No abstract

exo-planetary exploration (Young et al. 2000). MAVs can use flapping wing designs to take advantage of unsteady effects at low Reynolds numbers (Petricca et al. 2011). Flapping wing-style MAV designs could be improved with a better understanding of the governing physics of flapping wing motions. For example, more efficient flapping movements could provide longer battery life and hence increase flight duration. Of particular interest in the generation of an efficient motion is the fluid mechanical process by which lift is generated.One approach for understanding unsteady lift generation is to study simplified flapping motions. The simplest case experimentally is that of a purely translating wing. Dickinson and Götz (1993) used the relative motion of an accelerated wing in a stationary fluid to analyse the Wagner effect (see Sect. 1.3) at Reynolds numbers relevant to insect/MAV flight. They also discussed the inertial, 'virtual mass' effect which occurred during transients.1 Further work by Beckwith and Babinsky (2009) demonstrated that for small incidences, after the initial transient passes, Wagner's theoretical prediction converges with the measured force data. Later, PittFord (2013) demonstrated how the leading-edge vortex (LEV) circulation on a high incidence surging wing could be reasonably well modelled using Wagner's theorem and coupled with virtual mass to predict lift. It is not obvious that Wagner's theorem should apply at all since it is for thin aerofoils at small incidence and attached flow. Thus, here we want to investigate further the idea that Wagner 1 The non-circulatory effect occurs in an unsteady flow, i.e. when a wing is accelerated, a mass of fluid is accelerated with the wing, creating an inertial reaction force, which can contribute to the lift. There is no circulation associated with the production of this force. The effect is commonly called 'virtual mass '. Abstract Pitching flat plates are a useful simplification of flapping wings, and their study can provide useful insights into unsteady force generation. Non-circulatory and circulatory lift producing mechanisms for low Reynolds number pitching flat plates are investigated. A series of experiments are designed to measure forces and study the unsteady flowfield development. Two pitch axis positions are investigated, namely a leading edge and a mid-chord pitch axis. A novel PIV approach using twin laser lightsheets is shown to be effective at acquiring full field of view velocity data when an opaque wing model is used. Leading-edge vortex (LEV) circulations are extracted from velocity field data, using a Lamb-Oseen vortex fitting algorithm. LEV and trailing-edge vortex positions are also extracted. It is shown that the circulation of the LEV, as determined from PIV data, approximately matches the general trend of an unmodified Wagner function for a leading edge pitch axis and a modified Wagner function for a mid-chord pitch axis. Comparison of experimentally measured lift correlates well with the prediction of a reduced-order model f...

No abstract

The understanding of low Reynolds number aerodynamics is becoming increasingly prevalent with the recent surge in interest in advanced Micro-Air Vehicle (MAV) technology. Research in this area has been primarily stimulated by a military need for smaller, more versatile, autonomous, surveillance aircraft. The mechanism for providing the high lift coe cient required for MAV applications is thought to be largely influenced by the formation of a Leading Edge Vortex (LEV). This paper analyses experimentally, the influence of the LEV e↵ect for a flat plate wing (A = 4) under fast and slow pitch-up motions at Re =10,000 using a combination of dye flow visualisation and PIV measurements. It is found that a fast pitch over 1c shows a flow topology dominant LEV, while for a slow pitch case over 6c, the flow is largely separated. The development of the suction surface flow and the LEV was strongly correlated with the kinematics of the leading edge, suggesting that the e↵ective local angle of incidence at the Leading Edge (LE) is of considerable significance in unsteady pitching motions. NomenclatureA Aspect Ratio b Wing span, m c Wing chord, m c max Maximum number of chords travelled k Reduced frequency LE Leading Edge LEV Leading Edge Vortex Q Q-criterion vortex detection parameter s Distance travelled in chord lengths, m TE Trailing Edge TEV Trailing Edge Vortex t Time, s U inf Reference velocity, ms 1 ↵ Angle of incidence, ↵ Pitch rate, rad/s ✏ Error ci Swirling Strength ⇢ Density, kg/m 3

No abstract