Experimental And Theoretical Investigations On Internal Waves Generated By Submerged Bodies

In the ocean,the density is different in depth due to differences in temperature and salinity.This results in the formation of a stratified fluid with a pycnocline.When the submerged body such as a submarine moves in the ocean,internal waves can be generated by the submerged body and its wake.The flow field induced by internal waves can change the nature of the ocean background flow,making it possible for non-acoustic detection of the underwater object.Therefore,the investigation of internal waves generated by the submerged body is of the academic importance and the potential military application.On the basis of summarizing recent research situation for such subjects,the thesis presented a combined experimental and theoretical study for the formation mechanism and characterization of internal waves generated by submerged bodies in a stratified fluid.In charpter ?,techniques for both the self-propelled and towed models translating in the stratified fluid are developed,and then laboratory simulation methods for internal waves generated by such submerged models are established.Based on the real-time measurements of conductivity probe arrays which are arranged symmetrically in transverse section of the stratified fluid tank,measuring methods for the internal wave correlation velocity and its time history generated by such submerged models are developed.Furthermore,a method for analyzing symmetric and anti-symmetric components of internal waves generated by such submerged models is proposed by means of the odd-even function decomposition technique.In charpter ?,a series of experiments are performed for the influence of submerged depth and aspect ratio of towed models on generating internal waves.Results show that the transition between the body-generated and wake-generated internal waves is related to a critical Froude number Fr_c which is independent of submerged depths,but linearly dependent of the aspect ratios,and the variation law is abtained for the Froude numbers associated with the correlation velocities Fr_iw of wake-generated internal waves.For body-generated internal wave,the wave amplitude firstly increases with the growth of Fr until Fr reaches a certain value of Fr_p which is regardless of submerged depths but also linearly dependent of the aspect ratios,and then begins to decrease.For wake-generated internal wave,the wave amplitude linearly increases with Fr regardless of the aspect ratios,and the slope of the linear relation decreases with submerged depths and follow some exponential law.Moreover,the body-generated wave patterns are symmetric to the axis-symmetric revolutional body,while the wake-generated wave patterns are not symmetric to the axis-symmetric revolutional body.In particular,the present work demonstrates that it is not comprehensive to assume that the wake-generated internal waves for an axis-symmetric revolutional body are symmetric about the longitudinal section in center plane of the body.In charpter ?,an axis-symmetric equivalent mass source is adopted to simulate the excitation of the body-generated internal wave,while an oven-odd combination equivalent mass source is adopted to simulate the excitation of the wake-generated internal wave,and then a simplified theoretical model is established to calculate the vertical displacement field of the two kinds of internal waves,as well as a fast calculation method for the theoretical model is developed.Based on the theoretical model and experimental results,the method of determining the translating speeds and geometric sizes of the equivalent mass sources is proposed.The numerical results based on the proposed model are shown to agree well with the experimental data,indicating that the proposed model and its parameter setting method are reasonable and effective.In charpter ?,experiments are conducted for the time-space characteristics of internal waves generated by both self-propelled and towed models in a stratified fluid.Results show that the transition between the body-generated and wake-generated internal waves is related to some critical Froude number Fr_c for two cases.In particular,the Fr_c for the towed case is bigger than one for the self-propelled case,showing that the propeller effect makes the body-generated internal wave decay faster.For Fr<Fr_c,the internal waves are dominated by symmetric body-generated waves for two cases,and the wave amplitude for such internal waves generated by the self-propelled body is bigger than that one generated by the towed body,showing that the behavior of the propeller can also produce the body-forced effect.For Fr>Fr_c,the Froude numbers associated with the correlation velocities Fr_iw for wake-generated internal waves vary within some range for two cases,and the wave amplitude linearly increases with Fr for the towed case while the wave amplitude does not increase with Fr,but varies within some range for the self-propelled case.