Electromagnetic Force Turbulent Drag Reduction Mechanism Of The Wall

When Lorentz force is applied to the electrolyte solution channel flow, the distributions of mechanical quantities such as velocity and pressure in flow field can be changed. So the skin friction and vortex structures can be changed by appropriate Lorentz force. There is an increasing demand for energy conservation and emissions reduction at present, drag reduction can reduce energy consumption, raise speed and improve stability of equipments. So there is a broad prospect of this flow control.In this paper, numerical simulation technique is used to simulate the channel flow, and distribution of flow field controled by spanwise oscillating Lorentz force is received. In the simulations, Fourier transform and periodic boundary conditions are used in the streamwise and spanwise direction, Chebyshev transform and no-slip boundary conditions are used in the wall-normal direction. Then the time term in the N-S equation is discretized via difference method. And finally, the N-S equation is solved by applying both influence-matrix method and Chebyshev-tau method.After the results of the equations in different time are calculated, the distributions of flow field in different time are received. On this basis, The change of flow field distribution and the the influence of Lorentz force can be studied, and then the mechanism of skin drag reduction can be explained. The local skin friction can be decomposed into three parts, i.e., laminar, turbulent and other factors, the second of which is a weighted integral of the Reynolds stress distribution. It is reconfirmed that the near-wall Reynolds stress is primarily important for the skin friction of wall turbulence. In this paper, the distribution of streaks, vortex constructures, Reynolds stress, speed probability density and energy spectrum are calculated. By comparing the distributions in different time, the result that the drag from turbulent contribution can be decreased due to the decrease of streamwise velocity and wall-normal velocity by applying Lorentz force in the flow field is derived, which is the mechanism of the decrease of the local skin friction.