Investigation into the aerodynamics of gliding snakes

Abstract

Many animals use gliding as a form of locomotion, be it for foraging or to escape from predators. A key component for successful gliding is special appendages that can be used as wings to generate beneficial aerodynamic forces. However, flying snakes have a different strategy. Instead of using wings, flying snakes use full-body undulation. Complex aerodynamics are the result of this dynamic gliding behaviour and a detailed understanding is still missing. This thesis seeks to contribute to this area by providing tools and new insights into the aerodynamics of flying snakes. The aerodynamic performance is strongly influenced by the cross-sectional shape of an aerofoil. Yet, the cross-sectional shape of a flying snake has never been quantified. In a preliminary investigation, the cross-section of an airborne snake has been estimated using a 3D model, matching the hypothesised cross-section in the literature. For a more accurate investigation, a protocol is provided that uses a multi-view camera system and stereo-photogrammetry to create a 3D model with submillimeter accuracy.
Furthermore, two-dimensional computational models at Re=2,000 are used to investigate how dynamic movements affect the aerodynamic forces and the flow field experienced by a flying snake’s cross-section. It was found that a dynamic angle of attack with an pitch amplitude of 1° leads to an increase in lift-to-drag ratio of up to 3.5% while maintaining a separated flow structure as the primary vortex street. Adding horizontal and vertical movements increased the complexity of the vortex structure and widened the wake. To recapture these vortices, this could explain the need for the posteriorly increasing body displacement.
To validate these results, a robotic platform has been developed for particle image velocimetry (PIV), capable of performing the planar movements investigated in simulation. The robotic platform is designed for moving an aerofoil horizontally and vertically with an amplitude of up to 127mm at a frequency of up to 1Hz and 2Hz, respectively while simultaneously changing the angle of attack. Additionally, a stationary upstream aerofoil enables the investigation of aerodynamic interactions between dynamic aerofoils in tandem.
A critical step of the gliding cycle is cross-sectional morphing. How cross-sectional morphing could be achieved is addressed using an analytical model. The model describes the geometric specification and force requirements of a tendon-based mechanism which actuates ribs to evoke morphological changes. Retracting the tendon by 17.5% leads to flattening and lateral expansion of 35.9% and 687.6%, respectively.
The thesis generates new insights into the unique locomotion mode of flying snakes by simulating the aerodynamic phenomena generated during gliding and by developing new experimental methods for both biological measurement and robotic replication.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol University of the West of England
Supervisor	Andrew T Conn (Supervisor) & Jonathan M Rossiter (Supervisor)