This study investigated the control of Shock-Wave/Boundary-Layer Interactions (SWBLI) using jet actuators at several locations, both upstream and within the separation bubble. Such interactions are critical in high-speed aerodynamic applications, where flow separation can lead to performance losses. Using Large Eddy Simulation (LES), the effects of injection on the separation region are studied. Where both the time-averaged as unsteady effects are examined. The results show that size reduction of the separation bubble is not effective when the actuators are placed within the separation region. However the actuators were able to modulate the large scale motion of the separation independent of the injection frequency or location. Indicating that the instabilities of SWBLI are potentially inherent and can be modulated from within the bubble. The proposed driving mechanism behind this are the pressure disturbances created by the injection, which is supported by the results.

This research investigates the application of acoustic excitation as a novel method for transverse forcing to achieve friction drag reduction in a turbulent boundary layer. Traditional transverse wall motion, while effective in reducing drag, poses experimental challenges due to mechanical complexity and limitations at high Reynolds numbers. This study explores an alternative approach where transverse velocity gradients are induced through oscillatory acoustic fields rather than wall motion. An experimental setup was developed at the Delft University Boundary Layer Facility (DUBLF), incorporating a system of phase-synchronized speakers to generate controlled transverse acoustic forcing. Particle Image Velocimetry (PIV) was used to characterize the boundary layer and assess the effect of forcing across a range of Reynolds numbers. Results show measurable reductions in friction drag, with the optimal configuration achieving up to 6.02% drag reduction at Reτ = 1847. The study further reveals that the induced transverse velocity fields modulate near-wall turbulence structures, contributing to reduced turbulent kinetic energy and Reynolds stresses. This method provides a mechanically simpler and potentially scalable alternative for active flow control, with the potential to provide new insights into the mechanisms of transverse forcing in turbulent boundary layers.

The present Master's thesis investigated the low-frequency unsteadiness characteristic of highly separated transitional oblique shock wave-boundary layer interactions (hereinafter "OSBLIs"). Such phenomena, encountered notably in engine components operating at transitional Reynolds numbers, are relevant due to their impact on aerodynamic efficiency, structural integrity, and system reliability. The primary research question examined how variations in different parameters such as Mach number, Reynolds number and inviscid pressure jump influence a low-frequency shock oscillation mechanism which was previously identified in literature. To this end, experimental studies were conducted in the TST-27 transonic-supersonic wind tunnel at TU Delft, using high-speed and spark-light Schlieren visualizations capture the relevant flow phenomena. These recordings were processed using digital and spectral analysis.

The findings from this study revealed that the investigated transitional OSBLIs exhibited low-frequency shock oscillations strongly correlated with the periodic formation and disappearance of a Mach stem, which was denoted as the "dual domain" phenomenon. Through carefully chosen variations in Mach number and Reynolds number, it was shown that slight adjustments significantly impacted both the presence of the dual domain and the characteristics of the shock oscillations. Moreover, the Reynolds number regime identified as transitional for the natural flat plate boundary layer in previous research was validated.

Another aspect of this thesis involved the implementation of passive flow control techniques, specifically the introduction of thin two-dimensional steps (in height increments of 60 microns) designed to artificially trip the boundary layer. The experimental results demonstrated that even minimal boundary layer tripping significantly dampened the shock oscillations and modified the interaction dynamics of cases where the oscillation mechanism had previously clearly been identified. The frequency analysis confirmed this, as none of the oscillation peaks which had previously been identified were observed when tripping the boundary layer.

A non-dimensional analysis of the dominant oscillation frequencies indicated a consistent Strouhal number convergence around St = 0.33, particularly in cases where the "dual-domain" behavior and high oscillation amplitudes were observed. An increase in the Reynolds number consistently resulted in reduced laminar separation amplitudes and increased oscillation frequencies, which aligned with the theoretical expectations of accelerated boundary-layer transition dynamics.

In conclusion, this study was successful in identifying the main parameters that cause unsteadiness in transitional OSBLIs. It confirmed the existing transitional Reynolds number ranges which had previously been analyzed in the context of weak OSBLIs and the natural boundary layer of the flat plate which was used, and showed that even simple flow control methods with low 2D step heights can effectively reduce shock oscillations. Additionally, a meaningful non-dimensional scaling was done, which can aid in further research and comparison of the phenomena which were investigated in the present thesis. These findings provide a solid basis for future studies aiming to apply this knowledge to more general cases and improve the prediction and design of aerospace components affected by transitional shock-induced boundary-layer interactions.

As wind turbine rotors grow larger, compressibility effects near the blade tip become an emerging concern. This research experimentally investigated compressibility effects on the flow field and aerodynamic performance of the FFA-W3-211 wind turbine airfoil at Mach numbers within 0.5–0.65 and angles of attack from -4° to -11°. An experimental campaign was conducted at the TST-27 wind tunnel at TU Delft using Particle Image Velocimetry (PIV) and Schlieren techniques. Non-intrusive pressure field reconstruction and load determination methods are used to infer the aerodynamic loads from the flow field data.

Results show that compressibility strongly influences the mean flow field and aerodynamic loads of the FFA-W3-211 airfoil. Trends toward transonic flow and unsteady shock waves were identified with increasing absolute angle of attack and freestream Mach number. The results at α = -6° display a 30% reduction in the negative lift coefficient from cl = -0.43 to -0.31 and a 190% increase in drag coefficient from cd = -0.026 to -0.075, with increasing Mach number from M∞ = 0.5 to 0.65. For the same Mach number range, the lift coefficient results at α = -10° display a plateau around cl ≈ -0.65 to -0.67, and a 100% increase in drag coefficient, from cd = -0.085 to -0.174. At moderate angles of attack, the increased mean drag was influenced by the growth of trailing edge separation. Beyond M∞ = 0.6, the emergence of shock waves plays a greater role in inducing earlier separation and less efficient pressure recovery. The growth of the separation region decreases the mean negative lift for α ≤ -6°. For α ≥ -8°, conversely, the emergence of supersonic flow and lower pressures over the trailing edge counteract the effects of increased separation. For the phase-averaged analysis of the transonic buffet cycle at α = -10° and M∞ = 0.65, the phase-averaged lift coefficient varies periodically in a range of approximately ~22% of the time-averaged lift coefficient, while the phase-averaged drag coefficient varies up to ~80% of the time-averaged drag coefficient.

This work is part of the foundation for understanding the effects of compressibility in wind turbine airfoils and wind turbines. It identifies the complex interplay of shock waves, separation, and shock wave–boundary layer interaction (SWBLI) in the transonic regime. The results highlight and challenge current assumptions of incompressible flow for the design and operation of modern and future large-scale wind turbines that rely on incompressible aerodynamic polars.

Aerodynamic drag plays a critical role in high-speed sports, including running, where small performance margins can be decisive for victory in elite competitions. Despite its importance, research on running aerodynamics remains underexplored, with most studies relying on stationary mannequins or simulations that do not capture the dynamic behavior of flow around a moving runner. In other words, there is a significant research gap in experimentally visualizing the wake flow and accurately measuring the drag in real-world running scenarios. This study aims to address this research gap.

In recent years, the Ring of Fire measurement technique has emerged as a feasible option to visualise and analyse flow structures of transiting objects based on particle image velocimetry.
The technique has already been proven in sports like cycling and ice skating, but has not yet been applied to running. This study adapts the Ring of Fire measurement method for sprinting athletes, and the raw images are processed by Shake-the-Box (STB) Lagrangian Particle Tracking. This results in a three-dimensional, time-resolved velocity field in the wake of a runner with an uncertainty of less than 5\% of the runner's speed. This velocity field is used for qualitative flow visualisations, as well as for drag estimations, which are computed from a control volume approach by utilising the flow field before and after the passage of the athlete.

The experimental methodology involved nine junior athletes, wearing both standard sprint suits and aerodynamic suits, sprinting at constant speeds through a measurement setup including three high speed cameras, four LED arrays and Helium Filled Soap Bubbles (HFSB).

The results of the investigation include the visualisation of the full wake,
quantification of the velocity deficit, a qualitative vorticity analysis, some lateral velocity findings and the measured values for drag of one of the athletes.

There are multiple interesting findings about the flow around a dynamic runner in this report. One of them is the splitting of the wake into two side-by-side stream tubes in the far wake. The velocity deficit in a relevant region of the dynamic wake has also been quantified, and it is shown to vary with the inverse of the distance behind the runner, which is useful information for trailing runners.
Another interesting observation is that the vorticity field in the near wake generally follows the rules of finite cylinder flow, where the body parts of the athlete are finite cylinders with variable diameter.
The hypothesis that the flow should have a lateral oscillation related to the frequency of the runner's steps has also been confirmed.

This study demonstrates the applicability of the Ring of Fire system to running aerodynamics and offers the full visualisation of the three dimensional flow in the wake of a moving runner, bridging the research gap to pave the way for further advancements in the field.

Understanding atmospheric motion is crucial for analyzing wind-structure interactions, particularly within the Atmospheric Boundary Layer (ABL). Traditionally, wind tunnels have simulated unsteady ABL conditions using active and passive devices. Recently, multi-fan wind generators have emerged as a flexible, cost-effective alternative, allowing independent fan control to replicate wind conditions the wind turbines, airborne devices or civil structures are subjected to.

This thesis investigates the capability of a newly manufactured 3 x 3 multi-fan wind generator to produce idealized uniform flow, linear shear flow profiles, and streamwise sinusoidal gusts. The system consists of off-the-shelf computer fans, controlled via open-source software, offering a low-cost and adaptable approach. Particle Image Velocimetry (PIV) measurements show a 96% uniform core region with 11.5% turbulence intensity 1 m downstream from the system. There, the area of the core is reduced to 1/9 of the system’s area due to the outer shear layer. The system struggles to replicate linear wind shear, but can generate oscillatory gusts at 0.2 Hz, 0.4 Hz and 0.8 Hz, corresponding to the operating frequency of the fans. Further improvements should focus on extending the configuration to maintain a larger core region, and on reducing swirl dynamics and recirculation to enhance flow quality for wind engineering applications.

Carbon capture and storage (CCS) has emerged as a cornerstone technology for mitigating climate change, as highlighted by the Intergovernmental Panel on Climate Change (IPCC) and International Energy Agency (IEA). A key component of CCS involves the safe and efficient transport of CO2 between capture sites and storage facilities, often over long distances. Pipelines can be an attractive mode of transport, with CO2 typically maintained in a liquid or supercritical state to optimize density and viscosity. However, rapid depressurization events, whether due to intentional releases or accidental pipeline ruptures, lead to complex two-phase flow dynamics. Accurately modelling these phenomena is critical to designing pipelines that are both safe and cost-effective.
This thesis investigates various one-dimensional, unsteady, compressible two-phase flow models to simulate CO2 depressurization scenarios in pipelines. All models are implemented using a finite volume method and discretization is done with the HLLC approximate Riemann solver, enabling the resolution of shocks and dis- continuities. Thermodynamic properties are computed using equations of state (EOS), with a focus on the Span-Wagner (SW) EOS, to capture the CO2 unique phase behaviour under high-pressure conditions. The models are validated against existing experimental data from SINTEF’s depressurization facility, which provided high-resolution measurements of pressure and temperature during rapid phase transitions.
Three different models are initially looked at: DF3, DF4, and TF5. Key findings demonstrate that while the DF3 model provides accurate predictions of pressure variations over time, it underestimates the initial pres- sure drop by up to 20 [bar]. In contrast, the DF4 model, through the manual adjustment of the mass transfer term (Γ) using the relaxation parameter ,θ , more accurately captures the initial transient behaviour, aligning closely with experimental data. Both models ultimately converge to approximately the same state after 20 [ms], with no more than 4 [bar] difference between the models.
Temperature predictions, however, pose a greater challenge. The DF3 model exhibits a spurious downward temperature spike immediately following pipeline rupture, while the DF4 model predicts an even greater initial temperature drop, neither of which align with experimental observations. The delayed cooling effect observed in experiments is not captured by either model, highlighting limitations in the energy equation and the need for additional source terms to account for temperature drop delays.
Limitations of the models include challenges in simulating temperature variations near the critical point and an inability to accurately model delayed temperature effects for both the DF3 and DF4 models. Recommendations for future work include developing a generalized mass transfer model, incorporating implicit numerical schemes for stability near critical conditions. Furthermore, the TF5 model shows promise for improving temperature predictions over longer timescales.

Loss of control inflight is the most common cause of fatal accidents in aviation. Aerodynamic stall models are utilized in pilot training to enhance safety and prevent accidents. This research presents an advanced longitudinal stall model for the Cessna Citation II, achieved through innovations in modeling methodologies and experimental design. By introducing dynamic stall maneuvers with step inputs, the study mitigated $\tau_1$ and $\tau_2$ parameter correlation, enabling more reliable parameter identification. A separable nonlinear least squares method significantly reduced computational time for nonlinear stall model estimation, decreasing it from hours to seconds. This approach revealed two minimally correlated flow separation states, offering deeper insights into wing flow characteristics and improving model accuracy. The lift model was refined to incorporate pitch rate and elevator deflection effects, while the drag model was enhanced with a lift-induced drag component. Additionally, a center of pressure model was derived from pitching moment data, advancing the understanding of stability during stall. A novel structure for characterizing degraded elevator control effectiveness was also developed. These advancements resulted in substantial performance improvements, with mean squared errors for lift, drag, and pitch moment coefficients reduced by 32\%, 29\%, and 27\%, respectively. The models also demonstrated greater consistency across diverse maneuvers, evidenced by reduced variability in $R^2$ values. This work contributes to more accurate stall modeling, enhancing both aerodynamic understanding and aviation safety.

This thesis proposes techniques to produce wall-shear stress estimates from three-dimensional Lagrangian particle tracking (LPT). Several works have already faced the problem of determining near-wall velocity and in particular skin friction for the case of a flat surface. Here, the problem is translated to generic three-dimensional objects. The work makes use of an experimental database, recently produced, with LPT (Hendriksen et al. 2024), that provides in-situ registration of the object, for three shapes of increasing complexity: a cube, an airfoil and a cyclist. Four techniques are examined and compared, including interpolation method as well as local data regression. The uncertainty is evaluated a-posteriori, comparing the results with a local coin-stacking technique, where applicable. All techniques yield accurate and robust representations of the skin friction lines around three-dimensional objects, allowing for an insightful inspection of the near-surface flow topology. Instead, distinct differences are found when the skin-friction magnitude is estimated.

With the increased use of Advanced Driver Assistance Systems (ADAS) in modern cars, the  field of surface contamination saw a resurgence. To keep the various sensors working properly it is of utmost importance to keep them clean and free of contamination. One way to achieve this is to ensure they are mounted in areas where they can be kept free of water droplets that can cause issues themselves, or by depositing contaminants that they are carrying. Current research focuses on the resulting contami- nation pattern after a drive or test on the car, this thesis however aims to see whether state-of-the-art Particle Image Velocimetry (PIV) techniques can be used to track the water droplets themselves, and with it open up new ways of researching surface contamination. To achieve this two experiments were performed. The first one was a simple experiment to see whether the droplets are being able to be imaged with the PIV cameras. The second experiment introduced a car side mirror model to the flow and used the Shake-the-Box (STB) algorithm to track the individual water droplets. Both of these ex-periments were successful, and showed that this combination of water droplets and STB PIV has great potential for being used in surface contamination research. The main issues found were with experi-mental setups, especially in the windtunnel. As the droplets do not follow the airflow accurately, extra care needs to be taken to ensure the droplets are operating at the right conditions, and are present at the measurement domain. The actual behaviour of the droplets as tracked by the STB algorithm, seems to be accurate with respect to the expectations indicating that the tracking of the droplets is accurate. Further research should try to increase the scale of the experiments, and/or introduce contaminants to the droplets to see how, if at all, this affects the traceability of the droplets.

Wall-resolved large-eddy simulations are performed to study the interaction between a supersonic turbulent boundary layer and an impinging shock over a wall. The freestream conditions used were Mach 2 and a moderate friction Reynolds number of 𝑅𝑒𝜏 = 950. A passive control method was implemented using perforations in the interaction region of influence of the reference case (flat plate), with the addition of two separated cavities beneath the perforations. The cavities were designed to work as Helmholtz-like resonators at a separation length-based Strouhal number of 𝑆𝑡_𝐿𝑠𝑒𝑝 ≈ 0.03. For the reference case results consistent with the literature were obtained and a time analysis, based on the cross-correlations between dynamic properties allowed to suggest a sequence of events driving the unsteadiness of the interaction. The controlled case, resulted in a larger region of influence of the interaction, with the blowing-suction mechanism inside the cavities allowing a reduction of the separation length. Large oscillations were found close to the wall for thermodynamic properties, but also skin friction and wall-normal velocity. The topology of the recirculation bubble changed to become less symmetric resulting in a stronger reattachment compression fan. Regarding the unsteadiness of the controlled interaction, a tonal behaviour was found (at 𝑆𝑡_𝐿𝑠𝑒𝑝 ≈ 0.3) for the reflected shock motion associated with the resonant frequency found inside the cavities and also for the bubble volume variation. In general, the resonance within the cavities seemed to be able to affect the dynamics of the interaction, despite maintaining its global topology.

The thesis presents an analysis of unsteady vortex shedding and vortex induced vibrations (VIV) on wind turbine blades using a combination of Stereoscopic Particle Image Velocimetry (SPIV) and Computational Fluid Dynamics (CFD). A spanwise uniform blade with a symmetric NACA0021 airfoil section, 0.075 m chord, and 0.4 m span was tested experimentally for a static, plunging, and surging blade oscillating at 90 deg angle of attack, 5 Hz, and 2.5 Hz frequency, 1 chord oscillation amplitude. Equivalent CFD simulation cases were run to compare the results and validate the CFD setup, which was used to analyse additional cases of interest. The extent of the three-dimensional flow behaviour due to the tip vortex was described, along with determining the lock-in region for the two motions using additional simulations. The results indicated that the spanwise convection of the tip vortex and the transition to a ‘2D’ flow were only dependent on the blade aspect ratio and independent of the freestream wind speed, the type of motion, and the oscillation frequency. The lock-in region for the plunging and surging blades was presented, noting that all the surging cases were locked in, most likely due to the large oscillation amplitude. However, the surging cases had a positive aerodynamic damping and hence, VIV would not be excited for a freely vibrating blade. It is suggested as future work to test a flexible blade and study the onset and development of VIV.

Developments in computational capabilities and the always increasing demand for higher performance of internal flow applications has meant that Computational Fluid Dynamics (CFD) has become an essential tool within the design process. A key point of interest is to couple the fluid solver with numerical optimisation techniques in order to obtain a more automated design process that can handle a vast amount of different design aspects. The open source CFD suite SU2 has emerged as an enabler for aerodynamic shape optimisation involving a large number of design variables, due to its efficient, accurate and flexible discrete adjoint solver. 
For the adjoint-based aerodynamic design optimisation of internal flow applications the deformation of the volumetric mesh has to be performed in an robust and efficient manner. Often small wall clearance gaps and periodic domains are encountered in internal flow domains, which could potentially lead to the deterioration of the mesh. Sliding boundary node methods can be applied in order to maintain the mesh quality in case of small wall clearance gaps. Additionally, periodic boundaries can be displaced in a periodic manner following the applied deformation in order to prevent low quality cells near the periodic interface. Therefore, it would be of interest to implement the sliding boundary node methods and periodic conditions in a Radial Basis Function (RBF) interpolation method, one of the most robust mesh deformation methods available. 
Additionally, the computational efficiency should be considered, since high computational times should be prevented for large and complex three-dimensional cases with a high number of design variables. 
The aim of this thesis project is therefore to develop a robust and computationally efficient mesh deformation method suitable within the discrete adjoint optimisation framework of SU2 for internal flow applications by means of developing an implementation of the RBF interpolation method including sliding boundary node algorithms, periodic boundary conditions and data reductions methods.
The sliding is achieved by replacing the interpolation condition for the sliding nodes with a planar slip condition. Or alternatively, by freely displacing the sliding nodes based on the known deformation and subsequently projecting the nodes back onto the boundary. The periodic displacement of the boundaries is ensured by making the distance function of the RBF periodic. The periodic nodes are then treated as internal nodes to allow them to move.
The developed RBF-SliDe tool is able to generate higher minimum mesh qualities compared with the regular RBF interpolation method. The sliding of the boundary nodes reduces the degree of skewing of the mesh elements in case of drastic deformations, resulting in a higher minimum mesh quality. Furthermore, the introduction of the periodic displacement prevents low quality skewed or compressed mesh elements, as the periodic boundaries move along with the deformation. 
The Aachen turbine stator blade is considered as a realistic three-dimensional test case. For this stator blade an optimised geometry was available, which was obtained with an adjoint-based aerodynamic optimisation performed with SU2. Therefore, the resulting minimum mesh quality is compared to the one obtained with the more conventional linear elasticity equation method as used in SU2. The minimum mesh quality obtained with the RBF-SliDe tool is nearly three times higher compared to the minimum mesh quality of the linear elasticity equations methods. This highlights the potential of the periodic sliding RBF interpolation method in terms of preserving the mesh quality.

In recent years, research for turbulent boundary layer (TBL) has shifted towards developing novel approaches to reduce drag. This shift is driven by new industrial standards emphasizing fuel-efficient and environmentally friendly air transportation. As wall-bounded turbulence plays a significant role in the overall drag experienced by an aircraft, any understanding and control authority on associated flow structures becomes vital for more optimized and efficient flow control techniques to reduce drag. In this context, conventional passive flow control techniques have been widely used as they are simple and do not require any external power supply. However, active flow control techniques have demonstrated the potential for greater effectiveness, which offers more substantial drag reduction. Existing active control methods, such as wall oscillation, traveling wave, blowing, and suction, have been previously studied to reduce the viscous drag in the TBL. However, the efficacy of such control strategies tuned to outer layer large-scale motions (LSMs) at higher Reynolds number (Re) flows has emerged as a promising avenue for achieving significant drag reduction. In contrast, control techniques targeting the inner layer small-scales encounter limitations due to restricted control authority and the difficulties associated with miniaturizing hardware at higher Re flows.

LSMs convecting in the logarithmic region (log region) of TBL play a critical role in the dynamics of wall-bounded turbulence, as they carry a significant portion of the turbulent kinetic energy (TKE) [Abbassi et al., 2017]. The active wall oscillation technique tuned to the frequency of the LSMs has proven to be energy-efficient, as drag reduction increases with the frictional Reynolds number (Reτ ) and the contribution of LSMs to the TKE and wall shear stress increases with the Reτ [Marusic et al., 2021]. In this thesis work, the receptivity of LSMs to active large-scale control strategy is assessed via a spanwise array of wall-normal jets in a TBL. Multiple wall-normal jets manipulate the flow over a flat plate by introducing a spanwise traveling wave, which aims to mimic wall oscillation tuned to the large scales in the log region of the TBL.

To assess the efficacy of the suggested control strategy with spatial and temporal tuning, particle image velocimetry (PIV) and hot-wire anemometry (HWA) are employed to quantify second-order turbulence statistics and analyze the organization of the coherent patterns introduced in the flow via two-point correlations. In addition, the effect of different control cases derived from different tuning and control strategies is analyzed at Reτ = 2227. The TKE production plots show that control cases targeting the LSMs effectively attenuate the TKE production near the wall. Furthermore, based on the analysis of the spectral energy plot of streamwise velocity fluctuations, it is observed that the energy content associated with larger length scales convecting in the middle of the log region decreases in the controlled case compared to the uncontrolled base case. This finding suggests that certain control cases notably impact the organization of LSM and second-order turbulence statistics far downstream of the actuation point. Additionally, the reduction in TKE production near the wall represents a reduction in wall shear stress and viscous drag. However, additional research must be conducted in the future based on the current findings to reach a definitive conclusion regarding the efficacy of individual control cases.

Turbulent drag reduction (DR) is a crucial research area, as this can contribute to significant energy and emission reductions in various industrial and transportation applications. One such technique is the active flow control method of spanwise forcing. Spanwise forcing introduces a spatio-temporal spanwise wall oscillation as a boundary contrition. Consequently, a phase-varying spanwise velocity profile is generated, that poses useful interaction with the near-wall turbulence resulting in a drag-reduced flow state. DR values of over 40% have been found in both numerical and experimental realisations.

The objective of this thesis is twofold. Firstly, the idea is proposed that spanwise forcing can be realised passively by geometric surface modifications to create a spatially varying spanwise velocity profile. Secondly, supporting the first objective is the idea of investigating active flow control, in this case spatial spanwise forcing, which can aid in the development of passive techniques. The underlying philosophy is gaining understanding of the key factors and mechanisms that contribute to DR by studying active techniques. Subsequently, these leanings can be applied to advance the development of passive techniques. This research rationale is combined with a new pathway for turbulent DR. By actuating the outer-scaled low-frequency turbulent structures in the logarithmic layer, instead of the conventional approach targeted at high-frequency inner-scaled structures. Unlike the decreasing trend observed in inner-scaled actuation (ISA), outer-scaled actuation (OSA) exhibits a positive correlation between DR and Re_τ, making it particularly useful for practical applications that operate under high Reynolds number conditions, such as long-distance pipelines or aviation.

The first part of the thesis investigates the DR and OSA potential of passive techniques. Three techniques are considered: dimples, sinusoidal undulations and oblique wavy walls. Eight test plates are realised for direct force measurements and a detailed investigation into the scale-specific streamwise turbulence kinetic energy using hot-wire anemometry (HWA). From the direct force measurements, the DR trends with Re_τ are inconclusive due to uncertainties originating from a correction term and the results falling within the 95% confidence interval. A slight reduction of outer-scaled energy is observed for the sinusoidal undulations, hinting at effective OSA. Furthermore, one of the oblique wavy walls shows a broadband energy reduction in the wall-normal region 10 ≤ y⁺ ≤ 100.

The second part of the thesis is focused on active flow control, specifically, the method of spatial spanwise forcing. A research gap was identified for spatial forcing, with no experimental realisations to date and only a number of numerical studies. Furthermore spatial forcing is of interest since it shows the closest analogy to a passive counterpart. An experimental setup is proposed, titled the steady spanwise excitation setup (SSES). The experimental setup realises spatial forcing using a series of spanwise running belts that run in alternating spanwise directions so as to generate a spatial square wave forcing. The thesis presents a proof of concept of the aerodynamic working using a prototype setup with four belts. Stereo particle image velocimetry (PIV) measurements showt hat a significant flow control effect can be realised with this type of forcing, with a maximum DR value over 39%. Accordingly, the turbulent stresses are significantly reduced. The spanwise velocity profiles are in qualitative agreement with the model based on the laminar solution, with an almost exact match at y⁺ ≤10.

A series of numerical simulations was executed using the high-performance cluster at TU Delft's Aerospace Faculty. The primary objective was to gain deeper insights into the intricate dynamics of a sonic transverse jet in a supersonic crossflow with an oblique induced shock wave, especially concerning mixing. These simulations delved into the near and mid-field regions, illuminating how the position and intensity of an induced oblique shock wave affect the original jet in crossflow configuration.

In a simplified context, a box simulated the combustion chamber near the injector. A transverse jet was introduced through a lower wall hole, intersecting the incoming crossflow. Wedges on the upper wall generated induced shocks at varying distances from the jet exit. These experiments spanned different locations and strengths of shock impingements, all relative to the diameter of the jet's injection port hole.

The investigation aimed to comprehend how an additional shock wave influences mixing within the combustor. In the upstream scenario, the shock's proximity to the jet exit led to enhanced compression and the emergence of Kelvin-Helmholtz instability. Downstream impingement, on the other hand, disrupted the usual behavior of this instability. In both cases, the initial downward movement of fluid structures toward the wall was followed by ascent, expanding the region for crossflow-wall mixing downstream. Close-to-jet exit shock impacts intensified these effects, causing explosive disruptions and enhanced mixing.

Ultimately, the study revealed that closer and more intense shock impacts near the jet exit had a significant negative impact on volumetric energy, detrimental to combustion and propulsion potential. Optimal results were achieved when the shock impingement occurred farther downstream. The research also highlighted the need for improvements in simulation techniques and recommended further exploration of various parameters for comprehensive insights. This work serves as a foundation for future studies tailored to specific needs, promising more insightful outcomes.

Turbulent drag is the largest source of fuel consumption in aviation and forms a significant contribution to climate warming. Recent numerical studies have suggested the ability of surfaces with streamwise-preferential permeability to reduce turbulent friction, and a theoretical framework behind the working mechanism has been proposed. This work is the first experimental study on this concept in air and in which the permeability requirements for predicted drag reductions are met. Three different physical realisations of streamwise-preferential permeable substrates were manufactured: two seal furs, a substrate with unidirectional fibres, and a 3D-printed structure. They were investigated through wind tunnel experiments using direct force measurements and planar (2D-2C) particle image velocimetry (PIV). Results show an increase in drag for all test specimens and suggest that drag sources other than surface friction are non-negligible. One-point turbulent statistics show an overall increase in turbulence, mainly wall-normal velocity fluctuations enabled by the wall-normal permeability, which results in higher Reynolds stresses. No significant flow modulation in terms of turbulent events or change in coherent structures is observed. The measurements and results are a first of its kind and none of the theoretical framework predictions on drag reduction or flow modulation are found. Based on analytical derivations relating characteristic length scales to permeability, the validity of the framework and its assumptions in experimental settings is questioned. It is thought that there is an inherent mismatch between the pore size assumption and the virtual origin approach taken in the framework. All things considered, it is deemed unlikely that turbulent drag reduction by means of streamwise-preferential permeable surfaces is feasible in experimental settings. Nevertheless, given the relatively small increase in drag for the 3D-printed specimen, streamwise-preferential permeable surfaces might be interesting for other (flow control) purposes involving turbulent boundary layers.

The increasing importance of drag reduction in commercial vehicles, traditionally motivated by rising energy costs, and more recently accelerated through policymaking in a bid to reduce emissions, has resulted in the development of increasingly sophisticated add-on aerodynamic devices for the semi-trailer. Of these devices, the greatest uptake amongst trucking fleets has been for devices fitted to the underbody, primarily on account of allowing free movement of freight into and out of the trailer, hence incentivizing a better understanding of the flowfield in this region. The current research addresses the aerodynamics of trailer wheels in tandem, which form a key component of the semi-trailer underbody, by investigating the effects of wheel rotation, trailer side-skirts, and wheel cavity covers on the flowfield and drag. The configurations tested include a short side-skirt terminating before the front edge of the leading wheel, a long side-skirt covering both the wheels, wheel covers without openings (solid disks), and wheel covers with openings varying in coverage area and radial position.

Stereoscopic (2D-3C) PIV is applied to examine the flow topology in the near wake and on selected planes besides the wheels. The measured velocity data in the wake is further used to derive the pressure field (by solving the Poisson equation for pressure), which together are then used to calculate the drag using a control volume approach. The uncertainty in the derived pressure field and drag values is determined using linear uncertainty propagation.

The largest drag reduction from the baseline case, i.e. without a side-skirt or wheel covers, is seen for the long side-skirt, followed by the short side-skirt with wheel covers, short side-skirt only, and wheel covers only, in that order. The effectiveness of both the long side-skirt and wheel covers is seen to increase with wheel rotation, with an appreciable reduction in drag with the wheel covers fitted only seen for rotating wheels. Investigation of the flowfield in the wake shows significant differences between the stationary and rotating wheel, particularly within the region of the projected wheel profile, and indicates to an earlier separation of flow along the upper surface of the rotating wheel. The effect of the side-skirt on the velocity in the wake is seen primarily in a lower streamwise velocity deficit, narrower wake, and higher horizontal symmetry of the wake for the skirted configuration. Wheel covers show a comparatively limited effect on the velocity field in the wake, showing a marginally wider wake and a slightly higher velocity deficit for the uncovered wheel. On planes beside the wheels, the side-skirt and wheel covers show a greater influence than wheel rotation, considered to be a consequence of a static floor combined with a gap between the wheel and the ground. The non-skirted configurations here exhibit a larger region of separated flow and greater velocity deficit whereas the uncovered wheels show an outflow at the bottom of the wheel cavity and an inflow at the top. Finally, the wheel covers with openings show a behavior between that of a covered and an uncovered wheel, with the inflow/outflow depending significantly on the coverage percentage and radial position.

This master thesis investigates the aerodynamic response of the Hyperloop breach failure scenario. Using a scaled physical test setup, the air pressure inside a Hyperloop tube is lowered to 10 mbar, emulating Hyperloop conditions. By opening a breach hole inside the tube, air will flow into the tube due to the pressure difference with the outside ambient. A transparent PMMA tube is used to allow for background oriented schlieren(BOS) measurements. BOS is used to investigate shockwaves and pressure differences through the tube and to determine the feasibility of BOS for Hyperloop breach research. A ray tracing model based on Snell's law is created to relate physical test results to local pressure values inside the tube. Three tests are performed using two different sizes of breach holes. The first test is a low camera capture frequency test in which the entire pressure range from its initial 10 mbar to ambient 1000 mbar is captured. The second test is a shockwave detection test in which a high camera capture frequency is used in an attempt to capture shockwave effects on BOS images. The third test is a Hyperloop pod test, in which the effects of Hyperloop breach on a Hyperloop pod placed inside the test tube are measured. By comparing the results from BOS with measurements from pressure meters installed in the test setup, the feasibility of BOS is investigated. The results from the first test showed pixel displacement from BOS correlation images overestimate the pressure levels inside the tube while also showing pressure jumps compared to pressure values from the pressure meters. While data smoothing and correction operations could mitigate parts of the pressure overestimation and pressure jumps, error sources causing this remained. From the second test the presence of shockwaves could not be confirmed. While for the third test the pixel displacements around the pod could be visualized using a masking function over the Hyperloop pod, shockwave effects on the Hyperloop pod could not be visualized. From the experiment performed the use of background oriented schlieren for Hyperloop breach research is deemed feasible, however the results are sensitive to error sources.

Supersonic intakes are essential in assuring proper combustion for operating ramjet engines on cruise missiles. However, due to increasing threats and a rapidly changing service environment, today's intakes do not fit anymore to comply with the new set of requirements to tackle the above challenges. Therefore, a novel supersonic intake design is investigated to evaluate the possible integration of this design on modern cruise missiles.
The Submerged Supersonic Intake is a novel intake design that flushes with the external missile hull. An experimental model was investigated in a blow-down wind tunnel with a Mach 2.0 freestream flow. The analysis revolved around the occurrence of the pseudo-shock wave inside the intake's constant-area duct. The flow domain was investigated using Particle Image Velocimetry, Schlieren imagery and wall pressure measurements. Submerged intakes show comparable behavior to generic intakes and the Total Pressure Recovery resembles the results found from experiments in open literature.