Mosquitoes have slimmer wings, higher flapping frequencies, and much lower amplitudes than most other insects. These unique features signify special aerodynamic mechanisms. Besides the leading-edge vortex, which is one of the most common mechanisms of flapping-wing flight, mosquitoes have two distinctive mechanisms: trailing-edge vortex and rotational drag. In this study, the three-dimensional flow field around a hovering mosquito is simulated by using the immersed boundary method. The numerical results agree well with previous experimental data. Mechanisms unique to mosquitoes are identified from the instantaneous pressure and vorticity fields. The flow domains, containing several vortical structures produced by the flapping wings, are divided into different regions for quantitatively analyzing the contribution of vortical structures to the lift. Advection of the trailing-edge vortex and production of the leading-edge vortex each contribute peaks in lift. Passive deformation of the wings is also important, as it stabilizes delayed stall and decreases by 26% the maximum aerodynamic power required for hovering flight. In addition, the lift coefficient and power economy are improved as the Reynolds number increases, which explains the better ability of larger mosquitoes to seek and feed on hosts from the aerodynamic point of view.

Tunas are known for their extraordinary swimming performance, which is accomplished through various specializations. The caudal keels, a pair of lateral keel-like structures along the caudal peduncle, are a remarkable specialization in tunas and have convergently arisen in other fast-swimming marine animals. In the present study, the hydrodynamic function of caudal keels in tuna was numerically investigated. A three-dimensional model of yellowfin tuna with caudal keels was constructed based on previous morphological and anatomical studies. Vortical structures and pressure distributions are analyzed to determine the mechanisms of thunniform propulsion. A leading-edge vortex and a trailing-edge vortex are attached to the caudal fin and enhance the thrust. By comparing models of tuna with and without caudal keels, it is demonstrated that caudal keels generate streamwise vortices that result in negative pressure and reduce the transverse force amplitude. Moreover, the orientations of the streamwise vortices induced by caudal keels are opposite to those on the pressure side of the caudal fin. Therefore, caudal keels reduce the negative effects of the streamwise vortices adjacent to the caudal fin and thereby enhance the thrust on the caudal fin. A systematic study of the effects of variations in the Strouhal number (St), the Reynolds number (Re), and the cross-sectional shape of the body on the swimming of tuna is also presented. The effects of caudal keels are magnified as Re and St increase, whereas the cross-sectional shape has no major influence on the caudal keel mechanism.

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Caudal keels, a pair of lateral keel-like structures in tunas along the caudal peduncle, are a remarkable specialization. Although various hypotheses about the function of caudal keels have been proposed, our understanding of their underlying hydrodynamic mechanism is still limited. The penalty immersed boundary method was used to explore the hydrodynamics of a self-propelled flexible plate with a keel-like structure on the leading edge of the plate in an effort to understand the role of the caudal keel in nature. The clamped leading edge of the flexible plate was forced into a prescribed harmonic oscillation in the vertical direction but was free to move in the horizontal direction. For comparison, simulations without a keel were also performed. Vortical structures and pressure distributions were visualized to characterize the hydrodynamic benefits of the keel. The keel generates streamwise vortices that result in negative pressure and enhance the average cruising speed and thrust. The underlying propulsion mechanism was analyzed in detail by examining the phase of the heaving stroke. The average cruising speed and the propulsion efficiency are increased by more than 11.0% and 6.7%, respectively, by the presence of the keel. A parametric study was performed to determine the set of parameters of the keel that maximizes the propulsion efficiency η as a function of the reduced length (l/L) and the reduced height (h/L) of the keel.

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