Yoshiro Kumamoto

Acoustic streaming induced in focused Gaussian beams

Axisymmetric flow equations for a viscous incompressible fluid are transformed into the vorticity transport and Poisson’s equations. They are numerically solved via a finite difference method imposing appropriate initial and boundary conditions. A model source of 1‐cm radius and 5‐cm focal length with Gaussian amplitude distribution radiates 5‐MHz ultrasound beams in water. Numerical examples are shown for buildup of acoustic streaming along and across the acoustic axis. Evidently, hydrodynamic nonlinearity has an essential effect on the streaming generation in comparison with a linear flow case; the nonlinearity reduces the streaming velocity in the focal and prefocal region, whereas it tends to accelerate the flow in the postfocal region.

Harmonic generation in finite amplitude sound beams from a rectangular aperture source

Theoretical analysis and some experiments are performed on nonlinearly generated harmonic components in bounded sound beams emitted from a rectangular aperture source. The Khokhlov–Zabolotskaya–Kuznetsov equation, which takes account of nonlinearity, dissipation, and diffraction effects in the beams, is numerically solved by means of the alternating direction implicit difference method. Using a planar source of size 24×44 cm, axial sound pressures and beam patterns of the first three harmonics are measured in air for initially sinusoidal ultrasounds of 25‐ and 30‐kHz frequency, and are compared with the theory. They are in relatively good agreement. Deformation of the source face from circular to rectangular shape results in the unclear appearance of pressure peaks and dips with propagation. Within the framework of these studies, the harmonic pressure levels in the far field are almost the same as from a circular aperture source with equal face area and equal initial pressure, independent of the source le...

Nonlinearly generated spectral components in the nearfield of a directive sound source

Nonlinear propagation of sound waves generated by a directive ultrasound source in air is discussed theoretically and experimentally. The circular source of 21 cm in radius consists of 1410 small PZT bimorph transducers, whose resonance frequency is 28 kHz. For a single‐frequency wave excitation, sound pressures of the fundamental, second, and third harmonics are measured and are compared with the numerical results using a method of Aanonsen et al. [J. Acoust. Soc. Am. 75, 749–768 (1984)]. Extending their initial condition to the case of a two‐frequency wave excitation, propagation curves and beam patterns of the difference frequency sound are obtained and compared with the measured data. All observations quantitatively agree very well with the numerical calculation. Nonlinear attenuation of spectral components by increasing the source pressure is clearly confirmed.Nonlinear propagation of sound waves generated by a directive ultrasound source in air is discussed theoretically and experimentally. The circular source of 21 cm in radius consists of 1410 small PZT bimorph transducers, whose resonance frequency is 28 kHz. For a single‐frequency wave excitation, sound pressures of the fundamental, second, and third harmonics are measured and are compared with the numerical results using a method of Aanonsen et al. [J. Acoust. Soc. Am. 75, 749–768 (1984)]. Extending their initial condition to the case of a two‐frequency wave excitation, propagation curves and beam patterns of the difference frequency sound are obtained and compared with the measured data. All observations quantitatively agree very well with the numerical calculation. Nonlinear attenuation of spectral components by increasing the source pressure is clearly confirmed.

Time evolution of acoustic streaming from a planar ultrasound source

Eckart‐type acoustic streaming induced in confined sound beams from a piston source is examined in water theoretically and experimentally. Axisymmetric flow equations with a spatially distributed driving force in the beams are based on the continuity equation and the Navier–Stokes equation in a viscous, incompressible fluid. They are solved numerically by the stream‐function vorticity method [T. Kamakura et al., J. Acoust. Soc. Am. 97, 2740–2746 (1995)]. Experiments are conducted using a 5‐MHz planar transducer with a 9.5‐mm radius aperture. All measurements of the streaming velocities are carried out by a laser Doppler velocimeter and are compared with the numerical computations including the enhancement of the force due to finite‐amplitude sound distortion. These measurements agree well with the theoretical prediction. It is noted that diffraction of sound beams plays an important role in the generation of streaming, particularly in the early stage. Consistency between experiments and computations suggests that both acoustic and hydrodynamic nonlinearities should be taken into account in the present observation system.

Buildup of acoustic streaming in focused beams

Using a laser Doppler velocimeter, we measured the buildup velocity of acoustic streaming in water generated from a 2.8 MHz focusing source with a circular aperture along and across the acoustic axis. Steady streaming is established a few seconds after the beam is switched on, and a maximum velocity of about 4 cm/s is observed near the focus. The good agreement of theory and experiment suggests that the numerical calculation method previously developed by the authors is valid within the framework of the present source conditions.

UNIFIED DESCRIPTION OF SECOND-ORDER PHENOMENA IN SOUND WAVES

The generation mechanism of acoustic streaming and radiation pressure are discussed along with the KZK equation, which explains successfully the propagation of finite-amplitude sound waves. The energy loss of a sound beam in a viscous fluid generates the driving force of streaming which induces mass flow in the beam. The radiation pressure acts on the surface of an object whose acoustic impedance is different from the fluid. The effect of flow generated around the object on the radiation pressure is greater when the object is smaller. The main purpose of this paper is to describe the unified relationships among waveform distortion, acoustic streaming, and radiation pressure.

Nonlinear interaction of collinear sound beams in the nearfield

Many spectral components generated by nonlinear interaction of two collinear sound beams can be calculated using Aanonsens method [J. Acoust. Soc. Am. 75, 749–768 (1984)]. Aanonsens initial pressure condition is extended to the case where a radiating wave at the source consists of two adjacent harmonies. Numerical computations are performed for a nearfield of a Gaussian source in air. Propagation curves and beam patterns of the primary and secondary waves are given for various source levels. When the source intensity is increased, the higher frequency primary wave fades out more than the lower frequency wave, and the harmonics of the difference frequency increase. Parametric generation of the difference frequency and its second harmonic component for AM excitation are also considered.