Very Large-Scale Feature of Transitional and Turbulent Channel Flows: Dependence on Facilities

摘要

A recent investigation revealed that a low frequency peak in premultiplied energy spectra due to turbulent patch passes in a transitional channel flow continues in a low Reynolds number but turbulent flow. In this study, to denial that a peculiar vibration of the facilities causes this low frequency peak, hot-wire measurements were performed. Spectral energy distributions obtained in different air channel flow facilities clearly confirm very large-scale feature at low Reynolds number and its independence from peculiarities of the facilities. A flow visualization experiment conducted in a water channel illustrates that a cluster of characteristic streaks forms into a inclined stripe whose shape is similar to a turbulent patch in a transitional channel flow. The streamwise and spanwise sizes of the cluster are in agreement with length scales estimated from the energy spectral and a spanwise correlation of the streamwise velocity. The similarity to the turbulent patch and continuity of the low-frequency peak from the transitional Reynolds number infer that the very large-scale feature relates to the intermittency in the transitional flow in terms of its maintenance and occurrence mechanism.