Author(s): J. F. Fox; B. J. Belcher
Linked Author(s): James Fox
Keywords: Turbulence structure; Gravel-bed; Velocimetry; Turbulencedecomposition; Spatial field; Time-series
Abstract: Methods are compared for their ability to measure small-scale vortices shed from gravel particles or clusters of gravel particles at the bed and large-scale motion associated with alternating high-velocity and low-velocity regions of fluid that scale with the flow depth for gravel-bed flow experiments. Turbulence data collection and decomposition methods include proper orthogonal decomposition and Galilean decomposition of digital particle image velocimetry (PIV) data, triple decomposition of acoustic Doppler velocimetry (ADV) data, and triple decomposition of large-scale particle image velocimetry (LSPIV) data. Visualization results as well as length and frequency scale measurements are compared for the methods. The decomposed PIV data provide a streamwise-vertical view of smallscale vortices and large-scale motion from which estimates of scales can be made. The ADV and LSPIV methods were limited in their resolution and unable to measure small-scale vortices for the laboratory experiments. Decomposed LSPIV data showed characteristics of large-scale motion measured at the free-surface but deviated from PIV results of periodicity estimates. Decomposed ADV data shows good agreement with PIV time-series data generated using phase locking. Finally, the large and small-scale motions are tied to a structural model of turbulence separation at the bed of the river and the impact of separation upon the flow.
Year: 2009