Weijun Lin

Characteristics of group velocities of backward waves in a hollow cylinder

It is known that modes in axially uniform waveguides exhibit backward-propagation characteristics for which group and phase velocities have opposite signs. For elastic plates, group velocities of backward Lamb waves depend only on Poissons ratio. This paper explores ways to achieve a large group velocity of a backward mode in hollow cylinders by changing the outer to inner radius ratio, in order that such a mode with strong backward-propagation characteristics may be used in acoustic logging tools. Dispersion spectra of guided waves in hollow cylinders of varying radii are numerically simulated to explore the existence of backward modes and to choose the clearly visible backward modes with high group velocities. Analyses of group velocity characteristics show that only a small number of low order backward modes are suitable for practical use, and the radius ratio to reach the highest group velocity corresponds to the accidental degeneracy of neighboring pure transverse and compressional modes at the wavenumber k = 0. It is also shown that large group velocities of backward waves are achievable in hollow cylinders made of commonly encountered materials, which may bring cost benefits when using acoustic devices which take advantage of backward-propagation effects.

Surface tension and quasi-emulsion of cavitation bubble cloud.

A quasi-emulsion phenomenon of cavitation structure in a thin liquid layer (the thin liquid layer is trapped between a radiating surface and a hard reflector) is investigated experimentally with high-speed photography. The transformation from cloud-in-water (c/w) emulsion to water-in-cloud (w/c) emulsion is related to the increase of cavitation bubble cloud. The acoustic field in the thin liquid layer is analyzed. It is found that the liquid region has higher acoustic pressure than the cloud region. The bubbles are pushed from liquid region to cloud region by the primary Bjerknes forces. The rate of change of CSF increased with the increase of CSF. The cavitation bubbles on the surface of cavitation cloud are attracted by the cavitation bubbles inside the cloud due to secondary Bjerknes forces. The existence of surface tension on the interface of liquid region and cloud region is proved. The formation mechanism of disc-shaped liquid region and cloud region are analysed by surface tension and incompressibility of cavitation bubble cloud.

Acoustic field in a cased well with a sectorial crossing channel

To study the possibility of detecting the crossing channel by using a traditional logging tool, acoustic field generated by a monopole source in a cased well with a crossing channel of various angles is simulated by 2.5-D finite difference method for the first time. Snapshots of normal stress and synthetic time-domain waveforms are displayed. The two-dimensional spectrum in wave-number and frequency domains is also calculated, where the influence of the channel is clearer than that in the waveforms. Numerical study demonstrated that a crossing channel can be detected and sized if its angle is greater than 30 degree, and larger offset and lower frequency source are favorable to detect and size the crossing channel.The present invention is related to a speech synthesizer which includes a sampled signal storing device storing therein a sampled signal and outputting the sampled signal in response to an input signal, and a speech signal synthesizing circuit electrically connected to the sampled signal storing device, receiving an operation signal, having the sampled signal outputted by the sampled signal storing device be repeatedly operated in response to the operation signal, and then outputting a speech synthesized signal, wherein a frequency of the operation signal is higher than that of the input signal to allow the sampled signal to be repeatedly operated during a single cycle of the input signal. The present invention proceeds a plurality of times of operation for each entry of data in the storing device so that the synthesizing performance of the present synthesizer can be improved without increasing the storage amount of the sampled signals.

Simulation of 2.5-dimensional borehole acoustic waves with convolutional perfectly matched layer

A 2.5-dimensional method using the PDE package of the commercial finite element software COMSOL Multiphysics is developed to simulate wave propagation in a borehole. The computation is conducted in the frequency wave-number domain. A convolutional perfectly matched layer is implemented to eliminate the reflections from artificial truncation boundaries. Waveforms obtained in time domain are in good agreement with analytic solutions in a special model, which proves the validity of the method. A numerical modeling example is presented to illustrate the capabilities of the method. It is shown that this method can be used to solve a variety of non-axisymmetric borehole acoustic wave propagation problems.

Mechanism and dynamics of hydrodynamic-acoustic cavitation (HAC)

Bubble clusters in hydrodynamic cavitation, acoustic cavitation and hydrodynamic-acoustic cavitation (HAC) are investigated via high-speed photography. By introducing a cavitation state variable, a method for cavitation characterization is proposed. The periodic characteristics and intensity distributions of hydrodynamic cavitation, acoustic cavitation and HAC are quantitatively analyzed using this method. It is found that the range of HAC is evidently widened and the strength of HAC is significantly enhanced compared with hydrodynamic cavitation or acoustic cavitation. Furthermore, we developed a preliminary physical model describing the dynamics of a cavitation bubble in HAC and proposed a mechanism to explain the enhancement of the intensity in HAC.

Phase Relation of Harmonics in Nonlinear Focused Ultrasound

The phase relation of harmonics in high-intensity focused ultrasound is investigated numerically and experimentally. The nonlinear Westervelt equation is solved to model nonlinear focused sound field by using the finite difference time domain method. Experimental waveforms are measured by a robust needle hydrophone. Then the relative phase quantity is introduced and obtained by using the zero-phase filter. The results show that the nth harmonic relative phase quantity is approximately (n — 1)π/3 at geometric center and increases along the axial direction. Moreover, the relative phase quantity decreases with the increase of source amplitude. This phase relation gives an explanation of some nonlinear phenomena such as the discrepancy of positive and negative pressure.

SIMULATION OF AUTOMATIC SELECTION IN MULTI-TARGET MEDIUM USING TIME REVERSAL MIRROR

In high intensity focused ultrasound (HIFU) therapy, the focusing of acoustic waves in inhomogeneous media is a common and difficult problem to solve. The focusing can be strongly degraded by sound speed variation and density variation from geometrical distortion. The time reverse mirror is a solution to this problem when the target is a reflective one, even when the location of the target is unknown. The iterative time reversal mirror is also used to automatically select the strongest target in a multi-target medium. To simulate the acoustic fields of iterative time reversal mirror and evaluate its convergence, the FEM time-dependent model is used. Three cases of time reversal process in multi-target medium are presented. It is shown that in these three cases, the iterative time reversal process has the ability to focus on the strongest target. And the effectiveness of convergence is discussed in these three cases.

Design and performance evaluation of phased parametric array for the detection of geological conditions ahead of drill bit

The detection of the reflectors ahead of drill bit in oil drilling well is vital in ensuring the safety production and realizing well geosteering. In order to achieve the objective, a phased parametric array is designed and the performance is evaluated by simulating wave propagation in a drilling well with reflectors ahead of the drill bit. According to self-demodulation effect of sound field excited by parametric array and phased array method, a phased parametric array with particular parameters is designed by taking into account the conditions in the drilling well. Supposing that acoustic signals are emitted from the parametric array in the borehole, wave propagation of the signals in the drilling hole underground are investigated by using 3-D finite difference method in cylindrical coordinates, where the propagation mechanism of waves in the borehole, the formation surrounded and the reflectors ahead of the drill bit are studied in detail by analyzing the synthetic data. The numerical results and analysis show that geological information of the reflector such as the distance from the bit derived from the waveforms agrees well with the numerical model. The work indicates that a properly designed phased parametric array has the potential to be used in detecting geological conditions ahead of the drill bit.

Guided modes with multiple zero-group-velocity points in fluid-filled cylindrical pipes

It is known that guided modes in isotropic hollow cylinders exhibit backward wave propagation with negative group velocity. And interferences between backward and forward waves generate zero-group-velocity (ZGV) resonances with a finite wavenumber but vanishing group velocity. These ZGV resonances can be applied for non-destructive evaluation (NDE) of hollow pipes. In this paper, influences of a fluid-loading on ZGV resonances in pipes are studied for the possible application of integrity inspections of oil transportation pipelines. From numerically simulated frequency-wavenumber spectra of axisymmetric guided modes, in addition to the backward mode with a single ZGV point, certain branches change the sign of their slopes for twice (i.e., two ZGV points in one branch). Such multiple ZGV modes might be caused by the strong repulsion between the backward mode with a single ZGV point that is propagating in the hollow pipe and a number of longitudinal modes in the fluid cylinder. It is found that, from wave structure analyses, ZGV points correspond to relatively large displacement amplitudes at the pipe’s inner and outer interfaces. It indicates that guided modes with multiple ZGV points can be sensitive to the surface features of fluid-filled pipes, which is useful for NDE application.

Simulation of focusing high intensity ultrasound by time reversal method

Numerical simulations are used to investigate the validation of using time reversal to focus sound beams in high intensity ultrasound. Time reversal is a method based on linear and lossless sound field. But the high intensity of HIFU sound field brings with high nonlinearity and body tissues brings with thermoviscous absorption. So it is necessary to investigate whether time reversal is feasible in focusing sound beams in HIFU, whether it is preferable to normal methods. Nonlinearity and absorption is considered in numerical simulation based on KZK nonlinear parabolic wave equation. A time-domain algorithm with two-dimensional finite difference is used to solve the equation. Layered biological tissue model is created. Numerical results with various parameters are calculated to investigate the influence of nonlinearity and absorption on time reversal results respectively. Moreover, results are calculated both using time reversal and phased array method, to compare the differences on beam focusing behavior. It is founded that thermoviscous absorption has less effect on time reversal, while the influence of nonlinearity is greater. It is shown that in the presence of nonlinearity and thermoviscous absorption in HIFU, the ability of time reversal to focus the sound beams is also robust. Comparison results are shown that time reversal method is more accurate than normal method in inhomogeneous media considering nonlinearity and thermoviscous absorption. So time reversal method has a great prospect to focus sound beams in HIFU.