Flow turning effect and laminar control by the 3D curvature of leading edge serrations from owl wing

Abstract

This work describes a novel mechanism of laminar flow control of straight and backward swept wings with a comb-like leading edge device. It is inspired by the leading-edge comb on owl feathers and the special design of its barbs, resembling a cascade of complex 3D-curved thin finlets. The details of the geometry of the barbs from an owl feather were used to design a generic model of the comb for experimental and numerical flow studies with the comb attached to the leading edge of a flat plate. Due to the owls demonstrating a backward sweep of the wing during gliding and flapping from live recordings, our examinations have also been carried out at differing sweep angles. The results demonstrate a flow turning effect in the boundary layer inboards, which extends downstream in the chordwise direction over distances of multiples of the barb lengths. The inboard flow-turning effect described here, counteracts the outboard directed cross-span flow typically appearing for backward swept wings. This flow turning behavior is also shown on SD7003 airfoil using precursory LES investigations. From recent theoretical studies on a swept wing, such a way of turning the flow in the boundary layer is known to attenuate crossflow instabilities and delay transition. A comparison of the comb-induced cross-span velocity profiles with those proven to delay laminar to turbulent transition in theory shows excellent agreement, which supports the laminar flow control hypothesis. Thus, the observed effect is expected to delay transition in owl flight, contributing to a more silent flight.

A Comparison of Aerodynamic Parameters in Two Subspecies of the American Barn Owl (Tyto furcata)

“…Although this is different from the standard ornithological morphometrics [ 25 ], we felt that this way of measuring wing length is justified in the barn owl. The reason is that the wing is swept back distally (see also [ 4 , 17 ] so that the line from the tip of the wing (the tip of the tenth primary) to the base is almost parallel to the leading edge ( Figure 1 a,b).…”

“…Slow flight produces less noise. Moreover, a recently observed flow-turning effect in the boundary layer of the barn owl wing serves to attenuate crossflow instabilities and delay transition [ 17 ]. In other words, the data in [ 5 , 6 , 17 ] demonstrate that the owl wing has mechanisms to stabilize flow that improve aerodynamic behavior but also contribute to silent flight.…”

Interaction of barn owl leading edge serrations with freestream turbulence

“…What is missing so far is a more detailed investigation of how barn-owl shaped artificial serrations affect airflow, especially in the vicinity, and immediately downstream of the serrations and independently of the other adaptations to silent flight (Jaworski and Peake 2020). A first approach by Muthuramalingam et al (2020b) showed the surprising effect of an inboard flow turning induced by the serrations in a freestream flow, which was concluded to postpone transition to turbulent flow on the wings suction side during gliding flight, supporting the aeroacoustic noise reducing effect of the serrations as discussed above. This study used models of serrations based on digital reconstruction of the micron-size natural barb structures, producing serrations with yaw, pitch and twist, closely resembling the shape of natural serrations.…”

Trailing-edge fringes enable robust aerodynamic force production and noise suppression in an owl wing model

“…The silent flight of owls has long been a subject of interest and depends on a wide variety of factors [4]. In Graham's pioneering work [1], the unique owl-wing morphologies comprising leadingedge (LE) serrations [12][13][14][15][16], trailing-edge (TE) fringes [17][18][19][20], and velvety upper wing surfaces [21][22][23][24] were first systematically investigated. This scholar speculated that the three owl-wing morphologies may play complementary roles in contributing to the silent flight of owls.…”