リブレット面に沿う壁面噴流に関する実験的研究 : 壁面近傍の乱流構造の変化

概要

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Measurement is made on the near-wall turbulent structure in a wall jet over a riblet surface. Turbulent energy spectra, auto-correlations and probability density functions for the streamwise velocity fluctuation are measured, and they reveal that the turbulent structure near the wall is modified by the riblets. Two standard procedures are utilized to analyze the coherent structure over the flat and riblet surfaces. From comparative examinations of the bursting phenomena detected by the VITA technique, it is found that the variation of ensemble-averaged velocity is reduced and the bursting duration is decreased due to the riblets, but the bursting frequency is almost unchanged. The result of the four-quadrant method exhibits a subtle difference within the inner region. Two-point correlations of the streamwise fluctuating velocity are measured and the integral scales are computed, and it is shown that the turbulent structure near the wall is wider in the spanwise direction above the riblet surface in comparison with the flat surface. The spanwise spectrum is calculated from the spanwise correlation, which clarifies the difference in the near-wall structure in wall jets from a turbulent boundary layer or a channel flow. A disparity of the drag-reducing region of s^+ between the wall jet and the boundary layer or channel flow is attributed to the difference in the turbulent structure of these flows.
1995-08-25