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Electro-Magnetic Flow Control of Near-Wall Turbulence for Drag Reduction

Junguo Pang
School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, Nottingham, NG7 2RD, UK

Kwing-So Choi
School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD, UK


Results from an experimental investigation of Electro-Magnetic Turbulent Control (EMTC) in an open water channel are presented in this paper. With better understanding of the regeneration cycle of near-wall turbulence events and coherent structures, most of the turbulent control strategies are focus on how to effectively disturb this regeneration cycle. Spanwise-wall Oscillation Turbulence Control (SWO-TC) is one of these strategies, which has been studied intensively in the past decade. Spanwise Oscillating Electro-Magnetic Turbulent Control (SO-EMTC) has been developed as a result of these investigations. To make the water electrically conductive, CuS04 solution was injected tangentially to the flow in the near-wall region of the boundary layer through a rectangular slot. The conductivity within the boundary layer over the test plate has to be known in order to obtain accurate Lorentz force distribution on the EMTC arrays. Two different methods to get this concentration/conductivity profile were presented in this paper.
Our experimental data shows that the mean velocity gradients and the turbulence intensities in the near-wall region were reduced and the increases of higher moments of turbulence statistics like skewness and kurtosis near the wall suggested the thickness of viscous sublayer was increased by SO-EMTC. 45% skin friction drag could be achieved when St = 30πReτ/T+ = 280 in the present SO-EMTC conditions.
An "equivalent spanwise wall velocity" W+equi of SOEMTC, which is considered as a bridge to connect SOEMTC and SWO-TC together, was obtained from our calculation. The optimum Stuart number Stopt = 30πReτ/T+ derived from this W+equi was found slightly greater than the DNS results. Both of our experimental data and calculation suggested that St = 30πReτ/T+ is more likely to be the optimum St for SO-EMTC.