In the current study, selective forcing of cross-flow instability modes evolving on a 45• swept wing at Re = 2.17 · 10 6 is achieved by means of spanwise-modulated plasma actuators, positioned near the leading edge. In the perspective of laminar flow control, the followed methodology holds on the discrete roughness elements/upstream flow deformation (DRE/UFD) approach, thoroughly investigated by e.g. Saric et al. (1998);Malik et al. (1999) and Wassermann & Kloker (2002). The possibility of using active devices for UFD provides several advantages over passive means, allowing for a wider range of operating Re numbers and pressure distributions. In the present work, customised alternating current dielectric barrier discharge plasma actuators have been designed, manufactured and characterised. The authority of the actuators in forcing monochromatic stationary cross-flow modes at different spanwise wavelengths is assessed by means of infrared thermography. Moreover, quantitative spatio-temporal measurements of the boundary layer velocity field are performed using time-resolved particle image velocimetry. The results reveal distinct steady and unsteady forcing contributions of the plasma actuator on the boundary layer. It is shown that the actuators introduce unsteady fluctuations in the boundary layer, amplifying at frequencies significantly lower than the actuation frequency. In line with the DRE/UFD strategy, forcing a sub-critical stationary mode, with a shorter wavelength compared to the naturally selected mode, results in less amplified primary vortices and related fluctuations, compared to the critical forcing case. The effect of the forcing on the flow stability is further inspected by combining the measured actuators body-force with the numerical solution of the laminar boundary layer and linear stability theory. The simplified methodology yields fast and computationally cheap estimates on the effect of steady forcing (magnitude and direction) on the boundary layer stability.

Control of laminar-to-turbulent transition on a swept-wing is achieved by base-flow modification in an experimental framework, up to a chord Reynolds number of 2.5 million. This technique is based on the control strategy used in the numerical simulation by Dörr & Kloker (2015b). A spanwise uniform body force is introduced using Dielectric Barrier Discharge plasma actuators, to either force against or along the local cross-flow component of the boundary layer. The effect of forcing on the stability of the boundary layer is analysed using a simplified model proposed by Serpieri et al. (2017). A minimal thickness plasma actuator is fabricated using spray-on techniques and positioned near the leading edge of the swept-wing, while infrared thermography is used to detect and quantify transition location. Results from both the simplified model and experiment indicate that forcing along the local cross-flow component promotes transition while forcing against successfully delays transition. This is the first experimental demonstration of swept-wing transition delay via base-flow modification using plasma actuators.

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The influence of spanwise-uniform electro-fluid-dynamic forcing applied by a dielectric barrier discharge (DBD) plasma actuator on the growth of a plane mixing layer and the dynamics of large-scale spanwise vortices are investigated experimentally. A two-dimensional mixing layer formed between two streams of air with different velocities is employed for this study. Quantitative spatio-temporal measurement of the flow field is acquired using high-speed planar particle image velocimetry. The DBD actuator was constructed such that it imparts perturbations into the splitter-plate boundary layer formed by the high-velocity fluid stream, close to the trailing edge. Through this, the fundamental Kelvin–Helmholtz instability of the current mixing layer and its first sub-harmonic are forced. Forcing the fundamental instability results in the inhibition of vortex pairing due to the attenuation of sub-harmonic instabilities, and thus, mixing layer growth is halted in the vicinity of the trailing edge. With sub-harmonic instability forcing, neighboring vortices interact with each other and amalgamate together through mutual induction. This results in a higher growth rate compared to the unforced mixing layer at the streamwise location of this vortex interaction. Eventually, the growth rate of the forced mixing layers becomes similar to that of the unforced case. These results demonstrate the influence of the applied forcing on the spectral signature, growth, and stability characteristics of the plane mixing layer and the dynamics of the coherent vortical structures.

The influence of spanwise-uniform (linear) forcing applied by a dielectric barrier discharge (DBD) plasma actuator on the growth of a plane mixing layer and the dynamics of large-scale spanwise vortices within, are investigated experimentally. Quantitative spatio-temporal measurement of the flow field is acquired using high-speed planar particle image velocimetry. The DBD actuator is used to impart perturbations into the mixing layer to force the fundamental Kelvin-Helmholtz instability and its first sub-harmonic. Forcing the fundamental instability resulted in the inhibition of vortex pairing due to the attenuation of sub-harmonic instabilities. Correspondingly, the growth of the mixing layer is halted initially. With sub-harmonic instability forcing, two vortices interact with each other and merge together. This results in a higher growth rate compared to the unforced mixing layer at the streamwise location of this vortex interaction. Eventually, the growth rate of the forced mixing layer becomes similar to that of the unforced mixing layer. These results demonstrate the influence of the applied forcing on the growth of the turbulent mixing layer and the dynamics of the coherent vortical structures within. Nomenclature Symbols x Axis along the streamwise direction V ac AC voltage amplitude y Axis orthogonal to the streamwise direction f ac AC carrier frequency z Axis parallel to the splitter-plate trailing edge f b Burst modulation frequency uvw Velocity components along the xyz axes U 1 Streamwise velocity of low-velocity stream ... Time-average U 2 Streamwise velocity of high-velocity stream ... Fluctuating fields ∆U Velocity difference between two streams (= U 2 − U 1) ... N Normalized quantities U Average velocity of the two streams (= (U 2 + U 1)/2) r Velocity ratio (= U 1 /U 2) λ Velocity ratio (= ∆U/(2U)) y 0.5 y-coordinate where u = U 1 + 0.5 • ∆U δ w Vorticity thickness δ 0 Vorticity thickness at the onset of mixing layer c Growth rate fitting parameter R xy Reynolds shear stress (= −u v) f Frequency f 0 Frequency of the fundamental Kelvin-Helmholtz instability Φ γ Power spectral density of some quantity γ φ Phase along burst modulation cycle

In the present work, laminar flow control, following the discrete roughness elements (DRE) strategy, also called upstream flow deformation (UFD) was applied on a 45 • swept-wing at a chord Reynold's number of Re c = 2.1 • 10 6 undergoing cross-flow instability (CFI) induced transition. Dielectric barrier discharge (DBD) plasma actuation was employed at a high frequency (f ac = 10kHz) for this purpose. Specialized, patterned actuators that generate spanwinse-modulated plasma jets were fabricated using spray-on techniques and positioned near the leading edge. An array of DREs was installed upstream of the plasma forcing to lock the origin and evolution of critical stationary CFI vortices in the boundary layer. Two forcing configurations were investigated-in the first configuration the plasma jets were directly aligned against the incoming CF vortices while in the second the CF vortices passed between adjacent plasma jets. Infrared thermography was used to inspect transition location, while quantitative measurements of the boundary layer were obtained using particle image velocimetry. The obtained results show that the plasma forcing reduces the amplitude of stationary CF modes, thus delaying laminar-to-turbulent transition. In contrast to previous efforts [1], the plasma forcing did not introduce unsteady fluctuations into the boundary layer. The mechanism responsible for the observed transition delay appears to leverage more on localised base-flow modification rather than the DRE/UFD control strategy. Nomenclature Symbols V pp Peak-to-peak voltage x Axis normal to the leading edge f ac Carrier frequency y Axis orthogonal to the chord plane T x Thrust generated along x z Axis parallel to the leading edge c µ Momentum coefficient c Airfoil chord α Angle of attack uvw Velocity component along the x yz axes Abbreviations U Velocity magnitude CF Cross-Flow U ∞ Freestream velocity CFI Cross-Flow Instability Re c Chord Reynold's number UFD Upstream Flow Deformation ...

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