An Experimental Investigation Of Drag And The Turbulent Flow Structure In Simulated And Real Aquatic Vegetation.

摘要

The presence of aquatic vegetation in streams generates coherent structures at several length scales, that depend on the properties of both the vegetation and the flow. Stem- and leaf-scale wakes are generated as water moves within the canopy and the drag discontinuity at the top of the plants creates a free shear layerlike flow with coherent vortices that penetrate within the vegetation. Models to estimate velocity, turbulence, mixing rates, dispersion, and residence time within these complex, vegetated flows, require knowledge of the force exerted by the plants, often represented in terms of a drag coefficient, Cd , and yet its value is often left as a calibration parameter, to match numerical models against laboratory and field data. We present a laboratory, non-intrusive, drag measuring device. The drag plate is tested on two well documented cases: uniform flow over a flat plate, and flow around a rigid cylinder. The successful performance of the device proves it suitable for direct measurements of drag on more complex, single or multiple, rigid or flexible elements, which makes it an ideal device for studies on vegetated flow, natural rough-bed boundary layers, and coastal structures. We use the drag plate, coupled with quantitative imaging techniques, to capture the velocity field and obstructed frontal areas associated with it, and we generate an extensive data set for flow through submerged and emergent arrays of rigid cylinders, as well as submersed and emergent canopies of live, flexible stems of Eurasian watermilfoil (Myriophyllum spicatum ). Direct measurements of drag in flow through aquatic vegetation are still rare, since most research groups often estimate its value using a simplified momentum equation, which does not necessarily hold for all scenarios. Our direct approach allows us to compare those estimates against actual measurements, and to identify sources of errors in the estimated values. We use the measured values of drag in rigid cylinders, to obtain fitting parameters to predict Cd in canopies of live, flexible stems as a function of solid volume fraction, [phi], and a diameter based Reynolds number, Red = Ud/[nu] . For live stems, an effective diameter is proposed as the characteristic length scale, calculated from values of the volumetric frontal area, a (obstructed frontal area per unit volume, [L[-]1 ]), and the canopy density n (number of stems per unit horizontal area [L[-]2 ]), as de = a/n. The predicted values of Cd , and the newly introduced length scale, de , successfully perform at estimating the total drag, and balancing both momentum and turbulent kinetic energy budgets.