A comparative study of the turbulent flow field with and without a pebble cluster in a gravel bed river

摘要

This paper investigates the processes and spatial patterns of the turbulent flow around a naturally formed in situ pebble cluster and the interactions with large-scale coherent flow structures. Instantaneous high-frequency velocities were measured using the simultaneous deployment of four acoustic Doppler velocimeters, with the pebble cluster and after the cluster was removed. Results indicate a twofold increase in turbulent kinetic energy over the sampling area when the pebble cluster is present. Individual vertical component velocity spectra in the wake of the pebble cluster contain a marked peak which decreases in frequency from 1.8 Hz near the cluster to 1.0 Hz farther downstream. This peak is associated with eddies shedding from the shear layer in the lee of the cluster, and it agrees well with the values estimated using a Strouhal number of 0.18. Space-time correlations indicate the presence of coherent depth-scale macroturbulent flow structures. These structures in the intermediate flow region are relatively unaffected by eddies shedding from the pebble cluster. The results suggest the effects of the pebble cluster on the surrounding flow dynamics are quite localized and limited to the near-bed region.