Author(s): K. Blanckaert; S. J. Mclelland
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Abstract: Simultaneous velocity measurements were taken using Particle Image Velocimetry (PIV) and an Acoustic Doppler Velocity Profiler (ADVP) in a sharp open-channel bend with an immobile gravel bed. The PIV measures 3D velocity vectors in a vertical plane (～40cm x 20cm) at a frequency of 7.5 Hz, whereas the ADVP measures 3D velocity vectors in a vertical profile with a frequency of 31.25 Hz. The paper reports simultaneous measurements with both instruments positioned in the same location. Both instruments resolve accurately spatial structures of the complex mean flow fields characterized by small velocities of the order of 0.01 ms-1, such as the outer-bank secondary flow cell and the secondary flow cell in the zone of flow separation at the inner bank. PIV measurements of the mean velocities are of better quality near the flow boundaries and the spatial distribution of data allows investigation of the temporal behaviour of secondary flow structures. Power spectra and time-series of quasi-instantaneous velocities demonstrate that the ADVP measures turbulence accurately, whilst PIV measurements of turbulence suffer from the lower temporal resolution and the higher noise levels. The results presented in this paper demonstrate that the combined application of PIV and ADVP allows investigation complex 3D flows in greater detail than is possible from a single instrument.
Year: 2009