Water Impact: When a Sphere Becomes Flat.

A large hydrodynamic force accompanies the vertical impact of bodies on water. While added mass phenomena govern these forces for both spherical and flat impactors, the dynamics of a trapped gas layer critically alters the flat case, reducing the peak pressure below that predicted by water hammer theory. An impactor with a spherical nose cap looks increasingly flat as the nose curvature approaches zero. This causes one to ask at what curvature a spherical cap impactor transitions to flat impact behavior. We find this transition, relate limiting behaviors to theories, and dispel the long-held belief that the largest water impact forces occur for flat bodies.

A robust thin-film droplet-induced electricity generator.

The pursuit of cost-effective, high-voltage electricity generators activated by droplets represents a new frontier in hydropower technology. This study presents an economical method for crafting droplet generators using common materials such as solid polytetrafluoroethylene (PTFE) films and readily available tapes, eliminating the need for specialized cleanroom facilities. A thorough investigation into voltage-limiting factors, encompassing device capacitance and induced electrode charges, reveals specific areas with potential for optimization. A substantial enhancement in the open-circuit voltage (Voc) was achieved, reaching approximately 282.2 ± 27.9 V-an impressive increase of around 60 V compared to earlier benchmarks. One device showcased its capability to power 100 LEDs concurrently, underscoring its efficacy. Ten such devices created diverse luminous patterns with uniform light intensity for each LED, showcasing the practical potential of the approach. The methodology's cost-effectiveness results in a remarkable cost reduction compared to solution-based materials, paving the way for the widespread adoption of large-scale water droplet energy harvesting.

Maple samara flight is robust to morphological perturbation and united by a classic drag model.

Winged, autorotating seeds from the genus Acer, have been the subject of study for botanists and aerodynamicists for decades. Despite this attention and the relative simplicity of these winged seeds, there are still considerable gaps in our understanding of how samara dynamics are informed by morphological features. Additionally, questions remain regarding the robustness of their dynamics to morphological alterations such as mass change by moisture or area change by damage. We here challenge the conventional approach of using wing-loading correlations and instead demonstrate the superiority of a classical aerodynamic model. Using allometry, we determine why some species deviate from interspecific aerodynamic behavior. We alter samara mass and wing area and measure corresponding changes to descent velocity, rotation rate, and coning angle, thereby demonstrating their remarkable ability to autorotate despite significant morphological alteration. Samaras endure mass changes greater than 100% while maintaining descent velocity changes of less than 15%, and are thus robust to changes in mass by moisture or damage. Additionally, samaras withstand up to a 40% reduction in wing area before losing their ability to autorotate, with the largest wings more robust to ablation. Thus, samaras are also robust to wing damage in their environment, a fact children joyfully exploit.

Probing the mechanisms of enhanced crystallisation of APS in the presence of ultrasound.

Understanding the origins of the enhancement of crystallisation of a lipid (all-purpose shortening, APS) through the application of ultrasound is a fundamental pre-requisite for the exploitation of this technique in a wider context. To this end, we show here a number of measurements designed to probe the mechanisms responsible for this effect. For example, we show how the type of bubble cluster, produced at the sound source, alters the bubble population and residency time. In addition, to probe the various contributions to the enhanced crystallisation rate, isolation of the cluster environment below the piston like emitter (PLE) used as the ultrasonic source was shown to reduce the enhancement observed, but did not remove it entirely. This implied that the exposure of the liquid to pressure shocks and the environment around the cluster has a positive effect on the crystallisation kinetics. In turn the addition of extra seed crystals and mechanical agitation also enhances the rate of crystallisation. Finally, the time at which ultrasonic irradiation of the fluid is applied is shown to alter the kinetics observed. These observations suggest that two components are important: large bubble populations and mechanical effects on pre-existing crystals. These findings suggest that maximising these effects could be an eloquent way to enhance and control the material characteristics of materials produced in this manner.

Development of an optical flow through detector for bubbles, crystals and particles in oils.

The characterisation of bubbles or particles in an oil poses some unique challenges. In contrast to water solutions, the use of electrochemical detection approaches is more difficult in an oil. However, optical sensing systems have considerable potential in this area. Here we use a flow through channel approach and monitor the light propagation through this structure in an optical transmission sensor arrangement (OTS). This simple approach is demonstrated to be useful at detecting bubbles produced in the oil as a result of cavitation induced by high intensity ultrasound (HIU). The optical technique is shown to have an analytical basis. Bubble detection from an operating HIU source is shown to depend on position of the sensor with respect to the source. Critically, the bubble population can be followed for extended time periods after the ultrasonic source has been terminated. The detection of crystals is also demonstrated. Hence, this technique is ideal for the study of the effects of HIU on oils as they crystallise over extended time periods.

Cavitation upon low-speed solid-liquid impact.

When a solid object impacts on the surface of a liquid, extremely high pressure develops at the site of contact. Von Karman's study of this classical physics problem showed that the pressure on the bottom surface of the impacting body approaches infinity for flat impacts. Yet, in contrast to the high pressures found from experience and in previous studies, we show that a flat-bottomed cylinder impacting a pool of liquid can decrease the local pressure sufficiently to cavitate the liquid. Cavitation occurs because the liquid is slightly compressible and impact creates large pressure waves that reflect from the free surface to form negative pressure regions. We find that an impact velocity as low as ~3 m/s suffices to cavitate the liquid and propose a new cavitation number to predict cavitation onset in low-speed solid-liquid impact-scenarios. These findings imply that localized cavitation could occur in impacts such as boat slamming, cliff jumping, and ocean landing of spacecraft.

Enhanced crystallisation kinetics of edible lipids through the action of a bifurcated streamer.

The processing of healthy foods remains a challenge and any technology with the ability to tailor the physical properties of new materials is in demand. High-intensity ultrasound (HIU) has been identified as a useful processing technique for such activities particularly for edible lipids. HIU has been known to alter the crystallisation kinetics and in turn the resultant physicochemical properties for specific food applications. The role of cavitation dynamics during treatment of oils with HIU is of interest, with the knowledge gained allowing for insight into the complex and still undefined mechanism of action. To this end, the crystallisation kinetics of an edible lipid were investigated in the presence of several distinctly different cavitation conditions. Several cavitation clusters, including a bifurcated streamer (BiS), located on the surface of a piston-like emitter (PLE) were studied, each generated by a specific ultrasonic power level. Only samples crystallised at a low supercooling (ΔTSC) value display significant differences in induction time for each of the selected HIU powers, at least 5 minutes earlier than without exposure to HIU. Substantially better energy efficiencies were seen for the BiS regime (ΔTSC = 5 °C) which coincided with maximal crystal growth rates. An increase in melting enthalpy and elastic modulus is reported in the presence of HIU for all crystallisation temperatures, this effect is larger overall with increasing ultrasonic power. In addition, sonicated samples in the presence of the BiS event were composed of fewer smaller crystals compared to higher HIU powers after 60 minutes at 30 °C. Bubble dynamics recorded during a 10 s sonication period exhibited a greater acoustic attenuation effect for the highest ultrasonic power (75 W). The results suggest that the dynamics of the cluster and the presence of the BiS event are important in terms of energy efficiency and the physical properties of the crystallised lipid material.

Water entry impact dynamics of diving birds.

Some seabirds (such as northern gannets and brown boobies) can dive from heights as high as 30 m reaching speeds of up to 24 m s-1 as they impact the water surface. The physical geometry of plunge diving birds, particularly of the beak, allows them to limit high impact forces compared to non-diving birds. Numerically simulated data for one species (northern gannet) provides some insight into the impact forces experienced during diving, however, no reliable experimental data with real bird geometries exist for comparison purposes. This study utilizes eleven 3D printed diving bird models of three types of birds: plunge-diving (five), surface-diving (five) and dipper (one), with embedded accelerometers to measure water-entry impact accelerations for impact velocities ranging between 4.4-23.2 m s-1. Impact forces for all bird types are found to be comparable under similar impact conditions and well within the safe zone characterized by neck strength as found in recent studies. However, the time that each bird requires to reach maximum impact acceleration from impact is different based on its beak and head shape and so is its effect, represented here by its derivative (i.e. jerk). We show that surface diving birds have high non-dimensional jerk, which exceed a safe limit estimated from human impact analysis, whereas those by plunge divers do not.

Water walking as a new mode of free surface skipping.

Deformable elastomeric spheres are evaluated experimentally as they skip multiple times over a lake surface. Some spheres are embedded with small inertial measurement units to measure the acceleration experienced during water surface impact. A model for multiple impact events shows good agreement between measured acceleration, number of skipping events and distanced traveled. The experiment reveals a new mode of skipping, "water walking", which is observed for relatively soft spheres impacting at low impact angles. The mode occurs when the sphere gains significant angular velocity over the first several impacts, causing the sphere to maintain a deformed, oblong shape. The behavior is characterized by the sphere moving nearly parallel to the water surface with the major axis tips dipping below the water surface with each rotation while the shorter sides pass just above, giving the impression that the sphere is walking across the water surface.

Drop on a bent fibre.

Inspired by the huge droplets attached on cypress tree leaf tips after rain, we find that a bent fibre can hold significantly more water in the corner than a horizontally placed fibre (typically up to three times or more). The maximum volume of the liquid that can be trapped is remarkably affected by the bending angle of the fibre and surface tension of the liquid. We experimentally find the optimal included angle (∼36°) that holds the most water. Analytical and semi-empirical models are developed to explain these counter-intuitive experimental observations and predict the optimal angle. The data and models could be useful for designing microfluidic and fog harvesting devices.

Cavitation onset caused by acceleration.

Striking the top of a liquid-filled bottle can shatter the bottom. An intuitive interpretation of this event might label an impulsive force as the culprit in this fracturing phenomenon. However, high-speed photography reveals the formation and collapse of tiny bubbles near the bottom before fracture. This observation indicates that the damaging phenomenon of cavitation is at fault. Cavitation is well known for causing damage in various applications including pipes and ship propellers, making accurate prediction of cavitation onset vital in several industries. However, the conventional cavitation number as a function of velocity incorrectly predicts the cavitation onset caused by acceleration. This unexplained discrepancy leads to the derivation of an alternative dimensionless term from the equation of motion, predicting cavitation as a function of acceleration and fluid depth rather than velocity. Two independent research groups in different countries have tested this theory; separate series of experiments confirm that an alternative cavitation number, presented in this paper, defines the universal criteria for the onset of acceleration-induced cavitation.

The upside-down water collection system of Syntrichia caninervis.

Desert plants possess highly evolved water conservation and transport systems, from the root structures that maximize absorption of scarce ground water(1-5), to the minimization of leaf surface area(6) to enhance water retention. Recent attention has focused on leaf structures that are adapted to collect water and promote nucleation from humid air(7-9). Syntrichia caninervis Mitt. (Pottiaceae) is one of the most abundant desert mosses in the world and thrives in an extreme environment with multiple but limited water resources (such as dew, fog, snow and rain), yet the mechanisms for water collection and transport have never been completely revealed. S. caninervis has a unique adaptation: it uses a tiny hair (awn) on the end of each leaf to collect water, in addition to that collected by the leaves themselves. Here we show that the unique multiscale structures of the hair are equipped to collect and transport water in four modes: nucleation of water droplets and films on the leaf hair from humid atmospheres; collection of fog droplets on leaf hairs; collection of splash water from raindrops; and transportation of the acquired water to the leaf itself. Fluid nucleation is accomplished in nanostructures, whereas fog droplets are gathered in areas where a high density of small barbs are present and then quickly transported to the leaf at the base of the hair. Our observations reveal nature's optimization of water collection by coupling relevant multiscale physical plant structures with multiscale sources of water.

Elastic spheres can walk on water.

Incited by public fascination and engineering application, water-skipping of rigid stones and spheres has received considerable study. While these objects can be coaxed to ricochet, elastic spheres demonstrate superior water-skipping ability, but little is known about the effect of large material compliance on water impact physics. Here we show that upon water impact, very compliant spheres naturally assume a disk-like geometry and dynamic orientation that are favourable for water-skipping. Experiments and numerical modelling reveal that the initial spherical shape evolves as elastic waves propagate through the material. We find that the skipping dynamics are governed by the wave propagation speed and by the ratio of material shear modulus to hydrodynamic pressure. With these insights, we explain why softer spheres skip more easily than stiffer ones. Our results advance understanding of fluid-elastic body interaction during water impact, which could benefit inflatable craft modelling and, more playfully, design of elastic aquatic toys.

Mach stem formation in outdoor measurements of acoustic shocks.

Mach stem formation during outdoor acoustic shock propagation is investigated using spherical oxyacetylene balloons exploded above pavement. The location of the transition point from regular to irregular reflection and the path of the triple point are experimentally resolved using microphone arrays and a high-speed camera. The transition point falls between recent analytical work for weak irregular reflections and an empirical relationship derived from large explosions.

Determining 3D flow fields via multi-camera light field imaging.

In the field of fluid mechanics, the resolution of computational schemes has outpaced experimental methods and widened the gap between predicted and observed phenomena in fluid flows. Thus, a need exists for an accessible method capable of resolving three-dimensional (3D) data sets for a range of problems. We present a novel technique for performing quantitative 3D imaging of many types of flow fields. The 3D technique enables investigation of complicated velocity fields and bubbly flows. Measurements of these types present a variety of challenges to the instrument. For instance, optically dense bubbly multiphase flows cannot be readily imaged by traditional, non-invasive flow measurement techniques due to the bubbles occluding optical access to the interior regions of the volume of interest. By using Light Field Imaging we are able to reparameterize images captured by an array of cameras to reconstruct a 3D volumetric map for every time instance, despite partial occlusions in the volume. The technique makes use of an algorithm known as synthetic aperture (SA) refocusing, whereby a 3D focal stack is generated by combining images from several cameras post-capture (1). Light Field Imaging allows for the capture of angular as well as spatial information about the light rays, and hence enables 3D scene reconstruction. Quantitative information can then be extracted from the 3D reconstructions using a variety of processing algorithms. In particular, we have developed measurement methods based on Light Field Imaging for performing 3D particle image velocimetry (PIV), extracting bubbles in a 3D field and tracking the boundary of a flickering flame. We present the fundamentals of the Light Field Imaging methodology in the context of our setup for performing 3DPIV of the airflow passing over a set of synthetic vocal folds, and show representative results from application of the technique to a bubble-entraining plunging jet.