Turbulent flow within a random array of rigid, emergent stems : Laboratory characterization of the drag coefficient

Within the bioengineering framework, designing a non-erodible channel is a complex fluid dynamics problem as it involves knowing the drag exerted on the boundary, the drag exerted on the plant stems and the overall friction slope. Most of the existing design criteria employ resistance formulas such as Manning’s, calibrated ad hoc. Moving toward physically based design criteria, progresses have been made in the characterization of 3D flows over irregular boundaries, mainly due to the application of Double-Averaging methods (D-AM), which are a particular form of upscaling. Such methods are especially pertinent for the characterization of the flow within and in the near vicinity of plant canopies.This work is aimed at the determination of the drag coefficient associated to rigid emergent stems in turbulent flows. Specific objectives are 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, conditions similar to those found in nature were reproduced in laboratory facilities, using Particle Image Velocimetry (PIV).Since actual wetlands exhibit patchiness and spatial variability in stem density, the experiments featured a periodic distribution of stem densities with minimum and maximum values of 400 and 1600 stems/m2, respectively. The treatment of the data was done with the Double-Averaging methodology (D-AM), to account for the great spatial variability of the flow. The results reveal that the contribution of form-induced stresses, namely longitudinal and shear stresses is of the order of magnitude of the contribution of 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.