Abstract
It is well known that the motion of flexible vegetation leads to drag
reduction in comparison to rigid vegetation. In this study, we use a
numerical model to investigate how the detailed motion of kelp fronds in
response to forcing by surface gravity waves can impact the drag exerted
by the kelp on waves. We find that this motion can be characterized in
terms of three dimensionless numbers: (1) the ratio of hydrodynamic drag
to buoyancy, (2) the ratio of blade length to wave excursion, and (3)
the Keulegan-Carpenter number, which measures the ratio of drag to
inertial forces. We quantify drag reduction, and find that inertial
forces can significantly impact the amplitude of kelp motion and amount
of kelp drag reduction. Under certain wave conditions, inertial forces
can cause kelp fronds to accelerate more quickly relative to the wave,
which can lead to increased drag reduction and reduced wave energy
dissipation. In some conditions, frond motion leads to drag augmentation
in comparison to rigid fronds. Additionally, we discuss other features
of kelp motion, such as the degree of asymmetry, and their relationship
with enhanced drag reduction.