This thesis project examines the drag crisis trigger mechanism for double layer fabrics on cylindrical cross-flow. The primary goal is to discover why the double layer fabric is able to trigger the drag crisis much sooner than conventional surface roughness.

The methodology employed in this research follows an experimental approach, using balance measurements to determine the aerodynamic drag at varying Reynolds numbers for different configurations. Particle Image Velocimetry (PIV) measurements are performed to examine the boundary layer, flow separation point and other flow phenomena occurring near the cylinder surface.

Over the course of this research, eight different double layer configurations have been the subject of study, as well as six single fabric configurations and two reference configurations consisting of a bare cylinder and a cylinder with zigzag trips. Balance measurements have been performed on all of the configurations to determine which configurations are deemed relevant to be studied with PIV techniques. Thus, PIV measurements have been performed on two double layer configurations with a varying underlayer and the same overlayer, as well as the study of the individual fabrics employed to make up the two-fabric construction. That is to say, the two underlayers and one overlayer used have been studied on their own.

The balance results uncover that the minimum drag coefficient across all double layer configurations is achieved with the smallest rib spacing. Conversely, this minimum drag coefficient is located at the highest critical Reynolds number across all configurations. Additionally, a relationship between the critical Reynolds number and the underlayer rib spacing has been determined for the studied configurations. Furthermore, the PIV measurements provide insight into the normalized velocity fields and reconstructed pressure fields. With these results, the development of the boundary layer on the foreside of the cylinder with double layer fabrics can be studied. Examining the flow near the surface, it can be seen that the presence of the ribs results in a localized flow convergence (upstream of the rib) and divergence (downstream of the rib), these geometric effects accelerate and decelerate the flow locally, causing static pressure oscillations on the foreside of the cylinder. With the appearance of localized adverse pressure gradients on the foreside of the cylinder, flow instabilities are seeded eventually trigger the transition of the boundary layer to a turbulent state, thus allowing the flow to remain attached to the cylinder surface for longer, ultimately delaying separation and reducing pressure drag.

While the study has provided valuable insights regarding the trigger mechanism for the drag crisis on double layer fabrics on cylinders, it has also paved the way for further research regarding this topic and the specific effects of rib height, behavior and performance in unsteady flows and whether the two fabric construction is strictly necessary. These considerations are addressed at the end of the conclusions chapter.

As offshore wind energy expands into deeper waters, Floating Offshore Wind Turbines (FOWTs) are essential. This thesis investigates how complex floating motions affect wake behavior and recovery, addressing a key gap in experimental data. A 1:148 scale DTU 10MW turbine was mounted on a six-degrees-of-freedom Hexapod and tested in the Open Jet Facility at TU Delft. Using 3D Particle Tracking Velocimetry with Helium-Filled Soap Bubbles and high-speed imaging, detailed flow fields were reconstructed. Results show that the tested low-frequency surge motion improves wake recovery by 40% compared to static conditions at a distance of 5D, while high-frequency pitch has the least benefit. Turbulence, vorticity, and spectral analyses confirm enhanced mixing and earlier vortex diffusion under motion, offering valuable insights for farm layout optimization.

Growing interest in efficient short-haul and electric regional aviation has renewed focus on propeller-driven aircraft, although open rotors pose significant noise challenges. This study investigates how noise perception—quantified by the Effective Perceived Noise Level (EPNL)—influences propeller blade design, with particular attention to blade sweep. Aerodynamic modelling is carried out using a Vortex Lattice Method, while aeroacoustic analysis relies on Hanson’s frequency-domain tonal noise model.

A parameter study reveals that perceived noise plays a more critical role during take-off than cruise in guiding propeller design, and that blade sweep is more effective at reducing perceived noise than physical noise, especially at high rotational velocities.

In the second part of the study, multiple trade-off analyses are conducted through a series of optimisations that minimise EPNL under progressively loosened power requirements, all performed under take-off conditions. The results indicate that while reducing rotational velocity remains the most effective strategy for lowering both physical and perceived noise, the fundamental frequency becomes increasingly dominant in the total tonal noise. Consequently, optimisation for perceived rather than physical noise results in similar blade geometries, but with progressively greater reductions in perceived noise relative to physical noise because of the lowering Blade Passage Frequency, which the human ear is less sensitive to. Among various sweep-induced noise reduction mechanisms, fixing the blade radius—rather than the blade length—yields greater noise reduction, primarily due to reduced blade loading as the blade lengthens with introduced sweep.

A comparison of EPNL-optimised designs requiring 1% additional power shows that allowing the blade length to vary under a fixed radius enables a further 7.2 dB reduction in perceived noise. When examining the effectiveness of sweep versus twist under constant blade radius conditions, and again allowing 1% extra required power, blade twist achieves a 1.3 dB reduction in perceived noise, while blade sweep provides an additional 4.6 dB reduction.

Turbulent plumes play a crucial role in geophysical and industrial processes, yet their behavior in rotating environments remains insufficiently understood. This study experimentally investigates the influence of background rotation on negatively buoyant turbulent plumes using Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF). By varying the source Richardson and Rossby numbers, the effect of rotation on plume self-similarity, width and centerline velocity is examined.
Results reveal that rotation significantly alters plume dynamics, reducing entrainment and modifying self-similarity profiles at the edges of plumes. The evolution of plume half-width is inhibited, and centerline velocity decays faster in rotating environments. The curved area of both buoyancy and velocity self-similarity profiles do however overlap with a Gaussian curve.
These findings contribute to a better understanding of rotating turbulent plumes and provide insights relevant to oceanic convection, atmospheric dynamics, and industrial mixing processes.

In this thesis, the effect of a surging and pitching motion of a floating wind turbine on its wake as well as the effect of a surging, pitching and yawing motion on a stationary downstream turbine are analysed. Two experiments are conducted in order to address both points. The turbine models used are scale models of the DTU 10 MW turbine, and the upstream turbine is placed onto a kinematic robot which can move in all 6 degrees of freedom. The first objective is studied using Particle Tracking Velocimetry and the second objective using load cell measurements. It is found that pitch has a larger effect on the flow field in the wake than surge and that the effects of low frequency motion are more visible than those of high frequency motion. Moreover, it is found that the tracing particles are concentrated in specific areas, affecting the accuracy of the results. In future research, it should be examined why this is the case and how it can be improved. Additionally, low frequency pitch and surge had the greatest effect on the loads. A clear sinusoidal motion is visible and the mean thrust, torque, and power is increased. The effect of the high frequency motions is not visible in the results. In future studies, the wake behind a downstream turbine can also be examined using Particle Tracking Velocimetry. Moreover, the downstream turbine can also be moving rather than standing still and more movements of the turbine can be examined.

With a symmetric rotor, tip vortex helices develop regularly before interacting, following the leapfrogging instability. The latter can occur earlier when the helices are placed at an initial radial offset, which is realized by considering blades of different lengths. This study investigates the spatio-temporal development of near-wake behavior for rotors with a significant blade length difference. Large eddy simulations and actuator line method on a modified NREL 5MW turbine under various inflow conditions were conducted to evaluate the impact of blade length differences ranging from 5 to 30%. The development of tip vortex helices, the onset of leapfrogging, vortex merging, and ultimately their three-dimensional breakdown were monitored. The analysis is corroborated by using a simplified (two-dimensional vortex-filament model).
The results show that the leapfrogging process begins immediately downstream of the vortex release when blades of different lengths are considered. The instability growth rate obtained from the 2D vortex model agrees with the LES data. Although the rotor asymmetry accelerates the leapfrogging and, in some conditions, the vortex merging process alone, it proves insufficient to cause a large-scale breakdown of the helix system and enhance wake recovery. Under laminar and 5% turbulence inflow conditions, the rotor asymmetry has minimal effect on momentum deficit recovery. In other conditions, no appreciable variations of wake recovery can be observed.

Wind turbines play a crucial role in the worldwide effort to embrace sustainable energy, utilizing sophisticated aerodynamic principles to efficiently capture wind energy. A thorough comprehension of deep dynamic stall, a phenomenon that greatly impacts wind turbine performance, is essential for maximizing efficiency, maintaining structural integrity, and propelling the development of wind energy. This study presents an experimental exploration of deep dynamic stall phenomena through wind tunnel experiments carried out on a NACA643418 airfoil at TU Delft.

The study commences with the development of a comprehensive test matrix drawing from existing literature, with a focus on angles of attack of 40, 50, and 90 degrees. Through precise experimentation, the research team meticulously measures and corrects for wind tunnel effects, uncovering crucial trends in lift and drag coefficients. Significantly, the study identifies laminar separation bubbles and trailing edge separation as the main stall mode before the deep stall regime.

The analysis of static and dynamic pressure data offers valuable insights into the aerodynamic characteristics of the airfoil in deep stall conditions. Notably, significant variations in aerodynamic performance between the upstroke and downstroke are evident, particularly at high angles of attack surpassing 25 degrees. The data underscores the intricate interplay of pitching frequency, amplitude, and airflow separation, highlighting the pivotal influence of shedding frequency on dynamic stall phenomena.

Moreover, measurements of dynamic pressure reveal the intricate relationship between the frequency and amplitude of pitching and the patterns of vortex shedding. The research shows that vortex shedding is most significant at angles of attack close to 90 degrees, even during dynamic pitching. These discoveries emphasize the significance of comprehending deep dynamic stall to enhance the design and performance of wind turbines in various operational conditions.

The phase-averaged PIV images serve as a valuable complement to the pressure data, offering a visual confirmation of how flow dynamics impact aerodynamic performance in deep stall conditions. These images unveil clear disparities in airflow behaviour during upstroke and downstroke motions, shedding further light on the aerodynamic obstacles encountered by wind turbine blades during dynamic operation.

The aviation industry is exploring unconventional aircraft designs like the Flying V in its drive to reduce carbon emissions and improve fuel efficiency. The Flying V, featuring a crescent wing with leading edge kink, currently suffers from an unstable nose-up pitch tendency. This study investigates vortex control methods within this kink region to increase lift on the outboard wing and delay the pitch-break. A full-span, modular wind tunnel model is used to investigate the effects of parabolic and diamond juncture fillets, as well as full-chord fences, on the pitching moment characteristics. Stereoscopic Particle Image Velocimetry (SPIV) and oil flow visualization are used to analyse vortex behaviour and flow topology. Results reveal that the parabolic fillet outperforms the diamond fillet in generating lift at higher angles of attack due to its ability to promote vortex formation and delay breakdown. The installation of full-chord fences increases lift on the outboard wing and positively influences the pitching moment, though none of the tested configurations increased the pitch-break angle.

Leading edge roughness is an important challenge for the wind energy industry as blades are permanently exposed to the damaging impacts of the environment. The contamination due to impact of insects, ice accretion and dust; as well as the erosion of the blade due to rain, debris and abrasive airborne particles increase roughness upon the surface. Leading edge roughness leads to the deterioration of aerodynamic performance, including early transition from laminar to turbulent flow, increased drag, and reduced lift. These performance losses directly translate into a decreased annual energy production (AEP) and increased levelized costs of energy (LCoE). The aim of this study is to enhance the understanding of airfoil aerodynamics with LER, specifically focusing on the usage and impact of tripping tapes as a standard representation of LER in wind tunnel campaignsb by deviating proven zigzag tapes in non-standard setups. The research objectives include investigating critical roughness height, analyzing the impact of different tripping tape setups, studying drag generation in the boundary layer, and exploring the effects of real erosion.
Contrary to expected reference values, the critical height was found to be traceable with the Roughness Reynolds number for zigzag tape usage as Rek = 125, opposing to expected critical Reynolds number Rek = 200 in current literature. Most interesting application parameter after
critical condition, often related to height, is found as tape location. Drag and lift performance penalties behave as linear function of tape location for pre-stall drag bucket and linear lift region. In stall, the performance penalty is heavily related to locations closer to the leading edge with increased importance of tape height. Additionally, real roughness of reference studies has been more closely resembled by tripping tapes closest to the leading edge in terms of magnitude and as function of pitching angles when compared to industry standards. The boundary layer measurements have indicated that panel code simulation data has errors connected to transition location rather than boundary layer development, which reflects in polar data and transition location.

Overall, this study emphasized that ZZT is more versatile than the industry standard setup, highlighting the importance of considering different application variations for accurate assessments of LER impact, while indicating possible panel code improvements related to transition location
as observed in performance polars and boundary layer thickness.

Wind tunnel investigations of cycling aerodynamics are conducted in air streams that are designed to have minimum turbulence. This is not representative of the true, turbulent wind environment encountered by a track cyclist in an indoor velodrome. The goal of this thesis, broadly speaking, is to assess the effects of this turbulence on cycling aerodynamics.
To address this question, two experimental campaigns were conducted. The first consisted of track measurements, to measure the turbulence intensities and spectra encountered in an indoor velodrome, by an isolated cyclist and by a cyclist in the far wake of another. This turbulence data was collected using a three-hole pressure probe. In the case of an isolated cyclist, it was observed that the airflow was dependent on the cyclist's location on track. A spectral analysis also revealed the presence of periodic components to the airflow, a result of the cyclist's cadence. When riding in the far wake of another cyclist, the turbulence encountered was stronger when the distance of separation got smaller.
The second experimental campaign was conducted in the wind tunnel. First, the measured track turbulence intensities and spectra were simulated using grids, for small scale, high frequency turbulence, and large obstacles, for large scale, low frequency turbulence. Once the simulated turbulence matched the track turbulence to acceptable levels, its effects on the drag of a cylinder model, representative of the cyclist's limb, were assessed. These drag measurements were conducted using an external force balance. It was found that for a smooth surfaced cylinder, free-stream turbulence was enough to trigger flow transitions and cause a reduction in the measured drag. For a rough surfaced cylinder, increasing the turbulence triggered flow transition earlier, and led to an increased Reynolds number range for the critical flow regime. It was also seen that large scale turbulence did not have a significant effect of the drag of the cylinder.

Desert sand acoustic emissions are produced when a “sonic sand” is sheared locally or by a natural dune slipface avalanche, resulting in a brassy sound between 50 and 400 Hz. This type of sediment exhibits particular granulometric, shape and surface characteristics, due to the grains’ erosion and transport history, and emits sounds when the sheared grain layer vibrates in a synchronized manner, much like the membrane of a speaker. Recording such sand acoustic emissions on Mars (and perhaps other planetary environments) using rover microphones could thus become a new form of observable for scientists to estimate the surface sediment’s characteristics and history from a distance, but also the granular flow dynamics taking place. To determine whether this approach could be viable in the future, it is essential to evaluate how the Martian environment may affect sand acoustic emissions differently than on Earth. After showing that the muted Martian soundscape would likely allow rovers to detect such signals from a few tens of meters, the present thesis studies the impact of the interstitial air pressure within the sand bed on the sound emission mechanism of such sonic desert sands.

In this project, silent and sonic desert sand shear flows are induced under a range of pressure levels, from terrestrial ambient pressure to Mars-like pressure, within two separate, manually operated vacuum chamber setups: a smaller chamber shaken to create the sounds, and another longer chamber that better replicates avalanche-like sand flows. The motion applied and sound produced are measured using an accelerometer and a microphone inside the chamber. Metrics in the time and frequency domains are defined to analyse the changes in sound energy, amplitude, and frequency components produced at different pressure levels. Firstly, the silent sand tests are used to establish how the air pressure level within the experimental setup affects the regular sound of sheared sand (i.e. grains impacting one another) and more generally the sound emission of “normal” sounds, whose emission mechanisms do not depend on grain packing and synchronized motion. Then, a simplified theoretical model of how the sound pressure level (SPL) of a sound evolves with decreasing acoustic impedance, is derived and validated using the silent sand measurements performed. Finally, the sonic sand measurements are compared to the SPL model and silent sand measurement results, which are used as a baseline for nominal sound production behavior, to evaluate how the interstitial air pressure affects the amplitude and signal energy of the sheared sonic sand emissions. Furthermore, differences in the sand acoustic emissions’ frequency spectra and time duration across pressure levels provide information about the possible physical changes occurring in the granular flow dynamics of the sheared sonic sand.

In both experiments, the dominant frequency very closely follows the trend of the motion metrics used, as described in the literature, and remains very consistent across pressure levels. This suggests that the maximum sheared sonic sand layer thickness is independent of the interstitial air pressure. Then, the sonic sand emissions see an increase in the sound amplitude and signal energy related metrics from ambient pressure to 413.25 mbar, unlike the gradual decrease predicted by the SPL model and the trend of silent sand measurements with decreasing pressure. Below 413.25 mbar, the results suggest a stabilized behavior, with the acoustic metrics of the emissions following the model. Furthermore, in the avalanche-like emissions, a new frequency component slightly higher than the dominant frequency emerges as the chamber pressure decreases. These observations are evidenced in the time-domain, where the sand acoustic emissions seem to initiate earlier in the granular flow at 413.25 mbar and below, resulting in greater acoustic pressure levels being produced, compared to those at terrestrial pressure. It is hypothesized that more sheared sonic sand grains synchronize at 413.25 mbar and below (compared to terrestrial air pressure), and thus increase the amplitude of the sound wave produced. For avalanche-like flows, the new frequency component that appears with decreasing pressure level seems to suggest that the minimum sheared layer thickness threshold required to produce an emission is lowered at lower pressure, which leads to a higher frequency produced initially until the full layer forms, ultimately decreasing the frequency. Further research is required to confirm these preliminary findings and theories.

Surfaces that induce slip at the wall have been shown to reduce skin friction drag in the turbulent regime. While superhydrophobic and liquid-infused surfaces are capable of drag reduction in turbulent hydrodynamic flows, neither would be feasible in aerodynamic applications due to the added roughness and the minuscule slip lengths created by the unfavourable viscosity ratio between the liquid and air. This thesis explores the novel idea of liquid-infused superhydrophobic surfaces (LISHySs). These surfaces use liquid droplets held in superhydrophobic features to induce slip at the air-liquid interface while reducing resistance to the motion of the liquid within the cavity.

Since no framework existed on the drag-reducing mechanism of such a surface, this was devised and the parameters of importance were identified. These were seen to be the area of the air-liquid interface, the exposed and submerged areas of the droplets in the cavities, and the degree of superhydrophobicity of the surface. Two types of LISHySs were consequently designed - one with spherical droplets and another with spanwise cylindrical droplets. These were produced by 3D printing surfaces with cavities and applying a superhydrophobic coating, into which water was infused.

Direct force measurements on the surface showed an increase in the drag coefficient (between 10% and 17%) with respect to a smooth reference surface. While PIV for the flow over this surface showed an increase in mean streamwise velocity and a decrease in Reynolds stress in the overlap layer, the high degree of reflection produced by the surface meant that meaningful near-wall data could not be obtained. Observations of the air-liquid interface demonstrated that the hypothesised steady rolling motion of liquid droplets occurred at low freestream velocities, but this was observed to become more chaotic at higher velocities. Pressure drag due to droplets acting as roughness elements, among other reasons, further contributed to the total drag produced.

While the results showed that the current implementation of LISHySs was unsuccessful, the lessons from this first attempt point to the directions in which future studies could be made. Cylindrical droplets oriented in the streamwise direction offer high potential due to the creation of streamwise slip. Advancements in the fields of material science and hydrophobicity would allow for surfaces with finer cavities that exhibit higher superhydrophobicity to be produced. Measures such as these would pave the way for the passive reduction of turbulent drag through the novel concept of LISHySs.

Unsteady surface pressures shed light on the turbulent structures of boundary layer flow, which dictate for a large part the aerodynamic and aeroacoustic performance of aerodynamic bodies submersed in a flow. Remote microphone probes (RMP), e.g., pinhole probes, provide advantages compared to flush-mounted probes because of their reduced sensing area.
However, they feature a distinct transfer function (TF) that needs to be taken into account for accurate pressure measurements. Many empirical calibration techniques for such probes introduce spurious resonance into the calibration, which propagate to the measurements. In this study, a semi-empirical calibration method is proposed with the aim of removing the spurious resonance in a physics-driven manner that is less reliant on the operator. Bayesian inversion is used to fit an analytic model for the TF of the RMP to the empirical calibration data.

As market growth projections for Urban Air Mobility vehicles (UAMs) skyrocket, their presence in urban environments is likely to become increasingly prevalent, as will their noise. This creates a disturbance to both humans and wildlife, previously unaffected by aircraft noise. Further reinforcing the concern is the new multi-rotor designs, which introduce additional sources of noise.

Research on novel designs provides a limited understanding of the primary noise-generating mechanisms contributing to overall sound production. Among these innovative designs, complex geometries like open Contra-Rotating Propellers (CRP) and Shrouded Contra-Rotating Propellers (S-CRP) emerge. This study focuses on CRPs because they are anticipated to offer increased thrust with the same platform area, crucial for urban UAM operations. Additionally, shrouds are explored for their potential for increased thrust, acoustic shielding, and directivity manipulation, while also offering space for acoustic liners and increasing safety for ground operators.

The objective of this study is to isolate the aerodynamic and acoustic installation effects and identify the noise-generating mechanisms of the CRP and S-CRP configurations.

For this, the aerodynamics and acoustics of six propeller configurations are analyzed, including both the primary CRP and S-CRP geometries and sub-variants thereof. The exploration uses a hybrid numerical methodology, consisting of an aerodynamic flow solver based on the unsteady Reynolds-Averaged Navier-Stokes equations (uRANS) and a Finite Element Method (FEM) acoustic propagation solver. The latter generates acoustic sources from the aerodynamic solution using the source mode formalism.

This not only facilitates an aerodynamic understanding of the acoustic sources but can also detail near-field effects on acoustic propagation. While alternative methodologies achieving similar results necessitate high-fidelity simulations, the uRANS-FEM method can effectively capture aerodynamic and propagation effects within a moderate computational time. However, it also restricts analysis to tonal components of loading noise.

The CRP configuration showcased an improvement in efficiency metrics, marked by a 2.98% increase in FOM and a 57.53% increase in thrust per area, suggesting their higher efficiency and compactness compared to a single rotor. However, this came at the cost of increased noise levels, with amplifications ranging from 10 to 50 dB across various harmonics, attributed to the interaction of the contra-rotating blades. The study further noted a reduction in thrust for both the lead and rear propellers due to the interaction, particularly within the inner 75% radius of the rear propeller. A correlation between the azimuthal angle of the peak thrust and the angle of the highest noise generation was also observed.

When employing a shroud on a CRP, significant modifications in the performance of the shroud were not as anticipated in the literature. The shroud, while contributing to overall thrust, led to a considerable thrust reduction (over 54%) for both propellers due to separated flow. Acoustically, the shroud induced only a minor reduction in noise due to the aerodynamic effects, primarily due to decreased mean thrust. The dominant aerodynamic noise-generating mechanism in the S-CRP configuration is still the blade interaction...

Wind turbine noise is one of the grand challenges in the public acceptance of onshore wind farm projects. The field of psychoacoustics identifies the auralisation of wind turbine noise as a link between technical design and annoyance estimation. There is currently limited work on the auralisation of wind turbine noise, and none targets an application in psychoacoustic research.

This work investigates the auralisation of the aeroacoustics output of DTU's HAWC2 for use in annoyance estimation. A Gaussian beam tracing approach propagates the frequency domain output to observer locations. The resulting spectrograms are converted into sound signals by applying random phase and the inverse short-time Fourier transform. This work includes a binaural rendering module to enable future VR applications. The methodology's implementation results in the Wind Turbine Auralisation tool, WinTAur.

The noise signal output of WinTAur is validated using the HAWC2 model of a stall-controlled NTK 500/41 wind turbine and corresponding acoustic field measurements. Psychoacoustic sound quality metrics show significant differences between the auralised and measured noise. In the overall psychoacoustic annoyance metric, these differences mainly depend on the observer's position around the turbine. All metrics show this directionality dependence, while the loudness, sharpness and tonality metrics also indicate a dependence on wind speed. Differences in fluctuation strength show a minor dependence on the simulation case but are difficult to relate to a specific simulation parameter.
Spectral analysis of the simulation output samples reflects the limitations of HAWC2, demonstrating that it is the primary source of discrepancy. The analysis especially highlights the inaccurate prediction of the directionality and stall noise of the HAWC2 code. The choice of ground type is another probable source of discrepancy, as it does not accurately represent the measurement setup.
A subjective listening experiment demonstrates the significance of these discrepancies in human perception with generally high difference ratings between the simulated and recorded noise. The results illustrate a dependence on wind speed and the position around the turbine. These dependencies match well with the findings from the numerical validation.\\

Future work should focus on a sensitivity analysis of WinTAur since the case-independent parameters may be additional sources of discrepancy. Another recommendation is to investigate the unveiled errors in the underlying methodology. Lastly, better propagation modelling concerning the wind turbine wake and turbulence should be part of future wind turbine noise modelling.

Overall, using modelled wind turbine noise for the auralisation in psychoacoustic research has shown promising results. Validation with sound quality metrics provides good insights into the discrepancies found in subjective listening experiments. Eliminating the existing discrepancies through modelling improvements will allow this work to be applied in a fully modelled approach to estimate wind turbine noise annoyance.

In the last decades, the prevailing belief that smooth surfaces offer the lowest drag has been challenged often. Scholars have, for example, introduced rough and modified surfaces to reduce turbulent skin friction. One of the technologies proposed in the literature is an array of chevron-shaped protrusions; however, there is no academic consensus on the drag performance of this technique. Furthermore, although the theoretical working mechanism has been documented well, there is no experimental evidence in the literature to support the hypothesis around this mechanism.

In this study, the experiments from the literature are replicated, and new array configurations are tested to characterise the effect of individual parameters on the drag performance. The test plates are manufactured by applying vinyl protrusions of roughly 100 μm in thickness to an aluminium base plate. This thickness corresponds to 5δ_ν - 6δ_ν for the design Reynolds number of Re_τ= 1270. Direct force measurements are performed in the M-tunnel at a Reynolds number range of approximately 630 < Re_τ < 1850 to determine the drag performance. Furthermore, the coherent structures are characterised by means of 2D-2C PIV of a wall-parallel plane at a minimum distance of 17δ_ν from the wall.

The drag reduction reported in the literature could not be replicated, and the balance measurement results offer relevant insights into the drag performance of chevron-shaped protrusions. The results consistently show that the added roughness due to the presence of the protrusions is not the only parameter that determines the drag performance, confirming a meaningful interaction between the protrusions and the flow. Moreover, the results are found to be highly sensitive to the randomisation of the array. No substantial effect of the protrusions on the coherent structures close to the wall has been observed. In particular, no evidence has been observed that supports the working mechanism as proposed in the literature.

An analysis of possible causes to explain the discordance between this study and the literature is performed. Based on this analysis and the results from the aforementioned parametric study, an improved design is proposed, and recommendations for future research are postulated. This technology has inherent benefits for real-world implementations as it can easily be (retro)fitted to aircraft by means of a foil. Further research into this flow control technique is thereby deemed relevant due to the combination of the large drag reduction reported in the literature, the advantages in practical applications, and the novel opportunities for additional investigations.

With new rovers landing on Mars, like the Perseverance rover in 2020, the interest in Mars has grown recent years. The atmospheric conditions on Mars result in challenging conditions due to its low density atmosphere and extremely low temperatures. With a density on Mars that is 1% of that on Earth, creating sufficient lift for vehicles to fly becomes a challenge. The low density results in low Reynolds numbers. The low temperature has an effect on the speed of sound, due to which the Mach number is significantly higher at the same flow velocity compared to Earth. Airfoil data at these conditions, low Reynolds number - high Mach number, are sparse, but crucial for design of aerial vehicles. Next to the aerodynamic conditions, dunes on Mars are formed and migrate. The parameter of interest which defines the conditions required for transportation of particles is the threshold shear velocity. This parameter has been determined by different analytical expressions. However, the outcome remains a broad range, which point out the difficulty and inaccuracy of the results. Therefore, in this document, the design of a carousel wind tunnel is investigated to determine its feasibility to perform aerodynamic and aeolian measurements. The carousel wind tunnel consists of two concentric drums, of which the inner one rotates. The carousel wind tunnel is analysed by a computational fluid dynamics analysis with the k - ω turbulence model. The results indicate that due to secondary flow effects and the wake of a test object, no accurate aerodynamic measurements can be performed. Aeolian measurements are deemed feasible, with increased accuracy at sufficiently high rotational velocities.

The Advisory Council of Aviation Research has set the future goals of sustainable development in aviation by reducing pollutant emissions of CO2 by 75% and 90% in NOx emissions per passenger kilometre and noise emission by 65% of the perceived noise level compared with the measurements in 2000. This demand has led to the development of new aircraft concepts with alternative propulsive systems. The development of the all-electric and hybrid-electric aircraft concepts has regenerated the interest in distributed propulsion systems with propellers. Distributed-propeller systems are propulsive configurations with multiple propellers located along the wing or/and aircraft's airframe. Among the benefits of these systems is their electrical connectivity with the power generating sources or energy sources, like batteries, that make them attractive for vehicles performing electrical vertical take-off and landing for urban air mobility.
Previous studies' investigation of multi-rotor systems has been primarily focused on the aerodynamic performance changes in hover conditions. The thrust decrease of these propellers during their operation at a close distance is accompanied by oscillations associated with the flow structures interactions. This results in a noise emission increase attributed to the thrust fluctuations. The investigation, therefore, of the distributed-propeller system in forward flight will contribute to the in-depth analysis of the performance of these systems and illustrate the root of potential changes, assessing the changes in thrust as well as the unsteady loading of the blades. Also, the present analysis will study the mechanisms that lead to noise emission increase and the potential benefits of the relative phase angle variation between adjacent propellers. The effect of the relative phase angle has shown a positive impact on noise emission of multiple-propeller systems without considering the interference between adjacent propellers. Thus, the aim of the present computational study is to investigate the aerodynamic interference and aeroacoustic signature of a distributed-propeller configuration in forward flight, including the effects of the relative phase angle.
The comparison of the distributed-propeller model with the isolated propeller case showed minor differences in the time-averaged performance. The mean thrust coefficient increases slightly by 0.0004, whereas the time evolution plot reveals the thrust oscillation with amplitude equal to 5.3% of the mean value. The trailing vortex systems of adjacent propellers induce velocity components that alter the propellers' flow field upstream and downstream. These induced velocities result in axial velocity increase upstream of the propeller and in non-zero values of the tangential velocity, resulting in thrust oscillations. This results in a change of thrust as the propeller rotates. During the motion of a blade, its thrust is reduced when it approaches the adjacent propeller and subsequently increases during its retreat, resulting in an unsteady loading on the blades. The slipstream of the systems also presents differences from the respective one of an isolated propeller. The symmetric and circular shape of the wake flow behind the isolated propeller is broken and turned into a deformed wake flow behind the distributed-propeller system, as a result of the interference of the tip vortices. The tip vortices emitted by adjacent blades stay close with each other during their downstream motion, with their interaction results in change of trajectory, shape deformation and fast dissipation.
The noise emission of the distributed-propeller system shows a noise level increase in front of the propellers while along the propeller plane, this increase is smaller. The comparison of the middle propeller of the system with the isolated one reveals an increase of 13.5 dB along the propellers axis, while the increase along the plane of rotation is 1.8 dB. The increase along the propeller axis is associated with enhanced tonal components at frequencies up to the 5th blade passing frequency. The directivity of the noise emission and the augmentation of tonal components imply that the unsteady loading is the reason for the noise levels increase. The increase of the broadband component (3.2 dB) at oblique angles with respect to the propeller axis is attributed to the interaction of adjacent flow structures.
The variation of the relative phase angle between adjacent propellers results in blades passing from the region between adjacent propellers at different times. This has positively impacted the unsteady loading as it is decreased compared to the case without relative phase angle variation. The oscillatory behaviour of the thrust is reduced as the standard deviation of the thrust coefficient drops from 0.001 to 4.5e-04. On the contrary, the time-averaged performance of the propellers remains unaffected. The reduction of the unsteady loading results in noise emission reduction in the upstream direction by 5 dB, while at oblique angles in the downstream direction, there is a 1.8 dB increase. This increase is attributed to a different tonal component distribution than in the case without relative phase angle variation. Thus, the impact of the relative phase angle variation could be beneficial on noise emission angles normal to rotor plane due to the reduced unsteady loading.

Due to the rising demand for short-range air travel and the desire for aircraft driven by electric propulsion, there has been a renewed interest in propeller research. Despite the high potential aerodynamic efficiency of propellers, their excessive noise emissions prevent a widespread use on aircraft. A gradient based optimization study is performed to assess the trade-off between aerodynamic and aeroacoustic performance of propellers. A blade-element-momentum theory(BEMT) approach with a dependence to the effects of blade sweep is used. The BEMT-model is combined with a frequency domain approach for tonal noise prediction. The optimization study shows that the advance ratio and pitch are important operational parameters during the climb phase to induce a noise reduction. The application of blade sweep proves useful as a design measure to reduce noise, given that the propeller operates in high speed conditions.

Advancements in technology have made commercial unmanned aerial vehicles reliable and readily available, leading to an exponential rise in their market demand over the past few years. COVID-19 has further accelerated this growth through an increase in demand for contact-less delivery and crowd monitoring systems. However, despite these favorable conditions, their limited range, perceived threat, and concerns about noise pollution in urban environments have prevented them from being widely accepted by society. A recent study by NASA found that people perceive UAV noise to be more annoying than cars, and trucks at a similar sound pressure level, which highlights the need to understand the acoustic characteristic of these aircraft. These UAVs are generally powered by electric motors, making their propellers the most dominant source of noise. In the past, researchers have conducted several studies to understand and characterize the noise produced by aircraft propellers. However, these studies were limited to high Reynolds (>1,000,000) and Mach number operations for large commercial aircraft, creating a significant gap in the understanding of the aerodynamic and acoustic characteristics of propellers operating at low Reynolds (<200,000) and Mach number. This thesis aims to address the research gap by performing a high-fidelity computational simulation using Dassault Systèmes PowerFLOW®. The tool uses a lattice Boltzmann very large eddy simulation (LBM-VLES) based approach to compute the aerodynamic results and the Ffowcs-Williams and Hawkings (FWH) aeroacoustic analogy to calculate far-field acoustic values. The main objective of the thesis is: “To characterize and quantify the effect of non-axisymmetric inflow conditions on the aerodynamic and acoustic properties of propellers operating at low Reynolds numbers.” To meet the objective, a computational setup consisting of a twin-bladed propeller with a radius of 15 cm is designed in PowerFLOW®. The propeller is analyzed at 0º and 15º AoA, operating at 6000 RPM with a free stream velocity of 12 m/sec and the results validated against experimental data. Aerodynamic measurements and flow analysis revealed that the change in angle of attack (AoA) resulted in a 3.87% increase in the net thrust, and 1.16% increase in the net torque value of the propeller. Operating at an AoA, the propeller blade experiences asymmetric loading around the propeller plane, the loads fluctuate by 35% between the points of maximum and minimum loading. Further analysis of the propeller flow field is carried out by averaging the velocity field and performing a phase-locked analysis to visualize the vortex field. The analysis helps in understanding the effect of AoA on propeller wake and quantifies its a symmetric nature. Far-field acoustic data is acquired by two circular microphone arrays, with a polar angle resolution of 10º. The arrays are placed around the propeller plane and along the axial axis of the propeller. The change in AoA results in a 3 dB higher noise at an azimuthal angle (Ψ) of 90º and reduces by an equal magnitude at Ψ = 270º. The shift is attributed to the change in propeller tip Mach number and local blade AoA as a function of its azimuthal location and propeller AoA. Further analysis of the sound power level (PWL) produced by the propeller is carried out, showing a 1.5 dB increase in the PWL produced by the propeller blade at 15º AoA than 0º.