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Experimental Characterization of Drag on Arrays of Rough Cylinders

Author(s): A. M. Ricardo; M. Sanches; P. Martinho; M. J. Franca; R. M. L. Ferreira

Linked Author(s): Rui M.L. Ferreira, Ana Margarida Ricardo, Mário Franca

Keywords: Rigid vegetation; Patchiness; PIV; Double-Averaging Methodology; Drag coefficient

Abstract: Emergent vegetation, covering floodplains and wetlands, has an important role in fluvial ecosystems being able to control the fluxes of sediment, nutrients and contaminants. Proper understanding of flow resistance processes is crucial on the development of reliable tools for designing non-erodible channels. The main objective of this study is the quantification of the forces, per unit bed area, acting on the stems and the respective coefficient. Particular goals include a characterization and quantification of the flow within vegetated areas susceptible to be simulated by dense arrays of vertical emergent stems and a discussion of the dependence of the drag coefficient on parameters that characterize this kind of flows. To achieve the goals, experimental tests simulating rigid and emergent vegetation condition with varying and constant density of stems were performed. The data acquisition consisted mainly in 2D instantaneous velocities maps measured with a Particle Image Velocimetry system (PIV) and the data treatment was performed with the Double-Averaging methodology (DAM). The flow characterization shows an important contribution of form-induced stresses, namely longitudinal and shear stresses which are of the order of magnitude of Reynolds stresses. Hence, in general, these stresses should not be neglected within the balance for the flow resistance. The results show that the drag force seems uncorrelated with the density of stems but impacted by the longitudinal variation of this. For the range of investigated Re p and m, C D seems uncorrelated with Re p. C D is larger for lower relative flow depths, revealing the influence of the channel.


Year: 2014

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