The aviation sector faces growing pressure to reduce greenhouse gas emissions, while projections are suggesting that emissions could double by 2050 to account for increases in passengers. Liquid hydrogen fuel cell-electric (LH2FCE) propulsion offers a promising solution, with the potential to reduce climate impact by up to 90\% compared to conventional turboprop engines. However, significant challenges exist, including heat dissipation, increased drag, and increased weight penalties from heavy fuel cell stacks, cryogenic hydrogen storage and thermal management systems.
This thesis evaluates the impact on payload and range of retrofitting a regional turboprop aircraft with an LH2FCE propulsion system, by performing the preliminary design of the balance-of-plant systems, and assessing system performance by means of steady state analyses in take-off, top-of-climb and cruise conditions. A lumped parameter model was developed to simulate the fuel cell and balance-of-plant components, including an air supply and thermal management system and ram air ducts. Results show that payload reductions of approximately 58-77\% are expected for a 1500 km range compared to current turboprop aircraft, primarily due to increased mass and drag penalties, which reduce the propulsion system's specific power and lift-to-drag ratio. Sensitivity analyses were conducted, highlighting the effects of fuel cell operational parameters. It was revealed that adjustments in fuel cell temperature, pressure, and current density in the different operating conditions can enhance system performance. Additionally, it was found that most systems must be sized for top-of-climb, except for the ram air duct, which is constrained by take-off conditions. Incorporating a variable inlet design may eliminate ram air drag in cruise, although detailed drag analysis is recommended. Integration of a turbine and halving the rate-of-climb demonstrated that achieving a 1500 km range with a competitive payload (>3000 kg, 35 passengers) is feasible for retrofits, while reserving mass for non-modeled systems such as batteries. Moreover, projections for 2030 suggest that the performance of current turboprop aircraft could be matched by LH2FCE systems.
These findings highlight that LH2FCE propulsion is a viable and sustainable alternative for regional aviation, provided the reduction in payload is acceptable. With advancements in fuel cells, heat exchangers, electric motors, and liquid hydrogen storage, LH2FCE aircraft could achieve performance of current kerosene-powered turboprop aircraft by 2030.

This research addresses the challenge of improving aerodynamic performance by delaying flow separation using deployable vortex generators (VGs) actuated by shape memory alloys. These VGs combine passive and active elements to enhance mixing within the boundary layer, restoring momentum near the wall and minimizing separation in an Adverse Pressure Gradient (APG). A literature review identified a gap in understanding the dynamics of shape-adaptive VGs. Through Computational Fluid Dynamics (CFD) and experimental studies, the research optimized VG designs for aerodynamic efficiency. Results showed that counter-rotating vanes were most effective in controlling flow, while side load and yaw moment challenges necessitated robust actuation mechanisms. A novel design using rigid vane actuators and shape memory alloys for precise adjustments was proposed to address these issues. This study significantly advances the understanding and application of shape-adaptive VGs in flow control.

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.

Supersonic flows cause thin panels to flutter, which is characterized by high-amplitude self-sustained oscillations, increasing the risk of fatigue failure. Flutter is known to be exacerbated when a shock wave impinges on the panel, creating a shock wave/boundary-layer interaction (SWBLI) which promotes separation of flow and leads to increased aerodynamic and thermal loading on the panel. This novel fluid-structure interaction (FSI), known as shock-induced panel flutter, poses a risk to the structural integrity of components used in high-speed aerial vehicles, such as rocket nozzles and supersonic engine inlets.

Due to the complex coupled interaction between flow and structural dynamics in flutter, experiments provide a better option to investigate the underlying physical mechanisms, rather than computational studies, which usually have to compromise between computational cost and resultant accuracy. The ST-15 supersonic wind tunnel facility at TU Delft is employed to conduct experimental measurements of shock-induced panel flutter, using high-speed Schlieren imaging to capture flow structures and stereographic Digital Image Correlation (DIC) to record panel displacements in separate campaigns. Tests are done at Mach 2 with fully-clamped thin panels, and the flutter response is excited using different shock strengths and impingement locations.

In both campaigns, accelerometers are used to measure spurious vibrations around the wind tunnel test section. This helps reveal the existence of external vibrations inherent to the facility, which are also found to drive the frequency of the panel flutter: at 760-770 Hz without an impinging shock, and 605-640 Hz with an impinging shock. The latter frequency is detected in both - shock motion and panel displacements – which establishes coupling between flow and structure despite measurements being non-simultaneous. Changing the shock impingement location does not have a significant effect on the degree of flow separation caused over the thin fluttering panel, which is always higher than the separation on a rigid plate at the same shock strength, thus proving that fluttering panels are not viable means of shock-induced separation control. A stronger impinging shock produces increased shock-induced flow separation but results in less energetic panel flutter, which is attributed to the higher post-shock pressure rise suppressing the panel. Flutter is found to be most energetic, and consequently, the panel is most susceptible to fatigue failure, when the shock impinges at 60% of the panel length.

The turbulent boundary layer development under the influence of an air cavity is studied experimentally using planar PIV, with the aim of gaining insight and building upon the flow physics typically encountered in the application of air layer drag reduction. A detection technique based on correlation values is implemented to obtain an approximate shape of the air cavity and the location of the air-water interface. The technique was successful in identifying the maximum cavity thickness with sufficient accuracy. The leading and trailing edges of the cavity however, were harder to identify, the former owing to a limitation of the developed technique and the latter due to the dynamic nature of the flow and a slightly limited FOV. The ratio of the initial boundary layer thickness to the maximum thickness of the air cavity is 6.7, and as a consequence the boundary layer did not separate at the leeward side of the air cavity. The turbulent boundary layer is observed to feel the presence of the air cavity up to 8.5-9.5 cm upstream due to an adverse pressure gradient. Alternating streamwise pressure gradients are generated due to the curvature of the air cavity: from an adverse to favourable and back to adverse. Compared to solid bump studies in literature, additional perturbations due to a free-slip boundary condition and the unstable nature of air cavity increase the complexity of the current flow. The mean velocity profile and stresses are able to capture the effects of alternating streamwise pressure gradients and air injection, with variations mostly restricted to the inner region. Effects of streamline curvature in the outer region are found to be minimal, while potential effects of the free-slip condition were much harder to identify separately and further research would be needed to appropriately assess them. The mean velocity profile is found to deviate from the classic logarithmic behaviour at the apex of the air cavity, although the flow does not seem to relaminarise. Quadrant analysis shows differences in Reynolds stress producing events compared to the baseline turbulent boundary layer case hinting at possible alteration to coherent structuring of the turbulent boundary layer developing below the air cavity.

This research studies the fluid-structure interaction (FSI) of compliant surfaces in air flows, with an objective of finding possible turbulent viscous drag reduction. A compliant surface is a thin layer of viscoelastic material drawing inspiration from dolphin epidermis, which was thought to have drag-reducing capabilities by Gray (1936). Research into the drag reduction capabilities of compliant surfaces has been long going for more than five decades, starting with Kramer (1960), yet no firm conclusion has been reached. In addition, most of the experimental research has been focused on water flows, with air flows regarded as incapable of inertially forcing a compliant surface to deform. This research attempts to disprove this assumption by applying the proper inertial scaling to the FSI between the compliant surface and air flows. Compliant surfaces are characterised by the stiffness and thickness, which are presumed to be of first-order influence on the FSI. A parameter sweep of these compliant surface properties was conducted by drag delta measurements and flow visualisation by planar particle image velocimetry (PIV) in the M-Tunnel at the Low Speed Laboratory (LSL) Drag delta results confirm the possibility of turbulent viscous drag reduction by compliant surfaces, with a measured drag delta of -3.43 %. This result is further supported by the decrease in 1D turbulence intensity at the test plate's trailing edge from hot-wire measurements, and a smaller decay of the shape factor H from PIV. Quadrant analysis of the PIV data found evidence of a reduction in combined Q2 and Q4 events, further supporting the drag delta measured. Correlation between the compliant surface's viscoelastic properties and the drag delta found a negative correlation between the magnitude of complex shear modulus and a positive correlation between the loss tangent and drag reduction. The high loss tangent for the drag-reducing compliant surface and its subtle positive correlation with the drag reduction indicates that the viscoelasticity might have a greater influence on the FSI than expected.

Over the last few decades, the bypass ratio and nacelle diameter of modern turbofans have been increasing to achieve higher efficiencies. This trend has forced the underwing-mounted engines to be coupled more closely to the wing surface and the flap system, which significantly enhances the low-frequency noise, known as jet installation noise (JIN).

In this project, lobed nozzles are proposed and tested as a passive flow control technique for the reduction of the JIN. Measurements were performed using hot-wire anemometry and a linear array of microphones to investigate the effects of lobed nozzles on the near-field structure of a jet. The lobed nozzles were found to alter the convection velocities of the pressure fluctuations and the transition to turbulence of the jet. Moreover, far-field pressure measurements were performed inside the anechoic wind tunnel to examine the effects of the near-field structure on the perceived far-field noise. These findings can be used to assess the effectiveness of lobed nozzles for the reduction of the JIN in specific jet installation configurations.