Abstract
The flow of a weakly conductive fluid (i.e. seawater) can be controlled by electromagnetic forces, which holds promising applications in the drag reduction. In this paper, a fully developed weakly conductive turbulent flow controlled by a space-dependent electromagnetic force is investigated with direct numerical simulation (DNS) methods for different Reynolds numbers. With the application of electromagnetic force, the in-depth relations among characteristic structures in the flow field, mean Reynolds shear stress and the effect of drag reduction for different Reynolds numbers are discussed. The numerical results indicate that the well-organized quasi-streamwise vortex structures induced by electromagnetic force are generated with the optimal values of the amplitude and the wave number. The Reynolds shear stress is suppressed and the maximum of drag reduction is obtained with the effect of these structures. However, with the increase of Reynolds number, the wave number of optimal electromagnetic force increases, resulting in the decrease of scale of the well-organized quasi-streamwise vortex structures, which leads to the decrease of the height of these vortex structures. Therefore, the maximum rate of drag reduction decreases due to the decrease of the effective depth with the increase of Reynolds number.
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