From surrogate modeling to flow characterization: Investigating the mean flow and turbulence structure inside and above canopies of Eunicella singularis

 

Gorgonians are coastal megabenthic organisms facing the threats of destructive fishing activities and mass mortality due to thermal anomalies in Mediterranean Sea. They are engineering species playing a significant role in the maintenance of biodiversity by providing habitat to several marine species (Rossi et al., 2017). Gorgonians often have an arborescent geometry, and when their populations are dense enough, they form three-dimensional forest canopies similar to terrestrial ones.  Water flow is modified by the presence of the gorgonian canopy, with formation of a turbulent shear flow at the top of it. At this level, Reynolds stress is expected to present a maximum value suggesting an active momentum exchange and a significant mass transport (food and nutrients). Here, the gorgonian canopy flow is experimentally investigated in a flume using surrogates as in studies of aquatic flexible vegetated canopies (e.g., Sukhodolov et al., 2022). Laboratory experiments are conducted with artificial gorgonian canopies of different planar densities in a unidirectional open-channel flow. White gorgonians (Eunicella singularis) are mimicked by using 3D-printed surrogates with bending stiffness similar to the one of the real gorgonians allowing us to represent the drag force and the reconfiguration of the living organisms in water. Dynamic similarity between laboratory and in-situ conditions is ensured by using the same range of Reynolds number and the same Cauchy number. Simplified scaled symmetrical geometries are built respecting the geometrical aspect ratio and the main branching orders of the tree-shaped white gorgonians. 2D-2C PIV (Particle Image Velocimetry) flow measurements in vertical planes are performed to characterize the local flow conditions in and over gorgonian canopies. The high-spatial resolution of PIV measurements allows us to characterize most of the relevant flow scales; from the stem-scale wakes behind the tip branches of one colony to the canopy-scale turbulence forced by the vertical mixing layer near the top of the canopy, and finally to the much larger turbulent boundary-layer structures. Canopy-scale turbulence appears in high density canopies (H. M. Nepf, 2012), and thus characterizing the transitional regime between sparse and dense canopies is essential to define the minimum canopy density required for significant flow modification. This threshold is necessary to define the minimal conservation unit related to the canopy’s ecological functions. 

 

Nepf, H. M.: Flow and Transport in Regions with Aquatic Vegetation, Annu. Rev. Fluid Mech., 44, 123–142, https://doi.org/10.1146/annurev-fluid-120710-101048, 2012. 

 

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

 

Sukhodolov, A., Sukhodolova, T., and Aberle, J.: Modelling of flexible aquatic plants from silicone syntactic foams, Journal of Hydraulic Research, 60, 173–181, https://doi.org/10.1080/00221686.2021.1903590, 2022.