Hydrodynamics of turbulent flows within arrays of circular cylinders

In rivers riparian vegetation can have not only strong interaction with the flow but also with the ecological services of the aquatic system. In wetlands and along riverbanks often tree-like vegetation with quite stiff trunks can be found. The study of the flow in the space between plant stems is highly relevant since it influences the deposition of suspended sediment and the transport of pollutants or nutrients. With her comprehensive experimental research study Mrs Dr. Ana Margarida da Costa Ricardo carried out a novel detailed spatial characterization, at the inter-stem scale, of the turbulent flow within arrays of emergent, rigid and cylindrical stems, randomly placed with constant and varying density in a flume. Mrs Dr. da Costa Ricardo performed detailed measurement of instantaneous velocities with a 2D Particle Image Velocimetry (PIV) and a 3D Laser Doppler Anemometry (LDA). Part of the data was treated with Double-Averaging Methodology (DAM), used as an up-scaling technique to deal with heterogeneous flows. Mrs Dr. da Costa Ricardo could reveal that the drag force per submerged stem length is not correlated directly with the stem areal number density and the stem Reynolds number. The drag force depends on the longitudinal variation of the stem areal number-density. It could be observed that the decrease of the latter is associated to larger flow resistance and that the drag coefficient increases with the relative roughness, revealing an influence of the bed on the definition of the flow structure. The importance of vortex shedding and unsteady separation of the flow on the stems could be highlighted. Finally Mrs Dr. da Costa Ricardo gives a new insight in the dissipation rate of turbulent kinetic energy (TKE). She could contribute significantly to a better understanding of the complex flow within random arrays of rigid and emergent stems, at the inter-stem scale regarding flow resistance, budget of TKE and computation of dissipation rate of TKE.

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