Efficiency and Aerodynamic Performance of Bristled Insect Wings Depending on Reynolds Number in Flapping Flight

Felicity O’Callaghan,Amir Sarig,Fritz-Olaf Lehmann,Gal Ribak

doi:10.3390/fluids7020075

Felicity O’Callaghan, Amir Sarig + Show 2 more

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https://doi.org/10.3390/fluids7020075

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Abstract

Insect wings are generally constructed from veins and solid membranes. However, in the case of the smallest flying insects, the wing membrane is often replaced by hair-like bristles. In contrast to large insects, it is possible for both bristled and membranous wings to be simultaneously present in small insect species. There is therefore a continuing debate about the advantages and disadvantages of bristled wings for flight. In this study, we experimentally tested bristled robotic wing models on their ability to generate vertical forces and scored aerodynamic efficiency at Reynolds numbers that are typical for flight in miniature insects. The tested wings ranged from a solid membrane to a few bristles. A generic lift-based wing kinematic pattern moved the wings around their root. The results show that the lift coefficients, power coefficients and Froude efficiency decreased with increasing bristle spacing. Skin friction significantly attenuates lift production, which may even result in negative coefficients at elevated bristle spacing and low Reynolds numbers. The experimental data confirm previous findings from numerical simulations. These had suggested that for small insects, flying with bristled instead of membranous wings involved less change in energetic costs than for large insects. In sum, our findings highlight the aerodynamic changes associated with bristled wing designs and are thus significant for assessing the biological fitness and dispersal of flying insects.

Highlights

In the course of evolution, insect species have gone through a process of significant reduction in body size
Numerous previous experimental studies have focused on insect flight, most data investigating wing motion and aerodynamics were acquired from larger insects
Most wings that were previously tested in robotic apparatus or simulated numerically used flow past simple geometries of evenly spaced cylinder lattices to demonstrate how aerodynamic force production is reduced in bristled wings [13,14]

Summary

Introduction

In the course of evolution, insect species have gone through a process of significant reduction in body size. Many small insect species rely on flapping flight for locomotion, they suffer from their small size in different ways. They include a reduction in neurons for sensing and motor control and a decrease in flight muscle efficiency. It makes wing flapping more energetically costly because of added skin friction at low Reynolds numbers [2,3]. Very small insects avoid airfoil action and can be said to swim in air [4] This is surprising, given that rowing wings in flows near the change from high to low Reynolds numbers encounter high costs to overcome viscous friction. This friction is so pronounced that some of the smallest insects can use their wings as parachutes in order to delay their descent during wind dispersal [5]

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