Acoustic Scat From The Vortex In Water

The wake generated by the underwater structure can evolve the tail vortex with a special flow structure.When acoustic waves propagate through vortical flow,the flowsound interaction can affect the acoustic propagation,resulting in acoustic scattering phenomena.The spatial directivity of the acoustic scattering field of the wake vortex contains multiple inherent characteristic information of the tail vortex.By detecting and analyzing the characteristics of the scattering field over the vortical flow,the existence of the structure can be indirectly identified.Scattering from underwater vortical flow is of interest in exploring the mechanism of fluid-sound interaction,and it is also of interest in detection of underwater target.The time-domain and frequency-domain computation methods based on the acoustic perturbation theory are established for acoustic scattering from underwater vortical flow.The acoustic perturbation models of fluid-sound interaction field with vortical flow as background flow field are established for acoustic scattering from underwater vortical flow,including time-domain and frequency-domain models.The governing equations of the models are discretized on staggered grid and solved with finite difference method.The perfectly matched layer is used as the acoustic absorption boundary.The velocity and velocity gradient of the vortical flow are mapped into the acoustic field grid as the background flow field.After the calculation is finished,the acoustic scattering pressure in the acoustic field is extracted and analyzed.(1)The time-domain and frequency-domain calculation methods for acoustic scattering from vortical flow are validated using the steady vortex.The reasonable matching of the damping factor and the number of layers for the perfect matching layer is discussed.The results show that the thickness of the sound absorption boundary of the perfectly matched layer should be greater than 2.5 times the wavelength of the incident wave,and the damping factor is linearly related to the frequency of the incident plane wave The accuracy of the two calculation methods for different frequencies is analyzed.As the calculation frequency increases,the frequency calculation method can still better simulate acoustic scattering from vortical flow phenomena,but the result of the time-domain calculation method shows a large deviation.(2)Based on the above analysis,the frequency-domain calculation method is applied to simulate the acoustic scattering from a vortex in water.Effects of Mach number and dimensionless frequencies on the scattering amplitude are analyzed.Simulations indicate that forward acoustic scattering is much stronger than backward acoustic scattering.With the dimensionless frequency increasing,the form function of the main lobes increases,and the azimuth angle of main lobe approaches the incident wave propagation direction;and,the form function of main lobe grows linearly as the dimensionless frequency or Mach number increases.(3)Then,the forward acoustic scattering characteristics of the moving vortex are analyzed when the center of the vortex core is at different positions.The simulations show that the positions of the main lobes of the of the forward scattering form function at heading area and downstream area are symmetric about the position of the center of the vortex core.The orientation of the center of the vortex core can be calculated indirectly by the positions of the two main lobes.(4)Finally,the characteristics of acoustic scattering from the wake of a cylindrical structure at different time and different Reynolds numbers are analyzed.The simulations show that the number of peaks of forward acoustic scattering distribution curves is approximately equivalent to the number of the vortices in the wake over underwater two-dimensional cylindrical structures.The maximum value of the forward scattering pressure increases linearly as the Reynolds number increases.