Modification to first-order Born approximation for improved prediction of scattered sound from weakly scattering objects

When deriving the Fourier diffraction theorem based on the first-order Born approximation,the difference between wave number of the scattering object and that of the surrounding medium is ignored,causing substantial errors in sound scattering prediction.This paper modifies the Born approximation by taking into account the amplitude and phase changes between the scattering object and the water due to the wave number difference.By changing the radius and center position of the sampling circle in the Fourier domain,accuracy of the predicted sound scattering is improved.With the modified Born approximation,the computed far-field directional pattern of the scattered sound from a circular cylinder is in good agreement with the rigorous solution.Numerical calculations for several objects with different shapes are used to show applicability and effectiveness of the proposed method.

Parallel computation of unified finite-difference time-domain for underwater sound scattering

In this work, we treat scattering objects, water, surface and bottom in a truly unified manner in a parallel finitedifference time-domain （FDTD） scheme, which is suitable for distributed parallel computing in a message passing interface （MPI） programming environment. The algorithm is implemented on a cluster-based high performance computer system. Parallel computation is performed with different division methods in 2D and 3D situations. Based on analysis of main factors affecting the speedup rate and parallel efficiency, data communication is reduced by selecting a suitable scheme of task division. A desirable scheme is recommended, giving a higher speedup rate and better efficiency. The results indicate that the unified parallel FDTD algorithm provides a solution to the numerical computation of acoustic scattering.

Pure elastic resonance scattering for submerged elastic objects with separable geometries

Theoretically, the extraction of resonance scattering spectra is performed by a pure elastic scattering function, which is defined as the total scattering function subtracts an appropriate background term. In this paper, we derive a simple and explicit expression of the pure elastic scattering function for the separable geometries immersed in water. It depends on the modal mechanical impedance and acoustic impedance except a phase factor only relative to the geometry Analyses used the new expression leads to two Kinds of resonances with distinguishable character: the elastic-borne wave resonances and the fluid-borne wave resonances. The former depends mainly on elasticity of the object and the fiuid-loading has secondary effect. The later is related closely with the liquid-loading and vanishes if the liquidloading vanishes. This allows us to classify the family of individual resonance correctly. Taking into account the contributions of the fluid-borne wave resonances, we modify the conventional resonance scattering formula by use of the Singularity Expansion Method.

Study on the optical system for the record of the pulsed acoustic field and the scattering of the cylindrical-like objects buried in underwater sand

The response of the two working mode of the optical systems, the schlieren mode and shadowgraph mode, for taking of optical photograph of the pulsed acoustic field in liquid are studied. It shows that, the response of the optical intensity on the screen to the acoustic pressure is linear for shadowgraph mode and nonlinear for the schlieren mode. Because the function of shadowgraph mode has no limit on working frequency, it is suitable for the studies of the acoustic field of laboratory model of the seabed or the buried objects at low ultrasonic range. The ultrasonic pulse scattered by the cylindrical-like objects buried in underwater sand was studied experimentally by the shadowgraph mode at low ultrasonic frequency. There are five kinds of the scattered waves from the half-buried object and three kinds of the waves from the full-buried objects were recorded. The two kinds of creeping waves (the longitudinal wave and the shear wave of the object) appear in both of the two cases.

Annual Modulation in Intraday Variability Timescales of Extragalactic Radio Sources

Annual modulations in timescales of intraday variability (IDV hereafter) are discussed for six extragalactic sources: 0716+714, 0917+624, 0954+65, 1749+70, 1803+78 and 2007+77. The timescales calculated from scintillation theory are compared with the observational data . It is emphasized that systematic observations are required to identify the phenomenon and to determine the motion of the interstellar medium with respect to the Local Standard of Rest. In addition, significant deviations from the annual modulation could be due to intrinsic variations.

Optimum angle-cut of collimator for dense objects in high-energy proton radiography

The use of minus identity lenses with an angle-cut collimator can achieve high contrast images in highenergy proton radiography.This article presents the principles of choosing the angle-cut aperture of the collimator for different energies and objects.Numerical simulation using the Monte Carlo code Geant4 has been implemented to investigate the entire radiography for the French test object.The optimum angle-cut apertures of the collimators are also obtained for different energies.