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.

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.

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.