Low Aspect-Ratio Rigid, Flexible and Membrane Wings at Low Reynolds Numbers

Abstract

The interest in developing small, payload-carrying Micro Air Vehicles (MAVs)with a maximum linear dimension of six inches or below is as strong as ever,with both military and civilian bodies interested in the remote sensingcapabilities such a vehicle would provide. However significant challengesremain in developing such aircraft and the research presented here addressesspecific issues in the field of fluid-structure interactions for flexible wings.Wind tunnel tests were conducted on rectangular, aspect-ratio-two, rigid andflexible wings (including membrane wings) in an open-jet, closed-loop facility at the University of Bath. The chord Reynolds number for the experiments was in the range 34,000≤Rec≤69,000. Measurements of force, deformation and flow velocities were taken using a variety of techniques including Digital ImageCorrelation (DIC) and Particle Image Velocimetry (PIV).The key findings were that flexible wings experience different eigenmodevibrations and amplitudes. This was shown to be dependent on the incidenceangle of the wing to the incoming flow. Through measurements of the flow-fieldit was further shown that as the incidence angle increased, the behaviour of theseparated shear-layer was responsible for the changes in structural mode.There was a further change as the flow became fully separated and dominatedby bluff-body vortex shedding. Dynamic pitching of the wings, to simulate gustencounters, highlighted the complex nature of the fluid-structure interactions.Time lags between the flow features and structural response suggestedhysteresis effects also play a role.Tests conducted on free-to-roll wings demonstrated that self-excited ‘wing rock’ oscillations are possible for this configuration of wing at incidence angles either side of stall. These oscillations were driven by dynamic differences in thestrength and position of the two tip vortices. When subjected to forced pitchingmanoeuvres, synchronisation of the roll phase and frequency were observed.Given the right combination of pitch frequency and amplitude, this offered amethod of attenuating the self-excited rolling motion.

Date of Award	31 May 2013
Original language	English
Awarding Institution	University of Bath
Supervisor	Zhijin Wang (Supervisor) & Ismet Gursul (Supervisor)