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Laboratory Characterization of Turbulent Flow Within a Random Array of Rigid, Emergent Stems

Author(s): Ana M. Ricardo; Mario J. Franca; Rui M. L. Ferreira

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

Keywords: No Keywords

Abstract: Emergent vegetation covering floodplains and wetlands has an important role in fluvial ecosystems. The main characteristic of the flow through arrays of rigid stems is the great spatial variability that exists in the inter-stem space. Thus, to understand flow resistance phenomena in vegetated streams is necessary to resort to a formulation of the momentum and mass conservation equations that take explicitly into account flow variability. This work is aimed at i) the detailed characterization and quantification of the flow within vegetated areas susceptible to be simulated by dense arrays of vertical emergent stems and ii) the independent quantification of the forces, per unit bed area, acting on the stems and on the bed boundary. Both objectives concur for a better knowledge of the flow resistance in wetlands and vegetated areas in general. To meet the proposed objectives, two experimental tests were carried out with different densities of stems and a third test with no stems was performed to assess bed roughness. The measurements of velocity fields were made using Particle Image Velocimetry (PIV), a non-intrusive technique. The data treatment was done with the Double Averaged methodology (DAM) to account for the great spatial variability of the flow. It was developed a theoretical model for the calculation of the drag force exerted on the stems and on the bed boundary. The results reveal that the contribution of form-induced stresses is of the order of magnitude that Reynolds stresses. Hence, in general, this stresses cannot be neglected. The analysis of form-induced stresses helps to explain the increase of the drag coefficient when the stem density increases.


Year: 2010

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