Investigation On Flow Characteristics Around Blunt Bodies Controlled By Electromagnetic Forces

The cylindrical and hydrofoil structures are two typical forms of blunt structures in ship and ocean engineering. The vortex-induced vibrations due to ocean currents can result in fatigue damage and rupture accidents of the cylindrical structures. When hydrofoil structures sail with the ship by a certain attack angle, the hydrofoil structures will produce stall phenomenon for the cases where the attack angle is more than a certain critical value. Such stalls will lead to drags increase and the lift decrease of the hydrofoil structures, and directly affect the propulsion efficiency and control performance of ships. Based on the fact that both the vortex-induced vibration and stall phenomenon are mainly caused by the flow separation, the flow control of blunt bodies will then be an effectively way to not only reduce the drag and inhibit the lift of cylindrical structures, but also delay the hydrofoil stall. Therefore, investigations on the flow control of blunt bodies are of important academic significance and potential applied value for enhancing the safety of ocean engineering, as well as improving the propulsion efficiency and manoeuvrability of ships.Therefore, based on reviewing and summarizing the research progress on this subject, efforts are devoted to the investigations on the aforementioned two types of blunt bodies. The flow characteristics around the blunt bodies controlled by the electromagnetic forces are simulated by CFD method under different Reynolds numbers. The main work of the doctoral dissertation is given as follows:In Chapter II, the governing equations describing the incompressible flow with the electromagnetic force are expounded. According to the electromagnetic field equations, the finite element method is presented to solve the distributions of the electromagnetic forces near blunt bodies in a weakly conductive fluid. Furthermore, the DES method is presented to solve the flow characteristics around blunt bodies with high Reynolds numbers controlled by the electromagnetic force.In chapter III, the unsteady numerical method is carried out to simulate and analyze the flow structures around a circular cylinder and its lift/drag characteristics in a weakly conductive fluid under various combinations for electromagnetic interaction parameters and electromagnetic actuator widths when the Reynolds number Re ?200. The results show that for the case of small electromagnetic actuator widths, the separation point of the flow around a cricular cylinder is easier close to the back stagnation point, and the electromagnetic force has small influence on the total drag, but it has obvious influence on the pressure and friction forces. For the case of large electromagnetic actuator widths, the cylinder wake is easier to become jet flow, and the total drag decreases as the electromagnetic interaction parameter and the electromagnetic actuator width increase. Moreover, for the case where the electromagnetic force is insufficient to completely inhibit periodic vortex shedding, the lift amplitude decreases as the electromagnetic interaction parameter increases, however it first has an obviously reduction and then a slightly increase as the electromagnetic actuator width increases, and the lift pulsation frequency increases as electromagnetic interaction parameter and the electromagnetic actuator width increase.In chapter IV, the flow structures around a circular cylinder with the laminar boundary separation and its lift/drag characteristics due to the electromagnetic force are investigated at subcritical Reynolds numbers3 Re ?3.9?10 and4 Re ?5?10 base on the DES method. Moreover, the DES method with the turbulent separation(TS) is presented to simulate and analyze the flow characteristics around a circular cylinder with a turbulent boundary layer separation at subcritical Reynolds number51.4?10. The results show that the electromagnetic force can increase the fluid kinetic energy near the circular cylinder wall, weaken the intensity of the large-scale eddies in the streamwise and spanwise vortexes, decrease the drag force and the fluctuation amplitudes of the lift. Moreover, when the electromagnetic force parameter reaches a certain critical value, the separation angle disappears and the jet phenomenon appears in the wake of the circular cylinder so that the electromagnetic force produces the thrust action on the circular cylinder and the negative drag phenomenon occurs, as well as the lift fluctuation amplitudes decline to almost zero.In the chapter V, the DES method is presented to analyse the flow structures around a hydrofoil and its lift/drag characteristics due to the electromagnetic force at the Reynolds numbers51.63 10 LRe ? ? and62.97 10 LRe ? ?. The results show that the electromagnetic force increases the fluid kinetic energy around the hydrofoil surface, and can effectively improve the performance of the flow. The vorticity field characteristics near the hydrofoil surface are changed to positive when the force is large enough. The electromagnetic force reduces the pressure drag significantly, however increases the friction. Moreover, the electromagnetic force can significantly reduce the lift/drag fluctuation amplitudes and increase the lift-drag ratio, delay the stall phenomenon and improve the work performance of the hydrofoil.