Experimental Study On Coherent Structure In Open-channel Flow

Open-channel flows are an important class of boundary-layer flows confined to an artificial or natural channel form. Coherent structures in these flows play a key role in turbulence generation and dissipation, energy transportation, sediment suspension, pollutant dispersion, and drag reduction. Investigating coherent structures in open-channel flows is of great significance to both turbulence theory and engineering practice.This paper presents qualitative and quantitative experimental results of coherent structures in steady and uniform flows based on long-duration and high-frequency measurements conducted in a hydraulic flume system.An experimental system composed of a hydraulic flume and PIV was developed to enable automatic, real-time flow control and measurement as well as high-speed, large-capacity image acquisition and velocity calculation. The system's stability and reliability were verified through comparing measured turbulence characteristics (time-average velocity, Reynolds stress and turbulence intensity) in steady uniform flows under various conditions with widely acknowledged existing results.Analysis of the measured high-frequency velocity data shows that to achieve the same level of measurement accuracy, the required sampling frequencies as well as the sample durations increase progressively for evaluating mean velocity, turbulence intensity and Reynolds stress. The depth-averaged deviation from the benchmark data was quantified for various sampling frequencies and sample durations, and the required sample durations at three typical points along the flow depth were determined corresponding to different measurement accuracies (1% and 5%) and sampling frequencies(15Hz and 200Hz). These results provide reference data for the PIV community to better configure PIV systems and select measurement parameters.Description of the morphological characteristics of transverse vortex in continuous flow fields was presented, and based on which three modes of vortex structure were identified. It is found that transverse vortex and fluctuating velocities around them in the second-fourth quadrants often co-exist in the streamwise wall-normal plane and form coherent structures with identifiable structural characteristics. Explanation of the morphological and structural characteristics of vortex was given by analyzing spatial correlation of velocity and the distribution of Reynolds stress and turbulent energy in the four quadrants.An efficient algorithm to extract information on the vortex structure's position, rotation, characteristic scale and flatness has been developed which includes normalizing both velocity components, reconstructing the Mother wavelet, and determining the vortex circle. The novel algorithm for vortex extraction enables a more integrated description of the vortex structure compared to other existing algorithms that use circles to generalize vortex structures in the flow field and relate wavelet scales directly to actual vortex sizes. Based on analysis of the extracted vortex structures, information of occurrence frequency and their average characteristic scales of vortex was provided, and the influence of slope on relevant variables was also given. The relationship between the vortex structures and turbulence characteristics has also been analyzed.