A spanwise oscillating plate in a crossflow: Implication for mass transfer and locomotion

Abstract

Many aquatic processes (i.e., organism locomotion, macrophyte response to unsteady flow and mass/heat transfer) involve oscillatory or undulatory movements perpendicular to the direction of flow and perpendicular to the plane of oscillation (i.e., spanwise). The reduction of boundary layer (BL) thickness (δ) and increased turbulence have been identified as the mechanisms for faster heat and mass transfer on these moving surfaces. We examined the hydrodynamics of a submerged spanwise‐oscillating plate in a crossflow within turbulent open channel flow at different oscillation frequencies (f) and channel velocity (U). The BL over the plate was characterized using moving particle image velocimetry to understand the effects of f and U on boundary stress, vorticity, and transfer processes. Changes in the velocity gradient at the plate surface depend on the interactions between spanwise vorticities of opposite directions that reversed downstream along the plate. The vorticity structure reduced shear stress at the surface of the plate resulting in > 65% drag reduction near the upstream edge. Whereas δ was reduced in comparison to a stationary plate, vorticity increased the diffusive sublayer thickness true(δDSL) over the oscillating plate (≲1.75 thicker than stationary), which would reduce rather than increase mass transfer based on the film model. Conversely, oscillations would enhance the mass transfer by a factor of ≤ 7 due to periodic renewal of water at the DSL. Similar arguments indicate that renewal would lead to increased convective heat transfer along the plate. Results of this work will increase our understanding of the physical and biological processes that occur in environmental boundary layers.

Hydrodynamic Interactions Between Macroalgae and Their Epibionts

“…However, flexible macrophytes in water currents can flutter like flags, and such fluttering might increase the local water motion encountered by epibionts on distal regions of the host. Fluttering also stirs the water near macrophytes, thereby replacing depleted water with undepleted water (Koehl and Alberte, 1988;Koch et al, 2006;Jabbari et al, 2021). Another important aspect of the environments of epibionts is that macroalgae and seagrasses usually occur in aggregations (e.g.…”

“…When flexible hosts flutter like flags or flap back and forth in ambient water flow, they increase the rate at which old, depleted water is replaced by new water near their surfaces and the epibionts sitting on them (Koehl and Alberte, 1988;Hunter, 1989;Koehl, 1999;Koch et al, 2006;Jabbari et al, 2021). Water exchange near fluttering M. splendens is faster than near rigid structures of the same length at a protected site where net water transport is slow (Figures 5B, C).…”