Structure of Space-time Correlations of Bursting Phenomenon in an Open-channel Flow

Abstract

The present study is to investigate the structure of space-time correlations of bursting motions, such as ejections and sweeps in an open-channel flow, by a new conditional sampling method in which simultaneous measurements of the instantaneous Reynolds stress have been performed by two dual-hot-film probes. One probe was fixed near the edge of buffer layer and used as a detecting probe of the bursting motions, while the other probe was moved in both the streamwise and flow-depth-wise directions. The main conclusions obtained from the above are as follows : (1) The spatial and time scales of the streamwise turbulent velocity component are larger than those of the vertical velocity component. (2) The spatial and time scales of the sweep motion are also larger than those of the ejection motion. (3) The spatial scale of the ejection motion extends more widely downstream than upstream, and vice versa for the sweep motion. (4) The bursting motion is a kind of a large-scale eddy structure, and its coherent structure is fairly inclined downstream toward the wall. It is convected downstream with a longer life-time than the bursting passing-period, namely in the frozen-turbulence-like manner. Next, a qualitative model is proposed which attempts to explain the space-time structures of the bursting phenomenon, on the basis of the above anemometry information and other visual information.