Active control of laminar boundary layers with dielectric barrier discharge (DBD) plasma actuators (PAs) has made considerable progress in the last 15 years. First pioneering experiments have motivated numerous researchers to gain a deeper insight into the underlying working principles and corresponding quantification of the actuator performance. These investigations clearly show the strengths but also the weaknesses of the PA as a flow control device. Presently, the boundary-layer control (BLC) with PAs experiences the transition from lab studies to real flight applications. However, the PA community still struggles with the poor fluid mechanic efficiency and the limited momentum flux of the actuator. This review therefore addresses the question how applicable the actuator is as an energy efficient flow control device for future in-flight applications. Since any successful flow control requires detailed knowledge of the actuator’s control authority, this discussion is built upon a careful and comprehensive summary of performance evaluation measures and the interplay with various changes of thermodynamic and kinematic environmental conditions. Consequently, this review for the first time provides a comprehensive discussion of all required steps for successful DBD-based in-flight flow control spanning from the power supply to the achieved flow-control success in one coherent document.

In boundary-layer flows, one may reduce skin-friction drag by delaying the onset of laminar-to-turbulent transition via the attenuation of small-amplitude Tollmien-Schlichting (TS) waves. In this work, we use numerical simulations and experiments to compare the robustness of adaptive and model-based techniques for reducing the growth of two-dimensional TS disturbances. In numerical simulations, the optimal linear quadratic Gaussian (LQG) regulator shows the best performance under the conditions it was designed for. However, it is found that the performance deteriorates linearly with the drift of the Reynolds number from its nominal value. As a result, an order-of-magnitude loss of performance is observed when applying the computation-based LQG controller in wind-tunnel experiments. In contrast, it is shown that the adaptive filtered-X least-mean-squares (FXLMS) algorithm is able to maintain an essentially constant performance for significant deviations of the nominal values of the disturbance amplitude and Reynolds number.

Simultaneous measurements of the phase-averaged velocity distribution and the underlying discharge quantities of a dielectric barrier discharge plasma actuator (PA) are performed at $10~\text{k}\text{Hz}$ discharge frequency to investigate the interplay of the discharge and the surrounding flow. The underlying velocity information for the force estimation is obtained by means of phase-averaged particle image velocimetry; the discharge quantities are determined from a Lissajous-figure analysis. The results uncover a clear cause–effect relation between the phase-dependent effective discharge capacitance of the PA and the resulting spatiotemporal volume-force distributions. From this novel insight, it must be concluded that the instantaneous effective discharge intensity dominates the momentum-transfer rate rather than the formerly assumed operating voltage.

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A single dielectric barrier discharge plasma actuator is employed for flow control on the pressure side of a natural laminar flow wing section under free-flight conditions. A full sized motorized glider is equipped with the flow control device and data acquisition hardware to quantify the impact of the actuator on boundary-layer transition. A transition delay of approximately 3% chord is achieved, quantified by microphone and hot-wire measurements. Simultaneously, the influence of the variable ambient conditions on the flow control performance is characterized. A closed-loop control algorithm enables constant actuator performance, despite varying humidity, temperature and density throughout the test flights. The energy efficiency of the flow control approach is estimated, providing a positive outlook for further improvements and a net benefit of transition control.