The increasing population density in metropolitan areas, coupled with the faster pace of everyday life, calls for the development of Inter-Urban Air Mobility (UAM) solutions. Currently, there is a gap in the market since there is no competitor that can facilitate all market segments and also provide “last-mile transport” by air. This report aims to identify a promising Inter-UAM design, the Swing, focused on minimising the aircraft’s ground and environmental footprint. The small ground footprint will enable the vehicle to take off from smaller vertiports, helipads or even driveways, allowing it to penetrate urban areas and land where other UAM vehicles cannot.

This study is part of an ongoing research project, which aims to increase the physical understanding of the plume formation on Saturn’s icy moon Enceladus. The experimental setup presented in this study is the third iteration of the physical models aiming to recreate the main plume characteristics, where in this study the effects of the channel length, width, type (straight, converging/diverging or diverging), temperature and material are explored and linked to Enceladus’ crevasses. This is done by conduct- ing experiments with 7 different models, where the temperature and pressure are measured along the channels. The models are placed on top of a water-filled reservoir, where the reservoir conditions can be controlled to a limited extent by varying the heating power supplied to the reservoir water. The experimental setup is placed in the test section of the Hypersonic Test Facility Delft (HTFD), which functions as a vacuum chamber.

It is shown that the varying geometry of the channel imposes constraints on the maximum expansion angle before flow separation occurs, the sonic point location and the length/width combination in order to achieve a certain vent Mach number and mass flow rate, although this is also decided by the reservoir and ambient conditions. However, besides the physical properties of the model, there is evidence that the flow properties are dominated by the thermal processes occurring inside the setup. Condensation occurs only in the reservoir, releasing latent heat and making the isentropic flow assumption invalid by definition. It is demonstrated that it is likely that the thermal radiation from the test section of the HTFD onto the model is sufficient to thermally choke the flow. It is unlikely that the flows become choked due to the effects of friction alone. Cooling the models by 10-15°C did not result in significant changes in flow properties, and to have noticeable effects on the flow, the models would have to be cooled to much lower temperatures so that condensation occurs in the channel instead of the reservoir. Although the vapor remains unsaturated in the channels, there are signs of local temperature spikes in the ex- panding sections of the channels, near the vent, when either no or low heating power is supplied to the reservoir water and the pressure in the channel is reduced. This implies that either the temperature at the center of the channel is lower than what could be expected from the temperature measurements and heat is thus released by the deposition of the vapor, or the particles that condensed in the reser- voir partially evaporate after the throat of the channel, after which the evaporative cooling freezes the remainder of the particles, with the accompanying latent heat release. It is also not expected that a pressurized reservoir is necessary to create the plumes on Enceladus, nor is the presence of a geomet- rical throat, due to the combined effects of friction and condensing vapor. The results of one physical model are compared to a computational fluid dynamics model using the same geometry, and the main difference between the physical and numerical model is that the vent pressure of the numerical model is approximately half the vent pressure of the physical model, and the temperature of the numerical model dropped to about -50°C at the expanding section of the channel where the temperature only increased throughout the channel for the physical model. This, and the small heat spike near the vent under low-power conditions, has questioned the accuracy of the temperature measurement method and further research would be required to improve this accuracy.

In this study, plume experiments were conducted to mimic the thermodynamic conditions on Saturn's moon, Enceladus. The icy moon subsurface ocean and cracks in the surface have been simulated by using a liquid water reservoir and a narrow channel, while the low-pressure environment at Enceladus’ surface was achieved with a vacuum chamber. We aimed to examine how channel temperature affected the plume's behavior, testing two models with differing wall temperatures: room temperature and near 0°C. The colder model better replicated Enceladus' plumes, producing a saturated flow in which icy particles due to nucleation were seen. A conservative 1.5-3% minimum solid fraction is estimated from measurements and modeling. Pitot-tube measurements indicated Mach numbers around 1, with velocities between 400-500 m/s. Flow temperature and velocity closely correlated with wall temperature, indicating effective heat transfer. The study suggests that supersonic plume velocities observed on Enceladus can be achieved through thermal effects within the icy crust's crevasse, without requiring extreme expansion ratios. Additionally, the research provides evidence of the relationship between the crevasse's expansion ratio and the flow and wall temperatures.

The Cassini spacecraft, observing the Saturnian system for over 13 years, discovered aspects of the planetary system that were previously unseen. One such discovery is the eruption of geysers (plumes) from the Tiger Stripes on the surface of the icy moon Enceladus. An unexpected liquid water ocean exists underneath Enceladus’ icy crust (Postberg et al. 2018). A consequence of this finding was the complete revision of the habitability of the Solar System. This liquid ocean is propagated through conduits within the crust, and forms plumes when it reaches the surface. The plume material is believed to accelerate supersonically through nozzle-like channels (Schmidt et al. 2008) before being ejected at high speeds from the plume vents.

This thesis aims to improve the physical understanding of the interaction between the ocean, icy crust, and the plumes of Enceladus, by experimentally simulating such a plume in the wind tunnel laboratories of TU Delft, and monitoring and analyzing the dynamic physical processes taking place across the experimental setup. A physical analog, separated into regions simulating the ocean, crevasse, and vent of the plume mechanism, is monitored with pressure and temperature sensors, while plume particles are detected with optical tracing
techniques. These observations lead to estimations of the vapor mass flow rate, the outflow Mach number, and the fraction of the plume mass that is condensed. It is found that the ocean conditions can be easily controlled through an adjustable heating power supply. The vapor flow generated by the boiling ocean becomes choked in the crevasse and can attain supersonic velocities. The thermodynamic conditions at the vent of the plume exhibit a greatly varying behavior suggesting that the combined effect of the crevasse geometry and the
nucleation of vapor into liquid and icy particles results in considerable diversity in the plume characteristics. Thus, an isentropic description of the plume flow is found to be inadequate, while a Rayleigh flow is found to be feasible. Heat exchange phenomena appear to dominate locally the plume flow, as strong evidence of thermal choking in the crevasse is found. Particles with speeds of up to 426 ± 5 m/s are detected being ejected from the vent of the crevasse, and the maximum fraction of condensed plume mass is found to be 2.94% ± 0.15%. Finally, the possibility of a supersonic plume generated on Enceladus by a crevasse of constant cross-sectional area and cold walls is examined and found to be feasible.

Dimples are shallow surface indentations that have been recently considered as a passive viscous drag reducing technique for turbulent boundary layers. Studies so far were limited to incompressible low-Re flows and display little consensus on whether dimples can actually produce net improvements in total drag. Conversely, dimple geometry can be regarded as the inverse of transonic bumps, a matured wave drag reduction. In light of this, this thesis sets off to analyse the flow structures that arise over dimples at transonic speeds provide, serving as an initial assessment of their wave drag reducing potential.

A transonic flow was reproduced experimentally using an asymmetric nozzle. Then, the performance of five dimple designs was evaluated against the criteria adopted for transonic bumps. Measurements included Schlieren, surface pressure and PIV. Results reveal that small spherical dents seem to produce spanwise excitations that subdue the detrimental effects of expansion fans formed at their edges, and ultimately allow for a higher momentum retention across the interaction.

Drag in pipelines is composed almost solely of skin friction drag. The most common technique to achieve drag reduction (DR) is by adding drag-reducing agents. However, in the aerospace industry, various impressive passive drag-reducing techniques have been suggested to reduce skin friction drag in the past decades. Among these techniques, dimpled surfaces form a relatively unexplored terrain. Research into reducing skin friction drag in the turbulent regime by using dimples has been performed in the aviation industry since the '80s. The excessive amount of skin friction drag in pipelines forms an intriguing challenge to break new grounds. Several researchers investigated the potential of DR of turbulent flows using dimpled channels. Even though most of these studies that found DR are disputed, studies published at the National University of Singapore (NUS) obtained positive results time and again. NUS's exact test setup was reconstructed at Delft University of Technology, which has not been done yet as far as the author of this report is aware. Identical pressure measurements were performed, yielding an absolute DR of ≈ 5%, which is slightly less than what was obtained at NUS (>7%). Flat plates return a DR between 8-15%, dimpled test plates returned at DR between 12-20%. The test plates were covered for 99.5% with diamond-shaped dimples of 100 mm long and 50 mm wide. In total, 29 pressure taps were used to determine the change in drag in an 8 m long channel of 20 mm in height. Tests were done at Reynolds numbers, based on half channel width and centerline velocity, between 6,000 and 40,000. Accurate results were perceived up to a Reynolds number of 21,000, likely due to test-setup limitations. Multiple verifications such as two-dimensionality of the ow, comparison of theoretical and experimental skin friction coefficients, and instantaneous pressure tests were used to allow for an objective analysis. The pressure measurements were also supported by 1D hotwire anemometry (HWA) tests and surface oil ow visualizations (SOFV). The majority of the investigated boundary layers were absent of anomalies. It should be mentioned that the viscous sublayer could not be captured, neither a quantitative momentum analysis was performed. However, a shift in the velocity profile, acquired at the same test location, for different test plates was observed. Furthermore, an increased velocity near the wall was observed for dimpled test plates. Surface oil ow measurements did show similar ow patterns to what was recorded at NUS. However, the actual behavior is not investigated through HWA. Hence, it cannot be confirmed nor denied if the change in drag is a consequence of these near-wall ow mechanisms. SOFV did not show irregular ow structures such as ow reversal. Finally, tests were performed with a correction for test volume increase caused by the dimples. After this correction, the drag over dimpled plates increased instead of reduced. Considering other studies on dimpled surfaces that also found an increase in drag, it is strongly believed that the positive effect of dimples in turbulent channel flows does not stem from skin friction drag but rather from the increase in channel volume. Although the precision of the results was relatively high, the accuracy of the wind tunnel was insufficient. Additional research is required to narrow down the 8-15% error that was obtained while testing at plates.

NOx emissions cause harm to the human body, the world around us and our atmosphere, both directly and indirectly. Flameless combustion is a combustion regime that has the potential to reduce these emissions by more than 95%. Operation in this regime is possible when lowering the availability of oxygen and lowering the combustion temperature in the combustion zone. Key for this operation is the recirculation of flue gases using internal recirculation zones and mixing of these gases with the incoming combustion air and fuel. These aerodynamic characteristics have been researched experimentally and numerically for the DUT flameless combustor. Results show recirculation and mixing can be increased with decreasing the jet size, however with an increase pressure loss. Numerical investigation shows that simulation of this setup is challenging using RANS and that LES might be the only way to go forward.

The interest in manipulating particles, droplets and bubbles have garnered significant attention in recent years, owing to the advantages offered by micro-fluidics and the advancement in micro-fabrication technologies. These manipulation activities have found its applications in myriad fields of engineering, ranging from medical diagnostics to chemical industry to drug discovery. This has increased demand for the development of devices such as 'Lab on a chip', which performs laboratory-sized experiments and analysis on a single small chip, with the same speed and accuracy as its room-sized counterpart. However, manipulation activities carried out in these devices has fixed channels, designed to serve purpose for specific manipulation tasks. This makes the device suitable for a specific application. Addressing this aspect, a device designed without having any real channels would give an opportunity to integrate multiple functionalities onto a single-chip in the long run. As a first step towards reaching this 'bigger picture', it is necessary to explore the feasibility of manipulating particles, droplets and bubbles by generating so called 'virtual channels'.
The present thesis focuses on an attempt to manipulate particles without the use of any real channels or external field. Although such manipulation is desired in the micro-scale, a top down approach is preferred and hence, the manipulation is carried out in a scaled up model. First, a Hele-Shaw flow cell is designed with sources and sinks in the millimeter scale to deviate streamlines in the same range. Thereafter, four different velocity fields are studied under different combination of sources and sinks, which are then compared to the computational ones. The property of a Hele-Shaw cell that the averaged velocity over the height of the channel is irrotational, makes it possible to compute velocity fields by the use of potential flow theory. The same velocity fields are hence, computed using a discrete source based Panel Method. A good agreement is found between computation and experiment, making PIV measurements not a necessary option for evaluation of velocity fields under these sources and sinks. Finally, individual particle is inserted into the Hele-Shaw cell and manipulated using unsteady fields. The manipulation includes tasks such as diverting particles having same initial position to different end locations; trapping particle for different instances of time and then releasing them into different directions; flipping positions of two particles; and deflecting a particle by ninety degrees. The individual particle trajectory for the above manipulation activities are tracked down using a particle tracking code and then compared with the ones generated using the Panel Method. This, however, excludes the activities where particles need to be trapped because of particle fluctuation near stagnation point. Overall, the Panel Method serves well in predicting particle path-lines and can be used as a tool for manipulating particles.

This study was aimed at improving the accuracy of the model predictions for the minimum fluid and inner wall temperatures for cold, low-pressure start-up of wells that produce oil or gas. Due to the large pressure drop over the well head choke, the so-called Joule-Thomson cooling will give a very low temperature of the
expanding gas jet. Low-temperatures can give brittle fracture of the material in the piping downstream of the choke. Models are needed to verify whether the material temperature remains above the lower-design temperature. For the model validation, Imperial College in London (on request by Shell) has carried out lab experiments with argon gas that expands through an orifice from 120 bara to 1 bara. Awaiting the results of the lab experiments, detailed simulations were carried out in the present study using the Fluent CFD programme.

The 3D, steady, compressible Reynolds-Averaged Navier-Stokes equations were solved with the SST k −ω model for the turbulence. The considered configuration is the same as in the lab. It consists of an upstream chamber with argon at 120 bara, that expands through a 5 mm long orifice with 1.55 mm diameter, into a square outlet section with 50 mm sides and 500 mm length. The inlet temperature is -17 oC and the outlet pressure is 1 bara. The supersonic flow leaving the orifice reaches a maximum Mach number of about 9, just before a shock to subsonic flow is found. The jet reaches very low temperatures due to isentropic expansion, and reaches the isenthalpic expansion temperature of 196 K (or -77 oC) downstream of the shock. The jet reaches the sides of the outlet at a distance of about 100 mm.

The maximum Mach number of about 9 predicted by Fluent is higher than the value of about 6 found in a previous simulation study that used the STAR-CCM+ CFD programme. To verify the Fluent results, the distributions of grid cells was varied and the number of grid cells was increased. Also, a MATLAB programme was written that solved the inviscid compressible equations (Euler equations) for an axisymmetric jet.

This confirmed the Fluent results. In addition to the 3D square outlet section, also 3D and 2D Fluent simulations were carried out for a cylindrical outlet (using a hydraulic diameter of 50 mm). The maximum Mach number and the jet structure (velocity, temperature) are not affected by the side walls. This is because the side walls are sufficiently far from the jet.

Furthermore, the temperature and the heat transfer at the walls of the outlet section were investigated. Thereto both adiabatic and non-adiabatic walls were considered. The ambient temperature is 20 oC. Thermal boundary layers are formed along the side walls, that are exposed to a temperature of 196 K (the isenthalpic expansion temperature) in the centre of the pipe, up to a distance of about 1.5 m, where the outer edge of the boundary layer reaches the centre of the pipe. Thereafter the centre line temperature increases due to heat inflow from the ambient.

Current-day space vehicle are equipped with a passive Thermal Protection System (TPS), but these systems operate at the edge of their capabilities. The research into the next generation reusable space vehicle demands the use of active TPS, these systems are not limited to the capabilities of the used material but depend on the active cooling mechanism. A system of cooled metallic TPS is developed at the TU Delft resulting in an increase in the maximum allowable thermal load for a given TPS temperature, this system is the enhanced radiation cooling TPS. The research question is: "Can an active thermal protection system improve the flying characteristics and the flight performance of a re-entry vehicle?". The first step in answering this question is to verify the process behind the active TPS. This thesis aims to design an experiment to verify this process.

To answer the research question two tools were developed: a flight simulator and a thermodynamic analyser. The flight simulator provides insight in the behaviour of the rocket and shows the consequences of implementing the enhanced radiation cooling TPS on the rocket. The thermodynamic analyser is used to estimate the wall temperature of the system and shows the effect of the sensitivity analysis of the design of the cooling subsystem. The enhanced radiation cooling system is designed to be mounted on the T-Minus Dart and an experiment to verify the impact of the active TPS is derived using the nominal mission of the rocket. Sensitivity analysis resulted in thin outer shell and a insulation sphere filled with the coolant. The outer shell needs to be as thin as possible in order to maximize the cooling effect. For an outer shell of 0.5 millimetres, the maximum allowable thermal load can be doubled in comparison with an non-cooled flight. If an identical thermal loading is assumed for two return vehicles, the active cooled vehicle can be designed with a nose radius of around 30 % smaller that the non-cooled vehicle. This will result in a vehicle with a higher lift-over-drag ratio, extending the return trajectory which increases the safety of the re-entry flight.

The validity of performing an enhanced radiation cooling experiment is shown by the use of the tools developed in this thesis. The incoming heat flux can be doubled while maintaining the same wall temperatures if the cooling system is applied. This opens the possibility of redesigning re-entry vehicles to achieve a high lift-to-drag ratio.

Curved exhaust ducts are used in aero engine applications for different purposes, including thrust vectoring, shielding of parts from the exhaust or improving stealth properties. Their integration, however, regularly represents a design problem due to flow separation and high aerodynamic losses occurring in the bend. Curved duct flows for both compressible and incompressible conditions have been studied extensively in the past. However, no experimental results of a high Reynolds number flow through a turbine exit guide vane (EGV) followed by a curved duct have yet been published. An in-depth CFD analysis of the aerodynamic effects is therefore carried out, using the RANS solvers TRACE (at MTU Aero Engines) and SU2, to analyze the flow field and geometric sensitivities of a high Reynolds number flow through an EGV followed by a 90 degree bent duct. The geometry of interest is investigated at a Reynolds number of Re=10^6 and a ratio of bend radius to duct diameter of one. The inflow conditions are prescribed to closely resemble typical flow conditions at the low pressure turbine exit plane of a turbojet engine. After validating the solver with experimental data using the test case of a 90 degree bent duct, an initial CFD analysis of the combined exit guide vane geometry with curved duct is carried out to identify the dominant flow phenomena and mutual effects of the EGV and the bend. Three zones of flow separations are found, each at the convex and concave sides of the bend and one at the lower side of the plug. Secondary flows caused by the bend are found to have an effect on the flow upstream of the EGV, leading to a non-uniform flow inlet angle and aerodynamic loading of the individual blades. Separation downstream of the EGV is influenced by the presence of low velocity wakes from the EGV. Subsequently, a sensitivity study is carried out to find the effect of different geometrical parameters on the flow field. Main investigated parameters are the ratio of bend radius to diameter (R_c/D) and the distance between EGV and bend (l/D). Additionally, the aerodynamic effects of the circumferential EGV positioning, swirl and the plug shape are investigated. It is found that an increase in both bend radius and distance between bend and EGV improves aerodynamic efficiency, while swirl can decrease pressure losses in the duct for small bend radii of R_c/D<0.8. Improving the plug shape and rotating the EGV allows to further increase the aerodynamic efficiency without weight increase. For further optimization of the geometry, it is recommended to include duct geometries with a non-constant bend radius and outer duct diameter to increase the design space.