Victor Maldonado

Active vibration control of a wind turbine blade using synthetic jets

Active vibration control via an array of synthetic jet actuators was investigated experimentally in a wind tunnel. Using synthetic jets the flow over a small scale S809 finite wind turbine blade was controlled, resulting in reduction of the blades structural vibrations. The effectiveness of the synthetic jets was explored for a range of post-stall angles of attack at Reynolds numbers between 7.1 × 104 and 2.38 × 105. The blade vibrations were measured and quantified using a pair of strain gauges mounted at the root of the model. Using flow control, significant vibration reduction was observed for some post-stall angles of attack. A correlation between vibration reduction and the degree of flow reattachment, measured using Particle Image Velocimetry, was found.

Active Enhancement of Wind Turbine Blades Performance

The feasibility of using synthetic jet actuators to enhance the performance of wind turbine blades was explored in wind tunnel experiments. Using this technique, the global flow field over the blade was altered such that flow separation was mitigated. This, in turn, resulted in a significant decrease in the vibration of the blade. Global flow measurements were conducted, where the moments and forces on the blade were measured using a six component wall-mounted load cell. The effect of the actuation was also examined on the surface static pressure at two spanwise locations; near the blade’s root and near the tip. In addition, Particle Image Velocimetry (PIV) technique was used to quantify the flow field over the blade. Using synthetic jets, the flow over the blade was either fully or partially reattached, depending on the angle of attack and the Reynolds number. Furthermore, the changes induced on the moments and forces, as well as on the blades vibrations were found to be proportionally controlled by either changing the momentum coefficient, the number of synthetic jets used, or by the driving waveform. Finally, a proof-of-concept closed-loop control system was developed to test the ability of using synthetic jet actuators to restore and maintain flow attachment and reduce the vibrations in the blade during dynamic pitch. The synthetic jets were switched on when the root strain vibration spectrum exceeds a predetermined threshold at a given frequency. While the control system implementation used is simplistic, it demonstrated the ability of synthetic jet actuators to reduce blade’s vibrations (by restoring and maintaining attached flow) during the dynamic motion, analogous to the wind gusts seen in wind turbine operation.

Free-Stream Turbulence Effects on the Flow around an S809 Wind Turbine Airfoil

Two-dimensional Particle Image Velocimetry (2-D PIV) measurements were performed to study the effect of free-stream turbulence on the flow around a smooth and rough surface airfoil, specifically under stall conditions. A 0.25-m chord model with an S809 profile, common for horizontal-axis wind turbine applications, was tested at a wind tunnel speed of 10 m/s, resulting in Reynolds numbers based on the chord of Re c ≈ 182,000 and turbulence intensity levels of up to 6.14%. Results indicate that when the flow is fully attached, turbulence significantly decreases aerodynamic efficiency (from L/D ≈ 4.894 to L/D ≈ 0.908). On the contrary, when the flow is mostly stalled, the effect is reversed and aerodynamic performance is slightly improved (from L/D ≈ 1.696 to L/D ≈ 1.787). Analysis of the mean flow over the suction surface shows that, contrary to what is expected, free-stream turbulence is actually advancing separation, particularly when the turbulent scales in the free-stream are of the same order as the chord. This is a result of the complex dynamics between the boundary layer scales and the free-stream turbulence length scales when relatively high levels of active-grid generated turbulence are present.

The Role of Large Scales of Turbulence in Wind Turbine Blades at Various Angles of Attack

Two-dimensional Particle Image Velocimetry (2-D PIV) measurements were performed to study the effect of free-stream turbulence (FST) on the flow around a smooth and rough surface airfoil, specifically under stall conditions. A 0.25-m chord model with an S809 profile, common for horizontal-axis wind turbine applications, was tested at a wind tunnel speed of 10 m/s, resulting in Reynolds numbers based on the chord of Rec ≈182,000 and turbulence intensity levels of up to 6.14%. Results indicate that when the flow is fully attached, turbulence significantly decreases aerodynamic efficiency (from L/D ≈ 4.894 to L/D ≈ 0.908). On the contrary, when the flow is mostly stalled, the effect is reversed and aerodynamic performance is slightly improved (from L/D ≈ 1.696 to L/D ≈ 1.787). Analysis of the mean flow over the suction surface shows that, contrary to what is expected, freestream turbulence is actually advancing separation, at stall conditions, particularly when the turbulent scales in the free-stream are of the same order as the chord. This is a result of the complex dynamics between the boundary layer scales and the free-stream turbulence length scales when relatively high levels of active-grid generated turbulence are present.