Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully three-dimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.

Flow over a developed bottom topography in a bend has been investigated experimentally. The measuring section is in the outer-bank half of the cross section at 60° into the bend. Spatial distributions of the mean velocities, turbulent stresses, and mean-flow and turbulent kinetic energy are presented. The cross-sectional motion contains two cells of circulation: besides the classical helical motion (center-region cell), a weaker counterrotating cell (outer-bank cell) is observed in the corner formed by the outer bank and the water surface. The downstream velocity in the outer half-section is higher than the one in straight uniform flow; the core of maximum velocities is found close to the separation between both circulation cells, well below the water surface. The turbulence structure in a bend is different from that in a straight flow, most notably in a reduction of the turbulent activity toward the outer bank. Both the outer-bank cell and reduced turbulent activity have a protective effect on the outer ...