The blades of wind turbines are growing and length, while being constructed from lighter and more flexible material, making them susceptible to flutter and buffeting. Flutter is instability due to the negative dampening effects, which can occur in winds of sufficiently high speeds, whereas buffeting is caused by turbulence or other disturbances such as wakes in the incoming wind field. These phenomena can be a significant source of fatigue damage and undermine the structures serviceability. The ACTAGREEN (Aeroelasticity control for transportation and green energy) project was therefore established to seek common techniques for analysing and suppressing wind induced oscillation in large, flexible civil engineering structures. The initiative combined expertise in mechanics, aerodynamics and control and began by investigating the limits and potential of leading and trailing edge flaps in suppressing aeroelastic instabilities. Results showed that although stabilization for the system can be achieved relatively simply it is sensitive to wind speeds beyond the torsional divergence speed. This important finding sets a qualitative limit and benchmark for the performance of all controllers using leading-edge and trailing-edge winglets. In the case of long-span bridges it was also found that it was possible to significantly increase the structure’s performance by fitting the deck with controllable flaps along one third of the total length of the main span. Wind tunnel tests were conducted to confirm numerical findings and showed that leading edge and controlled edge flaps were effective in stabilizing the bridge. In addition, the use of passive systems, which did not require an energy supply, were used to devise the novel Flap Mass Damper (FMD), which absorbs energy from the vibrating structure. Researchers also showed that ′strip theory′ (whereby aerodynamic forces on a given section depend only on the flow field at that section) can be applied to long-span suspension bridges, regardless the scale of the turbulence. These results are also relevant for wings or rotor blades. ACTAGREEN will benefit the bridge design and construction industry and as well as the wind turbine industry. It will also benefit EU citizens by providing access to cheaper mobility and cheaper wind energy. The work also has important spin-off applications for the fluid-dynamic control of other flexible structures that operate within a flexible flow field.
Flutter, buffeting, wind turbine blade, ACTAGREEN, aeroelasticity, flap mass damper