Streaky structures have been reported in open-channel flows by various investigations in the inner and outer layer. Although the ubiquity of streaks is well recognized, it remains unclear how the streaky structures in the inner and outer layers are related to each other. This paper employs a unified algorithm to identify streaky structures which can be applied in both inner and outer layers with exactly the same subjective threshold. The characteristic scales of streaky structures are quantified, including the width and the spanwise spacing. The results reveal that the spatial scales increase from the near-wall region to the outer layer. The continuous linear growth trend indicates a close relationship between the streaky structures in the inner and outer layers, which can be plausibly explained with the attached-eddy hypothesis.

Low- and high-speed streaks (ejection, Q2, and sweep, Q4, events in quadrant analysis) are significant features of coherent structures in turbulent flow. Streak formation is closely related to turbulent structures in several vortex models, such as attached eddy models, streamwise vortex analysis models, and hairpin vortex models, which are all standard models. Vortex models are complex, whereby the relationships among the different vortex models are unclear; thus, further studies are still needed to complete our understanding of vortices. In this study, 30 sets of direct numerical simulation (DNS) data were obtained to analyze the mechanics of the formation of coherent structures. Image processing techniques and statistical analysis were used to identify and quantify streak characteristics. We used a method of vortex recognition to extract spanwise vortices in the x–z plane. Analysis of the interactions among coherent structures showed that the three standard vortex models all gave reasonably close results. The attached eddy vortex model provides a good explanation of the linear dimensions of streaky structures with respect to the water depth and Q2 and Q4 events, whereby it can be augmented to form the quasi-streamwise vortex model. The legs of a hairpin vortex envelop low-speed streaky structures and so move in the streamwise direction; lower-velocity vortex legs also gradually accumulate into a streamwise vortex. Statistical analysis allowed us to combine our present results with some previous research results to propose a mechanism for the formation of streaky structures. This study provides a deeper understanding of the interrelationships among different vortex models.

The interphase interaction between water flow and sediment and particle collision in sediment laden flow will modulate the flow turbulence. Due to the complexity of suspended sediment movement, the mechanism of water-sediment interaction has always been a difficult point in the study, especially the modulation law of water-sediment interaction on flow turbulence has not reached a consistent conclusion. It is of great significance for the study of sediment laden flow to optimize the construction of the numerical model of water and sediment. In this study, a Euler solid–liquid two-phase flow model was used to investigate the effects of drag force, density gradient, and particle collisions generated by natural sand and plastic sand on flow characteristics under the condition of different sediment concentrations for the case of equilibrium suspended-load transport, so as to determine the degree of influence of various factors in the numerical simulation process on the turbulent flow properties. Results showed that the presence of sediment particles changes the flow velocity, sediment concentration distribution, and turbulent energy distribution, and that such effects strengthen with increase in sediment concentration. The effects of drag force and particle collisions on the resistance coefficient and on flow velocity are dominant. The drag force tends to reduce the resistance coefficient and increase flow velocity, whereas particle collisions produce the opposite effect. The density gradient and particle collisions are the dominant factors affecting the turbulent diffusion coefficient of the suspended load and the vertical distribution of the sediment concentration. However, they produce opposite effects that partially cancel each other. With increase in sediment concentration, the effect of sediment particles on the turbulence of sediment-laden flow increases; the drag force and density gradient inhibit turbulence, and particle collisions promote turbulence.

Complex interactions between turbulence and sediment movement around bridge piers result in bridge damage. However, the scouring mechanism remains ambiguous owing to insufficient quantitative experimental analysis pertaining to scouring topographic characteristics and their relationships with turbulent flow. Hence, an experiment is performed in this study to clarify the relationships between turbulent vortex structures and scour topography. First, we measure the two-dimensional flow fields around a bridge pier using particle image velocimetry systems and then establish a three-dimensional scour topography using the structure-from-motion technique. Subsequently, according to the unified coordinate system, we perform an innovative quantitative analysis of the scouring topography and the distributions of the shear force and horseshoe vortex around the pier. The results show the maximum depth of the scour hole increases linearly with the flow intensity. For a single scouring hole, both the cross-sectional area and volume of the scour hole vary parabolically with the height from the pit bottom. The coupling of the flow and bed topography forms the maximum scour hole via shear stress, and the large streamwise vortices on both sides of the bridge pier result in the formation of long shallow grooves on both sides of the sand dune downstream.