Fluid-structure interactions for flexible and rigid tandem-wings at low Reynolds numbers

Force, Particle Image Velocimetry, Digital Image Correlation and hot-wire measurements were performed for two flat rectangular wings of semi-aspect-ratio 2 in tandem configurations. The wings were separated by a streamwise distance of 1.5 chord lengths at 30° angle of attack and Reynolds number 100,000. The crosswise wing separation was varied systematically from -1.5 to 1.5 chord lengths. A rigid-leading rigid-trailing (R-R) wing configuration and a flexible-leading rigid-trailing (F-R) wing configuration were considered. Crosswise wing separations for the F-R case between 0.0 to 0.4 chord lengths were shown to increase lift coefficient relative to the R-R case; the main benefit in lift generation was noted for the trailing-wing. For the F-R configuration, spanwise deformation of the leading-flexible wing shifted the impingement point of the trailing-edge shear layer on the trailing-wing thus affecting lift generation. Deformation measurements showed that the presence of the rigid-trailing-wing had a profound influence on the flow-induced vibrations of the leading-flexible-wing, peaking at a crosswise separation of 0.32 chord lengths which also provided the largest unsteady forces. Velocity spectra measurements revealed that cases producing increased unsteady forces possessed distinct inter-wing velocity fluctuations which were 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.