Preliminary Study Of Surface Characteristics Of Internal Waves Generated By A Horizontally Moving Source

The modulated surface characteristics of internal waves generated by a horizontally moving source (sphere) in a stratified fluid is studied preliminary by combining theory, particle image velocimeter(PIV) experiment and numerical model. The surface characteristics include hydrodynamical fields (velocity, 2-component velocity, gradient, divergence, vorticity, energy) and SAR images in laboratory experiment. Scientific grounds are provided for non-acoustic detection of underwater moving bodies based on the principles of dynamics of the internal waves.The works and main conclusions of this dissertation are describled as follows. The theory part:(1) Miles'(1971) classical theory and Xu and Sturova's(2006) on the linear theory of the generation of the internal waves by a horizontally moving source are compared. It is proved that they have the same basic equations, boundary conditions and dipole description. (2)Xu and Sturova (2006) obtained the internal wave fields produced by Rankine ovoid. Following their ideas, the internal wave fields generated by a horizontally moving sphere in stratified fluids are deduced. (3)Yuan's (1997) SAR physical model is simplified according to laboratory conditions, and the SAR image formula for the experiments is formed.The experiment part: The PIV technique is applied to obtain the high resolution surface flow fields in Laboratory of Physical Oceanography, Ocean university of China. Surface characteristics of the internal waves generated by a horizontally moving sphere has been analysed according to experimental data.Internal waves generated by a horizontally moving sphere in the pycnocline.(1) Velocity fields are irrotational at the surface.(2) Velocity fields,3-component gradient fields,energy fields,divergence fields have similliar crest lines as that of Kelvin wave's induced by ships. (3) Only diverging wave can be seen from velocity divergence fields, there is no transverse wave.(4) There is only one peak in divergence angles varying with internal Froude numbers.(5) There are two peaks in intensity of divergence varying with internal Froude numbers.(6) A series of SAR images are systematically obtained in experiment related to the divergence field for the first time.The angles of the V-shape in the SAR images are greater when the sphere is below the pycnocline than above the pycnocline at the same moving velocity.For linearized stratification, conclusions are made as follows:(1) SAR images are affected by wind.(2) In straitified fluid, bright, narrow V-shaped SAR images can be seen. But in well mixed fluid there are no V-shaped SAR images.The numerical model part: Based on Xu and Sturova's(2006) numerical model the point source-sink systems is changed to the dipole system. The linear model about the internal waves generated by a horizontally moving sphere can be obtained. The same data processing procedure is carried out as in the experiment part.The SAR images from the numerical model are compared with experimental SAR images and the results are as follows:(1) Numerical experiments showed that phase pattern of the velocity, 3-component gradient fields, divergence fields, SAR images are all contrary when the sphere is towed either above the pycnocline or below the pycnocline.(2) Numerical SAR images are similliar to the experimental SAR images when the sphere moving below the pycnocline with the same experimental parameters.