水中密排圆柱体群的超声多重散射参数

以水中紧密排列的平行圆柱体群为对象,研究平面超声脉冲经多重散射后的透射波性质,通过分析其中头波和散射波的特征获得对应的多重散射参数.对直径随机分布、位置无序排列、数量密度约100个/cm²、面积占空比约0.53的非接触圆柱体群,采用中心频率2.5 MHz的宽带脉冲波入射。为解决透射信号在时域表现出随机性的问题,将散射体尺寸、分布都相同但位置分布不同的多个模型仿真的透射波叠加平均后用于分析.在频域对头波的宽带衰减系数进行分析,并在时域研究散射波声强的时间演化曲线,获得了系统的弹性平均自由程、传输平均自由程等多重散射参数。经多重散射后,透射波中的头波表现出相干性,由不相干近似理论可对其对应的散射参数进行定性描述;散射波是不相干的,其对应的多重散射参数可近似利用扩散近似理论获得。

Ultrasonic multiple scattering parameters of densely packed cylinders immersed in water

To reveal the mechanism of ultrasonic multiple scattering by densely packed scatterers,the two-dimensional(2D) problem of wave transmission through densely packed and parallelly oriented cylinders is examined in water.By analyzing the characteristics of the ballistic wave packet and the scattered wave,the contributions of multiple scattering are identified.A group of non-contact cylinders,whose sizes are evenly distributed,locations are randomly determined,and has a density of~100/cm2 together with a surface fraction of~0.53,is considered.To better understand the characteristics of the transmitted signals where temporal randomness is expected,signals obtained from multiple models are averaged for analysis.As 2.5 MHz broadband ultrasonic pulses incident upon cylinder groups,the transmitted ballistic packets are centered at 0.5 MHz in the frequency domain,while temporal coherence of these packets are observed for different cylinder groups.The scattered waves arrive after the ballistic wave and their frequencies are similar to those of the incident waves,while temporal incoherence is found for different cylinder groups.The frequency dependent attenuation coefficient of the ballistic pulse qualitatively agrees with that determined from the independent scattering approximation.For the scattered wave,its temporal variation in acoustic intensity can be described using a diffusion model.