An Atmosphere-Breathing Electric Propulsion (ABEP) system is a novel concept in Satellite propulsion that involves capturing and compressing the rarefied atmosphere at Very Low Earth Orbit and using the captured gas to generate thrust using conventional electric propulsion methods. For such a system, the gas intake system needs to provide sufficient compressibility and mass flow to sustain the operation of the thruster. A numerical investigation into the flow characteristics and performance parameters of an active ABEP device has been conducted using the Direct Simulation Monte Carlo (DSMC) method. The Intake presented here comprises of a passive converging part and an active Knudsen Pump. The presented work initially recreates two studies one each of a passive intake and a Knudsen Pump from literature to validate the DSMC simulation methodology. Subsequently additional simulations are done based on the results of these benchmark cases.

Knudsen Pumps are usually operated in the transition flow regimes for its superior performance compared to the free-molecular regime. This study initially investigates the operation of Knudsen Pumps in free-molecular flow regime. This is done because in ABEP applications, the gases are usually in the free-molecular flow regimes. The channel lengths and the ratio of the size of the narrow channel to that of the wide channel is varied in the presented work. The variation of macroscopic properties along the axis of the Knudsen Pump is obtained from the results of the DSMC simulations. The results are compared to 1D analytical solution for some limit cases. It was concluded that increasing the channel lengths and maintaining a transition flow regime inside the wide channel of a Knudsen Pump brings up its compression characteristics while operating in a free-molecular flow regime.

In the second stage of this work, a hybrid intake consisting of a passive converging diffuse wall connected to an active Knudsen pump is investigated. A numerical DSMC model for such a setup is realised and verified using convergence studies. Two different configurations of single stage and five stage Knudsen Pump is studied. In the former case, the mass flow rate through the intake was varied using a given transmissivity for the intake exit and the temperature difference within each channels is varied. It was found that higher compressibility was observed when the total mass flow rate through the thruster was lower than the maximum thermal creep mass flow rate. Whereas the compressibility increased when the temperature difference was increased from Δ𝑇 = 500 to Δ𝑇 = 1000. However, further increase in temperature difference did not result in increased compressibility. In the five stage Knudsen Pump case, two different outlet transmissivities were studied. It was observed that the pressure and mass density drops down after consecutive stages of the Knudsen pump when the mass flow rate through the Knudsen Pump was higher than the maximum thermal creep mass flow rate. This resulted in no significant pressure or density compressibility. However, when the mass flow rate through the thruster was less than the maximum thermal creep mass flow rate, there was progressive increase in the mass density while the pressure continued to drop after consecutive stages. It was concluded that in-order to obtain significant compressibility the mass flow rate through the hybrid intake needs to be very low which in-turn might not yield sufficient thrust. A closed configuration where there is only mass flow during intermittent thruster firing could be ideal for obtaining higher compressibility using the Knudsen Pump

Enceladus, one of Saturn’s icy moons, exhibits dynamic cryovolcanic activity through plumes of water vapour and ice erupting from its south polar terrain. Understanding the variability of these plumes is essential for interpreting Cassini observations and planning for future missions. This thesis investigates whether temporal variations in plume activity can be explained by coupled processes of deposition, wall accretion and sublimation within subsurface channels. To address this, a two-phase computational fluid dynamics (CFD) framework was developed by extending an OpenFOAM solver to incorporate wall interactions. Parametric studies were performed across a range of boundary conditions. It demonstrated that wall accretion and sublimation act as competing mechanisms that progressively narrow or widen the vents over time. These geometric evolutions feed back into plume dynamics, modulating supersaturation and nucleation phenomena. Comparison with Cassini data suggests that vent-scale processes captured by this model can reproduce key features of observed variability.

This thesis presents the first systematic experimental study of boattail radius effects on Hammerhead Fairings (HHFs) in transonic flow, using the Coe and Nute Model 11 as a representative platform. Boattail radii were selected based on ratios of radius to boundary-layer thickness, and tested in two campaigns: high-speed schlieren imaging, oil-flow visualisation, and PIV at selected cases. Results show that increased radius generates a third expansion region, often with a lambda-shock, reducing separation length by 10–25% but increasing reattachment-point oscillations. Strong radius effects promoted shock-induced separation and lifted the shear layer at the shock foot. Two dominant unsteady modes were identified: a flapping mode linked to reattachment motion and shock features, and an undulation mode related to momentum injection and ejection from the separation bubble. Findings hint at trade-offs between separation reduction and increased unsteadiness, with implications for launch vehicle stability and structural loading.

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.

Reducing aerodynamic drag is crucial for lowering energy consumption and emissions in industries such asaviation and fluid transport through pipelines. In cruise conditions, skin friction drag is the dominant contributor to aerodynamic resistance and has therefore been the focus of numerous studies. A widely discussed technique for skin friction reduction is active forcing through spatio-temporal spanwise wall oscillations, which has been numerically shown to achieve drag reductions of up to 50%. This technique relies on creating a spanwise wall motion, generating a phase-varying spanwise velocity profile that suppresses near-wall turbulence. However, practical implementation remains challenging due to mechanical complexity and high energy costs, leading to interest in passive alternatives. Among these, dimples and Oblique Wavy Walls (OWW) are examples, as their surface deformations have shown to introduce a spatially varying spanwise shear layer into the flow. Through geometrical surface deformations pressure gradients are formed, which give rise to a spanwise component of the flow in the near-wall region. Although these techniques have demonstrated meandering flow structures, no significant drag reductions have been established previously. This motivates the investigation of sinusoidal undulations as a potential passive method for introducing a spatially periodic spanwise velocity component.

This thesis studies the flow topology of sinusoidal undulations to assess their ability to impose a spanwise shear layer. The geometry is characterised by a streamwise oriented sinusoidal groove, that displaces in the spanwise direction, having a dimple-like cross section. The influence of the geometry is assessed in terms of, spanwise displacement amplitude A, depth-to-diameter ratio d/D, and scale (while retaining geometric similarity). The flow over sinusoidal undulations was visualised by means of Particle Image Velocimetry (PIV) in (i) a stereographic PIV in the streamwise-wall-normal plane to quantify the spanwise velocity amplitude and the penetration depth, and (ii) a planar PIV in the streamwise-spanwise plane in the near-wall region (y = 1 mm) to examine the near-wall flow structure. These experiments were conducted in the M-tunnel of the Delft University of Technology.

Analysis of the experiments confirmed an alternating spanwise movement of the flow that aligns with the geometrical spanwise angle of the sinusoidal undulation. Simultaneously, the windward edges of the geometry act as diffusers, pushing part of the flow over the edge and out of the groove onto the flat area. These combined effects led to the identification of a converging-diverging flow structure, locally resembling dimple flow structure. Additionally, a relation was found between the undulation geometry and flow characteristics such as maximum spanwise velocity and penetration depth. While spanwise velocity oscillations were observed across all geometries, a larger geometrical displacement intensified the diffuser effect of the windward edges. The measured velocity magnitudes were comparable to other passive techniques, but lower than those achieved by active spatially periodic forcing with optimal actuation conditions for drag reduction. Furthermore, these velocities were associated with penetration depths into the buffer layer.

This study demonstrates that sinusoidal undulations are able to introduce a phase-varying spanwise velocity of w⁺≈ 1, or approximately 4% of the freestream velocity, forming a converging-diverging flow topology, bearing resemblance to dimples with comparable spanwise velocity amplitudes (w/U_∞ ∼ 3%). The measured spanwise velocity profiles exhibit similarities to the analytical solution of the spatial Stokes layer, though the penetration depth follows a different definition. The near-wall shear layer in the viscous layer resembles a horizontally mirrored Stokes layer, and is characterised by a secondary deeper penetration resultant from the imposed pressure gradient. To further assess the drag-reduction potential of sinusoidal undulations, drag balance measurements and numerical simulations are recommended.

An impinging Shock Wave-Turbulent Boundary Layer Interaction (SWTBLI) at Mach 2 was investigated experimentally while implementing two-dimensional Shock Control Bumps (SCBs). The aim was to investigate the changes in the unsteady dynamics while changing the bump ramp angle, tail angle, spanwise shape, and the impinging shock location. Schlieren and oil flow visualisations were used to identify these changes. Unsteadiness was quantified through a spectral analysis based on Welch’s method, and a Canny-based edge-detection algorithm was developed to track the impingement and separation location in the Schlieren images.

An uncontrolled SWTBLI could successfully be generated, and the implementation of the baseline bump showed a replacement of the unsteady separation shock by a steady compression ramp shock originating from the leading edge. Spectral analysis confirmed this behaviour since the characteristic low-frequencies of the uncontrolled separation shock seemed to be removed for this compression ramp shock. An increase in the bump ramp angle showed the progressive generation of a separation shock upstream of the bump with spectral content trending towards the low frequencies of the uncontrolled interaction. On the contrary, no alterations were observed for an increase in tail angle. An upstream impingement produced similar behaviour as the increase in ramp angle; a separation shock was generated upstream of the bump with increased low-frequency spectral content. The downstream impingement, however, did not show any alteration in the same region. Major factors influencing the unsteady dynamics are identified as the impingement location and the ramp shape of the bump.

The developed edge-detection algorithm proved unsuccessful in quantifying the unsteadiness although it could detect the impinging and reflection shock of the interactions. Spatial standard deviation distributions of the interaction revealed increased deviation values in the impinging shock suggesting that the impinging shock fluctuates. Rather, it is suspected to be a form of noise intrinsic to the experimental technique. Therefore, this affects the detection of the edges and the calculated impingement and separation location. Future research is suggested to improve the noise mitigation method in the algorithm. Additionally, the benefits associated with the 2D-SCB would be most noticeable in an integrated approach with an industrial application. Finally, numerical simulations and/or different experimental techniques are suggested for future research to obtain a better quantification of the interaction and unsteady dynamics.

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.

To meet the climate goals and reduce the negative effects of anthropogenic industrial activity, the human civilization must move toward sustainable energy sources. However, in hard-to-abate applications and to compensate for the intermittency of renewable energy sources, combustion will continue to be a crucial energy conversion mechanism. This requires modern combustion devices to have highly reduced emissions not only to abide by stringent regulations but also as a collective social responsibility. Usually, a reduction of NOx requires a lowering of flame temperatures, which leads to an increase in CO. However, Flameless/MILD combustion is a technology that has the potential for low NOx while attaining complete combustion. This is because of its unique requirements of having very high reactant temperatures (typically above autoignition temperatures) and having low oxygen concentration. The current work investigates the phenomenology of the combustion process in a jet-stabilized combustor, where this regime can be produced with fuel and oxidizer injected in a premixed manner. The work has three main components; the first deals with the autoignition chemistry of methane-air mixtures under exhaust gas vitiation conditions, the second investigates the flow field and turbulent interface of a turbulent-jet-in-coflow which is a canonical version of the flow conditions in a jet stabilized combustor and finally, experiments were done on a jet-stabilized combustor with methane-hydrogen blends to classify the flame stabilization mechanism and quantify emissions.

Autoignition characteristics of vitiated methane-air mixtures were studied by simulations in 0D reactor setup. Unlike most studies in literature, vitiation, in the context of flameless combustion, was generated as the hot combustion product of fresh reactants that also contained radicals that existed in equilibrium. Particularly, the effect of varying levels of vitiation and heat loss was studied on properties such as ignition delay time, reaction time scale, and the NOx and CO emissions. This revealed the most suitable conditions to achieve low emissions and distributed reaction zones for premixed reactants that are vitiated by exhaust gases. Further, a regime of multi-ignition was discovered where prior to the main ignition event, there is a pre-ignition event attributed to the initial pool of radicals in a vitiated mixture. The conditions of occurrence were mapped out, as well as the mechanism behind it was explained.

The mixing at the interface of the jet and the recirculation zone in a jet-stabilized combustor has an important role in determining the composition of the hot-diluted mixtures. Thus, the fluid mechanics of the turbulent-turbulent interface were studied in a canonical configuration of a turbulent-jet-in-turbulent-coflow. This is also a common configuration used to produce Flameless/MILD combustion under laboratory conditions. Although there is vast research on free turbulent jets, combustors operating in the Flameless Combustion regime would reach flow conditions where the ratio of coflow to jet velocity would increase. This work elucidates the evolution of such a flow field through Particle Image Velocimetry (PIV) measurements done along the axis of the jet in the range 0<x/D<42. Further, the interface is detected using an algorithm developed based on other image processing algorithms using vorticity as a criterion. This enables the assembly of conditional statistics with respect to the interface. The results show that the cases with higher coflow have a lower jet centerline velocity decay rate and reduced jet spreading. The mean axial velocity shows a region of deficit compared to the free jet near the interface region. Further, the case with higher coflow shows higher turbulence intensity and Reynolds Shear Stress close to the interface. The detailed results are presented as both unconditional and conditional statistics and the mechanism behind this effect is deduced.

Experiments were done on a jet-stabilized combustor capable of producing the Flameless Combustion regime. It was operated using methane-hydrogen fuel admixtures at varying equivalence ratios. The combustor performance was analyzed based on the stabilization of the flame zone and the emissions. This work presents a unique, comprehensive measurement of temperature, gas composition, velocity field, and chemiluminescence signal in a jet-stabilized combustor. The recirculation regions are visualized through PIV measurements and the recirculation ratio is quantified. The instantaneous flame images are used to identify flame kernels and construct probability density functions of the aspect ratio, rotation angle, and location along the combustor axis. An increase in hydrogen content in the fuel mixture shifts the stabilization mechanism from autoignition to flame propagation. There is also an increase in NO emissions. A similar effect is seen with the increase in equivalence ratio from lean to stoichiometric condition. Distributed reaction regimes with ultra-low NO and moderate flame temperatures are achieved at very low equivalence ratios. Such mixtures are stabilized better with the addition of hydrogen to the fuel mixture.

This thesis provides fundamental information on the chemistry and flow physics of the phenomenon in a jet-stabilized combustor followed by measurements from the operation of one. The data and conclusions are a suitable reference for future engineers designing jet-stabilized combustors for low NOx emissions and high combustion efficiency.

The 'hammerhead fairing' (HHF) launcher configuration uses a payload fairing that has a larger diameter than the launch vehicle. This allows for larger payloads to be carried using the same launcher components. However, the HHF leads to high aerodynamic forcing in the transonic regime called buffeting. If unaccounted for, buffeting can lead to launcher failure. The flow phenomena associated with buffeting were investigated on the Vega-E launcher, which is currently being developed by the ESA, using Pressure Sensitive Paint (PSP) in the TST-27 tunnel.

PSPs are a relatively new flow measurement technique that allows for quantitative measurements of surface pressure levels in a non-intrusive manner. High spatial resolution, which is equivalent to using millions of pressure taps, can be achieved using PSP. This allows for CFD-like surface pressure resolution to be obtained from wind tunnel experiments. However, PSPs had previously never been successfully implemented in Delft. Therefore, a test set up was devised for experiments in the TST-27. The final implementation was found to have a pressure uncertainty of at most 2.5 kPa, with a spatial resolution of 0.088mm.

First, validation measurements were done on the Coe and Nute Model 11. The Model 11 is a common HHF test case in literature that is used to study the buffeting phenomenon. Results obtained in the TST-27 were compared to historical data and showed good matching when accounting for blockage. After the validation, measurements were taken on the Vega-E launcher at different Mach numbers and angles of attack to quantify the effects of each variable on the pressure field. Lastly, the impact of protuberances on the model was studied. The protuberances that were used mimic those that will be present in the full-scale launcher.

The aerospace industry’s push for faster, lighter, and more durable designs highlights the critical challenge of panel flutter in supersonic flight, a poorly understood phenomenon intensified by shock-wave/boundary-layer interactions (SWBLI). This study experimentally investigates the effects of static and dynamic panel deformations on shock-induced panel flutter at Mach 2.0, using open, closed, and ventilated cavity conditions. Results show that cavity closure significantly influences panel response, with the open cavity causing greater oscillations due to strong pressure waves entering it, causing resonance behaviour; closing the cavity seems to filter out this resonance behaviour. Additionally, temperature changes and wind tunnel confinement were identified as factors affecting panel and flow behaviour. The results underscore the importance of understanding experimental conditions to accurately interpret fluid-structure interactions in supersonic conditions.

Contrary to conventional wisdom favouring smooth surfaces for drag reduction, the last decades have brought forth different textured surfaces showing drag reduction in turbulent boundary layers, of which riblets are the most studied.
Sirovich and Karlson (1997) introduced a different textured surface named chevron-shaped protrusions, indicating a drag reduction of 10% in turbulent channel flows.
Later studies questioned their efficacy as no credible reproduction of the results was obtained throughout the years.
The last credible reproduction study by Carrasco Grau et al. (2023) suggested that the reasons for this discrepancy could reside in the difference in model and test section size.
All reproduction studies were performed in facilities with external boundary layers instead of internal and with a covered area no longer than 0.8 meters, instead of the 8-meter-long channel flow which was fully covered in these chevrons from the original study.
This work will investigate just that, using a channel flow facility with dimensions more akin to that used by Sirovich and a model size of 2.4 meters.

The study assembles and characterises an improved channel flow facility with dimensions identical to those used by tay et al. (2011) at the National University of Singapore (NUS), measuring a total length of eight meters and a test section of 2.4 meters.
This facility used an array of 29 static pressure taps to determine the skin friction via the mean pressure gradient method, and utilised hot wire measurements in the midpoint of the test section for investigation of the flow mechanics of the chevrons.
The measurements performed on a flat plate were compared to the results obtained at the NUS to characterise the facility and validate its suitability for single-point drag measurements in the order of 5-10% increase or decrease.
Once this was confirmed the different configurations of chevron-shaped protrusions were investigated.

The key findings of this study are twofold.
Firstly, the developed channel flow facility at DUT proves proficient in generating canonical boundary layer profiles, enabling accurate skin friction measurements of textured surfaces.
The facility provides good hot-wire measurements without sensor vibrations capable of investigating the boundary layer characteristics in this facility.
Secondly, the study establishes that chevron-shaped protrusions are likely unsuited for reducing turbulent skin friction in turbulent channel flows.
Despite an inability to definitively disprove the working hypothesis, the observed increase in drag suggests that the technique's efficacy is not determined by the facility type and model size.

It is not considered worthwhile to conduct additional research into chevron-shaped protrusions as a potential technique for reducing drag in turbulent boundary layers.
However, the minimal drag penalty associated with this technique opens new possibilities for the application of this technique in the aviation sector, mainly in aiding with separation control and heat transfer.

A Tomographic Background-Oriented Schlieren (TBOS) technique is developed to aid in the visualization of compressible flows. An experimental setup was devised around a sub-scale rocket nozzle, in which four cameras were set up in a circular configuration with 30° angular spacing in azimuth. Measurements were taken of the overexpanded supersonic jet plume at various nozzle pressure ratios (NPR), corresponding to different flow regimes during the start-up and shut-down of rocket nozzles. Measurements were also performed for different camera parameters using different exposure times and f-stops in order to study the effect of measurement accuracy. Density gradients and subsequently two-dimensional line-of-sight integrated density fields for each of the camera projections are recovered from the index of refraction field by solving a Poisson equation. The results of this stage are then used to reconstruct two-dimensional slices of the (time-averaged) density field using a tomographic reconstruction algorithm employing the filtered back-projection and the simultaneous algebraic reconstruction technique. By stacking these two-dimensional slices, the (quasi-) three-dimensional density field is obtained. The accuracy of the implemented method with a relatively low number of sparse cameras is briefly assessed and basic flow features are extracted such as the shock spacing in the overexpanded jet plume.

TOP contoured nozzles, with large area-ratios, are commonly employed in rocket propulsion systems as they feature an excellent thrust-to-weight ratio. A significant shortcoming to this design is that, during the startup and shutdown transients of a LRE, the internal nozzle flow progresses through a series of overexpanded flow states - Free Shock Separation (FSS) and Restricted Shock Separation (RSS), which produce critical loads associated with SWBLI and asymmetric flow separation. Exacerbated by FSI, this operational phase is known to generate the highest vibroacoustic loads at which payload and vehicle structures are subjected to when the engine is operated at off-design conditions.

Motivated by the importance of understanding how the interaction between the developing flow and the vibrating nozzle walls has an effect on supersonic noise generation and propagation, this work has studied the effect that wall compliancy has on the vibroacoustic loading of TOP contoured nozzles. This is demonstrated by means of cold flow tests carried out in the High Speed Laboratories of the Delft University of Technology on a stiff-walled aluminum nozzle, which serves as a baseline test case, and on a urethane-based compliant walled nozzle. Tests are conducted under comparable flow conditions and test parameters are measured by means of acoustic and optical techniques. Simultaneous recordings are performed and include the nozzle-wall deformation, by means of stereoscopic tracking of tracers on the nozzle lip, the imprint of the near-field acoustic signature, by means of arrays of pressure-microphones, and Schlieren imaging of the jet plume.

Measurement data allows for a Fourier decomposition of the nozzle lip displacement and of the acoustic pressure field in azimuth. Reconstruction of the instantaneous plume development enables the identification of the main flow structures responsible for noise generation.

Comparison of results between the two test articles highlights a different spectral content and directivity pattern. Correlation between the structural displacements and the acoustic signal, together with the use of DMD, quantitatively aids the investigation of how FSI has an impact on the generation of an aeroelastic tone at 180 Hz. Findings suggest that its production is the result of the periodic thickening and thinning of the shear layer owing to the heightened flapping motion of the nozzle lip preceding RSS transition, driven by an intensified shock foot instability.

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.

The transonic regime is of great interest for designing aircraft wings because most conventional aircraft fly transonic. When flying in this regime, the wing of the aircraft tends to experience a phenomenon known as buffet, which is characterized by an oscillating shock wave. The surface pressure of a structure experiencing transonic flow is always changing; however, the average surface pressure can be computed. Pressure sensitive paint (PSP) is a flow measurement technique that determines the surface pressure and serves to compute its average. Using PSP, the average surface pressure of an ONERA OAT15A supercritical airfoil model can be determined in transonic conditions. This allows to study the effect of shock control bumps on the shock wave and surface pressure of the OAT15A model, as well as to determine the pressure coefficients of the airfoil.

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.

As the space industry continues to grow rapidly, the development of reusable launch vehicles has become crucial in achieving cost-effective and sustainable access to space. Payload capacity optimisation has been at the forefront of this effort, leading to a renewed interest in hammerhead or bulbous payload fairing (PLF) configurations. These designs feature a larger diameter in the PLF than in the rest of the launch vehicle, enabling the same structure to be used for large payloads. However, the transonic regime poses unique challenges for these PLFs due to their susceptibility to flow separation and strong pressure fluctuations.

This master thesis investigates the influence of nose and boat-tail geometries on flow phenomena, particularly shock wave generation, around hammerhead configurations in transonic conditions. The research was conducted through three distinct experimental campaigns in the transonic wind tunnel TST-27 at the high-speed laboratory of TU Delft, employing schlieren, oil flow, and Particle Image Velocimetry (PIV) techniques.

The study revealed notable insights into the aerodynamics of the hammerhead PLF configurations. Boat-tails set at five and 15-degree angles were observed to broaden the range of shock wave oscillations, introducing an additional shock wave that could occasionally merge with existing ones. Moreover, the conic nose design induced higher shock wave oscillations, while the bi-conic nose introduced an extra shock wave compared to the conic and ogive noses. The study also found that altering the nose shape while keeping the boat-tail constant, or viceversa, resulted in similar effects on flow dynamics.

These findings underscore the critical role that nose and boat-tail geometries play in shaping the aerodynamic behaviour of hammerhead PLF configurations. The results have implications for such configurations’ design and stability considerations in transonic conditions. It was observed that raising the Mach number heightened shock wave oscillations and flow detachment. The angle of attack disrupted model symmetry, primarily impacting reattachment patterns. Furthermore, the conic nose exhibited greater unsteadiness due to oscillations in the shock waves compared to the other geometries. The study also provided detailed insights into shock wave spectral characteristics and identified potential influences of pressure wave oscillations on shock wave behaviour. The conclusions lead to recommend designs with bi-conic nose, and avoid boat-tail angles around 15 degrees.

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.

This Master thesis consists in investigating the formation and behavior of vapor and ice plumes. These plumes can occur in icy moons of our solar system, such as Europa or Enceladus, which are widely believed to have a liquid water ocean beneath their crust. These plumes most likely consist of a sub- surface reservoir of liquid water fed by the ocean, a crevasse, and a vent at the ice surface. Under certain conditions, the liquid water starts evaporating and, due to the high reservoir pressure, the result- ing vapour starts flowing upwards through the crevasse. Due to the high pressure difference between the reservoir and the vent at near vacuum conditions, supersonic plumes are formed. This work stud- ies the reservoir conditions and plume physics, gathering different models available in literature and comparing them to available data, such as that from observations from Cassini spacecraft. First, a model capable of describing the condensation phenomenon is sought. The effects of the consequent release of latent heat to be absorbed back by the vapour flow are also studied. Grains will then start to nucleate and grow. These particles can either collide and stick to the walls or keep flowing mixed with the vapour. Some of the above mentioned latent heat can be absorbed back by the icy walls of the crevasse and generate more vapour by sublimation. Further, this project also aims to extend the model mentioned above so that it considers the effects of a fully multi-phase, multidimensional flow, checking for the effects of rarefaction and hence the limits of the continuum assumption. Finally, a similarity analysis is performed so that the influence of working with scales as different as the channels used in the laboratory at TU Delft or the real dimensions of the crevasses found in Enceladus can be fully tackled. Any progress on the ongoing investigations about the physical characteristics of these plumes could be crucial to deepen our knowledge on geological mechanisms in icy moons of our solar system. This, in turn, could trigger research on organic compounds present in these moons, perhaps even allowing for the existence of life. Further, the model used throughout this work can be applied to study power production devices where condensation might play an important role, such as steam turbines for light-water-cooled nuclear reactors or turbines proposed to be used in innovative organic Rankine cycle (ORC) configurations, natural gas supersonic separators, supercritical CO2 compressors for large-scale carbon capture and sequestration (CCS) and micro-nozzles.

The concept of a spaceplane, a launch system that is a combination of an airplane and a rocket, is one of the most promising concepts of sustainable access to space. Along its trajectory, it experiences most extreme conditions with frequently opposing design requirements, making it one of the most complicated challenges in aerospace engineering. This project aims at assisting in its design process by creating a KBE framework that combines a 3D geometry generation module, with automatic discretization and low-fidelity analysis using a panel method, ZONAIR. The results of the low-fidelity simulations are then correlated with CFD calculations using a multi-fidelity data fusion technique called co-kriging, to provide a good balance between high accuracy and low computational cost, making the framework applicable in the preliminary design phase. During the study of the accuracy of the proposed algorithm it was found that just 30 high-fidelity points is enough to generate surrogate models of aerodynamic coefficients with expected relative error below 5%.