Shreyas Mandre

Associate Professor
Mathematics Institute, University of Warwick

Waving marine grass

Realization of the synchronized wind-induced waving of grass in a flowing soap film.

I bet you have seen gusts of wind exciting synchronized waves on the surface of tall grass field just like observed on the beach.

Synchronized waving of grass in aquatic and terrestrial setting and its impact on environmental transport is immense. The waving affects hydrodynamics of flow, which in turn influence transport and mixing of fluid, nutrients etc above and below grass, hence can affect the ecological function of aquatic and terrestrial systems.

Common feature of these waving is generation and flow of vortices in stream wise direction at top of canopy (grass). The generation of these vortices is manifested due to presence of a hydrodynamic instability of flow experiencing different resistance within and above the grass canopy, similar to classical Kelvin Helmholtz instability. See this article for a mathematical description of the mechanism. In the experiment nylon filaments mimic the role of grass and flow in the film mimic flow or air/water in terrestrial/aquatic setting.


Singh, Bandi, Mahadevan and Mandre. Linear stability analysis for monami in a submerged seagrass bed. J. Fluid Mech. 786, R1 (2016).
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Abstract: The onset of monami – the synchronous waving of seagrass beds driven by a steady flow – is modelled as a linear instability of the flow. Unlike previous works, our model considers the drag exerted by the grass in establishing the steady flow profile, and in damping out perturbations to it. We find two distinct modes of instability, which we label modes 1 and 2. Mode 1 is closely related to Kelvin–Helmholtz instability modified by vegetation drag, whereas mode 2 is unrelated to Kelvin–Helmholtz instability and arises from an interaction between the flow in the vegetated and unvegetated layers. ... (read more)

Fluid Mechanics Biomechanics Environment