This laboratory experimental study investigates the temporal evolution of the size distribution of subsurface oil droplets generated as breaking waves entrain oil slicks. The measurements are performed for varying wave energy, as well as large variations in oil viscosity and oil‐water interfacial tension, the latter achieved by premixing the oil with dispersant. In situ measurements using digital inline holography at two magnifications are applied for measuring the droplet sizes and Particle Image Velocimetry (PIV) for determining the temporal evolution of turbulence after wave breaking. All early (2–10 s) size distributions have two distinct size ranges with different slopes. For low dispersant to oil ratios (DOR), the transition between them could be predicted based on a turbulent Weber (We) number in the 2–4 range, suggesting that turbulence plays an important role. For smaller droplets, all the number size distributions have power of about −2.1, and for larger droplets, the power decreases well below −3. The measured steepening of the size distribution over time is predicted by a simple model involving buoyant rise and turbulence dispersion. Conversely, for DOR 1:100 and 1:25 oils, the diameter of slope transition decreases from ∼1 mm to 46 and 14 µm, respectively, much faster than the We‐based prediction, and the size distribution steepens with increasing DOR. Furthermore, the concentration of micron‐sized droplets of DOR 1:25 oil increases for the first 10 min after entrainment. These phenomena are presumably caused by the observed formation and breakup oil microthreads associated with tip streaming.

The high-speed impact of a droplet on a bulk fluid at high Weber number (We) is not well understood but is relevant to the production of marine aerosol by raindrop impact on the sea surface. These splashes produce a subsurface cavity and a crown which closes into a bubble canopy, but a floating layer of immiscible oil, such as a crude oil slick, alters the splash dynamics. The effects of oil layer fluid properties and thickness, droplet size and impact speed are examined by high-speed visualization. Oil layer rupture and crown behaviour are classified by dimensional scaling. The subsurface cavity volume for impact on thick layers is shown to depend on the Reynolds number (Re), although canopy formation at high Re introduces a competing We effect since rapid canopy closure is found to retard cavity expansion. Time-resolved kinematic measurements show that thin crude oil slicks similarly alter crown closure and cavity growth. The size and spatial distributions of airborne droplets are examined using high-speed holographic microscopy. The droplets have a bimodal distribution with peaks at 50 and 225 µm and are clustered by size at different elevation angles. Small droplets (50 µm) are ejected primarily at shallow angles, indicating production by splashing within the first 100 µs and by breakup of microligaments. Larger droplets (225 µm) are found at steeper elevation angles, indicating later production by capillary instability acting on large ligaments protruding upward from the crown. Intermittent droplet release while the ligaments grow and sweep upward is thought to contribute to the size-dependent spatial ordering. Greater numbers of small droplets are produced at high elevation angles when a crude oil layer is present, indicating satellite droplet formation from ligament breakup. A crude oil layer also increases the target fluid Ohnesorge number, leading to creation of an intact ejecta sheet, which then ruptures to form aerosolized oil droplets.

No abstract

Droplet generation through spray breakup is an unsteady and non-linear process which produces a relatively dense, highly polydisperse aerosol containing non-spherical droplets with sizes spanning several orders of magnitude. Such variability in size and shape can lead to significant sources of error for conventional measurements based on laser scattering. Although direct imaging of droplets can potentially overcome these limitations, imaging suffers from a shallow depth of field as well as occlusions, which prevents the complete spray from being analyzed. In comparison, digital inline holography (DIH), a low cost coherent imaging technique, can enable high resolution imaging of the sample over an extended depth of field, typically several orders of magnitude larger than traditional imaging. In this study, we showcase an automated DIH imaging system for characterizing monodisperse and polydisperse aerosol droplet size and shape distributions in the 20 m -3 mm diameter range, over a large sample volume. The high accuracy of the technique is demonstrated by measurements of monodisperse droplets generated by a vibrating orifice droplet generator, achieving a resolution of ~14.2. Measurements of a polydisperse spray from a flat fan nozzle serve to establish the versatility of DIH in extracting a two-dimensional size-eccentricity distribution function, which indicates a strong semilogarithmic scaling between the two parameters that decays as the droplet migrates away from the nozzle. Due to its low cost and compact setup as well as high density of data obtained, DIH can serve as a promising approach for future aerosol characterization.

Interactions of currents and waves with a rippled seabed in the inner part of the coastal ocean bottom boundary layer are studied using particle image velocimetry, ADV, and bottom roughness measurements. Mean velocity profiles collapse with appropriate scaling in the log layer, but vary substantially in the roughness sublayer. When wave-induced motions are similar or greater than the mean current, the hydrodynamic roughness (z 0 ) determined from velocity profiles is substantially larger than directly measured values. The roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range, but decays in the log layer for larger roughness elements. Reynolds shear stress profiles peak in the lower parts of the log layer, diminishing below it, and gradually decaying at higher elevations. In contrast, wave shear stresses are negligible within the log layer, but become significant within the roughness sublayer. This phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave-induced motion. No single boundary layer length scale collapses the Reynolds stresses, but both the Prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation, especially near the seabed. Within the log region, profiles of shear production and dissipation rates of turbulence converge. Below it, dissipation rapidly increases, peaking near the seabed. Conversely, the shear production decays near the seabed, in agreement with the eddy viscosity model, but in contrast to both laboratory and computational rough wall studies.

No abstract

No abstract

No abstract