Secondary flow in sharp open-channel bends

Abstract

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 threedimensional 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.

A fluid-mechanics based classification scheme for surface transient storage in riverine environments: quantitatively separating surface from hyporheic transient storage

“…Jamieson et al (2010) showed that small outer bank vortices are centered above scour holes. Increased transverse Reynolds stresses and turbulent kinetic energy are the mechanisms by which curvature-induced secondary flow and small outer bank gyres scour and erode the outer bank (Blanckaert and Graf, 2001;Blanckaert and de Vriend, 2004;van Balen et al, 2009).…”

Experimental Study on Erosion and Sedimentation Patterns Downstream of a W-weir in a Sinusoidal Mild Bend

“…The water level decreases at the inner bank of a bend in the river, which creates a rotational flow. A secondary flow is therefore formed, with the faster fluid moving to the outside of the bend and the slower fluid to the inside of the bend [2]. Natural meandering rivers are quite unstable due to bank erosion downstream of concave banks.…”

Wall-Roughness Effects on Flow and Scouring in Curved Channels with Gravel Beds

“…The flow structure and related scouring effects in bends have been studied empirically (i.e., Blanckaert and De Vriend 2004;Blanckaert and Graf 2004;Abad and Garcia 2009;Sukhodolov 2012), numerically (i.e., van Balen et al 2009;Zeng et al 2008), and theoretically (i.e., Odgaard and Bergs 1988;Johannesson and Parker 1989;Blanckaert et al 2013). Fargue (1868) was probably the first to publish a formula for scour in river bends, based on field observation.…”