Study on calculation method of target strength in waveguide

Based on the physical acoustic method (the Kirchhoff's approximation) and planar element method, the problem of the echo intensity of the target in a shallow water waveguide is studied. The physical acoustic method is a kind of high frequency approximate calculation method, which ignores the contribution to the scattering field of the shadow of the target. The planar elements method is a method of surface integral numerical discretization based on the physical acoustic method, which is to approximate the target surface with a large number of small planar polygons (or plates and surfaces). In this paper, the target echo is decomposed into the direct wave, sea surface reflection wave and sea bottom reflection wave. The direct wave is obtained by using the planar elements method directly, which is mono-static scattering. The sea surface and sea bottom reflection wave is analyzed by the mirror image method firstly, and the reflection waves are obtained by using the planar elements method, which include mono-static and bi-static scattering. The total scattering field of the target in an homogeneous wave guide is the summation of all the waves mentioned above. The target strength of the BeTSSi-Sub (Benchmark Target Strength Simulation Submarine) simple submarine in free field, and in a shallow water homogeneous wave guide, are calculated at different directions. The BeTSSi-Sub model is a benchmark model and is created and meshed using Ansys soft. In the computational examples, we suppose that the water depth is 100m, the sea surface is the absolute soft interface, and the bottom is the absolute hard interface. The target strength of a submarine under water 20m to 80m was calculated at 1 kHz. The results show that the echo intensity of the target in a shallow water waveguide is different from that of the free field due to the influence of upper and lower interface, and the echo intensity of the target is continuously changing with the depth of target.