The objective of this paper is to show that time reversal invariance can be exploited in acoustics to create a variety of useful instruments as well as elegant experiments in pure physics. Section 1 is devoted to the description of time reversal cavities and mirrors together with a comparison between time reversal and phase conjugation. To illustrate these concepts, several experiments conducted in multiply scattering media, waveguides and chaotic cavities are presented in section 2. Applications of time reversal mirrors (TRMs) in hydrodynamics are then presented in section 3. Section 4 is devoted to the application of TRMs in pulse echo detection. A complete theory of the iterative time reversal mode is presented. It will be explained how this technique allows for focusing on different targets in a multi-target medium. Another application of pulse echo TRMs is presented in this section: how to achieve resonance in an elastic target? Section 5 explores the medical applications of TRMs in ultrasonic imaging, lithotripsy and hyperthermia and section 6 shows the promising applications of TRMs in nondestructive testing of solid samples.

https://iopscience.iop.org/article/10.1088/0034-4885/63/12/202/pdf

Time-reversed acoustics

IEEE ultrasonics symposium

Flow separation in an S-shaped intake diffuser and its control using vortex generator vanes or actuators were studied experimentally in order to generate a data set aimed at validating CFD turbulence models. The baseline and controlled data sets comprised static and dynamic pressure measurements on the S-duct surface and in the AIP. In a first step, the effects of several configurations of mechanical vortex generators were evaluated. In a second step active flow control data sets were acquired for a variety of control amplitudes, control frequencies and Reynolds numbers.

Flow Control in a High-Offset Subsonic Air Intake

In the present study a two-dimensional axisymmetric numerical model is developed for supercritical parametric phase conjugation of ultrasound in a solid active element of cylindrical shape and finite length. The pseudospectral time domain algorithm (PSTD) is used owing to its efficiency to model large-scale problems. PSTD solves elastic wave equation in time-dependent heterogeneous isotropic and axisymmetric anisotropic solids using FFTs for high order approximation of the spatial differential operator on staggered grid, and a fourth-order Adams–Bashforth time integrator. In order to truncate the computational domain absorbing boundary conditions are introduced with complex frequency shifted perfectly matched layers. This procedure is highly effective at absorbing signals of long time-signature. The free surface of the active ceramic rod is introduced through the method of images. A systematic study of the influence of lateral limitations of the active medium on parametric wave phase conjugation of sound ...

Two-dimensional axisymmetric numerical simulation of supercritical phase conjugation of ultrasound in active solid media

This paper investigates nonlinear B/A parameter imaging in ultrasound echo mode. First, the B/A estimation methods which can be extended to echo mode are identified. The finite amplitude approaches are found to be excellent candidates to assess the nonlinear parameter because of their experimental simplicity, supported by a strong mathematical background. Second, three nonlinear coefficient measurement methods, thus far proposed for applications in homogeneous media, are extended to heterogeneous media. In particular, the simulations show that the extended comparative method (ECM) offers the best results when the probe diffraction effects are taken into consideration. The first experimental images obtained by applying the ECM for two different phantoms are also presented, showing the feasibility of B/A imaging.

Extensions of nonlinear B/A parameter imaging methods for echo mode

Application of the supercritical magnetoelastic wave phase conjugation to harmonic imaging in acoustic C-scan microscopy is demonstrated. Second-harmonic generation by phase-conjugate wave is used for improvement of resolution of an imaging system. Possibility to compensate phase aberrations introduced in harmonic image by inhomogeneity of propagation medium is shown experimentally and explained theoretically.

Acoustic imaging by second harmonic of phase-conjugate wave in inhomogeneous medium

Godunov-type computation schemes are applied to numerical simulations of wave propagations in time-dependent heterogeneous media (solids and liquids). The parametric phase conjugation of a wide band ultrasound pulse is considered. The supercritical dynamics of the acoustic field is described for one-dimensional systems containing a parametrically active solid. The impulse response function, numerically calculated for a finite active zone in an infinite medium above the threshold of absolute parametric instability, is in a good agreement with the analytical asymptotic theory. The supercritical evolution of the acoustic field spatial distribution is studied in detail for parametric excitations in an active zone of a solid layer, loaded by a semi-infinite liquid on one side and free on the other.

Numerical simulation of acoustic wave phase conjugation in active media

The results of theoretical and experimental studies of application of parametric wave phase conjugation (WPC) for nonlinear detection of isoechogenic phantoms and for ultrasonic velocimetry of flows are reported. Experimental WPC harmonic imaging of isoechogenic phantoms in water was demonstrated on an example of cells filled with a mixture of water and methyl alcohol. Variation of methanol concentration allows obtaining some mixtures with varied nonlinear parameters while their linear acoustic impedance remains close to the water's one. Analysis of harmonics of phase conjugate waves (PCW) in automatically con-focal WPC-system provides reliable detection of the phantoms. According to the theoretical predictions the contrasts of images are comparable with, or even stronger, than the ratio of nonlinear parameters of a phantom and its surrounding medium. 3D-simulation of propagation of PCW in heterogeneously nonlinear and moving media is developed. Break of time reversal invariance of WPC in moving media is proposed as a principle of detection and velocimetry of flows. The results of experimental linear and harmonic imaging of flows in water are presented in comparison with theoretical description.

Nonlinear Acoustic Imaging of Isoechogenic Objects and Flows Using Ultrasound Wave Phase Conjugation

Observation of ultrasonic waves in liquid under overthreshold parametric phase conjugation in ferrite

The results of using of wave phase conjugation in Doppler velocimetry of liquid flows are presented. The increase of sensitivity and signal-to-noise ratio of phase shift measurements by means of nonlinear interaction of phase conjugate ultrasonic waves is demonstrated. Doppler effect, wave phase conjugation, nonlinear interaction

V. Preobrazhensky

Papers

Acoustic imaging by second harmonic of phase-conjugate wave in inhomogeneous medium

Numerical simulation of acoustic wave phase conjugation in active media

Nonlinear Acoustic Imaging of Isoechogenic Objects and Flows Using Ultrasound Wave Phase Conjugation

Observation of ultrasonic waves in liquid under overthreshold parametric phase conjugation in ferrite

Flow velocity measurements by means of nonlinear interaction of phase conjugate ultrasonic waves