Overcoming the difficulties associated with low Reynolds number flows has recently become a primary goal for aerodynamicists due to the growing importance of micro air vehicles (MAVs). The limiting size requirement of a six-inch wing span for MAVs combined with their inherent tendency to suffer stall due to gusts makes this significantly more challenging. The use of two-wing configurations, inspired by historical aircraft, could prove to be an effective method of overcoming this limitation. This thesis is primarily concerned with the fundamental aerodynamics associated with biplane and tandem wing configurations at a low Reynolds number using experimental evidence. Experiments were performed at a Reynolds number, based on wing chord, of Re = 10^5 in a return-circuit open-jet wind tunnel. The wing models were rectangular flat planform wings with a semi-aspect ratio of two. The effects of streamwise and crosswise wing separation, spanwise wing flexibility, angle of attack and decalage (the relative incidence of two wings) are considered.Experiments considering the effects of streamwise and transverse wing separation in rigid wings without decalage revealed that at post-stall angles of attack, lift performance improves and stall is delayed significantly for many two-wing configurations. For a given angle of attack, there are optimal transverse wing separations for which total lift coefficient is maximised. Particle image velocimetry (PIV) measurements reveal five characteristic flow regimes. The aerodynamic characteristics of two-wing configurations depend heavily on the nature of the accelerated inter-wing flow. This accelerated flow has a profound influence on the separated shear layers emanating from the leading and trailing edges of both wings. Unsteady forces intensify for certain two-wing configurations. High flow unsteadiness and large lift fluctuations are associated with either a switching between stalled and unstalled states over the trailing-wing's suction surface or a switching between merged and distinct wing wakes.Combinations of spanwise flexible wings with rigid wings for two streamwise wing separations, ΔX/c = 0.5 and 1.5, at an angle of attack of 30° were investigated with force measurements. The selected data compare rigid-leading rigid-trailing (R-R) wing configurations with flexible-leading rigid-trailing (F-R) wing configurations for ΔX/c = 1.5 (α = 30°). The transverse wing separation was varied systematically from -1.5 to 1.5 chord lengths. PIV, Digital Image Correlation (DIC) and hot-wire measurements were performed to further investigate the nature of these two cases. For the F-R case, transverse wing separations between 0.0 and 0.4 chord lengths exhibit increased lift coefficients relative to the R-R case; the main benefit in lift generation is for the trailing-wing. For the F-R configuration, spanwise deformation of the leading-flexible wing shifts the impingement point of the trailing-edge shear layer on the trailing-wing thus affecting lift generation. DIC measurements show that the presence of the rigid-trailing wing has a profound influence on the flow-induced vibrations of the leading-flexible wing. These flow induced vibrations reach the greatest amplitude at a transverse separation of 0.32 chord lengths, matching the largest unsteady forces. Hot-wire velocity spectra measurements reveal that cases producing increased unsteady forces possess distinct inter-wing velocity fluctuations which are coupled with the wing vibrations. The results demonstrate that the time-averaged forces and unsteady fluid-structure interactions are strongly determined by the crosswise wing separation and the spanwise flexibility of the leading-wing in tandem configurations.To investigate the effects of decalage (differing wing angle), experiments were performed for three cases in the stalled regime. Force measurements demonstrate that the use of decalage can strongly enhance the lift characteristics of two-wing configurations in the stalled regime. PIV measurements show that performance relies heavily on the augmentation of downwash and the occurrence of a secondary trailing edge recirculation region. Configurations producing enhanced aerodynamic and power efficiency are associated with reduced recirculation regions and increased downwash. Configurations yielding enhanced lift characteristics are associated with increased downwash as well as significant entrainment of the wake over the upper/leading wing due to the accelerated inter-wing flow. The stall of the upper/leading wing is consistently coupled with a secondary recirculation region due to the upper/leading wing's trailing edge shear layer. A significant transverse wing separation dependence occurs for a fixed leading wing incidence of 25° and trailing wing incidence of -10°. A very sharp discontinuity in time-average force coefficients between ΔY/c = -0.75 and -0.6 occurs for this case and a similar discontinuity is observed for a trailing wing incidence of -5°. Flow fields reveal a sensitive flow regime transition which is highly dependent on the transverse wing separation. Three configurations with increased unsteadiness were identified in the force data; instantaneous PIV measurements reveal subtle and intermittent inter-wing flows encroaching through the wake. Significant unsteady forces are only found to occur in the considered configurations (with decalage) when the wings' wakes are merged.
|Date of Award||6 May 2016|
|Supervisor||Ismet Gursul (Supervisor) & David Cleaver (Supervisor)|