A laboratory investigation on the advection of free surface fluctuations in turbulent open-channel flows

The advection characteristics of both water surface fluctuations and turbulence in turbulent open-channel flows were experimentally examined in this paper. We used a simultaneous image measurement method to obtain instantaneous velocity vectors and the corresponding water surface profiles in a vertical cross section of the open-channel. Time series of the water surface profiles indicated that the water surface fluctuations were propagated with a constant advection velocity in each experimental case. The space-time correlation analysis of the water surface fluctuations and turbulence disclosed that the advection velocity of the water surface fluctuations was always smaller than the mean bulk velocity, while that of the turbulence above the near-bed region was the same as the mean velocity, well-known as Taylor's hypothesis of the frozen-turbulence approximation. It was also found that the ratio of the advection velocity of the water surface fluctuations to the mean bulk velocity became smaller as the Froude number decreased.