The three-dimensional structure of the flow behind a heaving and pitching finite-span wing is investigated using dye flow visualization at a Reynolds number of 164. Phase-locked image sequences, which are obtained from two orthogonal views, are combined to create a set of composite images that give an overall sense of the three-dimensional structure of the flow. A model of the vortex system behind the wing is constructed from the image sequences. Variations of the Strouhal number, pitch amplitude and heave/pitch phase angle are qualitatively shown to affect the structure of the wake.

A visual study of the flow structures behind a heaving and pitching finite-span wing

A review on fluid dynamics of flapping foils

Driven by its importance in nature and technology, droplet impact on solid surfaces has been studied for
decades. To date, research on control of droplet impact outcome has focused on optimizing pre-impact
parameters, e.g., droplet size and velocity. Here we follow a different, post-impact, surface engineering
approach yielding controlled vectoring and morphing of droplets during and after impact. Surfaces with
patterned domains of extreme wettability (high or low) are fabricated and implemented for controlling the
impact process during and even after rebound —a previously neglected aspect of impact studies on
non-wetting surfaces. For non-rebound cases, droplets can be morphed from spheres to complex shapes —
without unwanted loss of liquid. The procedure relies on competition between surface tension and fluid
inertial forces, and harnesses the naturally occurring contact-line pinning mechanisms at sharp wettability
changes to create viable dry regions in the spread liquid volume. Utilizing the same forces central to
morphing, we demonstrate the ability to rebound orthogonally-impacting droplets with an additional
non-orthogonal velocity component. We theoretically analyze this capability and derive a We2.25
dependence of the lateral restitution coefficient. This study offers wettability-engineered surfaces as a new
approach to manipulate impacting droplet microvolumes, with ramifications for surface microfluidics and
fluid-assisted templating applications.

Morphing and vectoring impactingdroplets by means ofwettability-engineered surfaces

The collective locomotion of two tandem autopropelled flapping foils is greatly affected by the phase difference. Two distinct vortex interactions are observed---merging interaction and broken interaction---which respectively result in the highest efficiency for the follower and the leader.

Phase difference effect on collective locomotion of two tandem autopropelled flapping foils

In the present study, a fractional-step-based multiphase lattice Boltzmann (LB) method coupled with a solution of a magnetic field evolution is developed to predict the interface behavior in magnetic multiphase flows. The incompressible Navier–Stokes equations are utilized for the flow field, while the Cahn–Hilliard equation is adopted to track the interface, and these governing equations are solved by reconstructing solutions within the LB framework with the prediction–correction step based on a fractional-step method. The proposed numerical model inherits the excellent performance of kinetic theory from the LB method and integrates the good numerical stability from the fractional-step method. Meanwhile, the macroscopic variables can be simply and directly calculated by the equilibrium distribution functions, which saves the virtual memories and simplifies the computational process. The proposed numerical model is validated by simulating two problems, i.e., a bubble rising with a density ratio of 1000 and a viscosity ratio of 100 and a stationary circular cylinder under an external uniform magnetic field. The interfacial deformations of a ferrofluid droplet in organic oil and an aqueous droplet in ferrofluid under the external magnetic field are, then, simulated, and the underlying mechanisms are discussed. Moreover, the rising process of a gas bubble in the ferrofluid is investigated, which shows that the rising velocity is accelerated under the effect of the external magnetic field. All the numerical examples demonstrate the capability of the present numerical method to handle the problem with the interfacial deformation in magnetic multiphase flows. Published under license by AIP Publishing. https://doi.org/10.1063/5.0020903., s

Numerical investigation of magnetic multiphase flows by the fractional-step-based multiphase lattice Boltzmann method

Performance investigation of a self-propelled foil with combined oscillating motion in stationary fluid

An interface-compressed diffuse interface method and its application for multiphase flows

The collective hydrodynamics in fish schools and bird flocks, which includes self-organization of multiple dynamic bodies, is complex and lacks sufficient exploration. In this paper, we study the performance of multiple self-propelled foils in tandem formation, whose flapping motions are asynchronous with a phase difference. It is shown that a compact formation, in which all of the foils perform like a complete anguilliform swimmer, can be spontaneously formed by multiple foils via hydrodynamic interactions. Both velocity enhancement and energy saving can be achieved by multiple foils in anguilliform-like swimming. Furthermore, such anguilliform-like swimming behaviour can be observed over a wide range of parameters, including the number of foils, the phase difference, the initial distance, the heaving amplitude and the pitching amplitude. The results obtained here may provide some light on understanding the self-organization behaviour of biological collectives.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/9B0D59092B1F7B8FAC143C0810342220/S0022112019009546a.pdf/div-class-title-self-organization-of-multiple-self-propelling-flapping-foils-energy-saving-and-increased-speed-div.pdf

Self-organization of multiple self-propelling flapping foils: energy saving and increased speed

Flapping-wing-based propulsion is ubiquitous in Nature, and it is free in all directions. In this work, the hydrodynamic behaviour of an unconstrained flapping foil, which can self-propel in both longitudinal and lateral directions, is numerically studied. It is found that the flapping foil can keep self-propelling in a straight line along the longitudinal direction, together with a passive oscillation in the lateral direction. Moreover, the effects of multiple parameters on the performance of the flapping swimmer are investigated, including the flapping frequency and amplitude, the mass ratio between foil and fluid, and the thickness–chord ratio of the foil. It is shown that the propulsive speed, the power consumption and the lateral oscillating motion obey some simple scaling laws. The results obtained here may provide some light on understanding biological flapping-wing-based propulsion.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112020009556

Xingjian Lin

Papers

Phase difference effect on collective locomotion of two tandem autopropelled flapping foils

Performance investigation of a self-propelled foil with combined oscillating motion in stationary fluid

An interface-compressed diffuse interface method and its application for multiphase flows

Self-organization of multiple self-propelling flapping foils: energy saving and increased speed

Self-directed propulsion of an unconstrained flapping swimmer at low Reynolds number: hydrodynamic behaviour and scaling laws