Effect of different meander curvatures on spatial variation of coherent turbulent flow structure inside ingoing multi-bend river meanders

Abstract

In this paper, the effect of different curvatures on the spatial variation of coherent flow structure inside two physical models with both strongly curved and mild multibend meanders is investigated. Three dimensional flow velocities at three sequential meanders were measured using an Acoustic Doppler Velocity meter (Micro-ADV). Three dimensions of flow velocity are classified into two major classes and eight different bursting events. The contribution probability and transition probability of each zone is calculated from experimental data. The results indicated that the effect of curvature in sequential bends was important particularly for strongly curved bends. The contribution probability of the events for strongly curved meanders with relative curvature (R c /B) of 2.6 were found to be higher than for mild curved meanders with relative curvature (R c /B) of 4.43. The minimum contribution probability was found in external inward interaction event. In addition, analysis of bursting events showed that the highest values of transition probabilities occurred in the stable organizations for both models. The influences of different curvatures on distributions of the Reynolds shear stress, the turbulent kinetic energy, the streamwise velocity and the vertical velocity were also shown to be in good agreement with eroded bed.The above results can be useful for finding meandering patterns inside rivers and also in river training works.

Investigation of turbulence flow and sediment entrainment around a bridge pier

“…Jafari and Keshavarzi (2010) investigated turbulence flow and sediment entrainment over the ripples by using quadrant analysis. Esfahani and Keshavarzi (2011) analyzed the bursting events in meander channels with different curvatures to find the turbulent characteristics of flow and sediment motion in river bends. However, in spite of the importance of coherent structures of turbulence and in particular the bursting events around the bridge pier; their characteristics have not been investigated in detail.…”

Distribution of primary and secondary currents in sine-generated bends

“…However, two-dimensional models cannot sufficiently present the influence of curvature on turbulent characteristics in meanders (Blanckaert and De Vriend, 2005). The distribution of velocity may differ in various bends, and the situation of maximum tangential velocity moves in depth from near-bed to near the water surface by decreasing the curvature (Esfahani and Keshavarzi, 2011). In strongly curved bends with small values of width/depth ratio (B/H), contour lines with the highest tangential velocity are located near the bed (or intermediate region and near the bed) in sharp multi-bends (Blanckaert and Graf, 2001;Blanckaert andDe Vriend, 2004, 2005;Tilston et al, 2009;Esfahani, 2009;Esfahani and Keshavarzi, 2011).…”

“…The flow structure inside the river meanders is complicated and is influenced by various properties and conditions, for example, the curvature and properties of bends (multi-bend or single bend) (Esfahani and Keshavarzi, 2011). Then, the influence of curvature-driven and topography-driven secondary flow on the redistribution of primary flow are analysed separately.…”

“…They reported that velocity components, bed topography and flow structure are affected by changes in the curvature. The contribution to probability of events for strongly curved meanders with relative curvature (R c /B) of 2.6 was found to be higher than for mild curved meanders with relative curvature (R c /B) of 4.43 (Esfahani and Keshavarzi, 2011). The relative curvature of the flume in Termini (2009) is 2.31, representing strongly curved bends.…”

“…The ratio of curvature radius to river width (relative curvature) is a very important factor which plays the main role in morphological changes of river meander (Esfahani and Keshavarzi, 2011). On this basis, river bends can be classified into 2 classes: strongly curved bends with relative curvature smaller than 3, and mild bends which have relative curvature larger than 3 (Leschziner and Rodi, 1979;Rozovskii, 1961).…”