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.

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.

Using Direct Numerical Simulation (DNS) data of a fully developed turbulent channel flow, the probabilities and fraction contributions of vortical/non-vortical structures to velocity–vorticity correlations and the net force were assessed. Flow structures are classified into four groups, i.e., filamentary vortices (FV), non-filamentary vortices (NFV), non-swirling vorticity structures (NSVS), and irrotational structures (IS). Compared with the other three structures, FV occupy a small region of the domain but contribute the most to velocity–vorticity correlations and the net force. Changing the threshold for vortex extraction reveals that the contribution of FV is concentrated in a small region. Owing to the lack of one velocity component in two-dimensional (2D) in-plane velocity fields, fewer vortical structures are detected and a large proportion of three-dimensional (3D) NFV are regarded as 2D NSVS by using the 2D swirling strength method. However, 2D Particle Image Velocimetry (PIV) can still capture m...

Flows in river bend present complex patterns that play an important role for river regulation and control. Experiments were conducted to investigate the evolution of the main flow in a U-shape flume (both of the inflow and outflow segments are 11m long). A Particle Image Velocity system (PIV) was used to measure the streamwise-vertical velocity fields at seven sections along the flume. Three cases (flow depth=4, 5, and 6 cm respectively) have been studied. Three major features of the flow have been revealed. Firstly, the maximum velocity occurs near the inner-side of the entrance of the bend, and it turns to the external side later and remains so all the way to the bend end. The exact position of the deflecting position is logarithmically related to the flume width B, centerline radius R _C and water depth H. Secondly, the flow shows dramatic change at around the 1/5 of the bend, where the deflecting position and the maximum velocity occur near the inner bank. Thirdly, with the combined effect of water surface slope, gravity and centrifugal force, the axis of the maximum velocity goes down along the bend and this trend becomes stronger with the increase of H. Results in this study provide real data for verifying mathematical models and for reference in practical engineering.