Theoretical model of acoustic emission under mechanical loading of rocks in the maximum compaction region

A theoretical model of changes in acoustic emission activity in a geomaterial under continuous or stepwise mechanical loading is justified. Based on this model, the experimentally found laws of emission in the region of the maximum compaction of rock samples with different rates of mechanical loading of these samples are analyzed. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 145–152, July–August, 2006.     

Theoretical model for the thermal emission memory effect in rocks

A model for the thermal emission memory effect in rocks under cyclic heating with the temperature amplitude increasing from cycle to cycle is validated. The model is used to consider one of the possible mechanisms of the effect related to the temperature gradient on the faces of cracks dividing structural elements of a geomaterial. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 2, pp. 172–177, March–April, 2008.     

Kinematic Fields and Acoustic Emission Observations Associated with the Portevin Le Châtelier Effect on an Al–Mg Alloy

This paper presents experimental tensile test results obtained on flat aluminum magnesium alloy samples on a hard machine. The mechanical response, kinematic fields and acoustic emissions were simultaneously obtained in an experimental setup. Propagation instabilities associated with the Portevin–Le-Chatelier effect were observed as localized intense strain increment bands. Depending on the strain rate, A, B or C types were studied on the basis of stress drops, acoustic emission and strain fields. Then the band characteristics (position, orientation, width, thickness reduction, intensity, acoustic emission, principal strain direction) were presented in various strain rate conditions.     

Vibrational dynamics of modern lightrail modules

The vibrational frequency response of a modern lightrail module is considered in a simple model including dynamical features of the (typically aluminum) body with finite flexural rigidity, which inter alia reveals additional wheel–rail resonances. The model also allows to calculate the systems acoustic noise emission spectra, and to study the differences between aluminum and steel coaches.     

Fluid flows induced by focused ultrasound and their use in single-crystal growth

Particle image velocimetry was used to study the structure of stationary acoustic flows on a solid surface subjected to acoustic radiation along the normal to the prefocal and postfocal planes of a spherical concentrator. The results of model experiments were used for rapid growth of water-soluble single crystals. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 4, pp. 11–17, July–August, 2009.     

Balanced Atmosphere–Ocean Dynamics, Generalized Lighthill Radiation, and the Slow Quasi-Manifold

Three of Lighthill's many interests, (a) wave propagation in moving media, (b) acoustic streaming and related phenomena, and (c), in a very subtle and fascinating way, aerodynamic sound generation, are all turning out to be fundamental to understanding our atmospheric environment. Among other things there is the global-scale circulation that shapes the ozone layer and controls the rate of destruction of man-made chlorofluorocarbons (CFCs). Recent progress in this field is sketched, including progress in understanding the abstract structure of Hamiltonian theories of balanced motion, the so-called slow “manifold”. Here a generic phenomenon, “velocity splitting”, turns out to be intimately related to aerodynamic sound generation, particularly its generalization describing the spontaneous emission of inertia–gravity waves from unsteady vortical motions in stratified rotating flow. Received 5 January 1997 and accepted 2 May 1997     

Noise radiated by a non-isothermal, temporal mixing layer

The ability of Lighthill's analogy to predict the sound radiated by a transitional mixing layer is evaluated by means of direct numerical simulation (DNS). The specific case of low Mach number flows with density variations is investigated. In order to limit the global computational cost, the acoustic source information is based on numerical results where the sound waves have been removed. It is shown that the low Mach number approximation coupled with the acoustic analogy can lead to very accurate predictions for the radiated sound if the acoustic sources in Lighthill's equation are taken into account carefully. Results for the acoustic intensity deduced from a repeated use of the Lighthill's analogy over a wide range of Mach numbers allow us to discuss the adequacy of scaling laws proposed by previous authors (J. Sound Vib. 28(3), 563–585, 1973; 31(4), 391–397, 1973; 48(1), 95–111, 1976) for the prediction of noise from hot jets.     

Spontaneous acoustic emission from strong shocks in solids

Summary  The role of free electrons in the stability of strong shock waves in metals under spontaneous acoustic emission is investigated. For that purpose, a three-term form of the equation of state is employed in order to describe the cold pressure, the thermal atomic pressure and the thermal pressure of free electrons. The equation of state enables the calculation of the sound velocity behind the shock, which in turn is utilized in the Dyakov–Kontorovich criteria for the shock stability. The integral over the Fermi–Dirac distribution function that describes the specific internal energy of free electrons is replaced by a model algebraic function that possesses correct asymptotic limits at low and high temperatures. It is shown that strong shock waves in all metals are prone to instability under spontaneous emission. However, the threshold for that instability is shifted to higher Mach numbers if free electrons are taken into account. It is further shown that the stabilizing effect of free electrons is vastly overestimated if the expressions for degenerate electron gas are employed for temperatures that are larger than the Fermi temperature. Received 22 November 1999; accepted for publication 12 July 2000     

The Acoustic Analogy and the Prediction of the Noise of Rotating Blades

The acoustic analogy was introduced into acoustics by Lighthill in 1952 to understand and predict the noise generated by the jet of an aircraft turbojet engine. The idea behind the acoustic analogy is simple but powerful. The entire noise generation process is mathematically reduced to the study of wave propagation in a quiescent medium with the effect of flow replaced by quadrupole sources. In jet noise theory, Lighthill was able to obtain significant and useful qualitative results from the acoustic analogy. The acoustic analogy has influenced the theoretical and experimental research on jet noise since the early 1950s. This paper, however, focuses on another area in which the acoustic analogy has had a significant impact, namely, the prediction of the noise of rotating machinery. The governing equation for this problem was derived by Ffowcs Williams and Hawkings in 1969. This equation is a wave equation for perturbation density with three source terms, which have become known as thickness, loading, and the quadrupole source terms, respectively. The Ffowcs Williams–Hawkings (FW–H) equation has been used for the successful prediction of the noise of helicopter rotors, propellers, and fans. Several reasons account for the success and popularity of the acoustic analogy. First, the problems of acoustics and aerodynamics are separated. Second, because the FW–H equation is linear, powerful analytical methods from linear operator theory can be used to obtain closed-form solutions. Third, advances in digital computers and computational fluid dynamics algorithms have resulted in high-resolution near-field aerodynamic calculations that are suitable for noise prediction. We present some of the mathematical results for noise prediction based on the FW–H equation, including examples for helicopter rotors. In particular, we discuss the prediction of blade-vortex interaction noise and high-speed impulsive noise of helicopter rotors. For high-speed propellers, we briefly discuss the derivation of a singularity-free solution of the FW–H equation for a supersonic panel on a blade. Received 24 January 1997 and accepted 30 May 1997     

不同应力路径下盐岩破坏声发射时序特征研究

利用RMT-150B岩石力学多功能系统和声发射监测系统,对不同应力路径下的盐岩破坏过程进行试验研究。在试验结果基础上,分析了盐岩常规单轴加载、分级加卸载和多级加载过程中的声发射时序特征和变形特征。试验表明:盐岩常规单轴加载变形表现明显的阶段性特征,声发射信号阶段性特征也比较明显;分级加卸载试验中,存在比较明显的记忆效应,在低应力水平有较明显的声发射Kaiser效应,而在70%峰值应力卸载再加载时,则出现Felicity效应;多级加载试验表明,在低应力水平稳压对盐岩造成的损伤不大,很少有新生裂纹;而在高应力水平稳压时,会有大量由新生微裂纹和裂纹扩展产生的声发射事件。     

Modeling of plane-wave propagation in an anisotropic elastic medium

We propose an algorithm that reduces the process of numerical solution to successive calculation of elementary one-dimensional problems of the type of a system of acoustic equations. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 1, pp. 199–206, January–February, 1999.     

Dust acoustic shock waves in coupled dusty plasmas with nonthermal ions

Dust acoustic shock waves of the Korteweg-de Vries–Burgers (KdV–Burgers) equation and the modified Korteweg-de Vries–Burgers (MKdV–Burgers) equation are studied in strongly coupled dusty plasmas containing nonthermal ions and Boltzmann-distributed electrons. The effects of important parameters, such as nonthermal parameter, relative temperature, relative density and dust particles viscosity, on the properties of shock waves are discussed.     

Nonlinear effect of external low-frequency acoustics on eigen-oscillations in a supersonic boundary layer

A method of simulation and results of numerical calculation of the evolution of hydrodynamic disturbances in a supersonic boundary layer on a flat plate under the influence of external acoustic waves at Reynolds numbersRe=220–640 and Mach numberM=2 are described. The solution is constructed by the method of expansion with respect to the small parameter; the contribution of linear and quadratic terms to the solution is taken into account. The method developed allows one to estimate the admissible level of the acoustic field, which does not affect the development of eigen-oscillations in the boundary layer. Institute of Theoretical and Applied Mechanics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 99–105, September–October, 1999.     

Gas-dynamic approach in the nonlinear theory of ion acoustic waves in a plasma: An exact solution

An exact solution of the problem of the acoustic wave structure in a plasma is obtained. Both plasma component are treated as gases with specified initial temperatures and adiabatic exponents. The system of equations describing the wave profile is solved using an original method consisting of reducing the system to the Bernoulli equation. A numerical example of the obtained general solution of the problem of the wave profile for arbitrary parameters is given. Curves are constructed that bound the region of existence of a stationary solitary ion acoustic wave in the parameter space. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 5, pp. 3–11, September–October, 2007.     

A return toward equilibrium in a 2D Rayleigh–Taylor instability for compressible fluids with a multidomain adaptive Chebyshev method

We numerically simulate a single-mode Rayleigh–Taylor instability between compressible miscible fluids with a highly accurate self-adaptive pseudospectral Chebyshev multidomain method in two two-dimensional boxes at small aspect ratios. The simulations are started from rest and pursued until the return toward mechanical equilibrium of the mixing. Four regimes—linear and weakly nonlinear, nonlinear steady bubble rise, return toward equilibrium, and finally a system of acoustic waves—can be identified. We show that this one-dimensional system of stationary acoustic waves is damped by the physical viscosity. This provides a reference solution.      

Disturbance convection velocity in turbulent jets under aeroacoustic excitation

The velocity of propagation of toroidal and oblique vortices formed in subsonic and supersonic turbulent jets under longitudinal internal and transverse external excitation by finite-amplitude saw-tooth acoustic waves is studied experimentally. It is demonstrated that the convection velocity of vortices is not constant, and the character of its variation depends on the vortex shape. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 5, pp. 21–25, September–October, 2007.     

Reflection and refraction of a planar acoustic wave in an anisotropic inhomogeneous layer

The problem of reflection and refraction of a planar acoustic wave by an inhomogeneous elastic layer whose material possesses general-type anisotropy is considered. The equations of motion of the elastic layer are reduced to a system of ordinary differential equations. The boundary-value problem for this system is solved by two methods: by reduction to problems with initial conditions and by the method of power series. Analytical expressions that describe acoustic fields outside the layer are obtained. Calculation results of the transmission factor for transversely isotropic layers inhomogeneous in thickness are presented. Tula State University, Tula 300600. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 179–184, September–October, 1999.     

A Conceptual Study of Cavity Aeroacoustics Control Using Porous Media Inserts

In this study, an integrated flow simulation and aeroacoustics prediction methodology is applied to testing a sound control technique using porous inserts in an open cavity. Large eddy simulation (LES) combined with a three-dimensional Ffowcs Williams–Hawkings (FW–H) acoustic analogy is employed to predict the flow field, the acoustic sources and the sound radiation. The Darcy pressure – velocity law is applied to conceptually mimic the effect of porous media placed on the cavity floor and/or rear wall. Consequently, flow in the cavity could locally move in or out through these porous walls, depending on the local pressure differences. LES with “standard” subgrid-scale models for compressible flow is carried out to simulate the flow field covering the sound source and near fields, and the fully three-dimensional FW–H acoustic analogy is used to predict the sound field. The numerical results show that applying the conceptual porous media on cavity floor and/or rear wall could decrease the pressure fluctuations in the cavity and the sound pressure level in the far field. The amplitudes of the dominant oscillations (Rossiter modes) are suppressed and their frequencies are slightly modified. The dominant sound source is the transverse dipole term, which is significantly reduced due to the porous walls. As a result, the sound pressure in the far field is also suppressed. The preliminary study reveals that using porous-inserts is a promising technology for flow and sound radiation control.     

On solving the problem of reflection of linear waves in a fluid from a porous half-space saturated by this fluid

Solutions of the problem of reflection of a stepwise pressure wave in a linearly compressed fluid from a flat boundary of a porous medium of infinite length saturated by the same fluid are obtained in the acoustic approximation. Based on analytical solutions, a numerical analysis is performed to reveal the specific features of the reflected and incident waves, depending on porosity and permeability of the porous half-space. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 16–26, September–October, 2006.     

Determination of Bone Trabeculae Modulus—An Ultrasonic Scanning and MicroCT (μCT) Imaging Combination Approach

To date, there is no method to measure non-destructively the modulus of trabeculae within cancellous bone, whilst retaining its structural integrity. In this study ultrasonic scanning, coupled with microCT imaging, is employed to determine trabeculae modulus along the three major anatomical axes non-destructively. The proposed method allows cancellous bone specimens to remain intact, for possible use in subsequent studies. Volume rendering of the microCT images allows three-dimensional visualization of cancellous bone specimens to be tested. This facilitates trabeculae selection and accurate measurement of distance traveled by the ultrasonic wave, thus yielding a good degree of confidence in the acoustic velocity measured. For all the three principal anatomical directions, the measured acoustic speeds ranged from 2,115 to 3,077 m/s, giving an average of 2,505 m/s. Average wave velocities in the superior–inferior, medial–lateral and anterior–posterior anatomical directions were found to be 2,295, 2,469 and 2,754 m/s, respectively; the differences corresponding to the three directions do not appear to be significant. Subsequently, the modulus was then determined using elastic wave propagation theory.