Swirling strength, λ_ci, is an effective vortex indicator in wall turbulence, and existing vortex extraction methods introduce normalisation of λ_ci with its root mean square as the basis for defining a universal threshold. This study presents an improved criterion by normalising the vortex indicator for prograde and retrograde vortices, respectively, with its conditional mean. Finally, a single, universal threshold is selected for extracting prograde and retrograde vortices simultaneously. The improved method was compared with the previous method based on open channel flow data measured with particle image velocimetry. Results show that while the two methods are basically equivalent in extracting prograde vortices, the improved method extracts a higher fraction of retrograde vortices. The approach of separately normalising vortex indicators associated with prograde and retrograde vortices can also be applied to other vortex extraction criteria.

The dynamic importance of spanwise vorticity and vortex filaments has been assessed in steady, uniform open-channel flows by means of particle image velocimetry. By expressing the net force due to Reynolds’ turbulent shear stress, , in terms of two velocity–vorticity correlations, and , the results show that both spanwise vorticity and the portion of it that is due to spanwise filaments make important contributions to the net force and hence the shape of the mean flow profile. Using the swirling strength to identify spanwise vortex filaments, it is found that they account for about 45% of , the remainder coming from non-filamentary spanwise vorticity, i.e. shear. The mechanism underlying this contribution is the movement of vortex filaments away from the wall. The contribution of spanwise vortex filaments to the Reynolds stress is small because they occupy a small fraction of the flow. The contribution of the induced motion of the spanwise vortex filaments is significant.

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.

Time-resolved particle image velocimetry measurements with sufficient spatial resolutions combined with coherence spectrum analysis were adopted to investigate the geometric and kinematic features of wall-attached motions in open channel flows. Results indicate that the diagnosed streamwise wavelength of wall-attached motions λ_xWA based on a given near zero coherence spectrum threshold exhibits a constant streamwise wavelength/wall-normal distance ratio accompanied with a roughly constant inclination angle within y⁺ > ∼100 and y/h ≤ 0.7 [y⁺ is the inner-scale normalized distance to the wall y with y⁺ = y/(ν/u_τ) (ν is the kinematic viscosity and u_τ is the friction velocity) and h is the water depth], meaning that they are geometrically self-similar. However, in the free-surface region, where y/h > 0.7, the diagnosed wall-attached motions are non-self-similar with λ_xWA increasing more dramatically and reaching up to ∼20h at the free surface, which is comparable to the typical scales of very-large-scale motions therein. The wall-attached motions are demonstrated to be both energetic and stress active. Within y/h < 0.7, the wall-attached motions with the streamwise wavelength greater than λ_xWA (including both the self-similar and non-self-similar wavelength range portions) carry more than 40% of the streamwise turbulent kinetic energy (TKE) and 30% of Reynolds shear stress. Beyond y/h = 0.7, where the self-similar portions vanish, all the wall-attached motions are non-self-similar wall-attached motions, which themselves still maintain considerable strength even at the free surface and contribute to 25% of the streamwise TKE and 10% of the Reynolds shear stress therein.