Unsteady load mitigation through a passive trailing-edge flap

There are a wide range of applications in which it is desirable to mitigate unsteady load fluctuations while preserving mean loading. This is often achieved with active control systems, but passive systems are sometimes more desirable for enhancing reliability. This is the case, for example, for wind and tidal turbines, where unsteady loading limits the fatigue life of the turbine and results in power peaks at the generator. Here, we consider the unsteady load mitigation that can be achieved through a foil with a trailing-edge flap that is connected to the foil via a torsional spring. We develop a theoretical model and show that the preload can be tuned to preserve the mean foil loading. The spring moment that maximises the unsteady load mitigation is approximately constant, and the load fluctuation reduction is linearly proportional to the ratio of the flap to the full chord of the foil. We verify this relationship through water tunnel tests of a foil with a hinge at 25% of the chord from the trailing edge. As theoretically predicted, we measure unsteady load mitigation of up to 25%, without any variation in the mean load. In highly unsteady flow conditions, when boundary layer separation occurs, the unsteady load reduction decreases. Overall we conclude that passive trailing-edge flaps are effective in alleviating unsteady load fluctuations and their effectiveness depends on their size relative to the foil.