Study Of Low-frequency High Scattering Strength Objects

One of the ways to detect submarines is using active sonars. In order to keep submarines undetected, an effective way is to place some scatterers to interfere with active sonars. Nowdays active sonars have lower its working frequency range to several hundred hertz to several thousand hertz. Regular scatterers have so low scattering strength in such frequency condition that effective interference can’t be made. In this paper we analyse the scattering character of gas-filled cylinders based on the resonance of air bubbles in water. Then we point out that the scattering strength of gas-filled cylinders is relatively high in low frequency condition. In the mean time we analyse the scattering character and scattering strength when multiple gas-filled cylinders exist in water. At last we measured the scattering strength of gas-filled cylinders and gas-filled line array, proving that it has high scattering strength in low frequency condition.The first part of the paper analyses the scattering field of infinite long rigid and gas-filled cylinders. We compare the scattering strength of rigid and gas-filled cylinders, proving that a resonance peak exists in gas-filled cylinders’ scattering strength curve, and the scattering strength is much higher than that of rigid cylinders. Then we modify the equations into the version of finite long cylinders and analyse its scattering strength. In the mean time an simple high-pass model is introduced. At last we study the theory of bubble resonance and analyse the resonance frequency of spherical bubbles and prolate spherical bubbles using Kirchoff integral equation.The second part of the paper analyses the scatteing field when multiple parallel cylinders exist in water. The core of the theory is the coordinate change formula of the first kind Hankel function. A scattering field parameters matrix is created based on the formula and the relative location of different cylinders. The scattering field can be solved out by solving the linear equations set. The scattering strength of cylinders array is based on the form of the scattering field.The third part of the paper creat finite element models of cylinders using LMS Virtual Lab12. In order to test the accuracy of finite element models, we creat a rigid cylinder model and calculate its scatteing strength. Then we creat gas-filled cylinders line array and run a simulation to calculate its scattering strength. Finally we creat a gas-filled cylinder shell model to anaylse the effects of different shell materials on its scattering strength.In the final part of the paper we did an experiment to actually measure the scattering strength of a gas-filled cylinder and its array. The cylinder has a 9mm inner diameter and a 12 mm external diameter. After introducing the instruments and the procedure of the experiment, we processed the data of the experiment and carefully analysed the deviation in the experiment. The result of the experient proves that gas-filled cylinders have high scattering strength in low frequency condition. The maximum backscattering strength of a line array which is made up by 8 0.5m-length gas-filled cylinders is 0.45 d B and it matches the result of theory anticipation.