Quantification of the S817 airfoil aerodynamic properties and their control using synthetic jet actuators

Wind tunnel experiments were conducted at Rensselaer Polytechnic Institute's Center for Flow Physics and Control's subsonic wind tunnel, which experimentally quantified the aerodynamic performance of the S817 airfoil. This study has two main thrusts: Experimentally evaluate common aerodynamic properties of the S817 airfoil, and develop flow control strategies using continuously actuated and pulse‐modulated synthetic jets for future field testing to show the reduction of unsteady loading and increased aerodynamic performance. Quasi‐2D and finite span 3D configurations were utilized, where integrated aerodynamic loading, surface pressure, and stereoscopic particle image velocimetry data were collected to quantify the overall aerodynamic performance and stall characteristics of this airfoil. Experiments showed that synthetic jets, located at x/c=0.35 and angled at 45° with respect to the surface, increased the lift curve slope by 3.8%, the maximum lift coefficient by 10.5%, increased the L/D by as much as 39% at high angles of attack and delayed the stall angle of attack by 3°. Global particle image velocimetry measurements quantified the flowfield and showed flow reattachment could be achieved at various angles of attack using flow control where the flow would otherwise be separated. Near field measurements of the synthetic jet orifice yielded insight as to how synthetic jets interact with the cross‐flow in the time‐ and phase‐averaged sense. For very high angles of attack, a pulsed modulation technique was implemented, demonstrating flow reattachment in scenarios where a sinusoidal synthetic jet actuation scheme was unable to reattach the flow, with the benefit of achieving this with lower energy consumption compared with sinusoidal actuation.     

Load control on a dynamically pitching finite span wind turbine blade using synthetic jets

The feasibility of active flow control, via arrays of synthetic jet actuators, to mitigate hysteresis was investigated experimentally on a dynamically pitching finite span S809 blade. In the present work, a six‐component load cell was used to measure the unsteady lift, drag and pitching moment. Stereoscopic Particle Image Velocimetry (SPIV) measurements were also performed to understand the effects of synthetic jets on flow separation during dynamic pitch and to correlate these effects with the forces and moment measurements. It was shown that active flow control could significantly reduce the hysteresis in lift, drag and pitching moment coefficients during dynamic pitching conditions. This effect was further enhanced when the synthetic jets were pulsed modulated. Furthermore, additional reduction in the unsteady load oscillations can be observed in post‐stall conditions during dynamic motions. This reduction in the unsteady aerodynamic loading can potentially lead to prolonged life of wind turbine blades. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of acceleration on dynamic stall of airfoil in unsteady operating conditions

In this paper the effect of accelerated flow over a moving airfoil is considered and based on the flow field around the airfoil the dynamic stall is evaluated. In contrast to ordinary pitching motion, the dynamic stall evaluation in this study is performed with a different motion pattern, in which the airfoil has a heaving motion in one direction. This motion pattern is also similar to rotation of an element of blade in horizontal axis wind turbines (HAWTs). In present investigation, the Reynolds number is changed during simulation time and variations of this parameter from initial to final values are shown by acceleration parameter. The operating Reynolds number is more than 10⁶, and a S809 airfoil is selected to move with accelerations of 1, 4 and 6 m/s² in normal direction to free stream. To resolve accelerated flow filed in the two‐dimensional computational domain and to achieve results within a reasonable computation time, the unsteady Reynolds‐Averaged Navier–Stokes (URANS) equations are employed. The governing equations are discretized based on the finite volume approach and semi‐implicit method for pressure linked equations algorithm is used for pressure–velocity coupling. Furthermore, turbulence effect on flow field is accounted using shear stress transport (SST) k‐ω model. It is shown that the accelerated flow can significantly influence on the aerodynamic loads and dynamic stall trend. This study may introduce a new concept regarding dynamic stall and aerodynamic loads when the rotational acceleration is involved in HAWTs. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical simulation of high-power synthetic jet actuator flowfield and its influence on mixing control

Detailed two-dimensional unsteady numerical simulation is carried out to investigate a high-power synthetic jet actuator flow field and its design characteristic. Simultaneously, mixing control mechanism of coaxial jets with actuators is also studied. Firstly, excitation frequency （rotating speed）, piston displacement and its exit slot width have effect on the controlling ability and controlling efficiency of actuator. With the invariable model and con- cerned parameters, the actuator becomes more desirable as the rotating speed increases. Average velocity and maximal velocity at the actuator exit section increase as the piston displacement enlarges or the exit slot width decreases. But the actuator does not always exhibit good performance with the narrower exit. Secondly, the synthetic jets also have the ＂push＂ effect on the coaxial jets, which results in the fluctuation of vorticity and temperature distribution of mixing flowfield. Finally, the employment of synthetic jet actuator can achieve mixing enhancement significantly.     

Integrating a new flow separation model and the effects of the vortex shedding for improved dynamic stall predictions using the Beddoes–Leishman method

This paper aims at improving dynamic stall predictions on the S809 aerofoil under 2D flow conditions. The method is based on the well‐known Beddoes–Leishman model; however, a new flow separation model and a noise generator are integrated to improve the predictions in the load fluctuations, including those induced by vortex shedding on the aerofoil upper surface. The flow separation model was derived from a unique approach based on the combined use of unsteady aerodynamic loads measurements, the Beddoes–Leishman model and a trial‐and‐error technique. The new flow separation model and random noise generator were integrated in the Beddoes–Leishman model through a new solution algorithm. The numerical predictions of the unsteady lift and drag coefficients were then compared with the Ohio State University measurements for the oscillating S809 aerofoil at several reduced frequencies and angles of attack. The results using the proposed models showed improved correlation with the experimental data. Hysteresis loops for the aerodynamic coefficients are in good agreement with measurements. Copyright © 2016 John Wiley & Sons, Ltd.