十分発達したチャネル乱流における大規模流れ構造について

第2報, 安定温度成層の影響と自己相似性について

抄録

Our main purpose is to study the structures of the large (comparable to the boundary-layer thicknessc δ) and very large (more than 10 times larger than δ) scale motions observed at or above the logarithmic layer of fully developed wall-turbulence, using Large Eddy Simulation (LES). In the second report of the continued research, we impose stable thermal stratification at different Richard-son-number conditions on the fully developed open channel flow to investigate the interactions among the very small structures near the wall (typically sublayer streaks), the large, and very large scale motions. It was fond that the large scales are strongly affected by thermal stratification and the very large scales are suppressed at a certain weakly stable condition, contrary to the sublayer streaks that appear to be rather insensitive to thermal stratification. As a result of their suppression in the outer layer, the peak wavelengths of the streamwise velocity's premultiplied spectra recovers linear growth with the wall distance above the buffer layer, which implies that the very large scales are the main reason for the lack of self-similarity of the streamwise velocity in wall-turbulence.