Research

Active Projects

Holographic image sequence of spray droplets produced by a breaking (wave is breaking below the image field of view)

Spray Generated by a Mechanically Generated Breaking Waves

Abstract: Sea spray is known to dramatically enhance the transfer of mass, momentum, and energy between the ocean and the atmosphere. Previous investigations have reported measurements of temporally averaged droplet size distributions and fluxes at fixed heights in wind‐wave systems at sea and in laboratory‐scale experiments. It is commonly accepted that droplets are primarily produced by three breaking subprocesses: wind shear (primarily at the wave crest), splashing, and the popping of breaker‐entrained air bubbles that rise to the free surface. However, there are little experimental data that quantitatively link local droplet generation characteristics to these subprocesses.

Publications:

Image of four water wave surface profiles at the time of jet impact for CLEAN (top left), TX1 (top right), TX2 (bottom left), and TX6 (bottom right) surfactant concentrations

Effects of Surfactants on Plunging Breakers

Abstract: An experimental study is conducted to compare the free surface profile evolution in a deep-water plunging breaker in the presence of six bulk concentrations of soluble surfactant refereed to as the CLEAN case, and TX1 to 6 cases. The breakers are generated by a programmable wave maker that is set with a single motion profile that produces a dispersively focused wave packet. The wave profiles are measured with a cinematic LIF technique and surface pressure isotherms of water samples at each surfactant concentration are measured using a Langmuir Trough. We found that in clean (CLN) and high surfactant concentration (TX6) cases the the plunging jet and entrained air cavity appear very smooth up to jet impact. However, in the TX1 case the jet curls inward and the air cavity becomes highly irregular while in the TX2 case no plunging jet is formed and no air entrained cavity appears, resulting in a spilling breaker. We hypothesize that this effect is because in the CLN and TX6 cases the surface tension along the jet surface is nearly constant, which results in a smooth jet and air cavity. While in the TX1 and TX2 cases the surface tension varies along the jet surface, which results in Marangoni stresses that influence the breaking process.

Publications:

A whitelight movie of wind-forced breaking water waves generated in a laboratory wave tank

Speed and Acceleration of Droplets Generated by Breaking Wind-Forced Waves

Abstract: The production of spray droplets by wind-waves is studied in laboratory experiments at the University of Delaware's Air-Sea Interaction Laboratory. Wind-waves are produced at free-stream wind speeds of 9.7, 10.7, and 11.7 m/s with the addition of mechanically forced waves with central frequencies of 1.0, 1.2, 1.4, 1.6 and 1.8 Hz and sidebands at +/- 0.05 Hz as well as a case with wind only forcing. Drop positions and diameters (d > 60 μm) are measured for each experimental condition and over multiple wave periods using an in-line holography system positioned 6.4 cm above the mean water level and at a fetch of approximately 23 meters. Droplet statistics including total number, mean diameter, and size distributions are reported at the different experimental conditions. The relative importance of spray production mechanisms is discussed and correlated to wave-field characteristics including the frequency of breaking events and the wind-wave properties.

Publications:

Image of a spray nozzle (bottom left) injecting water droplets (white streaks) into a turbulent boundary layer

Transport of Inertial Droplets in a Turbulent Boundary Layer

Abstract: The transport of highly inertial droplets in the presence of a turbulent boundary layer is studied experimentally inside a wind tunnel. Water droplets are injected into the boundary layer at free-stream velocities ranging from U = 2.4 to 6.6 m/s with a boundary layer thickness ranging from δ = 11.4 to 12.5 cm. The size, speed, and acceleration of droplets are measured using a high-speed cinematic in-line holographic system positioned 162 cm downstream from the nozzle and at many wall-normal streamwise locations spanning the full thickness of the boundary layer. Water droplets with diameters ranging from 30 to 1000 μm are measured with corresponding Stokes numbers ranging from Stη = 0.11 to 160. Wall-normal droplet concentration and velocity profiles are reported. Droplet statistics like velocity and acceleration are discussed as a function of droplet size and wall-normal location. The importance of turbulence on the motion of highly inertial droplets is discussed and compared to similar statistics obtained from inertial droplets produced by breaking wind-forced waves.

Inactive Projects

Air Entrainment and Surface Fluctuations in a Turbulent Ship Hull Boundary Layer

Abstract: Air entrainment due to turbulence in a free-surface boundary layer shear flow created by a horizontally moving vertical surface-piercing wall is studied through experiments. In the experiments, the moving wall is created by a laboratory-scale device composed of a surface-piercing stainless steel belt that travels in a loop around two vertical rollers; one length of the belt between the rollers simulates the moving wall. The belt accelerates suddenly from rest until reaching constant speed and creates a temporally evolving boundary layer analogous to the spatially evolving boundary layer that would exist along a surface-piercing towed flat plate. We report cinematic laser-induced fluorescence measurements of water surface profile histories, cinematic observations and measurements of air entrainment events, and air bubble size distributions and motions.

Publications:

Fluid particle dynamics and the non-local origin of the Reynolds shear stress.

Abstract: The causative factors leading to the Reynolds shear stress distribution in turbulent channel flow are analysed via a backward particle path analysis. It is found that the classical displacement transport mechanism, by which changes in the mean velocity field over a mixing time correlate with the wall-normal velocity, is the dominant source of Reynolds shear stress. Approximately 20 % of channel flow at any given time contains fluid motions that contribute to displacement transport. Much rarer events provide a small but non-negligible contribution to the Reynolds shear stress due to fluid particle accelerations and long-lived correlations deriving from structural features of the near-wall flow. The Reynolds shear stress in channel flow is shown to be a non-local phenomenon that is not conducive to description via a local model and particularly one depending directly on the local mean velocity gradient. 

Publications