Effect of Mounting System Friction on Simultaneous Thrust and Power Production for an Airfoil in the Wake of a Cylinder

An airfoil interacting with the wake shed by an upstream circular cylinder is able to extract both net power and net thrust from the flow when allowed to react passively to oncoming vorticity. This has important implications for the design of small-scale passive energy harvesting devices; however such devices are engineering systems, and their real dynamics may include nonidealities such as friction. To characterize the effect of mounting system friction on the flow past the airfoil, and therefore on the thrust and power production of the system, two sets of experiments were performed. Firstly, the effects of friction were directly observed from a mechanically-mounted airfoil. Secondly, an airfoil was driven through an idealized sinusoidal trajectory, exhibiting no frictional effects. The power and thrust production for both systems are compared, and the flow structures giving rise to the observed characteristics are interrogated. Implications for small-scale energy harvesting devices are explored.