Wings with leading-edge (LE) tubercles have gained increasing attention over the past decade. Despite their impressive aerodynamic performance, the underlying flow control mechanisms of tubercles remain controversial. In this thesis, both experimental and theoretical approaches are employed to investigate the flow patterns of a tubercled wing at pre-stall and post-stall angles of attack (AoAs).

In the experimental study, 2D Particle Image Velocimetry (PIV) was used to measure flow patterns at cross-flow planes along the chord. At a pre-stall AoA, high-vorticity regions generated by the tubercles appear in an alternating pattern near the LE. A quantitative comparison was conducted to examine the similarities between a tubercle and a delta wing. The results show that tubercles cannot be regarded as small delta wings in terms of vortex generation. The leading-edge vortex (LEV) sheets are convected downstream, where they interact with laminar separation bubbles (LSBs), creating complex flow patterns in the downstream regions. At a post-stall AoA, stall cells (SCs) appear along the span, with their formation dependent on both Reynolds number (Re) and tubercle amplitude. However, the spacing of SCs is relatively independent of AoA, Re, and amplitude, consistently ranging between 5 to 7 tubercle wavelengths.

In the theoretical study, the lifting line theory (LLT) approach was first used to predict the LEV strength but proved ineffective due to the absence of thickness effects. A subsequent analysis using the panel method in xflr5 showed that the Kutta condition should also be applied to the leading edge (LE) rather than only to the trailing edge (TE). Crow’s model was adapted by taking LEVs into consideration. However, a global description of the instability was not obtained due to difficulties in representing LEVs and related mathematical challenges.

This thesis contributes to a further understanding of the tubercle’s role in flow control. The LEVs generated by the tubercles are identified as key factors influencing flow evolution, yet these effects are not captured by LLT-based models or a conventional panel method. Future reduced-order models (ROMs) should account for the influence of LEVs to provide accurate representations of tubercled wing flow dynamics.

This thesis investigates the role of winglet slots in reducing tip vortices, inspired by bird wingtips. The effects of slotting and wingtip flexibility were analyzed using multiple bio-inspired models tested in a low-speed wind tunnel at Reynolds numbers from 2.9 × 10⁴ to 8.8 × 10⁴. Three experimental techniques were employed: force balance for aerodynamic force estimation, motion tracking of wingtip movement, and stereo PIV analysis to measure cross-flow planes in the winglet wake. Non-intrusive methods were used to estimate drag components based on PIV data and wake integral approaches. The slots significantly reduced induced drag by breaking down the tip vortex into smaller, weaker vortices. This breakdown was visualized using s-PIV results and analyzed through velocity component contours. The streamwise vorticity breakdown was examined in relation to wingtip flexibility, and the vortex cores' location and characteristics were analyzed, along with the phenomenon of vortex wandering.

By the end of the thesis, it was concluded that although slotted winglets reduced induced drag, they led to a significant increase in profile drag, resulting in higher overall drag compared to unslotted winglets. Additionally, the slots caused a loss in lift. The overall aerodynamic performance, measured by the lift-to-drag ratio, was also higher for the
unslotted winglet. Among the slotted winglets, static bending was common, while wingtip vibrations were minimal. The most flexible slotted winglet had the lowest induced drag. However, the results indicate that at very low Reynolds numbers, slotted winglets deteriorate the wing’s aerodynamic performance compared to unslotted ones. Among the slotted winglets, those with intermediate flexibility demonstrated the best aerodynamic performance, highlighting the importance of optimizing wingtip flexibility.

Blades in rotor systems such as helicopters and wind turbines experience highly dynamic inflow conditions such as rapid pitching that can lead to dynamic stall. In turn, that causes large force fluctuations which translate into fatigue for the blade structure and controllability issues for the vehicles. Biomimetics brings a potential solution in the shape of leading edge tubercles inspired from the flippers of humpback whales. These possess impressive manoeuvrability for the size and their flippers pitch up and down rapidly. While the effect of this bio-inspired leading edge shape is well-documented in steady conditions, being known to delay stall and make its character more benign, there are very few studies investigating the effect in unsteady conditions. Therefore, the scope of this study is to analyse the effect of leading edge tubercles on the flow structure evolution throughout sinusoidal oscillations of finite wings and on their tip vortex strength.

This thesis combined two methodologies: one experimental and another computational. The experimental study focused on the wind tunnel analysis of two different rectangular finite wings with leading-edge tubercles against a straight leading edge finite wing subjected to sinusoidal pitching oscillations. The study uses 3D Particle Tracking Velocimetry (PTV) technique, with the state-of-the-art Shake-The-Box (STB) algorithm which uses Helium-filled soap bubbles as flow tracer particles in order to obtain the complex instantaneous flow field around the wings throughout the oscillation cycles. After obtaining the particle tracks, phase-averaging was used to improve the quality of experimental results by making use of the multiple oscillation cycles recorded. The computational study replicates the wind tunnel conditions. U-RANS simulations were run using the Ansys CFX solver and using a deforming mesh to model the pitching motion of the airfoil. Both the CFD and the experiment were conducted at a Reynolds number of Re = 3.3 × 10⁴.

The results of both the experimental and the CFD correlate well in terms of the flow structure, allowing for an interesting comparison between the two methods. These show that the wings with leading edge tubercles do show a more benign stall behaviour and quicker reattachment on the downstroke of the oscillations thanks to the compartmentalisation effect of the streamwise vortices shed by the tubercles. However, surprisingly the angle of attack at which the maximum lift coefficient is produced is not increased or delayed compared to the straight leading edge (SLE) wings. Furthermore, the tubercle wings exhibit reduced tip vortex strength throughout the oscillation cycle, thanks in part to the destructive effect of the streamwise vortices near the wing tip on the tip vortex. Finally, in close correlation with the tip vortex circulation, the tubercle wings have a lower induced drag coefficient compared to SLE wings, with a better span efficiency thanks to the compartmentalisation effect of the spanwise flow.

This research addresses the challenge of improving aerodynamic performance by delaying flow separation using deployable vortex generators (VGs) actuated by shape memory alloys. These VGs combine passive and active elements to enhance mixing within the boundary layer, restoring momentum near the wall and minimizing separation in an Adverse Pressure Gradient (APG). A literature review identified a gap in understanding the dynamics of shape-adaptive VGs. Through Computational Fluid Dynamics (CFD) and experimental studies, the research optimized VG designs for aerodynamic efficiency. Results showed that counter-rotating vanes were most effective in controlling flow, while side load and yaw moment challenges necessitated robust actuation mechanisms. A novel design using rigid vane actuators and shape memory alloys for precise adjustments was proposed to address these issues. This study significantly advances the understanding and application of shape-adaptive VGs in flow control.

Inspired by the leading-edge protuberances found on the flippers of humpback whales, tubercles have drawn significant interest for improving airfoil aerodynamic performance. In static conditions, airfoils with tubercles exhibit a softer onset of stall and increased lift in post-stall regime, though with a reduced maximum lift coefficient. However, their impact under dynamic conditions is less understood, particularly how they affect the formation and convection of the dynamic stall vortex (DSV), which is crucial to the dynamic stall process. Therefore, this study aims to investigate how tubercles affect the development and behavior of the DSV during dynamic stall conditions. To deliberately trigger the formation of the DSV and thereby force the airfoil into dynamic stall conditions, this study employed a pitch-up and hold motion starting at a zero angle of attack and increasing to final angles of 30 and 55 degrees. Two pitching rates of k=0.05 and 0.1 were investigated, providing a comprehensive analysis of how these conditions influence the aerodynamic characteristics of the airfoils across a wide range of dynamic stall scenarios.

To assess the impact of leading-edge tubercles on airfoil aerodynamics under dynamic stall conditions, a series of wind tunnel experiments was conducted, involving two tubercled airfoils and a smooth leading- edge airfoil. These experiments were performed at a Reynolds number Re=3.3x10⁴, determined by the chord length and free-stream velocity. Employing particle tracking velocimetry (PTV), the velocity field on the suction side of each airfoil was measured, while ensuring precise airfoil positioning within the flow-field through the use of white tracking markers. Detailed monitoring of the DSV core location and circulation was conducted using the normalized angular momentum (NAM) criterion, also referred to as Γ₁ method. This approach provided quantitative insights into the influence of the tubercles on the DSV. Due to the experimental setup limitations in directly measuring aerodynamic forces, computational fluid dynamics (CFD) simulations were conducted using OpenFOAM. These simulations employed a sliding mesh technique, which facilitated the consistent application of the same computational framework for various tubercle geometries and pitching conditions. The accuracy and reliability of the simulations were rigorously validated against the experimental data.

The results reveal that leading-edge tubercles significantly influence the aerodynamic performance of airfoils under dynamic stall conditions. It has been found that tubercles modify the onset and severity of dynamic stall by reducing the strength of the DSV and shifting its formation closer to the trailing-edge. These changes result in a weaker and shorter lift overshoot, facilitating a quicker transition to the deep stall regime where tubercles enhance the lift provided by the airfoil. This alteration in the dynamic stall process has been consistently observed across all tested pitching motions, with the effects of the tubercles found to be proportional to their amplitude. These findings suggest that tubercles serve as dynamic stall mitigation devices, potentially benefiting applications where dynamic stall frequently occurs and can compromise structural integrity.