The generation of coherent flow structures in a gravel bed river

Turbulence in rivers is not a simple random field: visualisation and multipoint measurements show it is possible to decompose complex, multi-scaled, quasi-random flow fields into elementary organized structures which posses both spatial and temporal coherence termed either eddies or coherent flow structures (CFS). Quantifying the kinematic (size, scaling, shape, vorticity and energy) and dynamic (origin, stability, growth, genesis into new forms and contribution to averages) characteristics of CFS in gravel-bed rivers are central to improving our understanding of turbulent flow, and the contribution of CFS to shear stress, and hence sediment transport. Much of our uncertainty in understanding CFS over gravel-beds stems from two fundamental shortcomings: i) previous studies have used Reynolds decomposition of Eulerian time series to quantitatively determine processes, which may be interpolated to examine the whole flow field, rather than studying the complete instantaneous flow field; and ii) whole flow field visualization provides a qualitative understanding, but very little quantitative information. Here, we demonstrate a new experimental methodology to quantify simultaneously both the kinematic and dynamic characteristics of coherent flow structures based upon combined planar Laser Induced Fluorescence and Particle Imaging Velocimetry (pLIF-PIV) over a gravel surface for a range of Reynolds numbers. Snapshot POD is applied to the PIV results to determine the initiation of the structures. Initial results agree with the model of Falco (1991) that divides the outer flow into two distinct types of motion; large-scale motions, which are clearly being detected by the pLIF, and smaller ‘typical’ eddies, which the PIV is detecting within these large-scale structures. These results also conform with classical boundary layer hydraulics, where the dominant motions of flow have been shown to be the large-scale regions of momentum deficit that are elongated in the streamwise direction, following ‘roller-type’ structures which produce the majority of the Reynolds stresses.