a b s t r a c tVertical axis wind turbine (VAWT) blades can experience large positive and large negative angles-ofattack that produce both inboard and outboard dynamic stall. Dielectric barrier discharge (DBD) plasma actuators can control dynamic stall and hence an inboard/outboard switching control technique was developed where encapsulated electrodes were deployed on either side of the blades of an H-rotor turbine. An electromechanical system, including a shaft-mounted micro-switch and high-voltage relays, was developed for the purpose of earthing the inboard encapsulated electrode of the upwind blade with the outboard encapsulated electrode of the downwind blade. The actuators were connected to a highvoltage source via slip-rings and were pulse-modulated to exploit flow instabilities in an on/off feedforward configuration. Turbine performance measurements showed that switching produced slightly larger improvements than either inboard or outboard actuation alone. The modest differences were traced to weak plasma being generated over the floating encapsulated electrodes, whose source was unavoidable slip-ring conductor proximity. Elimination of the floating electrode plasma resulted in larger performance increments for inboard versus outboard actuation due to the larger dynamic pressure relative to the blades in the upwind swept area of the turbine compared to that in the the downwind swept area.
The current work presents a 1D analytic model for a PV aeromechanical system and compares it with a 3D CFD model. The 1D model is based on the analogy between airflow and electric current. A PV aeromechanical system enables accurate positioning of thin, flexible substrates by creating an air cushion between the substrate and an accurate, rigid surface, having bi-directional aeromechanical spring-like behavior. Nozzle can be described as the relation they allow between flow (Q) and pressure drop (∆p): R ∝ ∆p/Qn where n depends on the characteristic behavior and (in this work) is between 1 and 2. The 1D model is computationally much cheaper than the 3D CFD model. Although the 1D model requires one CFD 3D model analysis for quantifying the exact resistance in the air cushion, it allows very fast calculations of performance when varying the other parameters of air gap, pressure/vacuum supply, and flowrate. The difference between 1D analytic model and full CFD analysis, in terms of air gap stiffness results was approximately 3%.
This paper presents results from a CFD analysis that highlights the effect of nozzles' characteristics on the performance of PV aeromechanical systems. PV aeromechanical systems enable accurate positioning of thin flexible substrate by creating an air cushion between the substrate and an accurate rigid surface, having bi-directional aeromechanical spring-like behavior. Nozzles can be described as the relation they allow between flow (Q) and pressure drop across them (∆p): ∆p ∝ Qn where n depends on the characteristic behavior and (in this work) is between 1 and 2. The characteristic behavior depends on the mechanism by which pressure is reduced. The mechanism can be dominated by inertial effects, by viscous effects, or by a combination of both inertial and viscous effects. It was found that aeromechanical performance is very sensitive to the nozzles' characteristic. An air cushion with high aeromechanical stiffness and constant flow rate is achieved by combining vacuum nozzles of exponent n=1 and pressure nozzles of exponent n=2.
Dynamic stall poses significant problems on vertical axis wind turbines (VAWTs) since the blades experience both large positive and large negative angles of attack. The resulting unsteady loads imposed on the drive-train and generator are primarily responsible for failures in the field. To control dynamic stall, a feed-forward control mechanism was designed and integrated with high-voltage, pulsed, dielectric barrier discharge (DBD) plasma actuators on a double-bladed VAWT. The actuators are pulsed to exploit flow instabilities in an on/off feed-forward configuration. A mechanical switch controlled a relay which switched between the earthed electrodes of the DBD plasma actuators which were placed on opposite sides of both VAWT blades. This enabled the high-voltage electrodes to generate plasma on alternating sides of the blades depending on which side was predicted to stall. As a result, dynamic stall was partially controlled by alternating actuators during continuous operation of the VAWT. Three types of tests were performed: actuation tests to find the output possible under different actuation conditions; tests to find the effective azimuth range of actuator operation; and transient tests to characterize the system's response. The results suggest that DBD plasma actuators have the potential to reduce unsteady loads and increase VAWT efficiency. NomenclatureA = projected turbine area C P = power coefficient ΔC P = change in power coefficient P C = change in power coefficient with floating electrode disabled P C = change in power coefficient with floating electrode not disabled c = chord lengthD = turbine diameter DC p = plasma duty cycle DC T = turbine duty cycle F + = reduced frequency, f mod · c / U rel f p = plasma ionization frequency f mod = modulation frequency H = turbine height N = number of plasma bursts P = turbine power output R = turbine radius Re = Reynolds number, U rel · c / ν RPM = turbine rotation speed in rotations per minute ΔRPM = change in turbine rotation speed, in rotations per minute T = torque applied by turbine T brake = torque applied by brake T mod = period of modulation frequency T p = period of plasma pulsation 1 Graduate Student, Faculty of Mechanical Engineering. 2 Associate Professor, Faculty of Mechanical Engineering, Senior Member. Downloaded by KUNGLIGA TEKNISKA HOGSKOLEN KTH on July 30, 2015 | http://arc.aiaa.org | 2 t = time t on = length of time plasma is on during a half-cycle U i = induced wind speed I U = induced wind velocity U rel = wind speed relative to turbine blade rel U = wind velocity relative to turbine blade U ∞ = freestream wind speed V p2p = peak-to-peak voltage input to actuators α = angle of attack Γ = error fraction θ = blade azimuth angle θ on = blade azimuth angles during which DBD actuators operate θ i = blade azimuth angle during which DBD actuators initiate operation θ t = blade azimuth angle during which DBD actuators terminate operation λ = ratio of blade speed to freestream wind speed (tip speed ratio) τ = time constant ω = turbine rotational s...
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