The active cancellation of Tollmien-Schlichting waves is a promising and energy-efficient laminar flow control technology with the goal of drag reduction by delaying laminar-turbulent transition. This paper analyzes, for the first time, the control mechanism in the direct vicinity of the actuator using particle image velocimetry. Artificial Tollmien-Schlichting waves are generated at a frequency of f(TS) = 250 Hz in a two-dimensional zero-pressure-gradient flat-plate boundary layer (U-infinity = 15 m/s). The Tollmien-Schlichting waves travel downstream and are attenuated with an unsteady dielectric-barrier-discharge plasma actuator force. Phase-locked particle image velocimetry measurements at an acquisition rate of f(acq) = 800 Hz in double-frame mode are conducted in the direct vicinity of the plasma actuator, resolving 16 phase angles of the wave cycle. The use of two cameras allows measurement of two full wavelengths of the Tollmien-Schlichting waves and a detailed analysis of the cancellation process. The phase-averaged velocity fields of the artificially generated Tollmien-Schlichting waves are validated with a linear stability theory. The wavelength, amplification, and shape of the Tollmien-Schlichting wave disturbances and the counterwaves generated by the plasma actuator are analyzed.

Active flow control in laminar boundary layers can reduce wall friction on a wing, if laminar-turbulent transition is delayed. In this thesis, (re-)active flow control for active cancellation of Tollmien-Schlichting (TS) waves in a two-dimensional laminar boundary layer is investigated. The TS-waves that initiate the transition to turbulence are attenuated with a DBD plasma actuator by superposition. Active wave cancelation requires the use of control algorithms and their stability is influenced by variable inflow conditions. Model-based (Linear-Quadratic-Gaussian) and adaptive control algorithms (filtered-x-LMS) are investigated under realistic (varying) inflow conditions in wind-tunnel and in-flight experiments. The further developed delayed-x-LMS algorithm allows a stable and robust controller operation for active wave cancelation in flight. All experimental results are validated by direct numerical simulations and linear stability theory. Furthermore, the challenges for the application of DBD plasma actuator arrays to delay natural transition are pointed out. The simplification of transfer paths for reducing the required computational power is discussed.

This work investigates the potential of infrared (IR) thermography for the dynamic detection of laminar-turbulent transition. The experiments are conducted on a flat plate at velocities of 8–14 m/s, and the transition of the laminar boundary layer to turbulence is forced by a disturbance source which is turned on and off with frequencies up to 10 Hz. Three different heating techniques are used to apply the required difference between fluid and structure temperature: a heated aluminum structure is used as an internal structure heating technique, a conductive paint acts as a surface bounded heater, while an IR heater serves as an example for an external heating technique. For comparison of all heating techniques, a normalization is introduced and the frequency response of the measured IR camera signal is analyzed. Finally, the different heating techniques are compared and consequences for the design of experiments on laminar-turbulent transition are discussed.

In-flight experiments for laminar boundary layer stabilization are conducted on a laminar flow wing glove, mounted on a manned motor glider. A single DBD plasma actuator is operated on the pressure side of the glove to delay natural laminar-turbulent transition while an infra red (IR) camera is mounted below the wing allowing a measurement of the spatial development of the transition. Besides the quantification of the flow control result, this article shows postprocessing strategies for perspective camera calibration with curved surfaces, image dewarping and for inhomogeneous surface heating.

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.

This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broadband Tollmien–Schlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a ‘black-box’ system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere.

Submitted for the DFD16 Meeting of The American Physical Society In-Flight Active Wave Cancelation with Delayed-x-LMS Control Algorithm in a Laminar Boundary Layer1 BERNHARD SIMON, Tech Univ Darmstadt, NICOLO FABBIANE, KTH Stockholm, TIMOTHEUS NEMITZ, Tech Univ Darmstadt, SHERVIN BAGHERI, DAN HENNINGSON, KTH Stockholm, SVEN GRUNDMANN, University of Rostock — This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TSwave cancelation. Active wave cancelation of two-dimensional broad-band TollmienSchlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a “black box” system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere.

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.

This work investigates the stability of a fxLMS controller for active wave cancelation of broad-band Tollmien-Schlichting disturbances in a flat plate boundary-layer with a single DBD plasma actuator. In particular the influence of a changing free stream velocity and the resulting off-design operation of the control algorithm is analyzed up to an unstable behavior. As the main reason for unstable controller operation in the off-design case the difference between actual and predicted phase angle of the disturbances at the position of the error sensor is identified. A method for an online adjustment of the secondary-path model to different free-stream velocities is presented. Finally a wall-bounded method based on the disturbances phase speed is developed that can cope with changes of the physical secondary path not only due to changes of the free-stream velocity but also due to changes of the pressure distribution. This method enables the extension of the stable operation range of the control system significantly. (C) 2015 Elsevier Inc. All rights reserved.

AbstractIn 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.

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. Finally, a numerical procedure is presented to quantitatively estimate the effect of dielectric barrier discharge actuation on boundary-layer transition to enhance the design of flow-control experiments.

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.