Influence of Hydropower Propellers on Open-Channel Flow

A streamflow velocity field affected by hydropower prototype propellers was investigated using an acoustic Doppler current profiler (ADCP) and an acoustic Doppler velocimeter (ADV) at an open channel flowing in the Adige River floodplain, Italy. Characterizing turbulent coherent structures using the ADCP is difficult because of (1) instrumental low acquisition frequency; (2) the monostatic configuration, which relies on a layer homogeneity assumption; and (3) the lack of stable deployment of the instrument from a drifting platform. These ADCP features frequently result in velocity vector contamination at the scale of the diverging beams' distance. ADV pointwise measurement overcomes these limitations by relying on a higher acquisition frequency and a bistatic principle, but its application usually is limited to shallow flows. Coupling of mono- and bistatic measurements provided velocity field maps which elucidated the coherent structures laying in the cross-section plane, and velocity time series that corroborated information close to boundaries at 10-Hz acquisition frequency. Recirculating flow structures triggered by the shape of the Biffis Channel were exacerbated in the wake of operating propellers, although the observed maximal amplitude of velocity oscillation was unchanged. The difference among concurrent estimations of vertical velocity provided by the ADCP's redundant transducer (i.e., the error velocity) was on the order of the velocity standard deviation of time and lower than the assessed velocity secondary components, which validated the observed flow structures.