This study examines the control capabilities of an array of spanwise-invariant roughness strips applied on a swept-wing boundary layer (BL) dominated by a cross-flow instability (CFI) that is forced by periodically spaced discrete roughness elements to a monochromatic wavelength. Several configurations of strip arrays are investigated, varying their height, width and chordwise periodicity. Infrared thermography is employed to track the impact on the BL transition location. Optimal configurations are identified, extending laminar flow by up to 10 % of the wing chord. Additionally, BL forced by patches of randomised surface roughness are considered, better representing realistic wing surfaces. In this scenario, the application of strip arrays with optimal geometry extends the laminar portion of the BL by almost 10 % chord and beyond when combined with a discrete roughness element array. Time-averaged particle image velocimetry (PIV) velocity fields are acquired to monitor the CFI amplitude for the various configurations. The BL spectral content in the spanwise direction is used to characterise the chordwise behaviour of individual disturbance modes, whose amplitude is found to be reduced by up to 17 % for the optimal strip configuration.

This work presents the first experimental characterization of the flow field in the vicinity of periodically spaced discrete roughness elements (DRE) in a swept wing boundary layer. The time-averaged velocity fields are acquired in a volumetric domain by high-resolution dual-pulse tomographic particle tracking velocimetry. Investigation of the stationary flow topology indicates that the near-element flow region is dominated by high- and low-speed streaks. The boundary layer spectral content is inferred by spatial fast Fourier transform (FFT) analysis of the spanwise velocity signal, characterizing the chordwise behaviour of individual disturbance modes. The two signature features of transient growth, namely algebraic growth and exponential decay, are identified in the chordwise evolution of the disturbance energy associated with higher harmonics of the primary stationary mode. A transient decay process is instead identified in the near-wake region just aft of each DRE, similar to the wake relaxation effect previously observed in two-dimensional boundary layer flows. The transient decay regime is found to condition the onset and initial amplitude of modal crossflow instabilities. Within the critical DRE amplitude range (i.e. affecting boundary layer transition without causing flow tripping) the transient disturbances are strongly receptive to the spanwise spacing and diameter of the elements, which drive the modal energy distribution within the spatial spectra. In the super-critical amplitude forcing (i.e. causing flow tripping) the near-element stationary flow topology is dominated by the development of a high-speed and strongly fluctuating region closely aligned with the DRE wake. Therefore, elevated shears and unsteady disturbances affect the near-element flow development. Combined with the harmonic modes transient growth these instabilities initiate a laminar streak structure breakdown and a bypass transition process.

The present work is dedicated to the investigation of the effect of an isolated roughness element on a swept wing boundary layer. In particular, the flow modifications incurred by a single cylindrical element applied on a swept wing model are measured, toward describing the nature of the perturbations introduced in the flow field, their development in the near and far wake region, as well as their eventual breakdown. The measurements are performed using infrared thermography, to achieve a general overview of the element wake origin and spatial spreading. Local quantitative characterization of the stationary and unsteady disturbances evolving in the flow is instead acquired through hot wire anemometry. When present in an undisturbed laminar boundary layer, isolated roughness elements are found to introduce flow disturbances, which lead to the formation of a turbulent wedge. As it develops downstream, the wedge undergoes rapid spanwise expansion, affecting the adjacent laminar flow regions. The wedge origin and development is mostly associated with the instabilities introduced by the shedding process initiated in the roughness element wake, comparably to the dominant flow features characterizing the transition of two-dimensional boundary layers conditioned by an isolated roughness element. Nonetheless, the presence of the crossflow velocity component in the boundary layer baseflow notably affects the overall flow development, introducing an asymmetric evolution of the main flow features.

The presented work introduces a cancellation technique, based on the linear superposition of stationary crossflow instabilities (CFIs) through the application of a streamwise series of optimally positioned discrete roughness element (DRE) arrays on a swept wing surface. The DRE arrays are designed and arranged with suitable amplitude and phase shift to induce velocity disturbance systems that destructively interact, ultimately damping the developing CFIs. The robustness of this technique is investigated for a smooth wing surface as well as in the presence of enhanced distributed surface roughness. The resulting flow fields are measured with infrared thermography and particle tracking velocimetry, allowing for the extraction of the laminar-to-turbulent transition front location and for the characterization of the local boundary layer development. The acquired data show that the superposition of suitably arranged DRE arrays can successfully suppress monochromatic CFIs, reducing their amplitude and growth and delaying the boundary layer transition to turbulence when applied on a smooth wing surface. However, the presence of elevated background roughness significantly reduces the effectiveness of the proposed method.

The effect of discrete roughness elements on the development and breakdown of stationary crossflow instability on a swept wing is explored. Receptivity to various element heights and chordwise locations is explored using a combination of experimental and theoretical tools. Forcing configurations, determined based on linear stability predictions, are manufactured and applied on the wing in a low turbulence facility. Measurements are performed using infrared thermography, quantifying the transition front location, and planar particle image velocimetry, providing a reconstruction of stationary crossflow instabilities and their associated growth. Measurements are corroborated with simulations based on nonlinear parabolised stability equations. Results confirm the efficacy of discrete roughness elements in introducing and conditioning stationary crossflow instabilities. Primary instability amplitudes and resulting laminar-turbulent transition location are found to strongly depend on both roughness amplitude and chordwise location. The Reynolds number based on element height is found to satisfactorily approximate the initial forcing amplitude, revealing the importance of local velocity effects in non-zero-pressure gradient flows. Direct estimation of initial perturbation amplitudes from nonlinear simulations suggests the existence of pertinent flow mechanisms in the element vicinity, active in conditioning the onset of modal instabilities. Dedicated velocimetry planes, elucidate the development of a momentum deficit wake which rapidly decays downstream of the element followed by mild growth, representing the first experimental evidence of transient behaviour in swept wing boundary layers. The outcome of this work identifies a strong scalability of the transition dynamics to roughness amplitude and location, warranting the upscaling of roughness elements to more accessible, measurable and spatially resolved configurations in future experiments.

This work presents the first reported experimental characterization of the flow field in the direct vicinity of discrete roughness elements (DRE), in a swept wing boundary layer. High magnification tomographic Particle Tracking Velocimetry (3D-PTV) measurements are used to acquire time-averaged velocity and standard deviation fields in a 3D volume directly aft of the DRE elements. The collected data detail the near-element flow topology, providing information on the developing wake and emerging flow structures, their organization and amplitude evolution. A transient growth behaviour is identified in the element wake, while onset and growth of crossflow instabilities is observed further downstream. As such, the near element flow is confirmed to be a fundamental part of the receptivity process, contributing in setting the initial amplitudes for the crossflow instability evolution.

A dielectric barrier discharge actuator (DBD) is considered and studied as a stall recovery device. The DBD is installed on the nose of a NACA0015 airfoil with chord × span 300 × 930 mm. The geometry of the exposed electrode has periodic triangular tips purposely designed for the case under study. Wind tunnel tests have been carried out over a range of airspeeds up to 35 m/s with a Reynolds number of 700 k. The flow morphology has been characterized by means of the particle image velocimetry technique, obtaining velocity fields and pressure coefficients. By exploring different planes along the model span, the three-dimensional effect of the DBD has been reconstructed, identifying the flow region mainly sensitive to the plasma actuation. Finally, the actuator effectiveness has been quantified accounting for the power consumption data, leading to defining further design improvements in view of a better efficiency.