A micro-scale flying insect has a unique wing configuration consisting of a central frame and several bristles. For the low-Reynolds-number regime in which the insect lives, the bristled wing utilizes a virtual fluid barrier inside gaps produced by strong viscous diffusion of shear layers to overcome its morphological limitations. Considering the unsteady flapping motion of such a wing, the aerodynamic characteristics of gap flow formation are investigated numerically using a two-dimensional bristled wing model for a wide range of Reynolds numbers. Inside a gap between bristles, the development of a stopping vortex during the deceleration phase and its effect on the extinction of an existing vortex generated at the same edge are dependent on the Reynolds number, which leads to a significant change in vorticity distribution at stroke reversal even with just a small change in the Reynolds number. As the Reynolds number decreases, the gap flow responds more rapidly to wing motion, and its pattern does not deviate significantly from the kinematics of the wing. A noticeable difference is also observed in the behavior of the aerodynamic force acting on each bristle at low and high Reynolds numbers. With regard to aerodynamic force generation by the bristles, each bristle behaves independently and produces similar force because of strong gap flows relative to the wing at high Reynolds numbers. Meanwhile, at low Reynolds numbers, each bristle experiences a different force depending on its relative position, which indicates the existence of collective interaction of bristles through a virtual fluid barrier.

Unlike the smooth wings of common insects or birds, micro-scale insects such as the fairyfly have a distinctive wing geometry, comprising a frame with several bristles. Motivated by this peculiar wing geometry, we experimentally investigated the flow structure of a translating comb-like wing for a wide range of gap size, angle of attack, and Reynolds number, Re = O(10) − O(10³), and the correlation of these parameters with aerodynamic performance. The flow structures of a smooth plate without a gap and a comb-like plate are significantly different at high Reynolds number, while little difference was observed at the low Reynolds number of O(10). At low Reynolds number, shear layers that were generated at the edges of the tooth of the comb-like plate strongly diffuse and eventually block a gap. This gap blockage increases the effective surface area of the plate and alters the formation of leading-edge and trailing-edge vortices. As a result, the comb-like plate generates larger aerodynamic force per unit area than the smooth plate. In addition to a quasi-steady phase after the comb-like plate travels several chords, we also studied a starting phase of the shear layer development when the comb-like plate begins to translate from rest. While a plate with small gap size can generate aerodynamic force at the starting phase as effectively as at the quasi-steady phase, the aerodynamic force drops noticeably for a plate with a large gap because the diffusion of the developing shear layers is not enough to block the gap.

This experimental study reports the collapse of an underwater bubble near a patterned structure with ridges and grooves. When a bubble is generated by a spark above a ridge, the entire bubble collapses toward the structure after its full expansion, or it is split into two smaller bubbles because of a radial jet induced by bubble contraction. These distinct collapse modes are dependent on the surface geometry of the structure and determined by the contracting speed of a bubble part inside the cross section of an adjacent groove. For a bubble that collapses in a groove, water flows induced from the tops of adjacent ridges collide with each other in the middle of the groove cross section, and this collision occurs if the effective width of the groove is small enough. For the bubble-splitting radial jet mode on the ridge and the collision mode in the groove, some energy of the bubble is lost during its contraction and, accordingly, the strength of the re-entrant jet toward the surface is weakened. Thus, these modes may be effective for reducing erosion on the structure surface, which is supported by our simple experiment for damage assessment.

In contrast to common continuous wings, microscopic flying insects such as fairyfly and thrips possess extraordinary wings that comprise several bristles. In the low-Reynolds-number environment inhabited by these insects, a bristled wing benefits from the formation of virtual fluid barriers that prevent flow leakage in the gaps of bristles. In this study, the optimal configuration regarding the aerodynamic performance of simple bristled-wing models is investigated numerically in terms of the Reynolds number and the distance between bristles, which are known to be important parameters for determining the effectiveness of a virtual fluid barrier. Inspired by the inherent characteristics of low-Reynolds-number flow, we provide new insights into the aerodynamics of a bristled wing by using a scaling parameter that combines these two independent parameters: effective gap width. In addition, the force ratio is scaled with the Reynolds number to better characterize the generation of drag and lift by a bristled wing. Despite the Reynolds number and the gap width varying by several orders of magnitude, the scaled drag force converges asymptotically beyond a specific effective gap width, and the lift force ratio has a peak within a very narrow range of the effective gap width. The trend of the lift with the effective gap width is correlated with changes in the asymmetric distributions of pressure and vorticity on the surface of the bristles. In addition, the drag and lift predicted theoretically, by invoking the linear superposition of creeping flows, agree well with numerical results, in particular for large effective gap width.

The stability and post-critical behaviour of periodic snapping are investigated experimentally for a buckled elastic sheet with two clamped ends under an external uniform flow. In addition to experimental investigations, low-order numerical simulations are conducted with the elastica model for the deformation of the sheet, which is coupled with the simple quasi-steady fluid force model based on Bollay's lift theory, in order to identify the deformed shape of the sheet in an equilibrium state and the critical velocity where the sheet begins to snap. Continuous exposure to fluid-dynamic loading induces snap-through oscillations from an initial equilibrium state. While the critical flow velocity for bifurcation is inversely related to the ratio of the streamwise distance of the sheet to its length, it is not significantly affected by the mass ratio of the sheet and the surrounding fluid, leading to divergence instability. In the post-equilibrium state, regular oscillations with the same dominant modes persist in the sheet for a broad range of the flow velocity. As the sheet crosses the midline in the snapping process, the bending energy stored in the sheet is released quickly, and the time for energy release is found to be lower than that required for energy storage. Because of the initial buckled shape, the minimum bending energy of the sheet over a cycle remains at least 40% of its maximum magnitude.