-dimensional, non-linear standing waves in a fluid-filled, rigid-wall tube. The tube is bounded on one end by a piston vibrating with periodic motion and terminated on the other end by an impedance boundary. A numerical procedure for calculations based on this model also is described. Calculations are carried out on a digital computer. The model and computer program are presently restricted to pre-shock conditions. Numerical results obtained for the special case of a rigid reflector and a sinusoidally moving piston are in excellent agreement with the results of Coppens and Sanders (private communication). The general approach is applicable to any one-dimensional system bounded by a velocity condition on one end and an impedance condition on the other end.     

Finite element simulation of non-linear acoustic generation in a horn loudspeaker

The loudspeaker is an electro-acoustic device for sound reproduction which requires the distortion as small as possible. The distortion may arise from the magnetic non-linearity of the york, the uneven magnetic field distribution, the mechanical non-linearity at the diaphragm suspension and the acoustic non-linearity due to the high sound pressure and velocity in the duct-radiation system. A horn is sometimes provided in front of the vibrating diaphragm radiator, which plays an important role to increase the efficiency by matching the acoustic impedance between the radiator and the ambient medium. The horn is in many cases folded twice or three times to shorten the length, which further degrades the reproduction quality. The sound intensity and velocity are apt to attain very high in the small cross-sectional area in the throat and in the folded regions, which may cause the distortion due to the non-linear effect of the medium. The present paper is to investigate the frequency characteristics of the loudspeaker numerically evaluating the generation of the harmonics and sub-harmonics. An axisymmetric folded horn is considered for which the wave equation with the non-linear term retained is solved by the finite element method. The solution is made in time domain in which the sound pressure calculated at the opening end of the horn is Fourier-transformed to the frequency domain to evaluate the distortion, while the wave marching in the horn is visualized.     

Investigation of the losses in a resonator with nonparallel faces

The relationship between the loss coefficient of lasers with partially reflecting lateral surfaces and the angle of nonparallelism of the resonator mirrors is theoretically treated. The relations obtained may be experimentally confirmed by means of the method developed previously for determining the internal losses in lasers from the time dependence of the generating process.In conclusion, the authors express their deep gratitude to Academician AS BSSR, B. I. Stepanov, for his interest in this work and discussions of the results.     

Sound-absorbing media with two types of acoustic losses

Sound-absorbing media with two types of acoustic losses, namely, inertial and deformation are considered. Such a medium is described by a complex density and a complex compressibility. It is shown that a certain relation between the latter quantities imparts some specific properties to the medium: a real wave impedance, complete absorption of sound waves at the boundary with the external medium, the properties of a viscous or “superviscous” medium, etc. The results are generalized to the case of a layered inhomogeneous medium with two types of losses.     

Saturation mechanism in clarinet-like instruments, the effect of the localised non-linear losses

The question of what limits the amplitude of the oscillations of clarinet-like instruments is investigated. The study is based on numerical simulations in which two kind of losses are taken into account: linear losses and non-linear losses localised at the open end of the tube. The influence of the amount of losses on the saturation process and on the playing range of clarinet-like instruments is shown. Results are confirmed by experiments using an artificial mouth in which the non-linear losses are varied using several terminations with different geometries.     

利用PASCO传感器研究管中声波的共振

利用PASCO传感器测量了共振管中的声压分布,结果与基于多重反射的理论分析符合很好.在此基础上,测量了共振管开口端对管内声波的反射系数及其对频率的依赖关系.     

Investigation of losses to magnetization in a superconducting helicoid

Energy losses to magnetization (hysteretetic losses) in a superconducting helicoid consisting of a flat-turn winding have been investigated. Each flat turn of the helicoid contained 15 parallel composite superconductors. The current-voltage characteristic of the helicoid was obtained by measuring the losses in the steady-state regime with a fixed transport current. The measurements of the losses in a dynamic regime made it possible to obtain a realistic estimate of the heat-transfer coefficient in liquid helium—an important thermophysical characteristic of the theory of the dynamic stability of superconductors. Comparing the losses in the helicoid with the losses in an individual constituent superconductor made it possible to determine the “size scale” of the helicoid as a superconducting composite structure. An expression for the losses to magnetization that describes the experimental results well is obtained in a geometric approximation of the helicoid by an infinite, isotropic, hollow cylinder with an azimuthal transport current. Zh. Tekh. Fiz. 67, 110–114 (February 1997)     

Optical visualization of non-linear acoustic propagation in cavitating liquids

Non-linearity effects on sound propagation induced by cavitation bubbles are investigated. The convergence of an acoustic wave due to the interaction with the microbubbles produced in the cavitation zone is shown experimentally. In these conditions the theoretical analysis shows that the self-focusing primarily depends on the effective microbubble volume fraction. This fraction turns out to be about 10^?6 with a corresponding self-focusing distance of about 9 cm in the Fraunhofer region of a plane circular transducer.     

The vibrations of glasses near the end of acoustic branches

The interest in measuring acoustic and optic vibrations of glasses at terahertz frequencies is explained. Techniques used for Brillouin spectroscopy at scattering vectors Q⩾1 nm⁻¹ are briefly described and compared. Experimental results, for example on vitreous silicas or glassy selenium, show that plane-wave acoustic excitations exhibit a crossover from propagation to strong scattering as Q is increased. Optical excitations at similar frequencies form a ‘boson peak’. These fairly local excitations can obey ‘molecular-like’ selection rules. The findings reveal a somewhat unexpected strong elastic inhomogeneity in the structure of glasses at the extended length scale of ∼5 nm, about which so little is known otherwise.     

Rational approximations of viscous losses in vocal tract acoustic modeling

The modeling of viscous losses in acoustic wave transmission through tubes by a boundary layer approximation is valid if the thickness of the boundary layer is small compared to the hydraulic radius. A method was found to describe the viscous losses that extends the frequency range of the model to very low frequencies and very thin tubes. For higher frequencies, this method includes asymptotically the spectral effects of the boundary layer approximation. The method provides a simplification for the rational approximation of the spectral effects of viscous losses.     

Investigation of a heterodyne receiver with open structure mixer at 324 GHz and 693 GHz

The performance of a submillimeter heterodyne receiver using an HCOOH laser local oscillator and an open structure mixer with a Schottky barrier diode has been optimized for 693 GHz. Working at room temperature a single sideband (SSB) system noise temperature of 7,300 K, a mixer noise temperature of 6,100 K and a conversion loss of 12 dB has been achieved. The same receiver system has been investigated at 324 GHz using an HCOOD laser local oscillator yielding a noise temperature of 3,100 K (SSB), a mixer noise temperature of 2,400 K (SSB) and a conversion loss of 10 dB (SSB). An acousto-optical spectrometer has also been constructed, with 1024 channels and a channel-bandwidth of 250 kHz. The system NEP per channel was 2.5×10^–17 W/Hz^1/2 at 324 GHz and 5.0×10^–17 W/Hz^1/2 at 693 GHz.     

Diffraction of a sound wave by the open end of a flanged waveguide with impedance walls

Diffraction of a plane sound wave by the open end of an impedance-wall waveguide connected to an opening in an impedance screen is considered. The plane wave is incident on the waveguide from a free half-space. Two versions of the problem are considered: for a semi-infinite waveguide and for a finite-length waveguide with a specified bottom impedance; the impedances of the walls, screen, and waveguide bottom can be different. The finite-length waveguide can be treated as an open cavity in the impedance screen. For the cavity of zero length, the problem is reduced to the diffraction by an impedance insert in the impedance screen. The solution in the external region determines the scattered field; the solution in the internal region allows one to determine the directional pattern of an array of receivers located in the cavity. The problem is solved using the integral Helmholtz equation with a specially selected Green’s function that provides the fulfillment of the boundary conditions. Formally, the problem is reduced to an infinite system of algebraic equations. The computational results obtained for bistatic and monostatic scattering patterns are presented.     

Modulation of the non-linear ion acoustic waves in a plasma consisting of warm ions and isothermal electrons

Summary Modulation of one-dimensional ion acoustic waves in a plasma consisting of a mixture of warm ions and isothermal electrons has been studied using the derivative expansion method. A non-linear Schr?dinger-type equation governing the complex amplitude of the perturbed ion density is obtained. The small-wave-number limit has been considered to compare it with the oscillatory solution of the Korteweg-de Vries equation obtained by Tagare and Lai using the reductive perturbation method. A good agreement has been found.     

Investigation of the relation between compression,relaxation, and end losses in the late phases of theta pinch discharges by means of a two-dimensional MHD computer programme

Chemical model reactions are discussed the steady states of which show the phenomenon of non equilibrium phase transitions. One example shows a phase transition of second order, another one shows a phase transition of first order. If diffusion occurs in the case of first order transition, coexistence of two phases in different domains is possible. For plane boundary layers between the domains the coexistence states are found by a construction analogous to the Maxwellian construction of vapor pressure of a Van der Waals gas. For spherical domains the coexistence dates change similarly as vapor pressure of droplets or bubbles with radius.