Electro-Magnetic Control Of Hydrofoil Wake

The electromagnetic control of hydrofoil wake is a technical means which can improve the hydrodynamic characteristic of the hydrofoil through the use of the Lorentz force to influence its boundary layer. It has a wide potential application background in the field of drag reduction, enhance or suppress vortex, flow separation or noise control etc., and has become the research focus of the flow control in fluid dynamic due to its in improvement of the propulsive efficiency of aerocraft, increase of the flight envelope and maneuverability, reduction of the fuel consumption and the enhancement of the flight stability.In the paper, the flow pass around a hydrofoil under the control of Lorentz force were studied both experimentally and numerically. Experiments were conducted in a high stabilization rotating annular tank which chose the potassium permanganate (KMnO4) solution as a marker for the flow visualization, and the lift and drag force is measured by the strain measurement system. Numerical simulations were carried out with the use of the dual time step method developed by Roger. To solve the incompressible N-S equations, the convective and viscous terms were differenced with a third-order flux difference splitting technique and a second-order central difference, respectively. In order to compare the numerical flow field with the experimental results, trace particles method is adapted to visualize the calculated digital flow. By introducing the boundary vorticity flux (BVF), a quantitative relation between the Lorentz forces, BVF and lifts is deduced and is used as a criterion to discuss the variation of the lift. The mechanisms of flow control under the action of Lorentz force have been elucidated. Both our experimental and numerical results indicate that:1) The flow fields of the hydrofoil are periodic and the same with the lift and drag variation.2) At a small or zero attack angles, the streamwise Lorentz force can increase the fluid momentum nearby the solid, suppress the flow separation and the wake, reduce the drag and increase the lift. While the reversed Lorentz force acts as the same with a big attack angle, it can reduce the fluid momentum in the boundary layer, decrease the capability of the flow to resist adverse pressure gradient, produce large scale of flow separation and complicated vortex configuration and result to the lift decrease and drag increase. 3) The Lorentz force field distribution is formed transiently with the action of electromagnetic node, however, the flow separation on the upper surface is not suppressed at the moment, the separation point moves along the surface from the leading edge to the tailing edge with certain speed. The tailing edge vortex disappears until the separation is suppressed completely.4) The flow separation at the leading edge can not be suppressed with a small Lorentz force, but the trailing edge separation can be suppressed. With the increase of Lorentz force, meanwhile, all the separation can be suppressed successfully. The lift raises with the increase of Lorentz force, however, the drag force is the reverse.5) The Lorentz force locating at the front edge can suppress the separation efficiently, however, with its location at the tail, the separation can not be suppressed. The effect of lift enhancement with the Lorentz force acted on the tail is better than the front, and the drag reduction is better with the Lorentz force applied to the front.