Session HN: Acoustics I

Numerical Analysis on Ultrasound Propagation and Temperature Rise in Focused Ultrasound Treatment with Navier-Stokes Equation

The linear and nonlinear approximation equations which have been usually used in the ultrasound field simulation, have the restrictions such that the higher order nonlinearity or the reflection and refraction cannot be reproduced. In order to overcome the restrictions, Navier-Stokes equation is applied to solve the ultrasound propagation. In the present work, it is assumed that the medium in HIFU treatment is water or liver. The unknown physical parameters which are not explicitly shown in the approximation equations, for example, shear and bulk viscosity, are set to reproduce absorption coefficient. The results of one-dimensional plane wave propagation show that the calculated absorption coefficient of the liver is 0.578 m$^{-1}$, which is almost equal to referenced value 0.574 m$^{-1}$ when the ultrasound frequency is 200kHz. The results of the two-dimensional focused ultrasound propagation show that the ratio of negative peak pressure to positive peak pressure decreases with the increase of the transducer drive level. The maximum temperature rise increases and the location of the maximum temperature comes closer to the transducer, when the ratio of bulk viscosity to shear viscosity increases with a constant absorption coefficient.   

Visualization of the flow and its vibration in Japanese traditional bamboo flute

Any wind instrument can sound due to the vibration of the air, expiration flow inside of the wind instrument. In case of a trumpet or a clarinet, a mouth or a reed helps to sound variable tones. In case of a flute, there is no mechanical vibration. The basic mechanism of the sound, i.e., the vibration, is well-known. A hot wire type flowmeter may be applied to measure the vibration and its frequency, but it can measure only at a certain point. We would like to investigate more detail about the flow and the vibration with sound inside and outside of the flute, in order to understand the mechanism of the wind instrument and to aid in the manufacture of the good instrument. In this report, a Japanese traditional bamboo flute was used in the experiment. We tried to measure the vibration multi-dimensionally by the Dynamic PIV. 2 kinds of experiments were done. At first, we measured the Argon-gas flow with different tone inside/outside of the bamboo flute at 5000Hz using a high frequency pulse laser. Oil mist was used as the tracer particles. Then, we also tried to measure the flow of bamboo flute when a human player played, using a CW-laser and the water-mist as the tracers. As a result, we successfully measured the oscillating flow. The flow near a hole of the bamboo flute went out from and came into the flute at about 500Hz dependent on the tone. The flow outside of the labium of flute was also measured.   

Flow-acoustic coupling in coaxial side branch resonators

Fully-turbulent flow over a coaxial deep cavity (side branch) resonator mounted in a duct is investigated using digital particle image velocimetry and unsteady pressure measurements. Interaction between the separated shear layers that form over the openings of the side branches is characterized in terms of instantaneous, phase- and time-averaged patterns of flow velocity, vorticity, streamline topology, and turbulence statistics. The effect of separated shear layer interaction on the generated acoustic power is investigated using calculated patterns of acoustic power production during several phases of the acoustic oscillation cycle. Generally speaking, the spatial structure of the acoustic source changes substantially as the interaction between the shear layers is increased. As the amplitude of the transverse flow oscillations increases, circulation of the large-scale vortical structures rapidly grows, and the region of the acoustic power production shifts upstream. Moreover, spatial structure and strength of the acoustic source also depend on the Strouhal mode of the separated shear layer oscillations.   

Imaging acoustic sources moving at high-speed

In the quantification of the noise radiated by a turbulent flow the source motion is important. It is well known that moving acoustic sources radiate sound preferrentially in the direction of motion in a phenomenon termed `convective amplification.' Modern acoustic theories have utilized this behavior in their predictions. In the inverse problem the imaging of noise sources, by techniques such as beam forming, the source motion is not explicitly taken into account. In this talk we consider the imaging of acoustic sources moving at speeds on the order of the the ambient speed of sound, as typical of high-speed jets, for which the D\"oppler shift approximation is not appropriate. An analysis will be presented that can be used to estimate the source motion based on the radiated acoustic field.   

Receptivity of a supersonic inviscid flow to periodic-in-time perturbations emanating from a wall

The receptivity of an inviscid supersonic flow past a flat plate to localized periodic-in-time perturbations emanating from the wall is revisited. The Euler equations are solved numerically with the help of a solver utilizing the CE/SE method. The computational results are projected onto the normal modes of the continuous spectra (fast and slow acoustic modes) with the help of the biorthogonal eigenfunction system formulated for the linearized Euler equations. The example illustrates how the biorthogonal eigenfunction system can be used to gain a physical insight into computational results. The considered problem also has an analytical solution written as an expansion into the normal modes. Comparison of theoretical and numerical results can serve for validating the numerical method.   

Acoustic chaos raised by Sommerfeld-Kononenko effect

The vibrations of an infinite plate in contact with an acoustic medium where the plate is subjected to a point excitation by an electric motor of limited power - supply are considered. The whole system is divided into two: ``exciter - foundation'' and ``foundation- plate - medium.'' In the system ``motor -foundation'' three classes of steady state regimes are determined: stationary, periodic and chaotic. The vibrations of the plate and the pressure in the acoustic fluid are described for each of these regimes of excitation. For the first class they are periodic functions of time, for the second they are modulated periodic functions, in general with an infinite number of carrying frequencies, the difference between which is constant. For the last class they correspond to chaotic functions. In another mathematical model where the exciter stands directly on an infinite plate it was shown that chaos might occur in the system due to the feedback influence of waves in the infinite hydro-elastic subsystem to the regime of motor shaft rotation. In this case the process of rotation can be approximately described as a solution of the fourth order nonlinear differential equation and may have the same three classes of steady state regimes as the first model.