Abstract
An array of 26 plasma synthetic jet actuators (PSJA) is flush-mounted on a NACA-0015 airfoil model to control the leading-edge flow separation at moderate Reynolds number (Re_c=1.7times 10^5). The stall angle of this airfoil is postponed from 15.5^circ to approximately 22^circ, and the peak lift coefficient is increased by 21%. PSJAs exhibit distinctive separation control mechanisms depending on the relative location between actuation and separation and reduced frequency of actuation (F^*). At an angle of attack of alpha =15.5^circ, the non-actuated flow separates approximately 4% chord length downstream of the jet orifices . Plasma synthetic jets (PSJs) applied at F^*ge 0.5 can displace the separation point downstream to mid-chord position, as a result of the energizing of the incoming boundary layer through mixing enhancement. As a comparison, with actuation frequency of F^*le 0.25, the separation point at alpha =15.5^circ remains near the leading edge and the zero-velocity line is periodically swept towards the suction surface by the convecting spanwise vortices generated from PSJ actuation, leading to a reduction of time-averaged backflow area. For the case of separation control at alpha =22^circ, the separation point resides always upstream of the actuation position, regardless of actuation frequency. The peak lift coefficient is attained at F^*=1, and the decreasing lift at high actuation frequency (F^*=2) is ascribed to the severe interaction between adjacent spanwise vortices at short spacing.Graphical abstract
Highlights
Flow separation can be found extensively in engineering applications including rotor blades, landing gears, trucks, inlets of supersonic aircrafts and so on (Simpson 1989)
When Plasma synthetic jets (PSJs) actuation is applied, the stall angle is postponed to approximately 22◦, and the hysteresis loop observed in the baseline case is completely eliminated, which is consistent with the observations of Post and Corke (2004b) where the same airfoil (NACA0015) is tested with SDBDAs at Rec = 1.6 × 105
These results indicate that plasma synthetic jet actuators (PSJA) can be potentially applied to increase the blade loads of vertical-axis wind turbines
Summary
Flow separation can be found extensively in engineering applications including rotor blades (pitching airfoil), landing gears (cylinder flow), trucks (blunt body flow), inlets of supersonic aircrafts (shock wave/boundary layer interaction, SWBLI) and so on (Simpson 1989). Pulsing at O(0.1%) of the main flow momentum, hydrodynamic instabilities of the separated shear layer are leveraged by unsteady jets to create coherent spanwise vortices. For the case when shear-layer stability is absent and natural transition has already ensued (e.g., trailing edge separation at high-Reynolds number), SDBDAs driven by nanosecond pulses effectively lose their control authority. The present work fills this gap and validates the control authority of PSJAs in leading edge separation at a chord-based Reynolds number of Rec = 1.7 × 105. This Reynolds number is lower from what is typically encountered in commercial aircrafts/ transportation trucks (O(106 − 107)), yet still relevant to small-scale unmanned vehicles and low-speed vertical axis wind tunnels (Timmer 2008).
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