Flow characteristics of aerial swimmers: flapping wing propulsion of tiny insects flying at extremely low Reynolds numbers

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Investigating the fluid dynamic processes of force production in complex organisms is central for any in-depth understanding of locomotion in flying and swimming animals. Within the past decade, the unusual and potent mechanisms of lift production in insects and birds have attracted numerous researchers in the field of biological fluid dynamics. Most studies on insect aerodynamics considered animal wings as simple, mostly rigid, flat plates, flapping at high Reynolds numbers between approximately 200 and 10,000. Flight of small insects with wingspans of less than a millimetre, by contrast, had been excluded from this analysis because tiny insects have abandoned altogether airfoil action and literally swim in the air. In contrast to conventional insects and birds, moreover, small insects exhibit unusual wings of comb-like planform, where up to 80% of the wing surface is replaced by long hairs. There is a pronounced gap in our understanding of the aerodynamic consequences of bristle wings for creeping flow propulsion including the question of how drag-based propulsion systems may generate vertical lift for body weight support. This proposal is thus concerned with the fluid dynamic phenomenon in smallest insects flying at Reynolds numbers below approximately 10. We aim to quantify flow structures of bristle wing fliers, scoring wing kinematics by high-speed video technique and quantifying flow structures by time-resolved digital particle image velocimetry (TRPIV). Employing robotic wings, we will determine the functional relevance of bristle wing design on both wake structure and force generation, and subsequently compare flow structures at flapping model wings with the data obtained from the flying insect. The results of this work might give us several new insights into animal locomotion, i.e. by estimating boundary layer structure and viscous drag propulsion in a miniaturized biological flight system and by providing aerodynamic key values for flight that may be used to determine power expenditures and flight efficiency in highly viscous environments.