Currently, there is a growing demand to improve the aerodynamic performance of Micro-Air-Vehicles for extended mission time, higher payload capacity and improved agility. Their wings are of small size and have to fly at low cruise speed. As a result, they have to operate within a challenging Reynolds number regime of Re = 10^4-10^5 which is known for its low energy content in the boundary layer, causing early flow separation and loss in lift production. Flexible wings, inspired from bats, could potentially address some of these difficulties. The aim is to promote shedding of vortical structures at the leading edge which can entrain energy close to the wing surface and enable flow attachment. The right selection of aspect-ratio together with wing compliance can contribute to the performance increase by forming tip-vortex structures close to the upper wing surface. However, the presence of these vortical structures comes at a price of increased drag and limitation in aerodynamic efficiency. This limitation could perhaps be address by designing MAVs that have flexible wings, but primarily operate in ground-effect (i.e. fly close to the surface) as this could combine the benefits of flexible wings with efficiency enhancement in ground-effect.
Therefore, two separate wind tunnel experiments are conducted to understand the impact of aspect-ratio and ground-effect on the fluid-structure interaction of membrane wings. Multiple measurement techniques are applied in high-speed, including load-cell recordings, photogrammetry, digital image correlation (DIC) and particle image velocimetry (PIV). The performance of rigid flat plates are compared to that of the membrane wings. The analysis involves time-averaged load, membrane deformation and flow quantities, their spatio-temporal evolution and correlation. Reduced order models are used to visualise highly energetic dynamics of the flow and the membrane deformations. Lower aspect-ratio membrane wings are found to extend operational range into higher angles-of-attack by delaying flow beneficial membrane dynamics. Membrane wings ability of static cambering and dynamic membrane oscillations are found to be beneficial in ground-effect, where the descent in height forces premature shedding of the leading edge vortex, however, this is accompanied by drag increase. Finally, the coupling between load-membrane-flow appears to change as the wing descends in to ground effect, especially the phase relationship between them where the ground tends to decouple the flow from the membrane as the lower side of the wing gains more importance.
Parts of this project are published in Journal of Fluids and Structures (see https://doi.org/10.1016/j.jfluidstructs.2016.02.005 and https://doi.org/10.1016/j.jfluidstructs.2016.12.001).
Figure: Part of the setup used in this project.
Robert Bleischwitz, Roeland de Kat, Bharath Ganapathisubramani
Fluid-structure interactions of membrane wings in free-flight and in ground-effect
Experimental Fluid Mechanics