Mean Flow and Turbulence in a Laboratory Channel With Simulated Vegetation
Much is unknown about the flow structure and turbulence characteristics in open channels with vegetative canopies. All models of open-channel flow through and above vegetative canopies require a quantitative measure of the ability of the plants to absorb momentum by form drag. This ability is commonly characterized by a drag coefficient. The present work experimentally investigates the flow structure and determines drag coefficients in a channel with simulated vegetation under uniform flow conditions. Vegetation is simulated by rigid and flexible cylinders placed in a laboratory flume. An acoustic Doppler velocimeter is employed to measure velocity and turbulence characteristics in and above the cylinder canopy and a new procedure is developed and used in the computation of drag coefficients. This procedure allows for the first measurements of the vertical profile of the vegetation-induced drag coefficient in an open- channel flow. Results for flow through rigid cylinders show that the drag coefficient is not constant in the vertical, as many models have assumed, but instead, reaches a maximum at about one-third of the canopy height. For flow through flexible cylinders, the shape of the drag coefficient profile is dependent on the amount of cylinder deformation in the channel and may take on one of two shapes: either a shape similar to that for flow through rigid cylinders when these are slightly deflected, or a shape which decreases with distance from the bed when the cylinders are highly deflected. Bulk drag coefficients and a shape factor are defined and computed and the effects of channel and flow parameters on the magnitude of these values are investigated. Measured drag coefficients are in good agreement with previously estimated values. In an open channel lined with rigid cylinders, the bulk drag coefficient is 1.13 +/- 0.2 and is not dependent on any of the flow parameters. In the presence of flexible cylinders, the bulk drag coefficient is significantly reduced when the cylinders become highly deflected. Primary investigators: Chad Dunn, Fabián López, and Marcelo H. García.