Turbulent Structure in Steep Open-Channel Flows

Abstract

Velocity measurement with a fiber-optic laser-Doppler anemometer were conducted in steep open-channel flows over smooth and incompletely rough beds. The channel slopes were changed between 1/500 and 1/25 by steps, and as a result, the Froude number changed from 0.5 to 3. The law of the wall and the distributions of turbulence intensity were examined in more detail. The von Kármán constant κ in the log law was verified to be unversally equal to 0.41. On the other hand, the integral constant A in the log law coincided with the usual value of 5.29 in subcritical flows, whereas it decreased with an increase of the bed slope in supercritical flows. This decrease of A was explained by a decrease of the damping factor B in the van Driest's mixing-length formula. The decrease of B was well correlated with an increase of the friction velocity and the roughness. The streamwise turbulence intensity became smaller in the near-wall region than the semi-empirical universal formula, as the friction velocity became larger.