An experimental study of the process by which dynamic stall occurs on a finite span S809 airfoil was conducted at the Center for Flow Physics and Control at Rensselaer Polytechnic Institute. Understanding the flow field around a dynamically pitching airfoil helped in controlling the dynamic stall process through active flow control via synthetic jet actuators. The three component, two dimensional flow fields were measured with a stereoscopic particle image velocimetry system. This study demonstrated that, through the introduction of periodic momentum near the leading edge of this model, the evolution of the dynamic stall vortex, which forms and convects downstream under dynamic conditions, could be delayed or suppressed in favor of the preservation of a trailing edge vortex that arises due to trailing edge separation and recirculation in the time averaged sense. This process seems to be the result of changing how the flow field transitions from trailing edge separation to a fully separated flow. In a phase-averaged sense, absent of flow control, this process is defined by the creation of a phase averaged leading edge recirculation region, which interacts with the trailing edge separation. Through the introduction of momentum near the leading edge, this process can be altered, such that the phase averaged trailing edge separation region is the dominant structure present in the flow. Additionally, a cursory investigation into the instantaneous flow fields was conducted, and a comparison between the phase averaged flow field and instantaneous fields demonstrated that while similar effects can be observed, there is a significant difference in the flow field observed in the instantaneous fields versus the phase averaged sense. This would imply that a different method of analyzing dynamic stall from PIV measurements may be necessary.
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.
The feasibility of active flow control to mitigate hysteresis loop due to a dynamically pitching finite span s809 blade was investigated experimentally at a Reynolds number of 220,000. Under normal operating conditions, hysteresis loop and tip vibrations exist, which with extended exposure would cause blade fatigue and eventually translate to a reduced lifetime of wind turbines. In this regard, active flow control via arrays of synthetic jet actuators was explored as a means to control flow separation over the finite span blade, which can lead to mitigation of these undesired unsteady loads. In the present work, a six-component load cell was used to measure the aerodynamic loading of 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 to the forces and moment measurements. It was shown that active flow control could delay or minimize dynamic stall through the reduction of the hysteresis loop of the aerodynamic loads. This implies less unsteady aerodynamic loadings on the blade, which can potentially lead to prolonged life of wind turbines. Nomenclature= Planform area of model = area of synthetic jet orifice = instantaneous angle of attack ̅ = mean angle of attack = amplitude of angle of attack during sinusoidal pitching oscillation AEP = Annual Energy Production c = chord length C D = drag coefficient C L = lift coefficient C m = pitching moment coefficient C µ = momentum coefficient COE = Cost of Energy DRF = Discount Rate Factor ICC = Installed Capital Costs f p = pitching frequency ̅ = time averaged jet momentum = linear reduced frequency LRC = Levelized Replacement Costs = free-stream dynamic viscosity n = number of jets O&M = Operations and Maintenance Costs Re c = chord based Reynolds number = free-stream density = density of fluid exhausted from synthetic jet during outstroke SPIV = Stereoscopic Particle Image Velocimetry t = time T = cycle period of a synthetic jet actuator τ = synthetic jet outstroke time U = streamwise velocity component U ∞ = free-stream velocity u sj (t) = jet exit velocity at the centerline V = normal velocity component W = spanwise velocity component x = streamwise direction (parallel to the free stream) y = wall-normal direction (perpendicular to the free stream) I. IntroductionWind energy is a rapidly growing sector of both the economy and research. At the end of 2009, the total capacity of wind turbines installed in the world accounted for nearly 160 GW [1]. However, in order to maintain and improve the economics of wind energy, three different factors must be addressed. These factors are the energy capture of the turbine over its lifetime, the capitol cost necessary to install the turbine, and operations and maintenance cost. This can be expressed as [2]:
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