The flapping of wings is a common locomotion strategy for small organisms in both aerial and aquatic environments. Cuvierina atlantica, a marine pteropod, employs a distinctive "overlap-and-fling" motion to propel itself underwater. To examine the hydrodynamics underlying this behavior, we perform high-resolution fluid-structure interaction simulations using the lattice Boltzmann method coupled with the immersed boundary method. The body geometry and wing kinematics are accurately reconstructed from high-speed experimental videos and validated through direct overlay comparisons. The simulations show that C. atlantica relies primarily on lift-based propulsion, with both leading-edge and trailing-edge vortices contributing to thrust generation. The overlap-and-fling motion enhances circulation, promoting the development of strong vortices and sustained low-pressure regions along the wing edges, while wing-wing interaction increases vertical lift by more than 20%. Parametric analyses demonstrate that higher fluid viscosity elevates total hydrodynamic force but markedly reduces the lift-to-drag ratio and overall propulsion efficiency due to stronger viscous effects. Increasing flapping frequency boosts force production but leads to saturation in lift-to-drag and pressure-to-shear ratios, associated with increasingly chaotic wake structures. When interpreted using the Strouhal number, efficiency in the neutrally buoyant case exhibits an optimal value near 0.6. However, under gravity, this relationship breaks down, as weight-induced posture adjustments modify pitching dynamics and alter the phase relation between force generation and wake evolution. Across all conditions, higher chordwise Reynolds numbers generate numerous small vortices that enhance energy dissipation and reduce swimming efficiency.

Antarctic krill (Euphausia superba) are a key species in the Southern Ocean food web and form various aggregation types, from diffuse swarms to organized schools. However, little is known about how environmental variables such as flow and light affect the organization of these aggregations. Here, we investigate the effects of light, flow and group density on the group-level behavior and organization of Antarctic krill in an annular flume. The experimental setup comprises a rotating inner drum and pumps with flow conditioners to adjust flow speed, overhead lighting with variable intensity and infrared backlighting for filming. An overhead camera measures group organization and a stereophotogrammetry system records three-dimensional krill swimming trajectories. We conducted experiments at Palmer Station, Antarctica, varying krill group density (1-19 krill l-1), flow speed (no flow, low flow and high flow) and light level (light versus dark). Without flow and regardless of density, krill were significantly more organized in the light than in the dark, indicating the important role of vision in school formation. However, regardless of density, high flow produced similar levels of increased group organization in both the light and the dark, indicating that hydrodynamic cues may be sufficient to organize a krill school via positive rheotaxis. Density strongly affected group organization, with maximum organization seen at group densities of 1 and 7-9 krill l-1 irrespective of light or flow. These findings illuminate environmental effects on krill collective behavior and may lead to a better understanding of field acoustic data and improved krill biomass estimates.

Sea angels (gymnosomatous pteropods) are small zooplanktonic shell-less marine snails inhabiting the meso- and epipelagic zones. They swim in an intermediate Reynolds number regime using highly flexible, wing-like parapodia in order to capture prey, avoid predators, and perform diel vertical migration. However, the kinematics and fluid dynamics of gymnosome swimming are not well understood, particularly for species residing in low-viscosity, subtropical waters. Here we use high-speed stereophotogrammetry and dual brightfield particle image velocimetry (PIV) systems to investigate the swimming of the rare subtropical species Pneumoderma atlantica, captured off the coast of Bermuda. In particular, we quantify wing kinematics for hovering and slow upwards swimming and compare our results with morphologically similar temperate and polar species, which can be up to twice as large and swim in water up to twice as viscous. Like tiny insects flying in a similar regime, the chordwise Reynolds number appears to be inversely related to the wing angle of attack and stroke plane. Thus both the small, warm-water and the large polar gymnosomes seem to use their wings more like paddles to generate upward forces while the temperate species seems to use its parapodia more like wings to generate lift. Further, we provide the first flow measurements of a swimming gymnosome, these at somewhat higher swimming speeds, which show that gymnosomes employ an unsteady flow interaction between the wings and body (similar to the clap-and-fling mechanism) twice during each stroke cycle which likely generates additional lift. These findings provide insight into how similar locomotion modes may be adapted to different viscosities and into the widespread use of lift-generating, clap-and-fling-like mechanisms among marine snails.

When a drop impinges onto a deep liquid pool, it can yield various splashing behaviours, leading to a crown-like structure along the free surface. Under high-speed impact conditions, the upper portion of the thin-walled crown may undergo necking and encapsulate a large bubble, which remains fascinating and is rarely discussed in the literature. In this work, we numerically study this physical process based on the volume-of-fluid and adaptive mesh refinement framework. Our meticulous observations have allowed us to unveil a spectrum of repeatable early-time jet behaviours, vorticity structures and crater evolution, underscoring the rich and complex nature of drop-impact phenomenon. We show that the interplay between aerodynamic pressure and surface tension on the liquid crown could play a significant role in its bending and surface closure. A regime map, incorporating both early-stage jet dynamics and overall bubble-canopy formation, is established across a wide parameter space. This study provides a comprehensive understanding of the diverse splashing regimes, offering insights into the fundamental characteristics of drop-impact phenomenon.

Understanding the gust response of free-falling bodies such as plant seeds and debris is critical in predicting their dispersal. Furthermore, gusts can significantly affect the performance and survivability of low-inertia aerial vehicles. However, current methodologies for studying common gusts, particularly transverse gusts, which are characterised by the sudden appearance of a flow velocity component orthogonal to the flyer’s velocity, are not applicable to untethered or free-falling bodies. This article introduces a novel approach that addresses this limitation through an accelerating reference frame generating a fictitious force that temporarily scales and redirects the gravitational force. This approach is demonstrated through a first-of-its-kind vertical wind tunnel that accelerates horizontally in the direction normal to the flow with the same acceleration as the gust. A preliminary characterisation of the facility is presented. The tunnel acceleration generates the same pressure gradient as irrotational, uniform transverse gusts, without introducing the shear layer typical of Küssner’s gusts. The gust response of a free-falling dandelion diaspore to a discrete transverse gust (Wagner type) is demonstrated, but the proposed approach is suitable for arbitrary time series of transverse gusts, including Theodorsen-type periodic gusts. For the first time, this novel approach will allow investigating the dynamic response of untethered bodies to transverse gusts, including micro- and nanodrones, unpowered microrobots, plant seeds, debris and more.

Metachronal swimming consists of the sequential stroking of multiple appendages or cilia, resulting in a wave of appendage motion traveling along the body. Further, metachronal swimming spans the viscous to inertial regimes as it is used across seven orders of magnitude of Reynolds numbers Reb, where Reb = VL/nu, and V , L , and nu are the characteristic swimming speed, body length, and fluid kinematic viscosity, respectively. Through analysis of morphological and kinematics data collected from the literature on a wide variety of metachronally swimming organisms, we examine how these factors affect swimming performance across Reb. Further, we find a strong relationship among the kinematics parameters, swimming speed, and fluid viscosity. This power law relationship, Reb similar to S w , where S w = omega AL /nu is the Swimming number (omega: average angular appendage tip speed, A : appendage tip excursion), is maintained for all flow regimes, explains why metachronal swimming is a successful locomotion mode at low Reynolds numbers, and may prove useful in designing bio-inspired robots. We also find that the Strouhal number St = fA/V , where f is beat frequency, is relatively constant across a wide range of Reb but suggest that S w better describes the underlying hydrodynamics of metachronal swimming.

Langmuir circulation is a common form of surface turbulence comprising a series of counterrotating horizontal vortex pairs. Upwellings and downwellings in Langmuir circulation may suspend and trap passive particles or active swimmers like zooplankton in regions known as Stommel Retention Zones. For zooplankton, Stommel Retention Zone formation depends on flow speed and animal swimming speed and direction. Here we explore this biophysical interaction using Daphnia magna in a laboratory model of Langmuir Circulation. The phototactic daphniids were induced to perform different levels of upwards swimming (mimicking diel vertical migration) via constant or intermittent illumination. Some daphniids were additionally exposed to chemically dispersed crude oil, which impaired swimming. We characterized the swimming speed and direction, trajectories, and spatial distribution of daphniids in still water and in response to Langmuir circulation-like flows of various strengths. In still water, constantly illuminated and oil-exposed daphniids swam upwards more often than intermittently illuminated animals. Greater levels of upwards swimming in still water corresponded to stronger daphniid aggregations in the downwelling when a Langmuir Circulation-like flow was present. However, at flow speeds exceeding their swimming abilities, daphniids were generally advected with the flow and uniformly distributed, an effect particularly evident for the weakly-swimming oil-exposed daphniids. An individual-based model was also used to investigate the effects of active swimming vs. passive behavior and of swimming direction on Langmuir circulation-associated aggregations. Our study of zooplankton Stommel Retention Zones generated in a laboratory facility offers insight into how and when Langmuir circulation-associated plankton aggregations may occur in the field.

The present work is devoted to the analysis of drop impact on a deep liquid pool, focusing on the high-energy splashing regimes caused by large raindrops at high velocities. Such cases are characterized by short time scales and complex mechanisms, thus they have received very little attention until now. The BASILISK open-source solver is used to perform three-dimensional direct numerical simulations. The capabilities of octree adaptive mesh refinement techniques enable capturing of the small-scale features of the flow, while the volume of fluid approach combined with a balanced-force surface-tension calculation is applied to advect the volume fraction of the liquids and reconstruct the interfaces. The numerical results compare well with experimental visualizations: both the evolution of crown and cavity, the emanation of ligaments, the formation of bubble canopy and the growth of a downward-moving spiral jet that pierces through the cavity bottom, are correctly reproduced. Reliable quantitative agreements are also obtained regarding the time evolution of rim positions, cavity dimensions and droplet distributions through an observation window. Furthermore, simulation gives access to various aspects of the internal flows, which allows us to better explain the observed physical phenomena. Details of the early-time dynamics of bubble ring entrapment and splashing performance, the formation/collapse of bubble canopy and the spreading of drop liquid are discussed. The statistics of droplet size show the bimodal distribution in time, corroborating distinct primary mechanisms of droplet production at different stages.

Chemical herders contract oil slicks floating on the water surface and will likely be a useful tool at high latitudes for condensing oil spills in ice laden waters for in situ burning. However, floating ice and other obstacles on the sea surface fracture the contracting oil slick and trap patches of oil, thus degrading herder performance and burning efficiency. Using Alaska North Slope crude oil and the herding agent OP-40, we performed laboratory scale experiments to investigate how a contracting oil slick interacts with multiple obstacles arranged at different packing densities and with a bottleneck-type gap between obstacles, with a focus on the fluid mechanics of this interaction. The surface and spatial distribution of the herded oil slick is optically measured over time, and particle image velocimetry is used to measure the oil slick flow around the obstacles. Obstacle groups containing large numbers of tightly packed obstacles acted collectively to trap large amounts of oil while widely spaced obstacles that are fewer in number retained smaller individual tails of oil that then break up via capillary instability. Small bottlenecks delayed the contraction of the herded oil slick and resulted in a jet of herder penetrating downstream of the gap whereas wider bottlenecks allowed faster contraction and thus more fragmentation of the herded slick. These results provided a greater understanding of how a contracting fluid film on the water surface interacts with and is fragmented by different types of obstacles and provide a foundation for future modeling efforts to better understand ice-herder-oil interactions.

Chemical herders are a promising technique for treatment of oil spills in icy waters at high latitudes. However, obstacles on the water surface such as floating ice affects or interferes with all spill response countermeasures including chemical herder operations by fracturing the contracting oil slick and trapping small oil patches. Here we experimentally investigate the fluid dynamics of oil slick contraction and the interaction of the contracting oil slick with single obstacles of different shapes and sizes in a laboratory scale basin using Alaska North Slope crude oil and OP-40 herding agent. Oil slick area and thickness, oil-herder interface contraction speed, and the spatial distribution of the slick were optically measured over time, and particle image velocimetry (PIV) was used to measure the flow of the oil slick around the obstacles. For all obstacle sizes and shapes, an oil tail in the obstacle wake was generated as the oil-herder interface passed, which subsequently fractured by a capillary-type instability. Larger and less streamlined obstacles retained greater amounts of oil in these tails. In addition, a widely used theoretical thin film spreading model was successfully used to describe the spreading of the chemical herder against the oil slick at equilibrium. These results provide a foundation for future investigation of synergistic trapping of oil by multiple obstacles as would be seen in the field.

Precipitation in the forms of snow, hail, and rain plays a critical role in the exchange of mass, momentum and heat at the surfaces of lakes and seas. However, the microphysics of these interactions are not well understood. Motivated by recent observations, we study the physics of the impact of a single frozen canonical particle, such as snow and hail, onto the surface of a liquid bath using a numerical model. The descent, melting, bubble formation and thermal transport characteristics of this system are examined. Three distinct response regimes, namely particle impact, ice melting and vortex ring descent, have been identified and characterized. The melting rate and air content of the snow particle are found to be leading factors affecting the formation of a coherent vortex ring, the vertical descent of melted liquid and the vortex-induced transportation of the released gas bubble to lower depths. It is found that the water temperature can substantially alter the rate of phase change and subsequent flow and thermal transport, while the particle temperature has minimal effect on the process. Finally, the effects of the Reynolds, Weber and Stefan numbers are examined and it is shown that the Reynolds number modifies the strength of the vortex ring and induces the most significant effect on the flow dynamics of the snow particle. Also, the change of Weber number primarily alters the initial phases of snow–bath interaction while modifying the Stefan number of the snow particle essentially determines the system response in its later stages.

Bubbles are ubiquitous in industrial and environmental processes, and bubble bursting is a widely studied and highly important physical process. Bubbles bursting at an air–water interface may produce high-speed gas jets and vortex rings. Here, the effect of liquid properties such as viscosity, density, and surface tension on these gas jets is examined using high-speed visualization. Bursting events are examined for 500 µm to 44 mm diameter smoke-filled bubbles in five different liquids with variations in viscosity (1–944 mPa.s), surface tension (23.3–70.7 mN/m), and density (789–1259 kg/m3). Gas jet speed generally increases with parent bubble size until a bubble Bond number Bo of approximately one and subsequently increases at a slower rate. Further, gas jet speed correlates with surface tension, with low surface tension liquids such as ethanol producing low-speed jets and high surface tension liquids such as water producing high-speed jets. Film retraction speed decreases with increasing bubble size and, in conjunction with bubble emergence, influences gas jet diameter. A new scaling law relating the pressure potential energy of the parent bubble to the kinetic energy of the emerging gas jet reveals that bubble size and surface tension control the gas jet size and speed whereas liquid viscosity and density do not significantly affect jet behavior. This scaling is compared to a previously developed scaling for the gas jet Reynolds number. The prevalence of jet drop production, which heavily depends on liquid viscosity, also is examined as a function of Bo and bubble Ohnesorge number Ohb.

Stommel Retention Zones (SRZs) associated with wind-driven Langmuir circulation (LC) facilitate the subsurface retention of particles and thus affect processes such as oil spill dispersion, sediment suspension, oil-particle aggregation, and plankton distribution. Since the effect of SRZs on these processes is difficult to study in the field, we present a novel laboratory facility designed to recreate the counterrotating vortex pair flow pattern associated with small-scale SRZs of various strengths in two dimensions (2D). We then inject an oil jet into this facility to study buoyant oil droplet retention in these SRZs. 2D particle image velocimetry and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation are used to characterize the flow. Large-scale oil droplet trajectories in the SRZs are visualized, and oil droplet statistics in the downwelling region and vortex core are measured using high-magnification brightfield imaging and a droplet detection algorithm. Values of maximum velocity (7.7-32.2 mm s(-1)), turbulent kinetic energy (10(-6) to 10(-4) m(2) s(-2)), and turbulent kinetic energy dissipation rate (10(-7) to 10(-5) m(2) s(-3)) in the downwelling region of the facility fall within the range of values observed in the field. Flow field strength strongly influences oil droplet trajectories, the shape and size of the SRZ, and oil droplet statistics over time, with stronger flows retaining larger oil droplets in the downwelling region over longer periods of time. Indeed, droplets of almost 400 mu m diameter are observed in the downwelling region after almost 10 min for the strongest flow. The facility may be used to study the behavior of microplastics, sediments, and zooplankton in LC in the future.

Mantis shrimp swim via metachronal rowing, a pattern in which the pleopods (swimming limbs) stroke sequentially, starting with the last pair and followed by anterior neighbors. A similar swimming pattern is used at various sizes, Reynolds numbers, and advance ratios by diverse organisms including ciliates, ctenophores, copepods, krill, and lobsters. Understanding this type of locomotion is important because it is widespread and may inspire the design of underwater vehicles where efficiency, robustness, and maneuverability are desired. However, detailed measurements of the flow around free-swimming, metachronally rowing organisms are scarce, especially for organisms swimming in a high Reynolds number regime (Re >= 10(4)). In this study, we present time-resolved, planar PIV measurements of a swimming peacock mantis shrimp (Odontodactylus scyllarus). Simultaneous kinematics measurements of the animal, which had body and pleopod lengths of 114 and 20mm, respectively, reveal mean swimming speeds of 0.2-1.9 m s(-1) and pleopod beat frequencies of 3.6-13Hz, corresponding to advance ratios of 0.75-1.84 and body-based Reynolds numbers of 23,000-217,000. Further, the animal's stroke is not purely metachronal, with a long phase lag between initiation of the first and fifth pleopod power strokes. Flow measurements in the sagittal plane show that each stroking pleopod pair creates a posteriorly moving tip vortex which evades destruction by the recovery strokes of other pleopod pairs. The vortex created by the anteriormost pleopod pair is the strongest and, owing to the animal's high advance ratio, is intercepted by the power stroke of the posteriormost pleopod pair. The vortex strength increases as a result of this interaction, which may increase swimming speed or efficiency. A relationship for vortex interception by the posterior pleopod is proposed that relates the phase lag between the interacting pleopods to the beat frequency, distance between those pleopods, and speed of the vortex relative to the animal. We describe this interaction with a novel parameter called the interpleopod vortex phase matching Strouhal number StIVPM which is equal to the phase lag between interacting pleopods. This new nondimensional parameter may be useful in predicting the conditions where a constructive interaction may occur in other species or in physical models. Finally, we relate the advance ratio to the Reynolds number ratio, the ratio between the body-based Reynolds number and the pleopod-based Reynolds number. The importance of these parameters in promoting the interpleopod vortex interactions identified here, in dynamically scaled experiments, and in wake signatures behind schooling metachronal swimmers is discussed.

Department of Ocean and Mechanical Engineering, Florida Atlantic University, Boca Raton, FL, United States, Harbor Branch Oceanographic Institute, Florida Atlantic University, Fort Pierce, FL, United States, Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, United States, Department of Mechanical Engineering, Pennsylvania State University, University Park, PA, United States, Department of Mechanical

Langmuir supercells (LS) are full-depth Langmuir circulations in unstratified shallow shelves. A Reynolds-averaged Eulerian formulation is developed resolving LS as a secondary component to the wind-driven mean shear current. This formulation is combined with Lagrangian particle tracking to investigate oil droplet entrainment induced by LS as a function of wind stress. Two cases are simulated, one in which 500-mu m oil droplets are released into a steady field of LS generated by a wind stress of 0.1 N m(-2) and waves of intermediate wavelength lambda = 6H where H = 15 m is the water column depth, significant amplitude of 0.6 m and period of 8 s. In the second case, the 500-mu m oil droplets are released into a steady field of LS generated by the same wave forcing but with a weaker wind stress of 0.025 N m(-2). It is found that the greater wind stress leads to LS able to spread the droplets throughout upwelling and downwelling limbs of the cells within the first 80 minutes after release. The weaker wind leads to weaker LS that, within the same time after release, limit the dispersion of the droplets to the downwelling limbs of the cells forming Stommel retention zones for a prolonged time.

Metachronal motion is used across a wide range of organisms for a diverse set of functions. However, despite its ubiquity, analysis of this behavior has been difficult to generalize across systems. Here we provide an overview of known commonalities and differences between systems that use metachrony to generate fluid flow. We also discuss strategies for standardizing terminology and defining future investigative directions that are analogous to other established subfields of biomechanics. Finally, we outline key challenges that are common to many metachronal systems, opportunities that have arisen due to the advent of new technology (both experimental and computational), and next steps for community development and collaboration across the nascent network of metachronal researchers.