The Impact of Unsteady Reconfiguration on Turbulence Structure within a Flexible Canopy: Experimental Study of a Model Seagrass Meadow

Abstract:

Flexible plants bend in response to fluid motion and this reconfiguration reduces drag forces, which protects against uprooting and breaking. The impact of reconfiguration can be described quantitatively by introducing a drag coefficient (C_D) that decreases as a power-law function of velocity (U) with a negative exponent known as the Vogel number (B). That is, C_D is proportional to U^B. In this experimental study, we explored the link between reconfiguration and turbulence dynamics using a model seagrass meadow. As the flow rate in the channel increased, both the mean and unsteady reconfiguration increased. The unsteady reconfiguration, called monami, provides visual evidence that stronger turbulent events cause greater reconfiguration, and we hypothesize that the greater reconfiguration leads to proportionally lower drag, as reflected in a lower drag coefficient, such that stronger turbulent events preferentially penetrate deeper into the meadow. The preferential penetration of stronger events is reflected in the skewness of the streamwise velocity, which increases as the penetration of strong events gain greater preference. The measurements confirmed that a more negative Vogel number led to a higher skewness peak within the canopy. Previous studies were used to link the Vogel number and the Cauchy number. In the transition between the asymptotic regimes of negligible (B = 0) and strong (B = -2/3) reconfiguration, there is a regime of weak reconfiguration that extends over Ca = 10 to 100. The Vogel number achieved its peak negative value (B = -1.1) within the weak reconfiguration regime. The peak skewness also occurred within the regime of weak reconfiguration.