Comparison of macroturbulence measured using decomposition of PIV, ADV and LSPIV data

Visualization and length-scale estimates are provided for turbulence in open channel flow over a gravel-bed. Results obtained using the digital particle image velocimetry (PIV) followed by post-processing with proper orthogonal decomposition showed higher and lower velocity regions aligned along a stagnation plane extending from the bed at approximately a 20 degrees angle. Periodicity of second and fourth quadrant events equalled 0.20 s and agreed well with the outer variable defined bursting period. Normalized turbulence length scales were measured after performing the decomposition of the turbulent fields collected with various instrumentation techniques. Acoustic Doppler velocimetry and PIV had turbulence length scales equal to the flow depth, and the good agreement highlights the ability of the former to estimate length scales in streams. Large scale PIV provided length scales equal to 1.5 times the flow depth, but care should be used if directly inferring scales of macroturbulence from the free-surface.