TURBULENT STRUCTURE IN UNSTEADY DEPTH-VARYING OPEN-CHANNEL FLOWS

Turbulence measurements over a smooth wall in unsteady depth-varying o pen-channel flows were conducted by the simultaneous use of a two-comp onent laser Doppler anemometer (LDA) and a water-wave gauge. In the pr esent study, velocity measurements in the viscous sublayer of highly u nsteady flow have been first conducted to evaluate the friction veloci ty independently of the log-law. The von Karman constant kappa was cal culated using this friction velocity. It was proven then that the valu e of kappa was really a constant of 0.41 even in unsteady open-channel flows. Next, statistical structures of mean velocity and turbulence c haracteristics were investigated across the whole flow from the near-w all region up to the free surface. In particular, turbulence measureme nts near the depth-varying water-surface zone of flood flow were condu cted successfully for the first time with the LDA; such measurements h ave been impossible with conventional velocity instruments such as hot -film anemometers. Mean velocity profiles and turbulence characteristi cs were revealed in both the rising and the falling stages of the floo d period.