Nearfield acoustic holography in a moving medium based on particle velocity input using nonsingular propagator

Nearfield acoustic holography in a moving medium is a technique which is typically suitable for sound sources identification in a flow.In the process of sound field reconstruction,sound pressure is usually used as the input,but it may contain considerable background noise due to the interactions between microphones and flow moving at a high velocity.To avoid this problem,particle velocity is an alternative input,which can be obtained by using laser Doppler velocimetry in a non-intrusive way.However,there is a singular problem in the conventional propagator relating the particle velocity to the pressure,and it could lead to significant errors or even false results.In view of this,in this paper,nonsingular propagators are deduced to realize accurate reconstruction in both cases that the hologram is parallel to and perpendicular to the flow direction.The advantages of the proposed method are analyzed,and simulations are conducted to verify the validation.The results show that the method can overcome the singular problem effectively,and the reconstruction errors are at a low level for different flow velocities,frequencies,and signal-to-noise ratios.

Avoidance of Blow-up by Moving Medium

This paper deals with two parabolic initial-boundary value problems in multidimensional domain. The first problem describes the situation where the spherical medium is static and the nonlinear reaction takes place only at a single point. We show that under some conditions, the solution blows up in finite time and the blow-up set is the whole spherical medium. When the spherical medium is allowed to move in a special space, we investigate another parabolic initial-boundary value problem. It is proved that the blow-up can be avoided if the acceleration of the motion satisfies certain conditions.

Spectral moment estimates of broadband backscattering acoustic wave in moving medium

In this paper, the signal and noise of broadband moving medium reverberation are supposed to be colored complex Gaussian processes. When the sample number is large, both the mean and variance of the first power spectral moment estimator by covariance techniques are obtained. The results obtained by D. S. ZRNIC[8], where the noise is assumed to be white,are generalized The transmitted signal is pseudo noise (PN) train and consists of two same subtrains between which the difference is only a time delay. In order to simplify the analysis,the center frequency and frequency bandwidth of both signal and noise are also assumed to be equal. It is shown that the mean of the first power spectral moment estimate is an unbiased estimate and that the variance of this estimate is dependent on (1) the correlation coefficient ρ between two subtrains, (2) the signal-to-noise ratio S/N and (3) △fTL, where △f, T and L are the frequency bandwidth, time duration and total number of pulse elements in a subtrain respectively. Three thousand groups of narrow-band-signal data obtained in field are analyzed.Then the range of ρ is determined. All these results are very useful in desiglling a broadballd acoustic Doppler current profiler (BBADCP).

SPECTRAL METHOD FOR CALCULATING MOVING INTERFACE BETWEEN FLUIDS IN POROUS MEDIUM

In the application of spectral method to the calculation of moving interface between fluids in porous medium there are two difficulties: the spectral calcula- tion of function defined by a singular integral and the numerical quadrature of highly oscillating function. This paper proposes a spectral method for calculating the problem and finds the way to overcome the two difficulties. Example calcula- tions show that the method can describe successfully interfacial motion and, with almost the same order of computational amount, is more accurate and stabler than the corresponding finite difference method.