Hydrodynamics inside Marine Animal Forests: Investigating the mean flow and turbulence using laboratory scale models and PIV measurements

Marine Animal Forests (MAFs) are underwater habitats formed by sessile benthic organisms, whose three-dimensional structure favors the presence of other species resulting in high biodiversity (Rossi et al., 2017). Marine Animal Forests have been severely impacted by recent marine heat waves and industrial fishing activity in the Mediterranean Sea. Conservation planning targeting MAFs requires the definition of a minimal forest unit. In the present study, we aim to define the minimal functional unit of a mesophotic gorgonian forest based on its ability to modify the current flow within the canopy. The effect of canopy density on the mean flow and on the generated turbulence is investigated experimentally in a 26 m open-channel flume. Multi-plane flow measurements within physical model canopies are taken using the telecentric 2D-2C PIV (Particle Image Velocimetry) method. The model canopies consist of 3D-printed scaled surrogates imitating white gorgonians (Eunicella singularis) with a simplified geometry. The four canopies tested here have frontal density λf = [0.033, 0.078, 0.136, 0.235]. Model canopies are tested at two different flow conditions with global Reynolds number Reg = 58*103  and Reg = 85*103, and local Reynolds number in the range of Rel = [64 - 287] and Rel = [132 - 592], respectively, where the local Reynolds number is based upon the model stem diameter. The incident flow is a uniform fully developed turbulent flow over rough bed, generated above a 15.86m array of solid cubes, as those used by Chagot et al. (2020), before reaching the model gorgonian canopy. Double-averaged (in time and in space) flow statistics are used here in order to account for the spatial heterogeneity inside the canopy, and to quantify all components of shear stress, i.e. turbulent and dispersive stress. The flow structure and the bed shear stress within and above the canopy are measured and compared to classical turbulent boundary layer models in and over vegetated canopies for different canopy densities. The presence of a shear layer close to the top of the canopy, defined by the measured deflected height, allows us to attempt to model the mean flow using the mixing-layer analogy for vegetated flows as a function of canopy density.

1. Rossi, S., Bramanti, L., Gori, A., & Orejas, C. (Eds.). (2017). Marine Animal Forests: The Ecology of Benthic Biodiversity Hotspots. Springer International Publishing. https://doi.org/10.1007/978-3-319-21012-4 

2. Chagot, L., Moulin, F. Y., & Eiff, O. (2020). Towards converged statistics in three-dimensional canopy-dominated flows. Experiments in Fluids, 61(2), 24. https://doi.org/10.1007/s00348-019-2857-4 

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