Assessment of Experimental Bias on Laboratory Studies of Vegetated Flows

Abstract:

Measurement of turbulence in vegetated channels helps reveal the physical processes resulting in drag, dispersion and transport in aquatic vegetation canopies. Acoustic Doppler Velocimeter (ADV), Ultrasonic Velocity Profiler (UVP) and Particle Image Velocimetry (PIV) are some of the widely used techniques for measuring flow through aquatic vegetation. While these techniques yield accurate results in sparse vegetation canopies, measuring within dense canopies is still challenging due to the physical scale of the instruments or the minimum clearance required between stems of vegetation to yield significant results. Experimentalists commonly clear a region within the patch and hypothesize that flow measurements within such gap are representative of flow inside the unaltered canopy. The accuracy of such assumption requires qualitative and quantitative assessments of the alteration in local and bulk flow statistics due to the gap. In this study we present an assessment of change in flow statistics upon introduction of gaps of varying dimension in a dense vegetation canopy. We performed 2D PIV to measure flow statistics within the gap and used high-resolution Large Eddy Simulation (LES) of flow within the canopy to quantify the bias introduced by the gap. It is found that flow statistics – velocity and turbulent kinetic energy (k) are significantly biased, and point to the need of finding correction factors for such measurements. We propose a simplified model to account for those effects, which can help assess optimal configurations not only for experimental studies, but also for planting and seedling strategies on vegetation restoration projects.