Drag Reduction In Turbulent Channel Flow Utilizing Spanwise Oscillating Lorentz Force

When Lorentz force is applied to the electrolyte solution, the hydrodynamic characteristics of turbulent flow can be changed by modifying the turbulence structures, so the skin friction increase/reduction, vortex structures enhancement/ suppression and noise elimination can be achieved. There is broad prospect of this flow control method to improve propulsive efficiency, maneuverability and stability of vehicles.In the paper, the channel flow controlled by span wise oscillating Lorentz force is studied both experimentally and numerically. Incompressible Navier-Stokes equations is solved in the numerical research, 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, semi-implicit back-differentiation formula method with third-order accuracy is used for the time derivative, influence-matrix method including a tau-correction is employed to solve the equations. The experiment is conducted in a closing-loop water tunnel, PIV system is employed for presenting the distribution of streaks and drag measurement system including a active bed is designed for measuring the skin friction drag.The turbulence bursting events is identified from the numerical simulation results by VISA method, varying tendency of bursting frequency and bursting intensity with Lorentz force parameters(amplitude St and period T+) and optimal Lorentz force parameters are discussed. The induced effect to the flowfield, the suppression to the turbulence and the change in skin friction drag by Lorentz force are revealed in experimental research. The periodical change of flow field and the mechanism underlying these changes are researched based on the experimental and numerical results. The time-space averaged effect of spanwise oscillating Lorentz force on flow field is also shown through the turbulent statistic.The results both from our experimental and numerical research indicate that:1)The bursting frequency and bursting intensity vary with the Lorentz force parameters. When the T+ (or St) is fixed, varying tendency of bursting frequency is opposite to that of bursting intensity with increasing in St (or T+), which suggests there is an optimal parameter to balance these two opposite factor for achieving maximum drag reduction. Equivalent spanwise wall velocity W+ can well describe the synthetic effect of St and T+ on the skin friction drag and the optimal W+ is 11 approximately.2)It is observed that the spanwise oscillating Lorentz force can induces the near-wall flow to oscillate and impose the streaks in near-wall region either to tilt and oscillate or to eliminate. The skin friction drag can be achieved and the maximum drag reduction is 8%. These experimental results is demonstrated by numerical simulation results.3)Streaks and streamwise vorticity influenced by spanwise oscillating Lorentz force undergo a periodical change, which is tilt and oscillation—obscurity—elimination—obscurity- tilt and oscillation in detail. When streaks and streamwise vorticity present an tilting and oscillating pattern, flow field still maintain at turbulent flow, but the additional negative spanwise vorticity generated due to the tilting streaks and streamwise vorticity induced by spanwise oscillating Lorentz force. These additional negative spanwise vorticity can shift the mean streamwise profile and decrease the bursting intensity to reduce the skin friction drag. When the turbulence is suppressed significantly, streaks and streamwise vorticity present an obscure pattern and even eliminated, which lead to a significant reduction in skin fricition drag.4) Turbulent statistic suggest spanwise oscillating Lorentz force can well suppress the turbulence.