In the upcoming decades, a substantial growth in the commercial aviation market is anticipated, accompanied by an increasing societal awareness of global warming. This circumstance necessitates a technological development for emission reduction in future transport aircraft. A method for aerodynamic efficiency improvement is the so-called Laminar Flow Control (LFC), which can potentially achieve a considerable boost in aircraft efficiency. The research described in this thesis focuses on the specialised strategy of Base Flow Modification (BFM) based on plasma actuators (PAs), a promising member in the LFC family. The results of this booklet is divided into three main parts. The first part explores the effects of unsteady PA-induced perturbations on a swept wing boundary layer dominated by stationary crossflow (CF) vortices, representative of typical cruise flight regimes. Key parameters such as forcing frequency and streamwise location of PAs are scrutinized, which have pronounced effects on the development of crossflow instabilities (CFIs). To focus on the PA-induced unsteady disturbances, PAs are operated at very low power to minimize the net BFM effect. The second part primarily aims to experimentally validate the reduction of CF component achieved by the plasma-based BFM. It also delves into the effects of the PA forcing on the base flow and developing CFIs in a swept wing boundary layer. Employing the BFM strategy, a simplified predictive model is constructed with linear stability theory analysis to infer CFI modes’ characteristics. The achieved CF reduction and the base flow direction are traced under various momentum coefficients, which are controlled by the applied high voltage amplitude. Moreover, the streamwise growth of stationary and travelling CFIs is investigated under the condition of PA actuation. The last part focuses on controlling CFIs and laminar-turbulent transition in an experimental swept wing model using the plasma-based BFM technique. A simplified model coupled with linear stability theory analysis predicts the net BFM effect on CFI modes. Experiments are conducted in a low-turbulence wind tunnel where PA is operated at constant input voltage and frequency to achieve the BFM control. Various parameters of the PA-based BFM technique are investigated, namely the Reynolds number, angle of attack and wavelength of dominated stationary CFI modes. The results generally confirm the stabilising ability of BFM on the swept wing boundary layer. More importantly, the PA-based BFM essentially renders the boundary layer more susceptible to travelling CFIs. In the presence of net BFM effect and intrinsic PA unsteadiness, the PA-based BFM technique achieves transition delay with specific combinations of Reynolds number, angle of attack and wavelength of dominated stationary CFI modes. @en

This work examines the control of cross-flow instabilities (CFIs) and laminar–turbulent transition on a swept wing, through the plasma-based base flow modification (BFM) technique. The effect of experimentally derived plasma body forces on the steady boundary layer base flow is explored through numerical simulations. Linear stability theory is subsequently used to predict the net BFM effect on CFIs. Based on these preliminary predictions, experiments are conducted in a low-turbulence wind tunnel where a spanwise-invariant plasma actuator is installed near the wing leading edge and operated at constant input voltage and frequency. Various flow parameters governing the plasma-based BFM technique are investigated, namely the Reynolds number, angle of attack and wavelength of excited stationary CFI modes. Stationary and travelling CFIs are quantified by planar particle image velocimetry while the transition topology and location are recorded by infrared thermography. The results confirm the stabilising effect of BFM on the swept-wing boundary layer. However, the plasma-based BFM is found to render the boundary layer more susceptible to travelling CFIs. In the presence of both net BFM effect and intrinsic plasma unsteady perturbations, the plasma-based BFM technique achieves transition delay with specific combinations of Reynolds number, angle of attack and wavelength of excited stationary CFI modes. The present findings provide insights into the fundamental principles of operating plasma actuators within the context of BFM control.@en

This work experimentally investigates plasma actuator (PA) forcing effects on the base flow and developing crossflow (CF) instabilities in a swept wing boundary layer. Spanwise-invariant plasma forcing near the leading edge is configured according to the base flow modification (BFM) strategy. A simplified predictive model is constructed by coupling an experimentally derived plasma body force and a linear stability theory and is used to infer the stability characteristics of the boundary layer subject to BFM. The base flow velocity is measured by stereo particle image velocimetry (PIV) at various PA operating conditions. Similarly, the developing CF instabilities, triggered through discrete roughness elements, are quantified by planar-PIV. The results demonstrate that a PA can reduce the boundary layer CF component, whereas the control authority shows a high dependence on the momentum coefficient. The dissimilar reduction between the streamline-aligned velocity and CF component leads to a local re-orientation of the base flow. Spanwise spectral analysis of the time-averaged flow indicates that stationary CF instabilities can be favorably manipulated whereas the BFM reduction effects depend on the corresponding initial amplitudes of stationary instabilities. An evident spanwise shift in the trajectory of stationary CF vortices is observed, which appears to result from the local alteration of the boundary layer stability due to the PA forcing. Despite the overall reduction in the amplitude of stationary CF instabilities, unsteady disturbances are found to be enhanced by the PA forcing. The current results shed light on the underlying principles of BFM-based PA operation in the context of laminar flow control.

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This work investigates the response of a transitional boundary layer to spanwise-invariant dielectric barrier discharge plasma actuator (PA) forcing on a 45 ° swept wing at a chord Reynolds number of 2.17 × 10 6. Two important parameters of the PA operation are scrutinized, namely, the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements is installed near the leading edge to promote and condition a set of critical stationary crossflow (CF) instability modes. Numerical solutions of the boundary layer equations and linear stability theory are used in combination with the experimental pressure distribution to provide predictions of critical stationary and traveling CF instabilities. The laminar-turbulent transition front is visualized and quantified by means of infrared thermography. Measurements of velocity fields are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the inherent introduction of unsteady velocity disturbances by the plasma forcing. It is shown that, depending on actuator frequency and location, these disturbances can evolve into typical CF instabilities. Positive traveling low-frequency type III modes are generally amplified by PA in all tested cases, while the occurrence of negative traveling high-frequency type I secondary modes is favored when PA is operating at high frequency and at relatively downstream locations, with respect to the leading edge.

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Plasma-based flow control poses a simple and robust technique for transition delay on swept wings. However, a clear understanding of how plasma actuators affect crossflow instabilities is necessary to develop and mature crossflow control based on plasma actuators. In this paper, the design of a new swept wing model optimised for the study of crossflow receptivity and stability under plasma actuator is described in detail. First, a 2D wing shape is designed, to match the nearing leading edge pressure distribution of a reference high-Reynolds number swept wing model (M3J) which has been used extensively in past investigations. The aerodynamic performance of this new shape is investigated using CFD simulations and the results show a good agreement for the pressure coefficient. In manufacturing design, the wing model features provisions to accept plasma actuators, such as non-conductive material as well as an appropriately designed recess for the actuator assembly. The new model in conjunction with a recently refurbished low turbulence windtunnel facility are characterized in a preliminary experiment. The uniformity and quality of the flow is identified using pressure measurements and the results confirm the new model achieved near-invariant spanwise conditions until 40% of the chord. Infrared thermography is used to capture the surface footprint of stationary primary crossflow vortices. Clear formations of stationary vortices created by discrete roughness are captured and no visible transition is observed. Finally, the effects of plasma actuation on crossflow instabilities are inspected by Infrared Thermography and PIV scanning. The results validate the prediction of Linear Stability Theory with respect to the most unstable stationary mode and traveling mode. The appearance of secondary crossflow instabilities is observed at relatively upstream chord locations even without transition detected. The outcome positively confirms the ability of this new model to reproduce receptivity and initial growth of crossflow instabilities of the reference model (M3J) under plasma actuation.

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