Anisotropy in the Turbulent Flow through Random and Emergent Rigid Vegetation on Rough Beds

In this study, we explore for the first time the turbulence anisotropy of flows through random and emergent rigid vegetation on rough beds, using the anisotropy invariant maps (AIMs). We present the results of an experimental campaign, based on Acoustic Doppler Velocimeter (ADV) measures, varying the bed sediment size (coarse sand, fine gravel, and coarse gravel), under the same hydraulic conditions. The evolution of the stress ellipsoid formed by the Reynolds stresses is discussed, together with the analysis of the anisotropic invariant function. We demonstrate that vegetation distribution is of paramount importance in the turbulence anisotropy evolution along the water depth. While in the case of a regular pattern the bed roughness is the key parameter in determining the shape of the stress ellipsoid in the near-bed region, in a random vegetation distribution this assumption is no longer valid. However, by increasing the bed roughness in such kind of arrangement, its effect on the turbulence anisotropy are visible in specific areas around the vegetation stem. This is confirmed by the analysis of the anisotropic invariant function calculated for different sections close to the stem itself.