An annular groove (AG) structure with depth gradient is proposed which can manipulate the spatial distribution of the acoustic scattering field for a finite rigid cylinder in water. An analytical analysis is given for better understanding the underlying mechanism of the abnormal scattered wave, which can be accomplished by using the phased array theory. When the plane acoustic wave is normally incident, the scattering acoustic wave in the transverse direction of the cylinder deflects, which is due to the interaction between the phase delay modulated by the AG structure with varying groove depths and the Bragg scattering of adjacent grooves. The finite element method is used to calculate the acoustic scattering field of a finite rigid cylinder with annular grooves and obtain the frequency and spatial distribution characteristics. How the structural parameters such as depth, gradient, and duty ratio of the annular grooves affect the acoustic scattering field is discussed in detail. The results show that the target strength in the transverse direction decreases linearly with duty ratio increasing while the target strength in the deflection direction of the acoustic wave increases with the duty ratio until
δ= 30%, after which it remains almost constant. When the incident acoustic wave is fixed, the acoustic scattering wave of the AG cylinder can be deflected by designing the gradient appropriately, and the deflection direction is independent of the frequency. Numerical and experimental results for a cylinder with multiple annular-groove units show that the spatial directivity of the scattering field of the grooved cylinder changes, and the target strength is enhanced at six pre-designed deflection angles. Meanwhile, the deflected acoustic wave has a certain width and the interference among periodic structures of the AG units exists, which makes the spatial directivity of the scattering field of the cylinder equalize and changes the scattering characteristics of the cylinder, thereby providing a theoretical basis for designing three-dimensional underwater objects each with an acoustic stealth.
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