Professor Vladimir Nikora is the Sixth Century Chair in Environmental Fluid Mechanics at the School of Engineering, University of Aberdeen, UK. Before coming to Scotland in February 2006, he was Principal Scientist and Manager of the Hydrodynamics Group at the National Institute of Water and Atmospheric Research in New Zealand that he joined in 1995. Dr. Nikora’s main research accomplishments relate to improved understanding of stream turbulence, development and applications of the double-averaging methodology for describing and predicting rough-bed turbulent flows, new sediment dynamics concepts related to erosion and transport of cohesive and non-cohesive sediments, new concepts of flow-biota interactions including those for periphyton, mosses, vascular plants, mussels, and fish communities. Vladimir Nikora is Fellow of the Royal Society of Edinburgh, Editor of IAHR Journal of Hydraulic Research, and a recipient of 2010 Hunter Rouse Hydraulic Engineering Award of the American Society of Civil Engineers that “recognizes outstanding contributions to hydraulics and waterways”.

Turbulence in open-channel flows: recent advances and implications for sediment transport, hydraulic resistance and flow-biota interactions

Vladimir Nikora, School of Engineering, University of Aberdeen, UK   

Open-channel flow (OCF, e.g., streams and rivers) occupies a special place in a family of turbulent flows. It may exhibit a set of properties that make this flow unique and exciting, i.e.: (1) flow boundaries (sedimentary bed and free surface) can be highly ‘deformable’ and dynamic, constantly changing in response to varying flow; (2) channel surface may exhibit a complex hierarchical structure covering scales spanning 7-10 orders of magnitude; (3) aquatic biota may significantly influence flow and its boundaries; and (4) flow submergence (i.e., ratio of the flow depth to the height of prevailing roughness elements such as sediment particles, their clusters, bedforms, or benthic organisms) may be as low as 1-4.

This talk will highlight the key features of OCF turbulence and briefly review similarities with other flow types such as boundary layers, pipes and closed channels. Then, the latest advances in studies of OCF turbulence will be discussed, including turbulence statistics and coherent motions, with particular focus on ‘superstructures’ (or ‘very large scale motions’ up to 40-50 flow depths in length) and their potential relation to OCF secondary flows. This will be followed by consideration of turbulence effects in sediment transport, flow-biota interactions, and hydraulic resistance.

Among other findings, it will be shown that particle entrainments on the channel beds are likely associated with interactions between flow superstructures (i.e., ‘collisions’ of superstructures, ‘meandering’ across the flow, generate regions of a particular velocity pattern leading to the particle entrainment). Effects of aquatic biota on the OCF turbulence will be illustrated using aquatic vegetation as an example (Fig. 1), considering interactions at multiple scales, from the leaf scale to the plant patch mosaic scale comparable to the flow width. Finally, a theoretical approach for quantifying key contributors to the OCF friction factor will be outlined and its applications will be illustrated for fixed-bed and mobile-bed conditions.

Figure 1. Left: deployment of the Aberdeen Field PIV System in the Urie River, Scotland. Right: flow filed around a moving plant Ranununculus where the plant is shown in gray, plant vertical velocity in green, and flow velocity field in yellow. Full details are in Cameron, Nikora, Albayrak, Miler, Stewart, Siniscalchi, Journal of Fluid Mechanics, 2013, 732, 345-372.