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

Abstract

A stereoscopic particle image velocimetry (PIV) system for use in shallow (∼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 (∼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 (∼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.

The impact of macroalgae on mean and turbulent flow fields

“…4). In typical mixing layers and flows over aquatic canopies, it has been observed that sweeps (Q4 events) dominate at the top of a canopy, whilst ejections (Q2) dominate above the canopy [6,15], and we expect that this is also true here. The spatial patterns of , , , and imply that the flows around the algae at our field site are of intermediate relative submergence [16].…”

“…A horizontal shear layer with a width comparable to the patch width [13] develops between the water-surface and a specific depth within the canopy [3], while a near-vertical shear layer may also form at the patch edge [14]. Coherent vortices in these shear layers may dominate the turbulent flux of momentum across the patch edge [14], but the three-dimensionality of the wake and high background turbulence levels should disrupt periodic vortex formation [15]. At the downstream edge of a patch, velocity profiles slowly return to the undisturbed upstream condition.…”

“…11,13,15], measurements of hydraulic variables have generally been limited to time-averaged at-a-point measurements that aim to approximate the depth-mean velocity. This is problematic because in spatially heterogeneous flows, point measurements are dependent upon the sampling location [16,17].…”

Very-large-scale motions in rough-bed open-channel flow

“…Three-component vector fields are reconstructed from the two-component vector fields obtained for each camera using the local view angles obtained from a camera calibration procedure. The four-camera PIV system used here permits two independent estimates of each velocity component, which we use when calculating bulk statistics, correlation functions and spectra to reduce the contribution of measurement noise (for further details see Cameron et al 2013). …”

Pressure forces on sediment particles in turbulent open-channel flow: a laboratory study

“…The Aberdeen PIV system has been developed in-house at the University of Aberdeen by Dr Stuart Cameron (Cameron 2011;Cameron et al 2013). The system comprises four cameras (DALSA 4M60, complementary metal oxide semiconductor sensor, 2352 pixel × 1728 pixel resolution at 62 frames per second (f.p.s.))…”