d形粗面上の乱流境界層の組織構造 : VITA法および4象限分割法による解析

抄録

Conditional sampling techniques have been used to investigate the coherent structure of the turbulent boundary layer over a d-type rough surface at a momentum thickness Reynolds number of R_θ≒5000 at which the skin friction coefficient took a constant value. In this study, the effect of imperfect spatial resolution on hot-wire measurement was removed and the operational parameters included in the detection scheme of the VITA technique were adjusted to the most probable values. The ensemble averaged patterns obtained from the VITA technique showed that there existed events associated with the bursting phenomena even in the present d-type wall flow, and that these structures were observed in the region of y⁺≤480(y/δ≤0.26). Although the mean bursting frequency depended on the distance from the wall across the inner layer for the present rough wall flow, its value scaled with the wall variables at y⁺ = 15 was almost the same as that found in smooth wall flow. In addition, the events contributing to the Reynolds shear stress were measured using the four quadrant method. Compared with the results obtained from the boundary layer over both a k-type rough surface and a smooth surface, the present results were found to differ significantly. The ratio of the fractional contributions of the ejection events to the stress to that of the sweep decreases with increasing Reynolds number.