Research On Wake Field And Free Surface Characteristics Of Submerged A Body In Stable Stratified Flow

When a submarine moves in a stable stratified ocean environment,the "hydrodynamic wake" and "cold wake" induced by its disturbance,as well as the changes in free surface are very different from the situation when it sails in homogeneous flow.The wake features can be called important targets for non-acoustic detection submarines.So,it is of great significance to study and analyze the flow characteristics of the stratified flow,as well as the wake field and free surface characteristics of the submerged body in it.From this point of view,the main research work of this doctoral dissertation is as follows:Numerical methods have been established based on the mathematical model of the ocenic typical two-layer stratified flow and continuous stratified flow in this dissertation.In detail,two-phase flow VOF method is used to capture the interface and the free surface in order to achieve a strong stratified environment.The state equation and the energy conservation equation are simultaned to establish a single-phase stratification of the fluid temperature or density with a continuous state.The SST k-ω turbulence model which takes into account the "buoyancy effect" is used to simulate the wake tubulence.Based on the established numerical methods,analysises were performed on the characteristics of the interface wave induced by a cylinder in a two layerd flow.The evolution characteristics of the wake flow field around a sphere in a continuous stratified flow under different dimensionless parameters are simulated.Analyze and summarize of the phenomena and characteristics for the wake flow of the submerged bodies with regular geometric shapes can used to compare the results with exsit experiments in order to verify the accuracy and effectiveness of the numerical methods to deal with the "buoyancy effects".Then,on the basis of summarizing the characteristics of the interface waves of a cylinder in two-layer flows,the established two-layer flow numerical method is used to research the spatial and temporal propagation rules of the interface wave and free surface geometry changes induced by a real scale Suboff submerine.It mainly analyzes the evolution process of interface wave and free surface wave caused by the hull volume effect.The influence of main navigation parameters such as submerine depth and speed on the characteristics of interface wave and free surface wave are also studies.The effects of stratification on the "converging-and-diverging"effect of the free surface are analyzed.The research results show that when the submarine is sailing near the free surface,the hull has a strong disturbance effect on the interface and the free surface.The interface wave and fluctuations caused by the submarine are mostly in a narrow"V" shape.The angle of the wave pattern is different from the surface ship wave.When the speed changes,the distribution of the transverse wave system and the scattered wave system in the interface wave changes significantly.The change of the submerine depth affects the volume displacement effect of the hull on the fluid,which makes the amplitude of the wave significantly changed.The types of peaks and troughs corresponding to the hull position have changed.Based on the established numerical simulation method of the continuous stratified flow,the density fluctuations of the hydrodynamic wake and free surface wave induced by the real scale Suboff moving within a pycnocline are researched.The development and evolution process of the wake flow are studied.The influencing factors are include the main stratification elements such as density stratification intensity,the relative depth of the submarine to the free surface,and the speed of the submarine.The results show that the buoyancy effect is very imoprtant to the hull volume displacement induced wake.At the same time,the evolution and development of the free surface wave of the still water caused by submarine are analyzed.Studies have shown that submarine can cause significant density fluctuations in the wake field.The fluid inside the pycnocline is affected by the buoyancy effect of the stratified fluid,the vertical movement is strongly suppressed,and a certain"blocking effect" appears in front of the submarine hull.The fluid around the stern of the submarine collapses,and it is easy to form a" trench-shaped" wake region.The intensity of the pycnocline affects the strength of the"buoyancy effect" of the stratified environment,which in turn affects the severity of density fluctuations behind the submarine.When the submarine is located at different positions in the pycnocline,the peaks and troughs of the density field fluctuations within the pycnocline are different,and there are different forms of density fluctuations such as "downward concave"type,"bilateral collapse" type,and "upward convex" type.Finally,the cold wake of the real scale Suboff moving in the temperature stratified flow is researched in view of the temperature fluctuations.The distribution characteristics of surface temperature differences were also numerically studied.The development and evolution of wake temperature fluctuation field in thermocline was given,and the stratifaction elemnts such as thermocline intensity were compared and analyzed.The influences of navigation factors of the submarine such as depth and speed on the temperature fluctuations of the stratified wakes are simulated.At the same time,the features of free surface temperature distribution caused by submarine are compared and analyzed.In addition,the influence of submarine propeller operation on the space-time characteristics of cold wake is further studied and analyzed.Based on this,the propagation and evolution characteristics of propeller "swirling wake" in thermocline flow are studied.The research results show that the cold wake and hydrodynamic wake induced by the submarine have certain similarities and correlations in the form of spacetime structure,but their free surface performance characteristics are quite different.When a submarine is sailing in a thermocline flow,the temperature fluctuation field of the stratified wake induced by its disturbance changes significantly,especially the "swirling wake" caused by the propeller operation,which is a typical "momentum wake".The characteristics of the cold wake behind the submarine are strong,but the free surface features of the cold wake can gradually weaken during the upload process in the presence of an upward uniform layer.