| The electromagnetic force (i.e. Lorentz force) generated by the electromagnetic field in the electrically conducive fluid can modify the flow boundary layer, which is an effective flow control method. The Electromagnetic Flow Control (EMFC) can be categorized into two groups, Magnetic Hydrodynamics (MHD) and Electromagnetic Hydrodynamics (EMHD) according to the control method adopted. The flow of fluid with certain conductivity, especially weakly conductive fluids such as seawater and weak electrolyte solutions, can be controlled utilizing the electromagnetic force by reconstructing and adjusting the boundary layer. This flow control method represents the comprehensive effects of nonlinear control, belonging to the category of EMHD. This flow control technique may reduce the drag, enhance or suppress vortex, and have broad application prospect in improving propulsive efficiency, maneuverability and stability of the underwater vehicles (e.g. submarines and ships).The objective of this dissertation is to investigate numerically the three-dimensional structures of the flow field around three types of bodies of revolutions, i.e. cylinder, sphere and submarine, and the control effects of the electromagnetic force on the flow field. The Navier-Stokes equations considering the source term of electromagnetic body force are solved by finite volume method based on the hierarchical structured mesh.Main conclusions are as follows:(1)In the process of vortex shedding from the circular cylinder, the spanwise Karman vortex loses stability gradually and the small scale streamwise vortex is formed. Base on the spanwise scale of the streamwise vortex, two typical instability modes are mode A and mode B. The spanwise length of the mode A instability is to be around3.3-5times the cylinder diameter, and the spanwise length of the mode B instability is about0.83time the diameter. When the boundary layer of the flow around cylinder is controlled by Lorentz force, the vortex shedding is suppressed efficiently. The pressure in the region around the rear stagnation point is increased and the drag is reduced.(2)Unlike the flow past the circular cylinder, which the separation bubble lose symmetry and turn into unsteady flow, the wake of the sphere transforms from vortex ring to double filaments vortex, and the double filaments vortex loses stability leading to the shedding of the hairpin vortex during the wake transition process. When the flow around sphere is unsteady plane-symmetrical, the drag and lift coefficients oscillate periodically with the same frequency and phase as the hairpin vortex shedding. When the Reynolds number is about400, the vortex shedding and the time history of force coefficients become quasi-periodic. When the Lorentz force is applied on the sphere boundary layer, the formation and shedding of the hairpin vortex is suppressed. The region of favorable pressure gradients near the rear stagnation point is reduced and the fluctuation of the force is suppressed.(3)Aimed at the submarine model with complex appendages, the disturbance caused by the appendage to the flow field around the submarine is analyzed. The flow field structure around the submarine at different Reynolds numbers and attitudes are also investigated. By control the local flow field around the fairwater utilizing the Lorentz force, the overall flow field and the hydrodynamic characteristics are improved. The results show that the effect of disturbance caused by the vortex shed from the fairwater on the flow around submarine increase as the magnitude of Reynolds number increase. With all other flow parameters being equal, the flow around submarine at yaw is more stable than the flow around submarine at straight course, and the flow around submarine at pitch down attitude is more stable than the flow around submarine at pitch up attitude. The vortex shed from the fairwater are suppressed by applying the Lorentz force in the near-wall region of the fairwater. For the flow around the body of revolution with hydrofoils, the Lorentz force may also suppress the vortex shedding around the hydrofoil, improving the maneuverability of the aircraft.By systematic study on the three-dimensional structures of the flow field around typical bodies of revolution and the control effect of the electromagnetic force on the flow field, the structure characteristics of the flow field around the submarine and the control parameters of the Lorentz force are obtained. The results have certain practical value in drag reduction and vortex shedding suppression, and will promote the research work in improving propulsive efficiency, maneuverability and stability of the underwater vehicles. |
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