Experiments to investigate lift production mechanisms on pitching flat plates

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...

Linking the Unsteady Force Generation to Vorticity for a Translating and Rotating Cylinder

“…It has also been shown that in contrast to the assumptions made in these models, bound circulation is small in rapidly accelerating wings at large incidence, where flow separates at the leading edge to form a vortex [4]. Therefore, numerous studies have investigated the effect of the leading edge vortex (LEV) as well as on developing reduced low order models to predict the force response [5][6][7]. Being able to quickly model the forces, will allow for improved safety as well as provide the tools to design broader operating windows.…”

Unsteady aerodynamics of porous aerofoils

“…Beckwith & Babinsky (2009) demonstrate experimentally that the impermeable Wagner function predicts well the lift history at long times for small angles of attack, where measured early-time responses overshoot the theoretical predictions due to delayed stall effects not present in the Wagner model. Larger impulsive changes to the angle of attack lead to persistent nonlinear flow structures that must be integrated into the aerodynamic lift model to improve its predictive capacity (Pitt Ford & Babinsky 2014; Li & Wu 2015; Stevens & Babinsky 2017). However, unsteady porous aerofoils may introduce important nonlinear flow features in real flows even at small angles, and an experimental campaign beyond the scope of the present work is needed to examine the relevant fluid mechanics of porous aerofoils and the predictive capability of the present model.…”

Micro Air Vehicle Scale Gust-Wing Interaction in a Wind Tunnel

“…Thus, it makes sense that some have analyzed a pitching motion in regard to unsteady flows and gust encounters. One such work by Stevens and Babinsky examines both the flow structure itself as well as the forces and moments generated by such a motion [72].…”