Interactions between aquatic plants and turbulent flow: a field study using stereoscopic PIV

A stereoscopic particle image velocimetry (PIV) system for use in shallow (${\sim }$0.5 m deep) rivers was developed and deployed in the Urie River, Scotland, to study the interactions between turbulent flow and a Ranunculus penicillatus plant patch in its native environment. Statistical moments of the velocity field were calculated utilizing a new method of reducing the contribution of measurement noise, based on the measurement redundancy inherent in the stereoscopic PIV method. Reynolds normal and shear stresses, their budget terms, and higher-order moments of the velocity probability distribution in the wake of the plant patch were found to be dominated by the presence of a free shear layer induced by the plant drag. Plant motion, estimated from the PIV images, was characterized by travelling waves that propagate along the plant with a velocity similar to the eddy convection velocity, suggesting a direct coupling between turbulence and the plant motion. The characteristic frequency of the plant velocity fluctuations (${\sim }$1 Hz) may suggest that the plant motion is dominated by large eddies with scale similar to the flow depth or plant length. Plant and fluid velocity fluctuations were, in contrast, found to be strongly correlated only over a narrow (${\sim }$30 mm) elevation range above the top of the plant, supporting a contribution of the shear layer turbulence to the plant motion. Many aspects of flow–aquatic plant interactions remain to be clarified, and the newly developed stereoscopic field PIV system should prove valuable in future studies.