Flow field and wake structure characteristics imposed by single seagrass blade surrogates

Abstract

Seagrass surrogates are commonly used to mimic the behaviour of seagrasses exposed to currents and their effects on flow fields. The interaction is highly dependent on the chosen mechanic and geometric properties of the surrogates and needs to be understood in order to design artificial meadows. The interaction of single surrogates in unidirectional flow fields is studied by means of physical modelling. Surrogates made of plastic materials with different flexural rigidities, buoyancies and geometries are exposed to varying flow velocities. The instantaneous velocity fields in the vicinity and wake of the surrogates are measured by stereoscopic Particle Image Velocimetry (PIV). All employed surrogates disrupt and interact with the flow field by changing their posture. An empirical relation is derived between the flexural rigidity, buoyancy and characteristic diameter of the surrogates and the imposed differences in the attenuation ratio of flow velocities. Further, the approaching flow velocity and distance behind the surrogate influence the estimated attenuation. The vortex shedding frequency imposed by artificial seagrass is lower than frequencies determined for infinite, rigid cylindrical structures. Three main characterizing properties: the modulus of elasticity, buoyancy and cross-sectional dimensions need to be taken into account for design of artificial seagrass meadows. Our findings advance knowledge of fluid-structure interactions of flexible materials and help to progress proper design of artificial seagrass meadows.

Keywords:

• Artificial seagrass
• submerged vegetation
• stereoscopic PIV
• flow-vegetation interaction
• flexible surrogates

Acknowledgements

We would like to thank our project partners Hannah Behnsen and Carmen Arndt for conducting the tensile and bending strength test and providing valuable results for the herein used modulus of elasticity. All figures were created with Matlab and the scientific colormaps of Fabio Crameri (Crameri, 2018) were used to prevent visual distortion of the data (Crameri et al., 2020).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work received financial support by the MWK Lower Saxony, Germany and the Volkswagen Foundation through the Project SeaArt (Grant No. ZN3187).