To alleviate the deterioration in wind turbine performance caused by dynamic stall, the flow control of a pitching NACA0012 airfoil is investigated through numerical simulation of an alternating current dielectric barrier discharge (AC-DBD) plasma actuator at a Reynolds number Re = 135 000. To avoid the harmonic oscillations of aerodynamic force caused by unsteady DBD actuation, this work focuses on improving the control potential for steady actuation. The control mechanisms of actuators at various positions are investigated using five groups of actuators mounted at 0%, 3%, 10%, 45%, and 80% chord lengths c above the upper surface of an airfoil. The actuator at 80% c performs more efficiently in terms of lift enhancement in the initial upstroke and the final downstroke. The actuator at 0% c suppresses the growth of the leading-edge vortex and maintains the suction of the dynamic stall vortex (DSV). After the shedding of the DSV, it suppresses the secondary separation to delay the onset of dynamic stall. At the flow reattachment stage, the actuators at 3% c and 10% c accelerate the boundary layer reattachment by momentum injection. From these results, a multi-DBD control strategy is proposed. The scheme selects the optimal actuator in operation at a certain stage of dynamic stall and takes advantage of actuators at different positions to enhance the average and maximum aerodynamic force, delay the onset of dynamic stall, accelerate flow reattachment, and avoid excessive energy consumption.
To alleviate the performance deterioration caused by dynamic stall of the wind turbine airfoil, the flow control by microsecond-pulsed dielectric barrier discharge (MP-DBD) actuator on the dynamic stall of a periodically pitching NACA0012 airfoil was investigated experimentally. The unsteady pressure measurements with high temporal accuracy were employed in this study, and the unsteady characteristics of the boundary layer were investigated by wavelet packet analysis and the moving root mean square (RMS) method based on the acquired pressure. The experimental Mach number was Ma =0.2, and the chord-based Reynolds number was Re = 870000. The dimensionless actuation frequencies F+ were chosen to be 0.5, 1, 2, and 3, respectively. For the light dynamic regime, the MP-DBD plasma actuator plays the role of suppressing flow separation from the trial edge and accelerating the flow reattachment due to the high momentum freestream flow being entrained into the boundary layer. Meanwhile, actuation effects were promoted with the increasing dimensionless actuation frequency F+. The control effects of the deep dynamic stall were to delay the onset and reduce the strength of the dynamic stall vortex (DSV), due to the accumulating vorticity near the leading edge being removed by the induced coherent vortex structures. The laminar fluctuation and K-H (Kelvin-Helmholtz) instabilities of transition and relaminarization were also mitigated by the MP-DBD actuation, and the alleviated K-H rolls lead to the delay of the transition onset and earlier laminar reattachment, which improved the hysteresis effect of the dynamic stall. For the controlled cases of F+ = 2, and F+ = 3, the laminar fluctuation was replaced by relatively low frequency band disturbances corresponding to the harmonic responses of the MP-DBD actuation frequency.
In order to obtain the performance of a pulse detonation turbine engine (PDTE) and compare the PDTE performance with the traditional turbine engine performance, analytical and experimental investigations were conducted in the present work. Ideal thermodynamic cycle analysis was firstly carried out to obtain the upper performance limit of both the PDTE cycle and the Brayton cycle-based engines. A simplified analytical model and a liquid-fueled dual tube PDTE system were built up in order to study the real performance of the PDTE. Thrust performance of the PDTE system was obtained under a self-aspirated mode. The calculated thrust performance based on the simplified analytical model was compared with the experimental data to verify the reliability of the simplified analytical model. Additionally, the specific thrust and specific fuel consumption (SFC) of the PDTE system were compared with the ideal performance of the PDTE cycle and the Baryton cycle-based engines. The results indicate that the ideal PDTE performance is much better than the traditional turbine engine performance. The dual tube PDTE system can be successfully operated at frequencies from 10 Hz to 20 Hz under the self-aspirated mode, indicating the feasibility of the PDTE concept. The simplified analytical model could estimate the real performance of the PDTE if proper losses were being taking into consideration. The specific thrust of the PDTE system is higher than that of the ideal Brayton cycle-based engines, while the SFC is much lower. When an ejector was used for thrust augmentation, the performance could be largely increased. It proves the performance advantages of the PDTE over the traditional turbine engine experimentally for the first time.
The dynamic properties of the flow induced by a dielectric barrier discharge (DBD) plasma actuator array are investigated from the Lagrangian perspective. First, numerical simulations based on a body force model are performed to obtain the flow field induced by unsteady plasma actuation in the burst mode. The numerical simulations capture the flow characteristics of plasma actuation well. Subsequently, the ridges of the finite-time Lyapunov exponent field are employed to identify the Lagrangian coherent structures (LCSs). Both the attracting and repelling LCSs organize the plasma-induced flow’s dynamic behaviors. The attracting LCSs visualize the plasma-induced vortices. The vortex formation, development, and merging processes in the unsteady plasma actuation are resolved well by the LCSs. The material transport in the plasma-induced flow is analyzed by tracing the fluid particle motions. Then, the influences of the actuation parameters, duty cycle, and burst frequency on the flow structures are explored via the attracting LCSs. The presented results enhance the understanding of plasma actuation flow physics and promote the optimal use of DBD plasma actuator arrays.
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