The present paper aims to gain deeper insight into the evolution of secondary flows in open channel bend. A U-shaped open channel with long straight inflow/outflow reaches was used for experiments. Efforts were made to precisely specify flow conditions and to achieve high precision measurement of quasi-three-dimensional velocities with a multi-pass, two-dimensional PIV (Particle Image Velocimetry) method. The experimental results show that the flow begins to redistribute before entering the bend and it takes a long distance to re-establish to uniform conditions after exiting the bend. Complex secondary flow patterns were found to be present in the bend, as well as in the straight inflow and outflow reaches. A “self-breaking” (process was identified, which correlates stream-wise velocity with the intensity of flow circulation.

Eigenvalues of the 3D critical point equation (∇u)ν = λν are normally computed numerically. In the letter, we present analytic solutions for 3D swirling strength in both compressible and incompressible flows. The solutions expose functional dependencies that cannot be seen in numerical solutions. To illustrate, we study the difference between using fluctuating and total velocity gradient tensors for vortex identification. Results show that mean shear influences vortex detection and that distortion can occur, depending on the strength of mean shear relative to the vorticity at the vortex center.

Swirling strength is an effective vortex indicator in wall turbulence, and it can be determined based on either two-dimensional (2D) or three-dimensional (3D) velocity fields, written as λ_ci2D and λ_ci3D, respectively. A comparison between λ_ci2D and λ_ci3D has been made in this paper in sliced XY, YZ, and XZ planes by using 3D DNS data of channel flow. The magnitude of λ_ci2D in three orthogonal planes differs in the inner region, but the difference tends to diminish in the outer flow. The magnitude of λ_ci3D exceeds each λ_ci2D, and the square of λ_ci3D is greater than the summation of squares of three λ_ci2D. Extraction with λ_ci2D in XY, YZ, and XZ planes yields different population densities and vortex sizes, i.e., in XZ plane, the vortices display the largest population density and the smallest size, and in XY and YZ planes the vortices are similar in size but fewer vortices are extracted in the XY plane in the inner layer. Vortex size increases inversely with the threshold used for growing the vortex region from background turbulence. When identical thresholds are used, the λ_ci3D approach leads to a slightly smaller population density and a greater vortex radius than the λ_ci2D approach. A threshold of 0.8 for the λ_ci3D approach is approximately equivalent to a threshold of 1.5 for the λ_ci2D approach.

This study focused on scour beneath and adjacent to submarine pipelines with spoilers on a cohesive seabed. Field data about the seabed bathymetry and seabed subbottom imagery along the pipeline length direction in the Hangzhou Bay, China were collected between 2005 and 2016 for analysis. Results showed that during the last 12 years, self-buried pipelines increased from 40 to 90%; while distributions of scour pits changed randomly with time. Statistical parameters (depth and length of free span) of the scour pits beneath the pipeline in this study were roughly consistent with the values published in previous studies. However, scour adjacent to pipelines was different from that beneath pipelines in that it was much larger, and the formation of it was related to the erosion of the silt loam of the seabed. Three factors (decreased sediment supply, seabed scouring, and installation of spoilers), which probably led to the scouring, were investigated. Sediment supply to the Hangzhou Bay was reduced remarkably during the last 30 years, and the decrease of sediment load in the Yangtze River was the primary cause. In the Hangzhou Bay, the seabed near deep channels was continuously eroded, and the extent of erosion near the pipelines was the largest. Spoilers on pipelines could increase the degree of local turbulence and stimulate the suspension and transportation of sediment. Under the measured spring tidal current condition, the dimensionless scour depth calculated by Goncharov’s equation was relatively large; for example, the 273.1-mm (10.75-in.) diameter pipeline as an example, it was in the range of 0.33–2.49. According to the easily erodible property of silt loam, a hypothesis about the random distribution of scour pits was presented.

This study focuses on detecting trends in annual runoff volume and sediment load in the Yangtze river-lake system. Times series of annual runoff volume and sediment load at 19 hydrological gauging stations for the period 1956–2013 were collected. Based on the Mann-Kendall test at the 1% significance level, annual sediment loads in the Yangtze River, the Dongting Lake and the Poyang Lake were detected with significantly descending trends. The power spectrum estimation indicated predominant oscillations with periods of 8 and 20 years are embedded in the runoff volume series, probably related to the El Niño Southern Oscillation (2–7 years) and Pacific Decadal Oscillation (20–30 years). Based on dominant components (capturing more than roughly 90% total energy) extracted by the proper orthogonal decomposition method, total change ratios of runoff volume and sediment load during the last 58 years were evaluated. For sediment load, the mean CRT value in the Yangtze River is about −65%, and those in the Dongting Lake and the Poyang Lake are −92.2% and −87.9% respectively. Particularly, the CRT value of the sediment load in the channel inflow of the Dongting Lake is even −99.7%. The Three Gorges Dam has intercepted a large amount of sediment load and decreased the sediment load downstream.