Estimating Bubble Plume Dynamics in Breaking Waves using the Thermal
Signature of the Residual Foam
Abstract
This study is motivated by the observation that after a wave breaking
event in the ocean, the residual surface foam left in the wake of the
breaker rapidly cools down. The relationship between the cooling foam
and the characteristics of the breaking wave such as bubble plume
dynamics, visible surface foam, and energy dissipation is investigated
experimentally. Previous studies have suggested that the decay time of
the visible foam can be used to determine the dynamics of the subsurface
bubble plume, and to estimate the energy dissipation by the breaking
process. But the foam decay process can be greatly affected by the
surfactant concentration in the ocean and this effect need to be
accounted for independently. We present a new approach that utilizes the
thermal signature of the cooling foam to infer the breaking
characteristics and is less sensitive to surfactant concentration. The
experiments are conducted in a wave flume that is equipped with a
piston-type wavemaker and is filled with salt water. In order to study
the effects of surfactants on the cooling of the residual foam, two sets
of experiments are carried out; In the first set clean salt water is
used and in the second set, Triton X-100 at a concentration of
approximately 200 mg/L is added to the water. Breaking waves are
generated using the focusing wavepacket technique and are designed to
cover a wide range of slopes and breaking intensities. The bubble plume
and the surface foam are imaged using visible cameras and the surface
temperature is captured using an IR camera with an overlapping field of
view with the visible foam camera. It is observed that the average
temperature of the foam initially increases after the passage of a
breaking wave due to the disruption of the cool skin layer, but the foam
starts to cool down soon after the bubble plume has subsided, and the
foam regeneration is not sustained by the bubbles anymore. It is found
that the time from the start of the breaking process to the onset of
cooling of the foam (τcool) scales with the decay time of the bubble
plume (τplume), energy dissipation, and the wavepacket slope of the
breakers. It is also observed that the foam decay time is prolonged
greatly by the presence of additional surfactants (consistent with the
literature), but the bubble plume decay time and the onset of cooling of
the foam are not significantly affected.