Session A16: Free-Surface Flows: General I

Splash Production of Single Jointed Olympic Diver Models

Olympic divers perform what is called a "rip" entry to minimize their splash production and achieve higher scores. While there are several techniques used to rip the entry of a dive, it is always characterized by a collection of bubbles rising to the surface and the sound of tearing paper. This study focuses on the pike save technique in which the diver rolls forward at the hips immediately after entering the water. Although this entry technique is widespread in competitive diving, there is little research on how the manipulation of the air-water interface leads to a smaller splash. We develop a simplified single jointed diver model and drive it into a pool of water. The impact is recorded at 6000 frames per second to analyze the underwater kinematics, air cavity advancement, and splash production. To understand how the air cavity dynamics are affected by how the diver initiates the pike, we vary the stiffness of the hinge and asymmetric angle. The results of these experiments will illuminate the fluid dynamics of this problem and later be used to validate computational fluid dynamics simulations.   

Skin friction on oscillating interfacial bodies

The hydrodynamic drag a body experiences when moving along a fluid interface is well studied in literature for bodies of large length scales, but is relatively unexplored for bodies at the capillary scale. It has been previously shown that the drag on centimeter-sized disks sliding on water, referred to as “sliders”, is dominated by skin friction due to the Blasius boundary layer that forms in the fluid beneath the body [1]. Here, we investigate the drag experienced by sliders forced to oscillate along an air-water interface.  Scaling arguments suggest that in the unsteady limit (i.e. high Strouhal number) the boundary layer beneath the slider can be modeled as a Stokes (oscillatory) boundary layer.  We validate our model via experimental measurement by embedding a slider with a permanent magnet and externally forcing it with an oscillating magnetic field.  We extract the response amplitude and phase shift of the slider and compare it to what is predicted by our boundary layer drag model.

[1] G. Pucci, I. Ho, and D. M. Harris.  Friction on water sliders. Scientific Reports. 2019; 9 : 4095.   

Interfacial pumping inspired by snails

The apple snail Pomacea canaliculata exhibits a unique feeding mechanism to collect food particles floating at the water-air interface: while underwater, it positions part of its flexible foot parallel to the water surface and generates rhythmic undulations. These undulations trigger a flow near the free surface that brings the food particles towards the mouth. With a robotic system that periodically deforms a surface, we mimic the traveling deformations observed on the snail foot. When placed underneath the air-water interface, the undulator gives rise to a net flux of liquid in the direction of the propagating deformations. As a result, particles floating at a distance are sucked into the robotic snail. By combining experiment and analytical modeling, I will discuss how this mechanism creates a pumping effect both in the viscous as well as the inertial regime that leads to large-scale transport of floating particles.    

Floating Crude Oils Slicks are Thickened by Chemical Herder and Fragmented by Obstacles Protruding through the Water Surface

Chemical herding agents may be applied around marine oil spills to contract and thicken the oil slick in preparation for in situ burning and will likely prove useful in ice-infested waters in the Arctic or Antarctic. However, the presence of obstacles (e.g. ice or rocks) reduces the effectiveness of chemical herders and subsequent burning by fragmenting the contracting oil slick. The fluid mechanics of oil slick contraction via chemical herders and oil slick fragmentation by obstacles is not well understood. Here we present controlled laboratory experiments investigating Alaska North Slope crude oil slick contraction by the chemical herder OP-40 in an oil spill basin (92×42×20 cm) within a fume hood. A high-speed camera is used to visualize the oil slick contraction after the application of herder. Using 3D-printed obstacles, the effect of obstacle size and shape on oil slick area, contraction speed, and spatial distribution is determined at multiple time points. The effect of obstacle density is determined by testing multiple obstacles in various arrangements. Particle image velocimetry also is used to measure spatial variations in the slick contraction speed. Obstacle size and shape did not affect slick contraction speed or final slick area but did affect the spatial distribution of the oil slick as quantified by the central normalized moment.   

Not Participating

The quality and reliability of cast metal parts is very sensitive to impurities such as air bubbles and oxide compounds entrained in the melt while pouring the mold. Such impurities can concentrate stresses and shorten the fatigue life of these castings, and therefore there is a great need to understand how these entrainment events occur. While a body of literature on how plunging liquid jets entrain air does exist, there is little understanding yet of how disturbances on these jets cause air entrainment events, with most literature focusing on jets that either lack disturbances entirely or allow them to passively arise from unforced flow noise. This work concerns a novel air entrainment research system that forces harmonic disturbances on a well-controlled jet, enabling exploration of the relationship between a well-defined jet disturbance and air entrainment onset, its destabilization mechanism, and the eventual entrainment rate. Results from those investigations will be presented, including the finding of a novel free surface instability driven by the harmonic jet disturbances.   

A Quantitative Analysis of the Vertical-Averaging Approximation for Evaporating Thin Liquid Films

Thin liquid films play a central role in coating processes and other industrial and natural applications. Efficient optimization of these processes requires an understanding of capillary leveling, Marangoni flow, evaporation, and related phenomena. Although mathematical models are useful for gaining such understanding, it can be difficult to extract physical insight as the number of phenomena considered increases, so simplifying assumptions such as the vertical-averaging (VA) approximation for multicomponent films are often employed. In this work, we consider two-component films consisting of a solute and volatile solvent, and use lubrication theory to examine the performance of the VA approximation for three common evaporation models: constant, one-sided, and diffusion-limited. Whereas the VA approximation typically assumes ε²Pe<<1, where ε is the aspect ratio and Pe is the Péclet number, we find that the VA approximation works well when ε²Pe<<t_p, where t_p is the shortest time scale of phenomena that significantly depend on vertical gradients. Applying the VA approximation outside of the regime in which it is valid results in drastically different film-height and solute-distribution predictions depending on the evaporation model.   

Small amplitude oscillatory extensional rheometry for Newtonian filaments

We describe a new method of characterizing multiple parameters simultaneously for a Newtonian filament. Rather than requiring breakup to evaluate one independent parameter -- surface tension or viscosity, we propose utilizing oscillatory deformation profiles that avoid breakup and impose an external timescale. When combined with image processing to determine the free surface at various phases, this procedure can be used to determine the surface tension to density ratio (Weber number, We) and the viscosity to density ratio (Reynolds number, Re) simultaneously.  We consider parametric strategies and examine the We - Re - aspect ratio parameter space to determine an optimal regime for measurement of both parameters under conditions of full and limited resolution.   

Gas transport in an interfacially-driven biochemical reactor

Gas transfer at interfaces is a limiting factor in the performance of many chemical reactors and bioreactors. The knife-edge surface viscometer (KEV) is a flow apparatus in which a thin, rotating ring at the surface of a liquid conveys shear and mixing to the bulk fluid primarily through the surface shear viscosity and secondary inertial flow.  Here we examine the effects of interfacial shear and surface shear viscosity on gas transfer using a KEV.  Simulations using COMSOL were compared to CO₂ transport measurments with varying Reynolds (Re) and Boussinesq (Bo) numbers. Experiments used a pH sensitive fluorescent dye to visualize CO₂ transport over time in the KEV with different amounts of steric acid, an insoluble surfactant forming a monolayer. Good agreement is found between the experiments and the simulations. Results show a monotonic increase in gas transfer with increasing Re and increasing Bo.  These results are relevant to applications and future studies in chemical reactors, bioreactors, and gas transfer in microgravity studied using the ring-sheared drop (RSD), a containerless bioreactor launched to the ISS in 2019, originally designed to study amyloid fibrillization without interaction with solid walls. As a bioreactor, the RSD allows for optimal gas exchange due to increased interfacial area.   

Life and death of a dense spray puff

An exhalation is, from a fluid dynamics point of view, an impulsive source of mass and momentum diluting in a quiescent environment. The mean, coarse-grained expansion rate of the momentum carrying cloud is well known and its connexion to the turbulent cascade dynamics obvious. We are interested here in the fine-grained aspect of the phenomenon, namely in situations where the puff is initially concentrated in a liquid form liable to fragment and evaporate in an outer gas phase.

Puffs formed from liquids with very different volatilities demonstrate that the liquid soon fragments into close-by droplets spatially concentrated in dense stretched regions separated by large voids. The lifetime of an individual droplet embedded in such a spray is much larger than expected from the usual {\it d-squared} law describing the fate of a single drop evaporating in a quiescent environment. By analogy with the way mixing times of scalars are understood from the coupling between stretching and diffusion, we show, thanks to unique in-situ measurements, that the spray boundary with the diluting environment is slaved to the dynamics of its saturating vapor concentration field, thus explaining the very substantial evaporation delay of the liquid in spite of its division into fine droplets. This scenario is furthermore compatible with the evolution of the transient droplets size spray content, of which we give a full description up to the complete liquid evaporation.