Dynamics of shedding motions in response to large-scale flow structures in turbulent gravel-bed rivers
The flow structure in a gravel-bed river is closely related to the presence of protruding clasts and of pebble clusters. It is well known that shedding motions from the lee side of large clasts and clusters are a recurrent process that explains the strong exchanges of momentum in river flows. However, shedding has yet to be fully characterised for high Reynolds number flows such as those found in gravel-bed rivers. Moreover, our current understanding of shedding mechanisms does not include the recent discovery that large-scale flow structures in the form of high- and low-speed wedges occupy the entire flow depth over a gravel-bed river. From two original experiments, this paper investigates the influence of these wedges on the nature of shedding in the lee of a pebble cluster. The interactions between the large-scale wedges and shedding may be a key element for understanding flow organisation at the river reach scale. The first experiment provides an analysis of the space-time correlation of velocity time series obtained downstream from a pebble cluster in a natural river. Two pairs of one-minute time series were sampled. The first series of each pair was located in the region of flow separation downstream from the obstacle whereas the second was located at its crest. Results show that a significant negative correlation occurs with a negative time lag for the downstream velocity component. This reveals that a strong downstream velocity vector at the crest of the obstacle is followed 1 to 4 seconds later by a strong upstream velocity vector in the region of flow separation. The strength of the recirculation motion responds to the velocity fluctuations above the cluster. This is a crucial process in the development of vortex shedding. The second experiment aimed at visualising the shedding motion downstream from an obstacle. An underwater camera was used to obtain images of fluid motion in the lee of a pebble 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 depict the development of flow structures that are shed into the flow. Despite the high Reynolds number of the flow, we have obtained good quality images revealing the presence of different modes of vortex shedding initiated in the region of flow separation. From the velocity records, it was possible to identify the large-scale flow wedges and to show that the type of vortex shedding is controlled by high- and low-speed wedges. Based on these results, we propose a model having two steps: when a high-speed wedge approaches the pebble cluster, the shedding motion develops vertically both towards the water surface and towards the bed as the structures convect downstream; when a low-speed wedge passes, the shedding motion advects mainly towards the surface and it conserves a stronger coherence. This response of the shedding motion to the type of flow wedge is a recurrent and fundamental phenomenon. The results and the model presented herein shed light on the complex nature of vortex shedding in flows at high Reynolds number such as those found in rivers.