Abstract
The smallest flying insects commonly possess bristled wings and use drag to provide flight forces. A bristled wing, with a wing area about 10% of that of a flat-plate wing, operating at the relevant Reynolds number of 5–15, produces a drag close to the plate wing. How this is done is not well understood. Here, detailed flows around each of the bristles are investigated numerically using simple model wings, and the following results are shown. (1) The drag production mechanism of the bristled wing is different from that of the plate wing: For the plate wing, the flow is blocked by the wing, giving a small positive pressure on the windward surface, and there exists a pair of weak vortices on the wing back, giving a small negative pressure on the leeward surface; the drag is due to the pressure forces (the frictional stress has almost no contribution). For the bristled wing, each bristle operates in a creeping flow and produces thick and strong shear layers. Strong viscous force generates a very large pressure difference between the windward and leeward surfaces of each bristle and very large frictional stress on the bristle surface, resulting in a large drag on each bristle, and the drag is equally contributed by the pressure and frictional forces. (2) Due to the flow-interference effect, when the bristle number reaches a certain value, a further increase in bristles has little effect on force production but has the disadvantage of increasing wing mass; this means that for a bristled wing of miniature insects, the distribution density of the bristles will not be too large, which agrees with observations.
Highlights
Most of the smallest flying insects have bristled wings, e.g., thrips,1,2 or partially bristled wings, e.g., the tiny wasp Encarsia Formosa3,4
(1) The drag production mechanism of the bristled wing is different from that of the plate wing: For the plate wing, the flow is blocked by the wing, giving a small positive pressure on the windward surface, and there exists a pair of weak vortices on the wing back, giving a small negative pressure on the leeward surface; the drag is due to the pressure forces
Each bristle operates in a creeping flow and produces thick and strong shear layers
Summary
Most of the smallest flying insects have bristled wings, e.g., thrips, or partially bristled wings, e.g., the tiny wasp Encarsia Formosa (for images of bristled wings, see Refs. 2 and 4). By carefully examining and analyzing the results, they found that the flow blocking effect of the bristled wing and the aerodynamic forces on the wing are determined by one parameter: effective gap width, which is the product of Red and the non-dimensional gap width These previous studies clearly show that a bristled wing operating in the biologically relevant Reynolds number regimes (Re, based on the wing chord length, about 5–15; and Red, based on the bristle diameter, about 0.01–0.05) produces an aerodynamic force close to that of a membranous wing. Based on the above discussion, we suggest that the force production mechanism of a bristled wing is as follows: each bristle is like a cylinder of very small diameter and operates at very small Reynolds numbers (Red smaller than 0.1), and very large force per unit area can be produced, just like the case of a body in creeping flow. Two-dimensional wing models can capture the relevant flow features. we preform our study in two dimensions using a cross section through the chord of the wing [Fig. 3(a)]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
