The capability of acoustic Doppler velocimeters to resolve flow turbulence is analyzed. Acoustic Doppler velocimeter performance curves (APCs) are introduced to define optimal flow and sampling conditions for measuring turbulence. To generate the APCs, a conceptual model is developed which simulates different flow conditions as well as the instrument operation. Different scenarios are simulated using the conceptual model to generate synthetic time series of water velocity and the corresponding sampled signals. Main turbulence statistics of the synthetically generated, sampled, and nonsampled time series are plotted in dimensionless form (APCs). The relative importance of the Doppler noise on the total measured energy is also evaluated for different noise energy levels and flow conditions. The proposed methodology can be used for the design of experimental measurements, as well as for the interpretation of both field and laboratory observations using acoustic Doppler velocimeters.

A high-speed video system was used to study saltation of sand in an open channel flow. High rates of image acquisition provided the information needed to resolve details of the Lagrangian properties of particle motion. Saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length, height, and streamwise particle velocity. Results are compared with available empirical data. Saltation height seems to be independent of particle size when made dimensionless with the particle diameter; however, the dimensionless saltation length appears to increase as the particle size decreases. Particle collision with the bed is also analyzed. Observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision-rebound type, which contradicts previous discussions on the subject. The estimated value of the dynamic friction coefficient is about half that proposed by Bagnold, i...