Free shear layers are building blocks of many flows of interest in applications, including jets, cavity flows, and separated flows. It was found several decades ago that free shear layers are unstable to small perturbations over a wide range of frequencies, that they are dominated by coherent large-scale structures (even at high Reynolds numbers), and that the dynamics of these structures dominate important processes: entrainment, mixing, momentum transport, and noise generation. These findings motivated extensive research activities to actively control their development using excitation of instabilities, but early experimental research focused primarily on the control of low-speed, low-Reynolds-number free shear layers. This extensive body of the literature is briefly reviewed. The authors' recent work using localized arc filament plasma actuators in jets shows that free shear layers respond to the excitation over a large range of conditions that have been explored: jet Mach number (up to 1.65), convective Mach number (up to 1), and Reynolds number (up to 1.65 × 10 6). However, the nature of large-scale structures, shear-layer growth rate, and generation of Mach waves all depend on the jet Mach number and compressibility level. The results clearly demonstrate the similarity of instability processes and development of large-scale structures in free shear layers, regardless of the Mach or Reynolds numbers. Nomenclature
The effects of perturbation-based active flow control on supersonic rectangular twin jets (SRTJ) over a wide range of nozzle pressure ratios (NPR = 2.77 to 6.7, corresponding to fully expanded Mach numbers Mj = 1.3 to 1.9) were investigated. The aspect ratio and design Mach number for the bi-conic, converging-diverging nozzles were 2 and 1.5, respectively. The flow and acoustic fields of SRTJ are known to couple, often generating high near-field (NF) pressure fluctuations and elevated far-field (FF) noise levels. Large-scale structures (LSS), or equivalently instability waves or wave packets, are responsible for mixing noise, broadband shock-associated noise, screech and coupling. The primary objective of this research was to manipulate the development of LSS in this complex flow to better understand and mitigate their effects. The organization and passage frequency of the LSS were altered by excitation of instabilities over a wide range of frequencies and modes. Key findings include: (1) the screech mode of each jet was flapping along its minor axis; (2) the jets coupled, out-of-phase primarily in overexpanded cases and in-phase primarily in underexpanded cases, along the minor axis of the SRTJ; (3) coupling has significant effects on the NF pressure fluctuations, but only minor effect on the FF noise; (4) standing waves were observed only on the minor axis plane of the SRTJ; (5) altering or suppressing coupling can significantly reduce NF pressure fluctuations; (6) two high-frequency excitation methods proved effective in reducing the FF noise; and (7) nonlinear interactions between the screech tones and excitation input were observed in controlled cases in which screech was only partially suppressed.
Dynamic stall is observed in numerous applications, including sharply maneuvering fixed-wing aircraft, biomimetics, wind turbines, and most notably, rotorcraft. The associated unsteady loading can lead to aerodynamic flutter and mechanical failure in the system. The present work explores the ability of nanosecond pulse-driven dielectric barrier discharge plasma actuators to control dynamic stall over a NACA 0015 airfoil. The Reynolds number, reduced frequency, and excitation Strouhal number were varied over large ranges: Re 167;000-500;000, k 0.025-0.075, and St e 0-10, respectively. Surface pressure measurements were taken for each combination of Reynolds number, reduced frequency, and excitation Strouhal number. Phaselocked particle image velocimetry measurements were acquired for select cases. It was observed that the trends of effect of St e were similar for all combinations of Reynolds number and reduced frequency, and three major conclusions were drawn. First, it was observed that low Strouhal number excitation (St e < 0.5) results in oscillatory aerodynamic loading in the stalled stage of dynamic stall. This oscillatory behavior was gradually reduced as St e increased and was not observed beyond St e > 2. Second, all excitation resulted in earlier flow reattachment. Last, it was shown that excitation, especially at high St e , resulted in reduced aerodynamic hysteresis and dynamic stall vortex strength. The decrease in the strength of the dynamic stall vortex is achieved by the formation of large-scale structures induced by the excitation that bleed the leading-edge vorticity before the ejection of the dynamic stall vortex. At sufficiently high excitation Strouhal numbers (St e ≈ 10), the dynamic stall vortex was completely suppressed.
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