With an unprecedented surge in the number of individuals utilizing electric vehicles for commuting across the globe, it is becoming critical to immediately deliver an adequate and range-friendly degree of thermal comfort in car cabins to help drivers remain focused and attentive. The research presented in this thesis is part of the development work carried out by the Aerothermal team at Tesla. The team aims to better understand high aspect ratio subsonic jets, particularly intersecting twin jets, to ventilate the vehicle's passenger compartment. The intersecting twin jet has recently captured significant attention from the Aerothermal team for several reasons. Firstly, it provides the ability to actively control the flow angle from the HVAC unit into the vehicle cabin without requiring direct interaction with the vent. Additionally, it facilitates the flushing of the vent outlets with the A-class surfaces in the vehicle interior, helps achieve a simple and minimalist design for the instrument panel and other interior surfaces that align with Tesla's design language The objective pursued in this thesis is to investigate the physics represented by the bulk flow characteristics of intersecting twin jets, and to scrutinize the installation effect on the merging jet, focusing on the influence of the installation surface offset height. This objective is achieved by characterizing the evolution of large-scale flow structures in High AR intersecting twin jets at moderate Reynolds numbers experimentally using planar Particle Image Velocimetry (PIV) in both installed and uninstalled conditions. Once accomplished, the experimental results are then used to help select, validate, and calibrate the appropriate RANS turbulence model. The experimental results show that the intersecting jets converge before merging and forming a single jet. This jet behaves like a single jet emanating from a more recessed origin. The variation of the flow ratio between the two nozzle outlets results in varying the jet angle. The study reveals that the placement of the surface impacts the jet angle and velocity and introduces a circulation zone that affects the jet's behavior and direction. The surface's impact depends on the offset distance, length, and profile of the surface. The results also suggest that accurate computational prediction of the jet characteristics requires high mesh resolution and an appropriate selection of turbulence intensity. Among the four turbulence models assessed, the k-omega SST model is found to be adequate for the application, although it exhibits some numerical hysteresis. Overall, the findings provide insights into the dynamics of jet-surface interactions and can guide the design of systems involving such flows.

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.

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 5δν - 6δν for the design Reynolds number of Reτ= 1270. Direct force measurements are performed in the M-tunnel at a Reynolds number range of approximately 630 < Reτ < 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 17δν 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.

The growing importance of renewable energy to meet the demands of growing population has driven much focus for research in the wind energy sector. Currently most of the power production in the wind energy sector is done using the Horizontal Axis Wind Turbine (HAWT). due to its higher efficiency and reliability as compared to Vertical Axis Wind Turbine (VAWT). However, due to limitations of HAWTs such as the complex yaw mechanism, higher positioning of the center of mass and relatively difficult up-scaling, VAWT are receiving more attention. Attempt to access high range of power from VAWTs at offshore locations may damage the turbine blades due to increased loads. This thesis is dedicated to reduce the periodic disturbances on the turbine blades of VAWTs without affecting the total power production in a rotation, thereby ensuring the reliability and safe operation of VAWT. A control technique called Subspace Predictive Repetitive Controller (SPRC) is used for the recursive identification to estimate the parameters of wind turbine model and further providing an optimal control law accordingly. Basis functions have been used to reduce the dimensionality of the system, following which the system identification has been performed in the lifted domain. Using the identified model, two types of control approaches have been applied. In the first approach, the objective function is to track a specific reference that indicates blade load. This reference ensures a reduction in peak loads of the VAWT by transferring the loads from their upstream to their downstream part, without comprising on the power production in one whole rotation of VAWT. In the second approach, a general control strategy is used in which the controller is given freedom to choose the pitch trajectory so as to reduce the blade loads. The controller is specified with a power reference that ensures the maintenance of total power. This strategy allows the turbine to be operated in different wind conditions, as a higher weighting is assigned to power production when wind speeds are less than rated wind speeds, and a higher weighting is assigned to the reduction of blade load when wind speeds are more than the rated wind speeds. . These results show a real potential of the data-driven SPRC approach in wind turbines, and highlights new mechanisms to reduce the turbine loads on c{VAWT}s. These mechanisms offer valuable insights into enhancing the functions of VAWTs in a more reliable and damage-free manner.

This thesis presents an experimental study of the unsteady phenomenon known as transonic buffet. Completely developed in the High Speed Lab of the Aerospace Faculty at TU Delft (The Netherlands), the experiments were conducted in the transonic/supersonic wind tunnel TST-27. In order to achieve time and spatial resolution of the phenomenon, high-speed Schlieren and time-resolved particle image velocimetry were used as the main measurement techniques over two aerodynamic models, the NACA0012 and the OAT15A. The aim of the project was to achieve fully developed buffet conditions, to study the time evolution of the flow and the dynamic characteristics of the features involved in the phenomenon, to investigate the wave propagation in the flow that maintains the self-sustained character of buffet and to study the origin and dynamic development of these waves. An spectral analysis of the shock wave motion was done over all the experimental test cases to analyze the influence of different aerodynamic parameters (Ma, Re, AoA) over the phenomenon and to come up with the cases were buffet is most fully developed. The results showed that, as expected, there is a range of values of these aerodynamic parameters under which buffet develops, and out of this range it just vanishes. Buffet was fully achieve for the OAT15A model at Ma=0.70, AoA=3.5º and fixed transition at x/c=7%, on the other hand, buffet about the NACA0012 was not fully developed, but it was found to be stronger at Ma=0.70, AoA=4º and free transition. From the PIV results, the instantaneous and the phase-average velocity fields showed how the actual flow development takes place and all the features involved in the buffet cycles. It was shown that under fully developed buffet conditions, massive separation takes place from the shock foot and extends beyond the trailing edge during the upstream motion of the shock wave, whereas reattachment occurs in that region during the downstream motion of the shock. Finally, a correlation analysis of the velocity fields demonstrated the presence of the so called upstream traveling waves shed at around 2000 Hz and traveling upstream at -80 m/s. Furthermore, it was demonstrated that the vortices shed from the shock foot when full separation occurs are not the cause for the upstream traveling waves, instead they are originated by another type of disturbances shed when reattachment occurs at the shock foot. This made possible a reformulation of the hypothetical dynamic evolution of the features involved in the buffet cycles, explaining the links between them and the physical mechanisms that cause their self sustained development.

In experimental aerodynamics, pressure is an important parameter which provides insight into the various features of the flow around the test object. By obtaining the surface pressure information, various inferences can be drawn such as flow separation, local flow velocity etc. Traditionally, the most common method of obtaining pressure has been through direct measurements by incorporating the test model with pressure taps, which makes the model expensive and complicated. However, often there arises a requirement to obtain qualitative information and visualise the aerodynamic features during the test. Particle Image Velocimetry (PIV) solves provides this qualitative information as well as quantitative information in the form of a flow field. Pressure can be further computed using this velocity field. This technique holds a number of advantages over traditional wind tunnel measurements, especially with reduction in resources for manufacturing, ease of handling and non-intrusiveness. However, the conventional PIV technique came with the disadvantage of only having 2D2C information. Other techniques like stereo PIV and tomographic PIV were developed to tackle these limitations and the most recent development was that of Coaxial Volumetric Velocimetry (CVV). Along with the advent of Helium Filled Soap Bubbles (HFSB), this novel technique allows for large scale 3D3C measurements. This research is conducted to assess the feasibility of this promising technique to obtain surface pressure. The flow field around an inverted wing in ground effect is investigated using CVV by robotic manipulation. Using HFSB as tracer particles, the experiment was carried out in the W-Tunnel of TU Delft. The wing was tested at two angles of attack, α = 5° and α = 8° and three ground clearance configurations (h/c= 1, 0.6 and 0.3 at α = 5°) at airspeed of 10 m/s. The wing is equipped with a total of 30 surface pressure taps along the chord-wise direction, both on the pressure and suction side. The velocity field is obtained using Lagrangian particle tracking algorithm, Shake-the-Box and time-averaged velocity field is obtained by ensemble averaging of the obtained particle tracks. Static pressure is calculated using Poisson’s equation using the information obtained from velocity field. The pressure thus obtained is then compared with the readings obtained from pressure taps. Such an assessment not only shows the feasibility of the technique but also highlights the shortcomings and potential improvements. To the best of the author’s knowledge through a literature study, this particular technique has not been used to obtain surface pressure information. Most of the work published in the field of CVV is to study flow features, mostly in the wake. Hence, it is expected that this research and the recommendations help to further development of such a technique in the future.

Passive root flaps and in particular the Gurney flaps have been employed for improving aerodynamic characteristics for various applications since the 1970's. The effects of passive flaps on two dimensional airfoils have therefore been studied extensively, however, research on their application to Horizontal axis wind turbine blades (HAWT) is lacking and scarce. This research is a part of ongoing efforts at the Delft University of Technology to fill this missing gap. The blade of a HAWT is generally divided into three different regions for aerodynamic analyses. The region closest to the rotational axis is defined as the root region (r/R=0 - 0.3) . This region experiences the lowest rotational speeds and requires higher structural strength, resulting in a thick airfoil section. Although this section does not contribute significantly to the power generation of the whole turbine, the flow from this region does however effect the performance of the rest of the blade. Improving the aerodynamic flow characteristics of this region, therefore increases the performance of the whole turbine. For this research, passive (i.e. stationary) flaps were considered to enhance the flow at the root region and experimental analyses carried out at the Open Jet Facility (OJF) wind tunnel of TU Delft. An LM388 wind turbine blade (of Nordex N80 wind turbines) was used as a reference to create a scaled blade. The Analyses consisted of load (power and thrust) measurements and calculations for various flap configurations on a scaled wind turbine model. Furthermore, to understand how the flow is effected by the augmentation of flaps a Stereoscopic Particle Image Velocimetry (SPIV) was carried out for the most effective flap configuration and also no-flap configuration for the sake of comparison.

The high efficiency of open rotor propulsion has led to a comeback of propeller propulsion systems in recent years. The main issue preventing wide application of the technology is its high noise level. One of the main contributors to this noise is the pressure fluctuations generated during the interaction between a propeller tip vortex and a downstream surface. The goal of this research project is to create a better understanding of the vortex- surface interaction, which would allow for more effective noise mitigation strategies to be developed. An experimental approach was adopted to quantify the effect of a number of governing parameters on the pressure fluctuations over the surface induced by the vortex. Furthermore, an attempt was made to develop a conceptual model describing the vortex-surface interaction in order to study the relative contributions of the sub-phenomena of the interaction. The accuracy of the conceptual model was assessed using the data obtained from the experimental campaign. From the results it was clear that the low pressure vortex core was the primary mechanism that generated fluctuations over the surface. The vortex path was clearly visible and dominant over a large part of the chord. In the slipstream region the wake generated the largest pressure fluctuations. The effect of the wake was contained at the leading edge, up to 4% of the wing chord. Large discrepancies between the unsteady pressure, both in magnitude and in spatial distribution, were observed. Three contributing factors to the discrepancies were identified, namely the overestimation of the induced angle of attack effect, the use of the Lamb-Oseen vortex model and the method of determining the vortex core radius. These discrepancies implied that the conceptual model could not be used for its intended purposes. The results of this study show that the advance ratio and incidence angle are the most critical parameters governing the vortex-surface interaction. Although the former is difficult to incorporate into noise mitigation strategies, the latter can be used as a parameter in the initial design phase. Interestingly, the geometry of the airfoil seems to have limited effect on both the magnitude and distribution of the interaction. Although the conceptual model discussed in this report is not accurate, the development of such a simplified model would still contribute a lot to the body of knowledge. It could be used to evaluate the relative contributions of the different vortex-surface interaction sub-phenomena, or to perform a 'parameter sweep' to investigate the effect of the governing parameters on the unsteady pressure distribution and magnitude.

In recent years, the aviation industry has faced escalating environmental scrutiny, prompted by the global shift towards heightened environmental awareness. This study explores a collaboration framework set up by airlines which envision to partially or totally shoulder the environmental impact costs generated by their airborne emissions. The methodology developed includes the production of detailed emissions data to be employed during the airline’s planning strategy. The model conceived introduces a taxation approach exploited by airlines to include and incentive more environmentally sustainable solutions and, a collaboration mechanism based on the possibility to share passenger demand between collaborating airlines. The environmental incentive and collaboration are implemented in a Linear Programming Network and Frequency Planning model with the goal of maximising airlines profitability. A case study is analysed focusing on two airlines, KLM and TAP, collaborating to achieve improved environmental and economic performance. The airlines engage in a scenario where the environmental impact externalities generated by airborne operations are shouldered by the airlines themselves. Employing the model developed, the airlines explore solutions that either reduce the environmental impact or that solely maximise profit of the two airlines. The results underscore the potential benefits of including collaboration in the airline’s planning strategy, with a total 1.33% reduction in environmental impact when prioritising environmental cost abatement. Collaboration also demonstrated to be a consistent tool to achieve a profit recovery, showing up to a total anticipated profit increase of 2.21% when prioritising profitability or 0.71% when the priority is given to environmental impact reduction. In conclusion, this study highlights the potential efficacy of collaboration in addressing aviation’s environmental and economic challenges.

This work presents an experimental and analytical investigation of unsteady loading of a low Reynolds number propeller and its relation to aeroacoustics. The propeller is positioned at an angle of attack with respect to the freestream which causes a variation in loading along the azimuth. The propeller is operated at 4000 RPM with an advance ratio of J = 0.4, corresponding to a free stream velocity of 8 m/s. The chord-based Reynolds number of the propeller is of the order of 104 and is investigated at two angles of attack: αr = 7.5° and αr = 15°. For reference, a steady case, αr = 0◦, is presented as well. Loads and noise measurements were carried out in the A-Tunnel of TU-Delft. Two microphone array configurations were used to mimic the observers positions above and underneath the flightpath of the propeller. Underneath the flightpath, an increase in SPL was found with increase of the rotor angle of attack. Above the flightpath, the opposite effect was observed, resulting in an asymmetrical directivity profile when the rotor is at an angle of attack. A quasi-steady loading model, based on the lifting line code Xrotor, is used to determine the azimuthal variation in loading. The variation in tip speed along the azimuth due to the propeller angle of attack is prescribed to determine the variation in loading in one rotation. Far-field loading and thickness noise are computed using an aeroacoustic model based on the Ffowcs-Williams & Hawkings’ equation to predict the harmonic noise in the frequency domain of a rotating dipole and monopole, respectively. The numerical predictions for the steady case show good agreement for both the aerodynamic and aeroacoustic performance when compared with loading and noise measurements. For the αr≠ 0° cases it was shown that unsteady loading noise increases the SPL for all harmonics, with a larger contribution at higher harmonics.

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.

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.

Accurate measurement of emissions (i.e. volume flow rate multiplied by concentration) is vital to the control and reduction of air pollution. We quantify the uncertainty of flow measurements by S-type pitot tubes in narrow exhaust stacks (diameter < 0.5 m) in support of recent European Union (EU) regulations for emissions from medium-size combustion plants. The contribution of blockage and wall effects to the uncertainty budget is evaluated. The aim of this thesis is to characterize flow uncertainty and to aid the development of more accurate measurement methods for volume flow rates in narrow exhaust stacks. This thesis will provide the scientific basis for implementation of EU regulations concerning emissions from medium-size combustion plants. Characterization of flow fields in narrow exhaust stacks is key to accurate volume flow rate measurements. We use numerical simulations to study the mean velocity profile and turbulence statistics of fully developed turbulent flow. To investigate the impact of wall effects on measurement uncertainty, we simulate the flow eld around an S-type pitot tube in close proximity to the stack wall. In addition, we conduct S-type pitot tube measurements at various wall-distances and bulk velocities. Throughout this thesis, we use several uncertainty quantification techniques to evaluate the uncertainty of numerical simulations and experiments. Studying spatial variation in measurement uncertainty gives insight in the most suitable locations for S-type pitot tube measurements in narrow exhaust stacks. The results of this thesis suggest that wall effects dominate over other sources of measurement uncertainty in the near wall region. Throughout the measurement plane, the combined uncertainty of measurement error sources exceed the impact of blockage. We recommend to determine volume flow rates in narrow exhaust stacks by S-type pitot tube measurements in the stack center. A correction factor can be determined to compute the volume flow rate in a narrow exhaust stack from the maximum flow rate in the stack center.

Hybrid laminar flow control (HLFC) reduces skin friction drag, by combining boundary layer suction with pressure gradient tailoring to delay boundary layer transition. This research addresses two key developments required to perform the aerodynamic design of suction-type HLFC components. First, an existing numerical transition prediction tool was made compatible with boundary layer suction, by incorporating suction in its boundary layer solver and by ensuring that an appropriate grid of disturbance frequencies is evaluated with linear stability theory. Second, the scalability of the pressure losses associated to the perpendicular flow through large-scale and actual-scale perforated sheets was investigated experimentally. This development is required to predict the pressure losses of generic HLFC surfaces. It was found that the scalability of these pressure losses is limited, because the frictional losses inside holes decrease continually with decreasing hole diameter. In contrast, the inertial losses across holes reach a steady value with sufficiently many holes.

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.

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