Dispersant Research in a Specialized Wave Tank: Mimicking the Mixing Energy of Natural Sea States

Abstract

Breaking waves play a crucial role in the dispersion of oil spilled on the surface of the ocean both in the presence and absence of oil dispersants. Breaking of waves occurs when the forward horizontal velocity of water in the wave crest is greater than the wave propagation speed. These waves cause velocity shear and hence result in the mixing of oil and dispersant. Velocity shear with its associated friction also causes the dissipation of kinetic energy of the fluid. Of interest is the kinetic energy dissipation rate per unit mass, 8, which varies both in time and space. We use velocity measurements to compute the shear and subsequently the energy dissipation rate. The effectiveness of chemical oil dispersants is typically evaluated at various wavelength scales ranging from the smallest (10 cm, typical of laboratory flasks) to the largest (10s to 100s m, typical of open ocean test conditions). This study aims at evaluating dispersant effectiveness at intermediate or pilot scale. The hypothesis is that the energy dissipation rate per unit mass, ε, plays a major role in the effectiveness of a dispersant. If one assumes that s adequately describes dispersion behavior at both wave tank and field scales, then differences in the overall effectiveness of a dispersant under field conditions should be resolvable. To quantitatively define the conditions needed for effective dispersion in the field using scientifically sound, reproducible techniques, a wave tank measuring 32 m long x 0.6 m wide x 2 m deep was constructed on the premises of the Bedford Institute of Oceanography, Dartmouth, Nova Scotia. Waves are generated using a flap-type wavemaker. Controlled breaking wave conditions are generated by operating the wave maker at a low frequency followed by a higher frequency. Experiments defining the velocity profile and energy dissipation rates in the wave tank were conducted at three different induced breaking-wave energies. Energy dissipation rates were measured with an Acoustic Doppler Velocimeter (ADV) coupled to a data acquisition system. This presentation summarizes those results as well as preliminary results of the first series of dispersant effectiveness experiments showing the important influence breaking waves have on oil droplet dispersion.