Measurement of eddy length scales using LSPIV, PIV, and ADV in a rough bed flume

This paper presents measurements of turbulent eddy length scales for flow in a rough bed flume at moderate Froude and Reynolds numbers. Particle image velocimetry (PIV), acoustic Doppler velocimetry (ADV) and large scale particle image velocimetry (LSPIV) datasets are analyzed with turbulent decomposition methods, including Galilean decomposition, proper orthogonal decomposition and triple decomposition, to measure length scales classified here as small-scale eddies, large-scale motion, and very-large-scale phenomena. Small-scale eddies estimated using the PIV dataset with decomposition and statistics ranged from 0.12H to 0.14H, where H is the flow depth. ADV and LSPIV were unable to measure the small-scale eddy length scales due to low data resolution relative to small-scale eddy frequency. Streamwise length scales of the large-scale motion ranged from 1.1H to 1.8H and all three velocimetry methods were able to measure large-scale motion. Very-large-scale phenomena were able to be measured with the ADV data and length scales measured 7.7H. PIV and LSPIV were not able to be used for visualization of the very-large-scale phenomena due to the PIV window size and the weakness of the phenomena at the free-surface. It is expected that the methods used here can be extended to multidisciplinary laboratory studies as well as field studies of turbulent scales.