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Insects rely on their olfactory systems to detect odors and locate odor sources through highly efficient flapping-wing mechanisms. While previous studies on bio-inspired unsteady flows have primarily examined the aerodynamic functions of flapping wings, they have largely overlooked the effects of wing-induced unsteady flows on airborne odor stimuli. This study aims to explore how flapping kinematics influence odorant transport. Computational fluid dynamics simulations were employed to investigate unsteady flow fields and odorant transport by solving the Navier–Stokes and odor advection–diffusion equations. Both two-dimensional (2D) and three-dimensional (3D) simulations were conducted to visualize the flow fields and odor concentration distributions generated by pitching–plunging airfoils. Our findings reveal that higher Strouhal numbers, characterized by increased flapping frequency, produce stronger flow jets that enhance odor advection and dissipation downstream, while reducing odor concentration on the airfoil surface. In 2D simulations, symmetry breaking at high Strouhal numbers causes oblique advection of vortices and odor plumes. In contrast, 3D simulations exhibit bifurcated horseshoe-like vortex rings and corresponding odor plume bifurcations. These findings highlight the intricate coupling between unsteady aerodynamics and odor transport, offering valuable insights for bio-inspired designs and advanced olfactory navigation systems. 

Flying insects are equipped with complex olfactory systems, which they utilize to seek food, identify mates, and evade predators. It is suspected that insects flap their wings to draw odor plumes toward their antennae, a behavior akin to mammals' sniffing, aimed at enhancing olfactory sensitivity. However, insects' wing kinematics change drastically as their flight speed increases, and it is unknown how these changes affect the insect's odorant perception. Addressing this gap in knowledge is crucial to a full understanding of the interplay between insects' aerodynamic performance and sensory perception. To this end, we simulated odor-tracking hawkmoth flight at 2 and 4 m/s using an in-house computational fluid dynamics solver. This solver incorporated both the Navier–Stokes equations that govern the flow, as well as the advection-diffusion equation that governs the odor transport process. Findings indicate that hawkmoths enhance odor intensity along their antennae using their wings, with peak odor intensity being 39% higher during 2 m/s flight compared to 4 m/s flight. This demonstrates there is a trade-off between rapid transport and olfaction, which is attributable to differences in wing kinematics between low- and high-speed flights. Despite literature suggesting hawkmoths are limited to steady forward flights at speeds below 5 m/s—about half of what is theoretically predicted based on body mass—this study reveals that slower flight speeds improve their olfactory capabilities during navigation. Our findings offer insights into the evolution of flight and sensory capabilities in hawkmoths, as well as provide inspiration for the development of bio-inspired odor-guided navigation technologies. 

Flying insects exhibit remarkable capabilities in coordinating their olfactory sensory system and flapping wings during odour plume-tracking flights. While observations have indicated that their flapping wing motion can ‘sniff’ up the incoming plumes for better odour sampling range, how flapping motion impacts the odour concentration field around the antennae is unknown. Here, we reconstruct the body and wing kinematics of a forwards-flying butterfly based on high-speed images. Using an in-house computational fluid dynamics solver, we simulate the unsteady flow field and odourant transport process by solving the Navier–Stokes and odourant advection-diffusion equations. Our results show that, during flapping flight, the interaction between wing leading-edge vortices and antenna vortices strengthens the circulation of antenna vortices by over two-fold compared with cases without flapping motion, leading to a significant increase in odour intensity fluctuation along the antennae. Specifically, the interaction between the wings and antennae amplifies odour intensity fluctuations on the antennae by up to 8.4 fold. This enhancement is critical in preventing odour fatigue during odour-tracking flights. Further analysis reveals that this interaction is influenced by the inter-antennal angle. Adjusting this angle allows insects to balance between resistance to odour fatigue and the breadth of odour sampling. Narrower inter-antennal angles enhance fatigue resistance, while wider angles extend the sampling range but reduce resistance. Additionally, our findings suggest that while the flexibility of the wings and the thorax's pitching motion in butterflies do influence odour fluctuation, their impact is relatively secondary to that of the wing–antenna interaction. 

Abstract Ctenophores swim using flexible rows of appendages called ctenes that form the metachronal paddling. To generate propulsion, each appendage operates a power stroke that strokes backward, followed by a recovery stroke that allows the appendage to readjust its position. Notably, strokes of most metachronal swimmers are asymmetric, with faster power strokes while slower recovery strokes. Previously, the material properties are assumed as isotropic. So, the faster power stoke will lead to more pronounce deformation and the slower recovery stroke will lead to less deformation. However, this contradicts with the observations that power-stroking ctenes have the least deformation and recover deforms more, indicating an anisotropic material behavior. Such anisotropic material is hard to be manufactured, but the anisotropic behavior may be achieved by making the initial structural shape curved. The pre-curved ctene, that bending towards downstream, will be straighten in power stoke while easy to bend during recovery stroke. Our study aims to demonstrate the feasibility of using pre-curved shapes to achieve anisotropic material properties during metachronal swimming. Treating it as fluid-structure interaction (FSI) problem, we integrate our in-house computational fluid dynamics (CFD) solver with a finite element method (FEM) solver, utilizing strong coupling methods for convergence. By comparing the performance of pre-curved ctenes with straight ones, which represent isotropic material properties, we found that the curved ctenes exhibited 26.05% to 65.69% higher cycle-averaged thrust compared to the straight one as stiffness is lower. However, as stiffness increased, the pre-curved ctenes produced 3.92% to 30.58% less thrust than the straight ones. Similar trends were observed in propulsive efficiency, with the pre-curved ctenes demonstrating 46.97% better efficiency at the lowest stiffness but dropping to 34.02% less efficient as stiffness rise. Thus, while the pre-curved initial shape led to better performance at lower stiffness, exceeding a certain stiffness threshold resulted in worse performance compared to straight ctenes. The thrust enhancement from pre-curve shape is due to the drag reduction during recovery stroke, where the curved shape mitigate part of force to point more downward. 

Abstract Ctenophores employ flexible rows of appendages called ctenes that form the metachronal beating pattern. A complete cycle of such paddling consists of a power stroke that strokes backward to produce propulsion and a recovery stroke that allows the appendage to recover its initial position. Effective locomotion in these creatures relies on maximizing propulsion during the power stroke while minimizing drag in the recovery stroke. Unlike rigid oars, the ctenes are flexible during both the power stroke and the recovery stroke, and notably, their strokes are asymmetric, with faster movement during the power stroke. As previous research assumed uniform material properties. This assumption will eventually make the ctene deform more intensively in the power stroke than the recovery stroke due to the asymmetrical hydrodynamic forces. However, observations contradict these assumptions. One explanation posits that ctenes stiffen during the power stroke, enhancing their propulsive force, and become more flexible in the recovery stroke, reducing drag by minimizing the water-countering area. This study focusses on the influence of asymmetric stiffness on their propulsion mechanism. Inspired by nature, we conducted three-dimensional fluid-structure interaction (FSI) using an in-house immersed-boundary-method-based flow solver integrated with a nonlinear finite-element solid-mechanics solver. This integrated solver uses a two-way coupling that ensures a higher accuracy regarding the complexity due to the involvement of the multiple ctenes in a ctene row. The preliminary results show that the anisotropic stiffness of the ctene have better accuracy of deformation as compared to the deformation recorded by the high-speed camera. The asymmetric properties of the ctene material allow both the spatial and temporal asymmetry of the ctene beating pattern. Our investigation suggests that while symmetrical beating can only generate negative net thrust, a slightly asymmetrical beating can make the thrust positive. We find that power stroke period that cost 30% whole period can generates the highest thrust. As multiple ctenes involves, the interaction among ctenes can amplified the effects of the asymmetrical beating, so that the thrust generation is enhanced by 9 to 13 times because of it. 

Odor-guided navigation is an indispensable aspect of flying insects' behavior, facilitating crucial activities such as foraging and mating. The interaction between aerodynamics and olfaction plays a pivotal role in the odor-guided flight behaviors of insects, yet the interplay of these two functions remains incompletely understood. In this study, we developed a fully coupled three-way numerical solver, which solves the three-dimensional Navier–Stokes equations coupled with equations of motion for the passive flapping wings, and the odorant advection–diffusion equation. This numerical solver is applied to investigate the unsteady flow field and the odorant transport phenomena of a fruit fly model in odor-guided upwind surge flight over a broad spectrum of reduced frequencies (0.325–1.3) and Reynolds numbers (90–360). Our results uncover a complex dependency between flight velocity and odor plume perception, modulated by the reduced frequency of flapping flight. At low reduced frequencies, the flapping wings disrupt the odor plume, creating a saddle point of air flow near the insect's thorax. Conversely, at high reduced frequencies, the wing-induced flow generates a stagnation point, in addition to the saddle point, that alters the aerodynamic environment around the insect's antennae, thereby reducing odor sensitivity but increasing the sampling range. Moreover, an increase in Reynolds number was found to significantly enhance odor sensitivity due to the synergistic effects of greater odor diffusivity and stronger wing-induced flow. These insights hold considerable implications for the design of bio-inspired, odor-guided micro air vehicles in applications like surveillance and detection. 

Odor-guided navigation is fundamental to the survival and reproductive success of many flying insects. Despite its biological importance, the mechanics of how insects sense and interpret odor plumes in the presence of complex flow fields remain poorly understood. This study employs numerical simulations to investigate the influence of turbulence, wingbeat-induced flow, and Schmidt number on the structure and perception of odor plumes by flying insects. Using an in-house computational fluid dynamics solver based on the immersed-boundary method, we solve the three-dimensional Navier–Stokes equations to model the flow field. The solver is coupled with the equations of motion for passive flapping wings to emulate wingbeat-induced flow. The odor landscape is then determined by solving the odor advection–diffusion equation. By employing a synthetic isotropic turbulence generator, we introduce turbulence into the flow field to examine its impact on odor plume structures. Our findings reveal that both turbulence and wingbeat-induced flow substantially affect odor plume characteristics. Turbulence introduces fluctuations and perturbations in the plume, while wingbeat-induced flow draws the odorant closer to the insect’s antennae. Moreover, we demonstrate that the Schmidt number, which affects odorant diffusivity, plays a significant role in odor detectability. Specifically, at high Schmidt numbers, larger fluctuations in odor sensitivity are observed, which may be exploited by insects to differentiate between various odorant volatiles emanating from the same source. This study provides new insights into the complex interplay between fluid dynamics and sensory biology and behavior, enhancing our understanding of how flying insects successfully navigate using olfactory cues in turbulent environments. 

Abstract Metachronous rowing is a swimming mechanism widely adopted by small marine invertebrate like comb jellies, in which rows of appendages perform propulsive strokes sequentially in a coordinated manner with a fixed phase difference. To simulate metachronous rowing at intermediate Reynolds number, in this paper, a row of flexible cilia models was placed inside the flow field, with their roots stroke at a sinusoidal function of time and a fixed phase difference. A fully coupled two-way numerical solver was developed, which solves the Navier-Stokes equations for the fluid field coupled with the differential equation for the flexible cilia model. This numerical solver is applied to investigate how the row of cilia models are deformed by the hydrodynamic forces (pressure and shear) and momentum and thus impact hydrodynamic performance. Results show that the passive deformation of cilia potentially improve the hydrodynamic performance compared to the rigid cilia. With the metachronous rowing mechanism, the cilia generate the thrust to move forward. The approach used in this study presents a general way to explore the fluid dynamics of complex fluid-structure interaction problems. 

Abstract Insects rely on their olfactory system to forage, prey, and mate. They can sense odorant plumes emitted from sources of their interests with their bilateral odorant antennae, and track down odor sources using their highly efficient flapping-wing mechanism. The odor-tracking process typically consists of two distinct behaviors: surging upwind at higher velocity and zigzagging crosswind at lower velocity. Despite extensive numerical and experimental studies on odor guided flight in insects, we have limited understandings on the effects of flight velocity on odor plume structure and its associated odor perception. In this study, a fully coupled three-way numerical solver is developed, which solves the 3D Navier-Stokes equations coupled with equations of motion for the passive flapping wings, and the odorant convection-diffusion equation. This numerical solver is applied to resolve the unsteady flow field and the odor plume transport for a fruit fly model at different flight velocities in terms of reduced frequency. Our results show that the odor plume structure and intensity are strong related to reduced frequency. At smaller reduced frequency (larger forward velocity), odor plume is pushed up during downstroke and draw back during upstroke. At larger reduced frequency (smaller forward velocity), the flapping wings induce a shield-like air flow around the antennae which may greatly increase the odor sampling range. Our finding may explain why flight velocity is important in odor guided flight.