In the European Union greenhouse gas emissions of heavy duty vehicles make up 30% of the total caused by road transport. By placing vehicles in a platoon configuration the aerodynamic drag can be heavily reduced. The effect of platooning has been studied on both American and European type vehicles. However many of these studies only disclose the drag reductions and do not fully explain the flow behaviour between the models which in the end is what is causing the reduction in drag. Next to this studies done on European type vehicles mostly used time-averaged simulations so a next step is understanding the effect of unsteady flow on the vehicles. In this study the flow field between two vehicles in a platooning configuration is analysed numerically and experimentally for varying intervehicle distance and front- and rear drag reduction devices. The GETS model was used in this study which is a simplified model of a European heavy duty vehicle. For the numerical analysis Exa PowerFLOW is used which is a transient CFD package based on the LBM. Turbulence is modelled using VLES together with a RNG of the κ - ϵ equations. A study of the mesh size showed the sensitivity on the drag coefficient of the single model. The experimental analysis was performed in the OJF of the TU Delft at a Reynolds number of 3.9 x 105 based on the square root of the model. Next to balance measurements CVV measurements in the gap between the two models were taken in order to visualize the flow field. The single model showed a toroidal recirculation region in which a vortex ring can be seen. The drag force fluctuates at a Strouhal number of 0.073, side and lift force at 0.058 and 0.102 and 0.130 and 0.160. These fluctuations are caused by pressure fluctuations in the wake where the highest magnitudes are seen in the shear layer. The models with a smaller front radius saw a rise in drag with flow separation occurring for the smallest radius for the numerical results and for both smaller radii in the experiment. The addition of a boat tail lowered the drag of the models. With increasing tail angles the drag reduction also increases which is caused by the increase in base pressure. The tails also decrease the force fluctuations due to decreased pressure fluctuations in the shear layer. At the closest spacing of 0.10 times the vehicle length all tested configurations benefit from the platoon. The flow between the two models is made up of a toroidal recirculation region much smaller in size compared to the single model. In general the flow takes an S-shaped path between the two models. From the underbody of the leading model it either stagnates on the front of the trailing model or moves into the gap where it ends up in the upper of lower vortex or stagnates on the front or rear of one of the models. When a sharper front edge radius is applied to the trailing model more flow is deflected into the gap leading to lower pressure vortices and a higher stagnation pressure. A small vertical misalignment of the trailing model, which was seen during the experimental campaign, leads to a higher stagnation pressure on the bottom of the trailing model, also here more flow is deflected into the gap. At this spacing the side-force fluctuations are a bit higher due to the vortices that leave the domain passing over the rounded edges of the model. At the middle spacing of 0.45 times the vehicle length not all configurations benefit from the platoon. The trailing models with the baseline frontal radius saw an increase in drag due to the lack of thrust generated by the rounded edges. The other models did see a drag decrease. At this distance the wake of the leading model is quite similar to that of the single model. Flow enters from the top, bottom and sides of the model and ends up in the recirculation region or it stagnates on the rear of the model before it leaves the wake and stagnates on the front of the trailing model or flows over the rounded edges where it accelerates and leaves the gap. The effect of a sharper radius applied to the trailing model has little effect on the flow field. When a tail is applied to the leading model the recirculation region has been reduced as was seen for the single models. Due to the upwash the stagnation pressure on the bottom front has been increased but the drag has decreased compared to the platoon without a tail due to the increased suction of the rounded edges. At this distance the force fluctuations are much higher compared to the single model. The magnitude of drag and side force can go up to 2 and 6.5 times the values of the single model depending on the configuration. This is due to the stronger vortices from the leading model which pass over the trailing model. For the last vehicle spacing of 0.91 s/L all the configurations benefit from the platoon again. For the baseline models the gains are only a few percent but for the trailing model with a sharper front radius the gains are higher than those at the middle intervehicle distance. This is due to the reduced stagnation pressure and almost undisturbed suction coming from the rounded edges. At this distance any effect on the wake of the leading vehicle has vanished. The unsteady forces still show an increased amplitude compared to the single model however compared to the middle intervehicle distance they are much lower. The comparison between the results from the numerical and experimental analysis is quite good. The drag and flow field results are very similar. For the pressure and the unsteady forces this is less the case. The mismatch seen in the pressure comparison can be caused by alignment errors that were observed during the experiment. Next to this while reconstructing the flow field from the experimental measurements the resulting coordinates were outside the expected range. A manual coordinate shift was applied which may have resulted in additional errors. For the unsteady forces it is assumed that the ground plate and its interaction with the model as well as additional vibrations are the cause of the much higher fluctuations seen from the experimental data.

The near wake of the Ahmed reference body in the presence of steady cross-wind is investigated experimentally by means of large-scale three-dimensional Particle Image Velocimetry (PIV). A novel experimental technique is employed which exploits coaxial imaging and illumination while relying on Helium Filled Soap Bubble (HFSB) seeding as well as on robotic actuation. A 50% replica of the Ahmed body with a 25° slant angle is, therefore, considered in different configurations, among which the headwind condition and two different yaw angles, β1 = +4° and β2 = +8°. A height-based Reynolds number of 1.15 × 10^5 is chosen together with an airspeed of 12 m/s and the Open Jet Facility (OJF) Wind Tunnel of TU Delft is employed to perform the experiment.The relevance of the proposed work lies in the importance of experimental data concerning the Ahmed reference body as well as in the shortage of it with respect to the cross-wind case. The Ahmed body, indeed, once studied to improve the understanding of the flow field generated by road vehicles and to achieve a substantial drag reduction, is nowadays used as Computational Fluid Dynamics (CFD) benchmark and validation tool for automotive applications.To the best of the author’s knowledge, no work is present in the literature which investigates the flow field around the Ahmed body by means of three-dimensional quantitative measurement techniques such as the employed robotic volumetric velocimetry. The proposed study is, therefore, expected to be a significant contribution to the body of knowledge, supplying new valuable data to be used for validation purposes and to inspire future works.

In the context of the ongoing research regarding boundary layer transition, this project aims to study the effects of isolated cylindrical roughness elements on the transition from laminar to turbulent flow in a swept wing boundary layer. The project entails experimental campaigns: the 45 degrees swept wing model is placed inside the test section of the Low Turbulence Tunnel (LTT) and millimeter-sized roughness elements are attached to it, so that their interference with the boundary layer flow can be studied. The main goal is to investigate the flow features generating aft the roughness elements and their development into turbulent streaks. Moreover, this project aims to understand and quantify the effects of different roughness sizes on transition mechanisms, in the specific case of a three-dimensional boundary layer. This is achieved by means of different experimental techniques, such as PIV, HWA and thermographic flow visualization. The obtained results lead to a comprehensive description of this type of flows, with several phenomena changing their characteristics due to a change in parameters (freestream speed, roughness diameter and roughness height). Velocity field, spectral analysis of the signal, wedge width evolution, instability modes and velocity fluctuations in time were investigated with different parametric conditions, in order to understand how the flow was affected by them. Furthermore, the study allowed to evaluate the use of non-dimensional parameters such as roughness Reynolds number and aspect ratio for the prediction of the flow topology. The complexity of the phenomena involved is underlined in the conclusions of this research, with the interaction of several flow features making it a broad and interdisciplinary field of research.

Laser diagnostic techniques such as Particle Image Velocimetry and Particle Tracking Velocimetry to measure the flow of a fluid around an object have been in prevalence for a few decades. Typically, the fluid, whose motion is of interest, is seeded with micron scale particles and illuminated with a laser. Due to scattering inefficiencies of these micron scale particles the maximum measurement volumes were about a few litres. Development of the neutrally buoyant sub-millimetre scale Helium-Filled Soap Bubbles allows for much larger scattering cross-sections thus enabling large-scale measurements in air (Caridi et.al. 2015). Large scale Particle Tracking Velocimetry has been used to study the flow in the wake of a cyclist at typical time-trial speeds. A novel PTV algorithm known as Shake-The-Box (STB) (Schanz et.al. 2013) has been used to obtain the particle tracks from the images acquired in a thin volume in the cyclist’s wake using the wake-scanning method. The Lagrangian particle tracking technique was found to be more accurate than the Time Resolved Tomographic PIV through another experiment. Particularly for large field of views involving scanning at different positions, it produces whole-field results free from the boundary effects that arise between different positions of the scans. This yields accurate velocity components and their gradients that are crucial for pressure reconstruction. Flow-field results from 4D-PTV show that the wake structure is similar to those found in the literature. Two large regions of momentum deficit are present, one behind the thighs and one near the lower legs and the wheel axis. Furthermore, they reveal the presence of some vortices which were not reported previously. Based on the observed signs of the vortices the points of origination of these vortices are identified based on separation mechanisms. There is a good agreement in the flow-field results with the literature. Pressure fields reconstructed from the velocity flow-field show low pressure pockets in the regions of the vortex cores and separated flow over the lower back. Wake flow-field is utilised to compute the drag using the control volume approach (van Oudheusden 2007) and compared with the drag forces obtained from an external balance measurement to obtain the accuracy of large scale 4D-PTV. In total, five measurements at different velocities in the range of 13m/s to 15m/s are analysed. The accuracy of the drag estimates from 4D-PTV is obtained within 5%. Out of the three terms of the drag force, the momentum term contributes approximately 95% and the contribution from the pressure term is less than 0.3%. Rest is contributed by the Reynolds stresses in streamwise direction. Almost all the variation in the drag force comes from the momentum term.

As the world grapples with the challenges of global warming, there has been a significant push for the development of new methods of harnessing renewable energy. Energy harvesters have been a new avenue to generate electrical power locally for remote applications. An energy harvester is a device that taps into aeroelastic phenomena to convert vibrational energy into electrical energy. Additional benefits of harvesters include being lower in complexity, ease of maintenance, and the resulting cost-effectiveness. The splitter plate represents a wake control device that alters the wake to make the energy capture more efficient and practical when attached to a cylinder. However, the flow mechanisms under different conditions have not been fully understood. The motivation of the present work is to bridge this gap by characterizing the mechanisms and wake dynamics of a cylinder with a flexible splitter plate. This will help advance the understanding of the wake behavior of these devices to make safe and efficient devices. To characterize the behavior of the wake, measurements are made at different flow speeds. A preliminary analysis of the splitter plate motion showed that increasing the Reynolds number produced four distinct regimes to be studied further: crossover, baseline, reduced amplitude, and chaotic regimes. The initial analysis also revealed a condition named burst, defined as a period of resonant, two-dimensional, high amplitude oscillations of the splitter plate. All regimes are compared to a bare cylinder with no splitter plate to ascertain differences in wake behavior. The use Helium Filled Soap Bubbles (HFSB) in flow diagnostic tools such as Particle Image Velocity (PIV) experiments has permitted large-scale measurements. A Lagrangian Particle Tracking (LPT) experiment was performed in which the HFSB is tracked individually with the help of an algorithm. The present study employs Shake-The-Box (STB), a novel LPT tracking algorithm that uses time-resolved experimental data to predict future tracer particle locations. The benefit of such an algorithm includes reduced ghost particle detection and computational efficiency. The details of this experimental method have been covered extensively in this thesis. The raw experimental data was processed to perform statistical analysis, proper orthogonal decomposition, and spectral analysis to provide insights to meet the thesis objective.

Reduction of skin-friction drag over a fully developed canonical zero-pressure gradient turbulent boundary layer (ZPGTBL) subjected to spanwise oscillation is measured using planar particle image velocimetry (PIV). The experiments are conducted at Reθ of 1000 and 1800, the chosen range of spanwise oscillations amplitude and frequency are around the optimum reported in literature studies (T+osc = [100-700], A+osc= [50, 150]). A high-resolution planar PIV apparatus is employed to measure the drag reduction directly by wall shear stress estimates on the oscillating wall. The measurement uncertainty of the drag estimates from PIV measurements is examined. The results show drag reduction in the order of 15% after 6 boundary layer thicknesses from the beginning of the oscillating section. Variations of the drag reduction follow the trends reported in literature. The PIV measurements enable the analysis in terms of Reynolds shear stresses, turbulence production and allow visualising vortex packets by vorticity. A pronounced drop of turbulence production in the range y+ = [5, 30] is observed. The vorticity analysis indicates a distortion from the well-known hairpin-packet arrangement, suggesting that the mechanism of hairpin auto-generation may be inhibited by spanwise wall oscillations

Junction flow denotes the fluid phenomenon where the flow on a flat surface encounters a protuberance. This type of flow, which is highly three-dimensional, turbulent and unsteady, usually leads to the generation of a Horseshoe vortex at the obstacle's leading edge. Existing extensive experimental and numerical studies focus on the inception of such vortex, as it exhibits a bi-stable behaviour which persists downstream in the vortex legs. In a practical situation such as wing-fuselage juncture, the created vortex interacts with the flow around the aircraft. Consequently, the interference drag increases and the wake can reduce the effectiveness of the stabilizers. To tackle these effects, control devices are implemented to alleviate the strength of the vortex, being leading edge fairing the most widely used device. In this research project the junction flow is subjected to the influence of different control devices: leading edge fairing, vortex generators and the novel antifairing are tested. The flow field is captured with the state-of-the-art large-scale tomographic PTV technique. The aim of this project is to characterize the vortical structures associated with each passive control device. The time-averaged flow field results show the fairing is the most efficient device in reducing the presence of the Horseshoe vortex, but it is also the one which contaminates a larger wake area with the vortex wake. The vortex generators are the worst performer both in the mitigation of the vortex and the turbulence level in the wake. Nevertheless, the performance of such control devices highly depends on the location of the vortex generators. Thus, it is speculated that an optimal position exists which can yield better results. The antifairing shows minimal differences in the velocity field and the vortical structures compared with the reference case, but with a slightly lower turbulence level. An auxiliary experiment with stereoscopic PIV is performed to the wake of the junction, in which the momentum deficit, a good indicator of the drag, is calculated for the different configurations. The leading edge fairing shows minimal drag reduction associated with the HSV of just about 1-2%. The VG once again have proven not being very effective with an increment of 4%. A reduction of more than 15% is measured for the antifairing case. Although the associated flow topology assimilates to the reference case. This study investigates the HSV system around the junction as a volume. In comparison with most of the previous experimental studies on the subject, performed mostly with planar PIV and limited to just a few planes, the volumetric measurement can deliver more information with a single measurement. Therefore, it can better define the flow topology and vortical features and facilitate the understanding of the control devices working mechanisms.

This study explores the flowfield around a pusher propeller-wing configuration using Lagrangian Particle Tracking (LPT). It assesses LPT’s capability to characterise propeller-wing interactions, emphasising its ability to resolve complex flow dynamics. The experiment successfully achieves high particle concentrations downstream of the propeller, with propeller rotation enhancing the uniformity of the particle distribution. Velocity measurements are precise and feature low uncertainty, and vortex dynamics shed by the propeller are effectively resolved. Furthermore, the study confirms LPT’s robustness in characterising wing-propeller flow characteristics, particularly in thrust and torque calculations, highlighting its non-invasive nature for performance measurements. The investigation reveals how the wing influences the slipstream’s vortex structure and identifies the pylon’s effects. The method captures the propeller-induced effects on the wing, providing insights into upwash, downwash, and angle of attack variations. In conclusion, this research demonstrates the utility of LPT in comprehensively characterising propeller-wing interactions, offering valuable insights for aviation and propeller innovations.

The flow near the surface, and the acoustic emissions of trailing edge serrations are investigated in this work. The use of this family of aerodynamic devices on airfoils is intended for the reduction of turbulent boundary layer-trailing edge noise (TBL-TE noise). This purpose has been well demonstrated in wind tunnel and numerical experiments. Particularly, their use in the wind turbine industry has been of great interest in recent years. A growing number of field measurements have shown that a noticeable noise reduction of TBL-TE noise in state-of-the-art blades is also obtained. A full explanation on the mechanism of how noise is reduced is nevertheless lacking. Existing experimental research on serrations offers only a limited characterization of the relevant flow parameters. Fundamental concerns pertaining to the conditions at which that data has been previously gathered are furthermore recurrent. The persistent use of flow-misaligned serrations creates a situation in which flow structures may be observed and misinterpreted as necessary for the attainment of noise reduction. This circumstance complicates the discussion and isolation of the relevant noise reduction mechanism...@en

The research presented in this thesis focuses on boundary layer separation and, more specifically, on the related physical mechanisms, such as laminar to turbulent transition, with a vision towards effective separation control. The work is divided in three parts, each corresponding to a different separation scenario. The supporting data is experimental, obtained by means of particle image velocimetry. The first and major part regards the phenomenon of laminar separation bubbles. The spatial and temporal response characteristics of a flat plate laminar separation bubble to impulsive forcing are first investigated in order to shed light on the processes of flapping and bursting. The impulsive disturbance is introduced twodimensionally with a dielectric barrier discharge plasma actuator. The disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding strength in the aft portion of the bubble is reduced. Duration of the recovery of the bubble to its unperturbed state is independent of the forcing amplitude. At the same time, linear stability analysis shows that the growth rate and the frequency of the most unstable mode decreases for increasing forcing amplitude. Throughout recovery, growth rates are directly proportional to the shape factor, indicating that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. It is found that the stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop.@en

A flying aircraft consumes fuel to overcome air resistance during its motion. A significant part of this consumption (55%) is due to turbulent skin friction arising at the interface of the aircraft surface and the air. The current work aims to develop and investigate relevant turbulent skin-friction reduction techniques and assist in advancing technologies that will contribute to achieving a reduction in turbulent skin-friction drag and, consequently, fuel consumption and the associated emissions. The thesis examines active flow control techniques derived from the spanwise wall oscillation concept. The latter involves introducing a time-dependent spanwise motion to the wall over which a turbulent boundary layer is present. The current work relies on the experimental investigation using particle image velocimetry to quantify the effect of the active control techniques.....@en

Particle image velocimetry (PIV) is the state of the art for quantitative, full-field, 3D flow diagnostics. Despite the maturity of the technique, two bottlenecks are identified which are addressed in this thesis: the achievable measurement volume size, and the optical access to geometrically complex objects. Both aspects are well illustrated when considering the human body in sports action. Characterising the aerodynamic flow topology around an athlete demands measurement volumes on the cubic-meter scale, whereas the simultaneous illumination and imaging of the flow near the athlete’s body is challenged by the geometric complexity of the human body and the sports equipment. Focusing on sport performance, especially in timed disciplines, it is recognized that due to the shape of the human body, the aerodynamic resistance is often dominated by pressure drag. Therefore, a third element addressed in this thesis is the PIV-based pressure evaluation in the flow and on an object surface. To overcome the identified measurement constraints, a PIV system for the 3D diagnostics of large-scale and low-speed flows has been developed, synthesizing advancements in PIV imaging and illumination hardware, automation technology, tracer particle generation, and particle tracking algorithms. The so called robotic volumetric PIV concept is proposed in Part I of this thesis, along with dedicated data analysis methods to retrieve the shape of the test object, the total pressure in the fluid flow, and the aerodynamic pressure on the object surface. Part II features applications of the proposed tools in the context of sport aerodynamics, with specific examples in cycling and swimming. @en

A new type of tracer is making its entry in the scenario of wind-tunnel measurements: helium-filled soap bubbles (HFSB). The present work discusses the main fluid-dynamic and optical properties of HFSB to evaluate their use for quantitative measurements in aerodynamic experiments. In the past three decades, particle image velocimetry (PIV) has become a standard measuring technique in experimental fluid mechanics. Advances in both hardware components and software analysis have allowed achieving many milestones in flow diagnostics, mainly time-resolved and instantaneous volumetric measurements. In particular, the extension to the third dimension in space, i.e. tomographic PIV and 3D particle tracking velocimetry (PTV), has been used to provide quantitative visualizations of the coherent structures occurring in various turbulent flows and have provided insight in the spatial organization of the turbulent motions at different scales. The extension of the aforementioned techniques towards industrial practice in wind tunnel testing requires the development of a more efficient approach in terms of scaling and versatility. The present dissertation tackles the upscaling of PIV experiments towards industrial wind tunnels with the use of HFSB as tracing particles. The reasons and motivations behind this choice are addressed in the first chapter and followed by a description of the state-of-the-art of PIV. The second chapter aims at familiarising the reader with the working principles of PIV, which will be later recalled when presenting the advances towards large-scale experiments. Information on the mechanical behaviour of tracer particles and on the underlying physics are discussed in the third chapter, where also the case of HFSB is examined for use during quantitative measurements in the low-speed flow regime. The problem of seeding in wind tunnels is discussed in chapter 4, where a system for the injection of HFSB in a large wind tunnel is presented. Here, the relationship between HFSB production rate and the resulting spatial concentration and dynamic spatial range (DSR) are discussed. Specific experiments that examine the tracing fidelity of sub-millimetre HFSB tracers are presented in chapter 5. The behaviour of HFSB is compared to micro-size droplets, yielding a characteristic response time in the range of 10 μs. The latter milestone opens up to the applicability of HFSB tracers for quantitative velocimetry in wind tunnel flows. In chapter 6, a specific case of interest is presented whereby HFSB tracers are used to measure the flow velocity within steady vortices such as those released at the tip of wings. A dedicated experiment shows that the neutrally or slightly buoyant HFSB return a rather homogeneous spatial concentration within the core of vortices, solving the long-standing issue encountered for small heavy tracers, such as fog droplets, that are systematically ejected from highly vortical regions. An analysis of the light scattering by HFSB was conducted with theoretical and experimental approaches, as described in chapter 7. The light intensity scattered by the HFSB is characterised by two source points: the glare points. The overall scattered light appears to be 104-105 times more intense with respect to the oil-based micro-size droplets. This information is used to retrieve the maximum size of the measurement volume for a given light source. Chapter 8 closes this dissertation presenting a survey of all the experiments that have been conducted during this PhD research. The scale of experiments varies from the more academic case of a circular cylinder up to the one of a ship model installed in one of the large industrial wind tunnels operated at the German-Dutch Wind Tunnels laboratories (DNW), going through the visualization and quantification of large structures in the rotor region of a vertical axis wind turbine (VAWT).@en

The research presented in this booklet focusses on the cross-flow instability. Applying traditional and advanced flow diagnostics, the boundary layer evolution is studied in detail. The topology and evolution of both primary and secondary instability mechanisms is revealed with unprecedented detail for experimental research paving the way for new advanced-diagnostics investigations. Important confirmations of the outcomes of past experimental, numerical and theoretical studies are achieved together with the description of a newly-reported flow phenomenon. The latter consists of a low frequency motion of the "stationary" primary vortices. While this phenomenon is considered not relevant for the transition evolution, it is deemed important for experimental investigations as it encompasses very high levels of turbulent kinetic energy. Advanced flow control experiments based on alternating current dielectric barrier discharge plasma actuators are also performed following different instability control approaches. The primary instability is conditioned by the external forcing either in the wavenumber spectrum (by inducing selected spanwise modes) or in intensity (by weakening or enhancing the cross-flow velocity). The secondary instability modes are conditioned in the frequency spectrum and phase. These efforts achieved the intended scopes. Although, when selected stationary modes were forced, the boundary layer fluctuations were enhanced. These fluctuations can directly cause the turbulent breakdown vanishing the beneficial effect of the performed instability control. The cross-flow forcing, making use of newer actuators reaching higher frequencies, resulted successful yielding transition promotion or delay depending on the forcing direction. @en

The market growth expected for commercial aviation in the coming decades and the increasing social awareness regarding the effects of global warming are driving significant technological developments necessary for emission reduction in future transport aircraft. From the aerodynamics perspective, a significant increase in aircraft efficiency can be obtained by applying Laminar Flow Control (LFC) techniques. The objective of LFC techniques is to reduce the skin-friction drag component by delaying the laminar-turbulent transition through the stabilisation of boundary-layer instabilities. Relevant to high-subsonic transport aircraft is the development of Crossflow (CF) instability, which manifests as a series of co-rotating vortices inside the boundarylayer flow on swept aerodynamic surfaces...@en

Particle Image Velocimetry (PIV) relies upon the introduction of particle tracers that scatter sufficient light and follow the flow accurately. The use of submillimetre helium-filled soap bubbles (HFSB) as flow tracers for PIV is investigated for the purpose of enabling velocity measurements in large-scale industrial wind tunnels. That soap bubbles reflect more light than scattered by small liquid droplets or solid particles, allowing larger volumes to be illuminated, is a long known fact and has caught the attention of aerodynamicists since the 1930s. The difficulty encountered during initial efforts on using soap bubbles for accurate measurements revolves around the lack of control during the generation of these tracers, and the failure in presenting evidence that they could accurately follow the flow. Proof of concept that HFSB could be used for accurate flow measurements in wind tunnels was presented in the year that preceded the beginning of this work. In this thesis, the generation and control of HFSB and their tracing fidelity are studied through a series of experiments and simulations, bringing large-scale PIV using HFSB to the technology maturity level required for industrial measurements. High-speed shadowgraphy at the bubble generator exit revealed the main regimes of bubble generation. A regular, periodic and controlled generation bubbles of monodisperse size distribution, namely, the bubbling regime, was obtained by properly tuning of the input flow rates. The relation of the later with the bubble size and production rate was also obtained from these visualizations. Measurements of the HFSB velocity in the stagnation region ahead of a cylinder, obtained with Particle Tracking Velocimetry (PTV), relative to the flow velocity (slip velocity) were used to retrieve the HFSB time response and the ratio of helium to soap flow rates that satisfy the neutral buoyancy condition, in which the soap bubble density equals that of the surrounding air flow. Simulations of the particle motion in a rectilinear oscillatory flow was used to quantify the importance of the unsteady forces acting on a particle and to derive empirical relations for estimating the HFSB slip velocity in flows where the unsteady forces are relevant. In this case, the particle slip velocity is shown to depend on three parameters: the particle Reynolds number, the ratio of particle-to-fluid density and the flow time-scale. These cannot be combined into a single non-dimensional Stokes number. The validity of the empirical relations were extended for the analysis of the slip velocity of a particle travelling around an object. Based on the later, a method for deriving the density of a nearly-neutrally-buoyant particle that comprises the effects of unsteady forces and allows mismatch of acceleration between the particle and the flow was described. The tools developed for slip velocity analysis using the simulations were applied to assess experimental data from large-scale PIV measurements performed at the Low-Speed Tunnel (LST) of the German-Dutch Wind Tunnels (DNW). The experiments were realized in the flow around an airfoil of 70 cm chord at free stream velocity up to 70 m/s, reaching a chord-based Reynolds number of 3.2 million. PIV measurements using HFSB at this speed and Reynolds number were unprecedented. The results have indicated variations of the bubble density (20-30%) occurring post-generation. The tracing fidelity of HFSB in wall-bounded turbulence is investigated by comparing measurements in a turbulent-boundary layer of the mean velocity and Reynolds stress profiles, with those obtained with micrometre oil droplets (reference) and submillimetre air-filled soap bubbles (AFSB). The results have shown that the statistics of the first and second moments of velocity are well captured by all three investigated tracers, even by the heavier-than-air AFSB, which were shown to be poor tracers in the stagnation of a cylinder. Mechanisms of preferential concentration in turbulence were attributed as the cause of the better traceability observed. The thesis is concluded with a successful industrial application in the Large Low-Speed Facility (LLF) of DNW (9.5×9.5 m2 test section) around a tiltrotor aircraft in three flight modes, hover, transition and cruise, and tunnel speeds up to 60 m/s. The bubbles were introduced into the flow using a 3×3 m2 seeding rake, containing 400 bubble generators. The PIV measurements were performed in stereoscopic configuration in a field-of-view of 1.1×1.1 m2.@en

Experimental aeroelasticity has been hindered by the intrusivity of measurement equipment and complexity of the experimental setups. Advancements in hardware development have been encouraging optical tracking techniques to replace cross correlation approaches in experimental aerodynamics. However, digital image correlation (DIC) is still the standard optical technique in structural diagnostics. The first experiment of the dissertation assesses the accuracy of an established tracking technique, such as Shake-the-Box, for tracking surface markers on a moving panel, comparing it to DIC. Despite being outperformed in terms of spatial resolution by DIC, surface marker tracking resulted in the same order of accuracy, making structural tracking suitable for large-scale applications. Such results imply the feasibility of characterizing experimentally aeroelastic problems by tracking, in a simultaneous manner, flow tracers and surfacemarkers....@en

Wings operated at low and moderate Reynolds number such as the ones of UAVs or the blades of small turbine and of compressors, can be the source of an aero-acoustic phenomenon called laminar boundary layer instability noise. The narrow band noise can be attenuated using trailing edge (TE) serrations (Chong et al. 2013, Moreau et al. 2012) but the mechanism behind tonal attenuation is yet to be investigated in detail. An experimental study (1.32X105 < Re < 5.30X105) is conducted in an open-jet low turbulence wind tunnel using NACA 0018 airfoil (c = 0.20m) with modified TE. The acoustic emission in the far field is recorded using far-field microphones whereas the developing field near the TE is studied using Time resolved planar particle image velocimetry and oil flow visualization techniques. Study showed the serration-3 (2h = 20mm, λ = 10mm) has maximum tonal noise attenuation and effect the point of separation of laminar separation bubble. The span-wise change in flow characteristics in case of serrations can be attributed to the effective chord-length of the wing at each span-wise position. Further span-wise correlation of chord-wise flow fluctuations show the flow being turbulent upstream of serration-3 TE modification.

Vortex shedding occurs from commonly encountered objects in ows such as blunt and divergent (truncated) airfoil trailing edges [12], landing gear of aircraft [37], etc. In addition to undesired aero-elastic effects such as vortex-induced vibration and uctuating aerodynamic forces, tonal acoustic noise is emitted due to the regular and periodic nature of the shedding. With the aim of eliminating this emitted noise, an experimental study was carried out on a D-shaped bluff body model using appropriately designed and implemented plasma actuator configurations to achieve active three-dimensional vortex shedding suppression.