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Turbulence statistics and blade deformations of flexible emergent canopies impinged by water flows were experimentally investigated across a range of Reynolds numbers Reb=Ubb/ν (where Ub is the bulk incoming flow velocity, b is the blade width, and ν is the water kinematic viscosity) and blade aspect ratios AR=h/b (h is the blade length). Time-resolved particle image velocimetry was used to characterize both the deformation of flexible blades and the surrounding flow fields. Results showed that the blade deformation increased with the growth of both Reb and AR, with higher blade bending causing stronger variations in vertical profiles of streamwise velocities and Reynolds stresses. The drag produced by the presence of flexible canopies was identified as the dominant fluid loading balancing the pressure gradient. This term exhibited distinctive reduction near the water surface region with high blade deformation due to the large local blade inclination angle. Interestingly, in contrast to fully submerged flexible blades where the flow-induced drag increases monotonously with flow speed, a critical Reynolds number Reb,cri was observed, beyond which drag decreased with increasing flow speed until the blade became fully submerged. This phenomenon was explained with theoretical interpretations, which exhibited reasonable agreement with experimental results. Further analysis of unsteady flow dynamics revealed that Reynolds stress within the canopy was dominated by ejection events due to the absence of shear layer at the top of emergent canopy. Additionally, streamwise velocity spectra indicated that flow fluctuations inside the canopy were governed by periodic vortex shedding from blade.
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Seagrass deformation affects fluid instability and tracer exchange in canopy flow
Abstract Monami is the synchronous waving of a submerged seagrass bed in response to unidirectional fluid flow. Here we develop a multiphase model for the dynamical instabilities and flow-driven collective motions of buoyant, deformable seagrass. We show that the impedance to flow due to the seagrass results in an unstable velocity shear layer at the canopy interface, leading to a periodic array of vortices that propagate downstream. Our simplified model, configured for unidirectional flow in a channel, provides a better understanding of the interaction between these vortices and the seagrass bed. Each passing vortex locally weakens the along-stream velocity at the canopy top, reducing the drag and allowing the deformed grass to straighten up just beneath it. This causes the grass to oscillate periodically even in the absence of water waves. Crucially, the maximal grass deflection is out of phase with the vortices. A phase diagram for the onset of instability shows its dependence on the fluid Reynolds number and an effective buoyancy parameter. Less buoyant grass is more easily deformed by the flow and forms a weaker shear layer, with smaller vortices and less material exchange across the canopy top. While higher Reynolds number leads to stronger vortices and larger waving amplitudes of the seagrass, waving amplitude is maximized at intermediate grass buoyancy. All together, our theory and computations develop an updated schematic of the instability mechanism consistent with experimental observations.
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- Award ID(s):
- 1756279
- PAR ID:
- 10450541
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41598-023-30401-9
