Physics-informed laboratory estimation of Sargassum windage

Abstract

A recent Maxey--Riley theory for \sarg raft motion, which models a raft as a network of elastically interacting finite-size, buoyant particles, predicts the carrying flow velocity to be given by the weighted sum of the water and air velocities $(1-α)\mathbf{v} + α\mathbf w$. The theory provides a closed formula for parameter $α$, referred to as \emph{windage}, depending on water-to-particle-density ratio or buoyancy ($δ$). From a series of laboratory experiments in an air--water stream flume facility under controlled conditions, we estimate $α$ ranging from 0.02 to 0.96\pct. On average, our windage estimates can be up to 9 times smaller than considered in conventional \emph{Sargassum} raft transport modeling, wherein it is customary to add a fraction of $\mathbf w$ to $\mathbf{v}$ chosen in an ad-hoc piecemeal manner. Using the formula provided by the Maxey--Riley theory, we estimate $δ$ ranging from 1.00 to 1.49. This is consistent with direct $δ$ measurements, ranging from 0.9 to 1.25, which provide support for our $α$ estimation.