ABSTRACT: New insights into the effects of cluster bedforms on flow structure

CGRG Bibliography of Canadian Geomorphology
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Recent work on the effects of pebble clusters on the flow has shown that the turbulent structure developed in the lee of the particles is highly complex, intermittent and dynamic. It is clear, however, that shedding motions from the lee side of pebble clusters is a key mechanism for understanding the strong momentum exchange in flows over gravel beds. This mechanism has yet to be fully characterised at high Reynolds number flows typical of natural gravel-bed rivers. Moreover, our current understanding of shedding mechanisms does not incorporate recent discoveries on the presence of large-scale flow structures in the form of high- and low-speed wedges occupying the entire flow depth. This paper investigates the influence of these structures on the nature of shedding associated with pebble clusters. We have developed a methodology that allows to visualise the shedding motion downstream from an obstacle and to register the passage of the large-scale flow structures in a gravel-bed river. An underwater camera was used to obtain images of fluid motion in the lee side of the cluster while three electromagnetic current meters measured streamwise and vertical velocity fluctuations along a vertical profile downstream from the obstacle. A white tracer was injected in the region of flow separation to detect the development of flow structures shed into the ambient flow. The velocity fluctuations allowed to identify the passage of large-scale flow structures. Despite the high Reynolds number of the flow (150 000), we have obtained clear images which reveal the presence of a shedding mechanism initiated in the recirculating region. Flow returning towards the cluster is intermittently pushed upward against the particles forming the cluster. This vertical movement interacts with the accelerated flow at the top of the cluster and it initiates an instability that will propagate into the flow. The combined analysis of video sequences and velocity time series suggests a close relationship between large-scale flow structures and the onset and nature of this mechanism. Based on these results, we propose a model having two modes: when a high-speed edge occurs, the shedding motion develops vertically both toward the water surface and toward the bed as the structures convect downstream; when a low-speed wedge passes, the shedding motion advects mainly toward the surface and it conserves a stronger coherence. These represent new mechanisms for shedding motions in high Reynolds number flows and reveal the role of the structured incoming flow on the development of flow structures in gravel-bed rivers. These results thus highlight the feedback processes between bedforms and the turbulent flow field and are critical in understanding the dynamics of gravel-bed rivers.

Bibliography of Canadian Geomorphology