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1. Interspecific variation in bristle number on forewings of tiny insects does not influence clap-and-fling aerodynamics

   https://doi.org/10.1242/jeb.239798  

   Kasoju, Vishwa T.; Moen, Daniel S.; Ford, Mitchell P.; Ngo, Truc T.; Santhanakrishnan, Arvind (September 2021, Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT Miniature insects must overcome significant viscous resistance in order to fly. They typically possess wings with long bristles on the fringes and use a clap-and-fling mechanism to augment lift. These unique solutions to the extreme conditions of flight at tiny sizes (<2 mm body length) suggest that natural selection has optimized wing design for better aerodynamic performance. However, species vary in wingspan, number of bristles (n) and bristle gap (G) to diameter (D) ratio (G/D). How this variation relates to body length (BL) and its effects on aerodynamics remain unknown. We measured forewing images of 38 species of thrips and 21 species of fairyflies. Our phylogenetic comparative analyses showed that n and wingspan scaled positively and similarly with BL across both groups, whereas G/D decreased with BL, with a sharper decline in thrips. We next measured aerodynamic forces and visualized flow on physical models of bristled wings performing clap-and-fling kinematics at a chord-based Reynolds number of 10 using a dynamically scaled robotic platform. We examined the effects of dimensional (G, D, wingspan) and non-dimensional (n, G/D) geometric variables on dimensionless lift and drag. We found that: (1) increasing G reduced drag more than decreasing D; (2) changing n had minimal impact on lift generation; and (3) varying G/D minimally affected aerodynamic forces. These aerodynamic results suggest little pressure to functionally optimize n and G/D. Combined with the scaling relationships between wing variables and BL, much wing variation in tiny flying insects might be best explained by underlying shared growth factors. 

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2. Avian whiffling-inspired gaps provide an alternative method for roll control

   https://doi.org/10.1088/1748-3190/ac7303  

   Sigrest, Piper; Inman, Daniel J (June 2022, Bioinspiration & Biomimetics)

   Abstract Some bird species exhibit a flight behavior known as whiffling, in which the bird flies upside-down during landing, predator evasion, or courtship displays. Flying inverted causes the flight feathers to twist, creating gaps in the wing’s trailing edge. It has been suggested that these gaps decrease lift at a potentially lower energy cost, enabling the bird to maneuver and rapidly descend. Thus, avian whiffling has parallels to an uncrewed aerial vehicle (UAV) using spoilers for rapid descent and ailerons for roll control. However, while whiffling has been previously described in the biological literature, it has yet to directly inspire aerodynamic design. In the current research, we investigated if gaps in a wing’s trailing edge, similar to those caused by feather rotation during whiffling, could provide an effective mechanism for UAV control, particularly rapid descent and banking. To address this question, we performed a wind tunnel test of 3D printed wings with a varying amount of trailing edge gaps and compared the lift and rolling moment coefficients generated by the gapped wings to a traditional spoiler and aileron. Next, we used an analytical analysis to estimate the force and work required to actuate gaps, spoiler, and aileron. Our results showed that gapped wings did not reduce lift as much as a spoiler and required more work. However, we found that at high angles of attack, the gapped wings produced rolling moment coefficients equivalent to upwards aileron deflections of up to 32.7° while requiring substantially less actuation force and work. Thus, while the gapped wings did not provide a noticeable benefit over spoilers for rapid descent, a whiffling-inspired control surface could provide an effective alternative to ailerons for roll control. These findings suggest a novel control mechanism that may be advantageous for small fixed-wing UAVs, particularly energy-constrained aircraft. 

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3. Hybrid Metachronal Rowing Augments Swimming Speed and Acceleration via Increased Stroke Amplitude

   https://doi.org/10.1093/icb/icab141  

   Ford, Mitchell P; Ray, William J; DiLuca, Erika M; Patek, S N; Santhanakrishnan, Arvind (June 2021, Integrative and Comparative Biology)

   null (Ed.)

   Synopsis Numerous aquatic invertebrates use drag-based metachronal rowing for swimming, in which closely spaced appendages are oscillated starting from the posterior, with each appendage phase-shifted in time relative to its neighbor. Continuously swimming species such as Antarctic krill generally use “pure metachronal rowing” consisting of a metachronal power stroke and a metachronal recovery stroke, while burst swimming species such as many copepods and mantis shrimp typically use “hybrid metachronal rowing” consisting of a metachronal power stroke followed by a synchronous or nearly synchronous recovery stroke. Burst swimming organisms need to rapidly accelerate in order to capture prey and/or escape predation, and it is unknown whether hybrid metachronal rowing can augment acceleration and swimming speed compared to pure metachronal rowing. Simulations of rigid paddles undergoing simple harmonic motion showed that collisions between adjacent paddles restrict the maximum stroke amplitude for pure metachronal rowing. Hybrid metachronal rowing similar to that observed in mantis shrimp (Neogonodactylus bredini) permits oscillation at larger stroke amplitude while avoiding these collisions. We comparatively examined swimming speed, acceleration, and wake structure of pure and hybrid metachronal rowing strategies by using a self-propelling robot. Both swimming speed and peak acceleration of the robot increased with increasing stroke amplitude. Hybrid metachronal rowing permitted operation at larger stroke amplitude without collision of adjacent paddles on the robot, augmenting swimming speed and peak acceleration. Hybrid metachronal rowing generated a dispersed wake unlike narrower, downward-angled jets generated by pure metachronal rowing. Our findings suggest that burst swimming animals with small appendage spacing, such as copepods and mantis shrimp, can use hybrid metachronal rowing to generate large accelerations via increasing stroke amplitude without concern of appendage collision. 

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4. Fluid-structure Interaction of Flexible Flapping Wings at High Altitude Conditions

   https://doi.org/10.2514/6.2020-1781  

   Sridhar, Madhu; Kang, Chang-Kwon; Landrum, D Brian; Aono, Hikaru (January 2020, AIAA Scitech 2020 Forum)

   The long-range migration of Monarch butterflies extends over 4000 km. Monarchs experience varying density conditions during migration. Monarchs have been spotted at 1200 m during migration and overwinter at 3000 m, where the air density is lower than at the sea-level. Furthermore, Monarch butterflies have large flexible wings which deform significantly during flight. In this study, we test the hypothesis that the aerodynamic performance of the Monarch wing improves at reduced density conditions at higher altitudes. A design space with air density and stroke plane angle as design variables is constructed to evaluate the effects of fluid-structure interaction at high altitudes in the Reynolds number regime of Re = O(10^3). The effects of chordwise wing flexibility and the aerodynamic and structural response at varying densities are investigated by solving the Navier-Stokes equations, fully coupled to a structural dynamics solver at the Monarch scale. The lift, thrust and power are calculated in the design space. Our results show that lift increases with the stroke plane angle and the air density, whereas the thrust remains close to zero. The mean power required reduces with the altitude, eventually becoming negative at 3000 m. These results suggest that at lower altitudes near sea level, Monarchs can leverage the relatively large magnitude of their lift and thrust forces. At higher altitudes butterflies can fly while minimizing the power. 

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5. Propulsive efficiency of spatiotemporally asymmetric oscillating appendages at intermediate Reynolds numbers

   https://doi.org/10.1088/1748-3190/ad7abf  

   Herrera-Amaya, Adrian; Byron, Margaret_L (October 2024, Bioinspiration & Biomimetics)

   Abstract Many organisms use flexible appendages for locomotion, feeding, and other functional behaviors. The efficacy of these behaviors is determined in large part by the fluid dynamics of the appendage interacting with its environment. For oscillating appendages at low Reynolds numbers, viscosity dominates over inertia, and appendage motion must be spatially asymmetric to generate net flow. At high Reynolds numbers, viscous forces are negligible and appendage motion is often also temporally asymmetric, with a fast power stroke and a slow recovery stroke; such temporal asymmetry does not affect the produced flow at low Reynolds numbers. At intermediate Reynolds numbers, both viscous and inertial forces play non-trivial roles—correspondingly, both spatial and temporal asymmetry can strongly affect overall propulsion. Here we perform experiments on three robotic paddles with different material flexibilities and geometries, allowing us to explore the effects of motion asymmetry (both spatial and temporal) on force production. We show how a flexible paddle’s time-varying shape throughout the beat cycle can reorient the direction of the produced force, generating both thrust and lift. We also evaluate the propulsive performance of the paddle by introducing a new quantity, which we term ‘integrated efficiency’. This new definition of propulsive efficiency can be used to directly evaluate an appendage’s performance independently from full-body swimming dynamics. Use of the integrated efficiency allows for accurate performance assessment, generalization, and comparison of oscillating appendages in both robotic devices and behaving organisms. Finally, we show that a curved flexible paddle generates thrust more efficiently than a straight paddle, and produces spatially asymmetric motion—thereby improving performance—without the need for complex actuation and controls, opening new avenues for bioinspired technology development. 

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