Shock-wave/turbulent boundary-layer interaction (STBLI) is a ubiquitous phenomenon in high-speed aerodynamic applications, such as rocket nozzles, engine inlets, airplane wings and control surfaces. The interaction between a shock wave and a turbulent boundary layer often leads to flow separation, severe wall-pressure fluctuations, and unsteady thermal loads, which can significantly degrade aerodynamic performance and structural integrity. Mitigating these adverse effects remains a long-standing challenge in compressible flow research. While numerous active and passive control strategies have been proposed, many of them suffer from practical limitations, including high energy consumption, geometric complexity, or strong sensitivity to installation location.

This dissertation investigates the potential of spanwise-heterogeneous roughness as a purely passive control strategy for STBLI, with a particular focus on convergent–divergent riblets and streamwise-homogeneous ridge-type roughness. Using wall-resolved large eddy simulations combined with an immersed boundary method, a Mach 2.0 impinging shock-wave/turbulent boundary-layer interaction is systematically studied over smooth and rough walls. The numerical framework enables a detailed analysis of both the mean flow and unsteady characteristics of the interaction, as well as the underlying physical mechanisms governing roughness-induced flow modification.

The first part of the study examines the control effects of convergent-divergent riblet patches. It is shown that the riblets induce organized secondary flows in the form of counter-rotating streamwise vortices, which significantly modify the incoming turbulent boundary layer prior to the interaction. These secondary flows lead to a spanwise redistribution of momentum, resulting in a corrugated separation topology and an attenuation of wall-pressure fluctuations near the separation shock foot, while simultaneously causing an upstream shift of the interaction onset and an enlargement of the interaction and separation regions. Owing to the localized nature of the induced vortices, whose influence is expected to decay in the streamwise direction, the overall control authority remains inherently limited, while an additional pressure-drag penalty is inevitably introduced.

Motivated by these limitations, the second part of the dissertation focuses on streamwise-homogeneous ridge-type roughness, which offers greater robustness and reduced sensitivity to installation location. The results demonstrate that ridge-type roughness induces Prandtl’s secondary flows of the second kind, leading to systematic modifications of the upstream turbulent boundary layer. When the ridge spacing is comparable to the boundary-layer thickness, strong downwash motions locally energize the turbulent boundary layer, thereby suppressing flow separation while simultaneously increasing wall-pressure fluctuations. For smaller ridge spacings, a pronounced subsonic region forms within the incoming boundary layer, resulting in a less-full velocity profile. This modification weakens the streamwise wall-pressure gradient and smears the separation shock foot, leading to a substantial attenuation of wall-pressure fluctuations over a broad frequency range, albeit at the cost of an enlarged separation region. Parametric studies further reveal that increasing the ridge height amplifies the attenuation of wall-pressure fluctuations by enhancing the roughness-induced modification of the upstream boundary layer.

Finally, the influence of Reynolds number on the control performance is examined. The results show that wall-pressure fluctuations near the separation shock foot comprise a low-frequency component associated with shock motion and a high-frequency component associated with shear-layer turbulence, with their relative contributions strongly dependent on the Reynolds number. At low Reynolds numbers, the high-frequency component dominates, whereas at higher Reynolds numbers the low-frequency component becomes prevalent. In this high-Reynolds-number regime, where low-frequency shock unsteadiness governs the interaction dynamics, ridge-type roughness remains effective and yields an even stronger attenuation, with peak wall-pressure fluctuations reduced by up to 27%. Spectral analysis and cross-correlation studies support a downstream-influence mechanism for the low-frequency unsteadiness, while dynamic mode decomposition reveals the presence of large-scale Görtler-like vortices downstream of the interaction region.

Overall, this dissertation demonstrates that spanwise-heterogeneous roughness, if properly designed, can serve as a robust and practical passive control strategy for mitigating STBLI unsteadiness in high-speed flows, albeit at the cost of a moderate increase in skin-friction drag. The findings provide new physical insights into the interplay between roughness-induced secondary flows, Reynolds number effects, and low-frequency STBLI dynamics, and offer guidance for the design of roughness-based flow control concepts in future high-speed aerodynamic applications.
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This dissertation studies the aerodynamic behavior of turbulent flow over acoustic liners— permeable surfaces installed inside aircraft engine nacelles to reduce noise. While these liners are highly effective at attenuating sound, they are also known to increase drag. Most prior research has focused on their acoustic performance, often simplifying or overlooking their aerodynamic impact. This work shifts that focus, using fully resolved, high-fidelity direct numerical simulations (DNS) to study flow over realistic liner geometries. Unlike many earlier studies that rely on simplifying assumptions such as impedance boundary models, this study avoids those simplifications by directly resolving the geometry of the acoustic liners.

The study explores key questions: which geometric features of acoustic liners most influence their aerodynamic behavior, how do these surfaces compare to traditional rough walls surfaces, and what additional effects are introduced by acoustic excitation. Although acoustic liners are flush with the surface and lack protrusions, we find that they still behave like canonical rough surfaces due to their permeability. The aerodynamic impact is governed by the non-linear Forchheimer permeability—a parameter that we show is closely linked to strong wall-normal velocity fluctuations in the near-wall region. These fluctuations are the primary driver of the drag penalty: the higher the wall-normal velocity fluctutations, the higher is the drag penalty compared to the reference smooth wall case. Importantly, the findings show that by limiting these wall-normal motions through geometric modifications—such as tapered orifices, or alternative shapes like elliptical orifices—it is possible to reduce drag. Tapered holes in particular show potential, as they decrease permeability without significantly affecting sound absorption. More aggressive changes, like parallel slots, tend to degrade acoustic performance, highlighting a necessary trade-off. However, certain designs, such as perpendicular slots, appear to offer a favorable balance.

Using the first fully resolved spatially developing turbulent boundary layer simulation over an acoustic liner array, this dissertation further shows that, for the conditions studied, acoustic excitation—modeled as a planar upstream-propagating monochromatic wave—does not significantly affect aerodynamic behavior. However, this does not rule out more complex interactions under realistic engine conditions, where acoustic fields are broadband and multidirectional. Limitations in the numerical setup, particularly in acoustic modeling, mean that the full impact of sound waves remains an open question.

The work also touches on broadband acoustic liner geometries, which are becoming increasingly relevant. These designs are more permeable—not just in the wall-normal direction—but across multiple directions. Higher permeability typically correlates with higher drag, and this trend holds for acoustic liners as well. Still, the study shows that with careful design, broadband liners can be engineered to avoid additional drag penalties, achieving comparable aerodynamic performance to conventional designs.

In summary, this dissertation offers a detailed aerodynamic analysis of flow over acoustic liners, explaining the mechanisms behind drag increase and establishing the central role of permeability. It shows that aerodynamic optimization is possible without compromising acoustic effectiveness and highlights the need for fully resolved simulations when studying such complex surfaces. The findings lay the groundwork for the design of next-generation acoustic liners that better balance noise control and aerodynamic efficiency.
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We address a fundamental challenge in modern propulsion and energy systems: accurately modeling combustion under transcritical conditions, where operating pressures exceed the critical pressure of the fuel but still lower than cricondenbar values of the air-fuel mixture, leading to a complicated thermophysical mixture behavior. Transcritical regimes are characterized by strong non-idealities and the possible coexistence of vapor and liquid phases. The main objective of this work is to develop a high-fidelity, physically consistent, and computationally efficient simulation framework that captures these phenomena within a Large Eddy Simulation (LES) context, enabled by a Multiphase Thermodynamics (MT) approach. ... Read more

Hypersonic flows regard a special class of flows where dire conditions for spacecraft may be found. Simulation of these flows requires taking chemistry and thermal nonequilibrium into account as to acquire the correct wall quantities for the vehicles. Flux reconstruction -- a higher order method -- enables fluid simulations with a coarser grid than finite volume, as well as with easier shock discretisation for the practitioner. Computational Object-Oriented Libraries for Fluid Dynamics (COOLFluiD) allows the simulation of hypersonic flows with flux reconstruction, but not yet with thermochemical nonequilibrium due to positivity issues encountered. A survey of the literature for positivity methods in higher order methods, together with a presentation of flux reconstruction, the chemical model used and COOLFluiD were given. An entropy based exponential filter and a Fejér filter were implemented in COOLFluiD based on a pre-existing framework of filter based positivity and tested against the previous positivity method in place. The exponential filter was found to be equivalent for a 2D simulation with the Euler equations to the previous method while the Féjer filter behaved in a more destabilising manner. The entropy based exponential filter was implemented for cases with multiple species and multiple temperatures, with modularity for species sets. The derivation of entropy for this case was given. A new positivity method based on least squares optimisation was developed to target density positivity issues where the filter fails. The case was tested against two cases from literature in P0, ensuring positivity and validated against literature. The cases were found not to be yet fully spatially resolved, as expected in P0 simulations. P1 simulations were tested for the same cases and found to have issues with the time integration method and the artificial viscosity. An inspection of the code base was executed, revealing that the artificial viscosity was not fully implemented for the cases tested and that the current base implementation of backwards Euler cannot be used for these cases, requiring being solved through the already implemented Newton iterations method. The artificial viscosity was fully implemented for the tested cases. The P1 simulations were retested. The artificial viscosity was found to be active and to diffuse the shock wave. No positivity failures of the method were found. A stringent CFL condition was found to exist, and possibly linked to the stiffness of the equations. The exponential filter was found to lead to a growth of the temperatures just upstream of the shock wave, where the filter is active. This behaviour was linked to a possible interaction with the highly nonlinear model or with the artificial viscosity. While positivity was found to work, further issues in testing it are still found and expected to be part of future work. Possible extensions or alterations to the code were found to possibly be required in order to allow further testing. Thus, positivity was found not to be the only issue with running thermochemical nonequilibrium simulations, being only part of the problem, requiring further research in order to fully test it in the context of COOLFluiD. Future work was defined to focus on studying the exponential filter interactions, on enabling future tests, on expanding on these testes based on presented literature, and on expanding on the model used to include more robust but complicated methods. ... Read more

Rotating Detonation Engines (RDEs) are a type of pressure-gain combustion system based on detonation waves traveling around a cylindrical combustion chamber igniting the fresh gases. Compared to classical combustors, detonative combustion offers an increment in thermodynamic efficiency of the engine due to rapid heat release and lower entropy rise. The development of this technology could bring more compact and efficient combustors with applications to energy generation, aviation, and rocket propulsion.
The objective of the present work is to develop a robust set up to simulate an RDE employing the DLR TAU code to obtain physical solutions to investigate the flow field within the engine and its performance. The impact of different modeling decisions and their influence on the flow physics shall be addressed.
First, a set of 1D shock tube simulations have been conducted to evaluate the best solver parameters to capture detonation dynamics. Later, results of 2D simulations based on a test case from literature were performed and the modeling decisions were re-evaluated for this more realistic case. Lastly, two different 3D simulations have been performed and compared with the respective experimental results.
The results showed that a resolution of 200 microns was enough in 2D simulations to capture the main flow features. Moreover, the chosen chemical reaction mechanism was from Ó Conaire et al. 2004, and the upwind flux that performed the best was the AUSMDV (Wada et al. 1994) solver. Moreover, the time step employed was of the order of ten to the power of minus eight seconds. Different inlet boundary conditions were studied, finding the Dirichlet type more suitable to uncouple injection and detonation dynamics. In addition, different ignition strategies were evaluated, proving that the strategies were successful and achieved a stable mode of operation.
This work presents a robust set up to perform 2D and 3D RDE simulations employing the DLR TAU code. It also provides many insights into the impact of different modeling decisions on the flow field and evolution of the engine performance.
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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. ... Read more

In distributed electric propulsion (DEP) arrays, complex flow phenomena and strong interaction between the internal flow of the engines and the external aerodynamics demands the use of advanced numerical methods. Separated and unsteady flow during transition and high angle of attack flight, as well as off-design conditions such as engine failures and wind gusts, make scale-resolving methods like Large Eddy Simulation (LES) the most suitable modelling approach. A GPU-accelerated wall-modelled LES (WMLES) solver was successfully employed for eVTOL aerodynamic analysis. Previous work focused on external aerodynamic phenomena, such as transition from hover to cruise flight and hover in ground effect, using a volume source term to model the engines. However, to accurately characterise safety-critical flight conditions, such as the effect of engine failures on adjacent engines or the performance penalty caused by steady wind or gusts during hover, the whole aircraft and multiple engines with resolved turbomachinery components need to be simulated. Since numerous flight conditions need to be analysed with high accuracy to characterise the aerodynamics of an aircraft with DEP, it is key to maximise the computational performance of the solver. In the present work, the main bottlenecks of the solver were identified and removed, which resulted in improvements in the dynamic load balancing, voxelization and the update of the signed distance field. A detailed comparison of the computational performance on four simulation setups shows up to 86% runtime reduction. An eVTOL aircraft is simulated in two off-design tailwind conditions, for which the flowfield and engine performance is analysed. Finally, a comparison with wind tunnel measurements is presented. Results confirm that GPU-accelerated WMLES is a suitable approach to simulate DEP arrays both regarding accuracy and computational cost. ... Read more

Design an affordable eVTOL ambulance that provides emergency medical assistance for small and medium communities in Central Europe and is a scalable competitor to emergency helicopters, and a substitute for ground-based ambulances. ... Read more

Aerodynamics significantly influences race car performance, with wheels contributing 35–50% of total drag and affecting underbody downforce. This study investigates the flow around a rotating motorsport slick wheel using CFD to evaluate various turbulence models, validated against wind tunnel measurements. Simulations are conducted in a wind tunnel configuration using RANS, URANS, and DDES models in OpenFOAM. Results show that k-w SST outperforms EB Lag k-e in predicting near-wake features. In addition, DDES offers the highest accuracy overall compared to RANS and URANS, especially in capturing flow separation. Indeed, a trade-off must be made between solution accuracy and computational cost. Three main effects are analyzed in RANS. First, the wind tunnel rig induces wake asymmetry, increasing both lift and drag. Second, increasing Reynolds number delays flow separation from the wheel top, raising lift and reducing drag. Third, yaw angle alters wake symmetry and vortex strength, increasing drag and causing non-monotonic changes in lift. ... Read more

Explicit algebraic subgrid-scale models (EASSMs) have shown better performance compared to eddy viscosity models (EVMs) due to the improved tensorial alignment to the subgrid-scale (SGS) stress tensor. Under the EASSM framework, the dissipation term requires closure. A number of models exist but some are unsuitable to be applied under the EASSM framework, due to their non-linearity in the SGS anisotropy tensor. The isotropic dissipation model is commonly applied model but studies have contradicted the isotropic assumption. We propose to discover a closure model suitable to be implemented under the EASSM framework using gene expression programming (GEP) as a symbolic regression tool. Techniques such as wall scaling constraints and numerical constant optimization from other works are also incorporated in the present work. In addition, we attempt to mix training data from homogeneous isotropic turbulence (HIT) and turbulent channel flow (TCF) within the training process in order to obtain a more robust model. Discovery of a scalar dissipation model is an intermediate step towards a dissipation tensor model, a priori evaluation shows better performance relative to a reference model. Validation of the scalar model was performed on a channel flow case which also showed improvements. Tensor model discovery produced a few models which provide better tensorial alignment only in selected cases relative to there being no model at all. A critical review of the present work recommended the investigation of implementing an alternate timescale for non-dimensionalization and a study on the dissipation effects that are contained within the pressure-strain model implemented in the framework. ... Read more

This thesis presents a theoretical and numerical investigation of how a surface forward-facing step alters the stability and transition mechanisms of a laminar incompressible swept-wing boundary layer dominated by stationary crossflow instabilities (CFI). The results elucidate the processes that cause significant transition advancement due to the step and, in turn, challenge the classic paradigm in fluid mechanics that rapid, localised surface-geometry variations are universally detrimental to laminar flow. Through Direct Numerical Simulations (DNS) and new theoretical modelling, this thesis shows that, under specific conditions, a forwardfacing step stabilises pre-existing stationary crossflow vortices and thereby delays laminar-turbulent transition... ... Read more

Diesel engines find application in a large number of sectors of modern industries, such as the automotive and maritime. The wide adoption of Diesel engines, along with the rise of environmental concerns created the needs for optimization of fuel efficiency and power output as well as minimization of exhaust gas emissions. These targets must be met while retaining the reliability of Diesel engines. One technique that is used to significantly enhance the reliability of Diesel engines is piston cooling. The working fluid is engine oil which is injected in the cylinder towards the lower side of the piston. The objective of the current study is to simulate the process of cooling with the aid of CFD. The numerical methodologies that are used in the current study are the Reynolds-Averaged Navier-Stokes (RANS) equations for the continuous phase, while for the disperse phase the modelling is conducted with the Lagrangian modelling. The numerical model which is built focuses on the effect that the mesh resolution has on the results of the simulation, especially on the disperse phase of the flow. It can be inferred that there is dependence between the quality of the results and the mesh resolution. It is shown that the grid has to be fine enough to produce quality results, but if there is excessive refinement, the quality of results drops because assumptions of the Lagrangian modelling are not satisfied. Following this phase of the study, a CFD model is built to simulate the phenomenon of droplet impingement on a solid wall. In this set of simulations, the goal is to compare the impingement model that is used in the software with published experimental results of droplet impingement. The results indicate agreement between the CFD model and the experimental process. In the final stage of the thesis, there is the numerical study of heat transfer involved in the process of spray impingement on a hot wall. A CFD model is built to compare the heat transfer which is predicted by the software model and results from relevant published experimental studies. It can be concluded that there is divergence between CFD and published experimental results, stemming from the modelling assumptions of the software. The results of the present thesis can be used in future studies of the company, either as a continuation of the spray cooling topic, or for novel studies such as gallery cooling. ... Read more

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. ... Read more

Nature has been optimizing for millions of years the aerodynamics of dragonflies. The main goal of the present thesis is to understand these mechanisms so that they can be further applied in future bio-inspired designs.

To start with, the experimental and numerical methodology is described (chapter 3). Next, a single flapping wing is studied under different conditions (chapter 4). Reynolds number, angle of attack, wing shape and corrugation effects are characterized. It is shown that dragonflies leading
edge vortex is responsible for a great amount of lift production. Leading edge vortex circulation increases with Reynolds number, and so does lift. However, drag is also found to be a crucial contributor to the force balance that sustains dragonflies hovering. Additionally, corrugation effects improve aerodynamic efficiency in the studied flow regime.

Finally, wing-wing interaction effects are studied numerically in a whole dragonfly (chapter 5). It is illustrated that phase changes between hindwings and forewings can maximize force production or be tuned to achieve a more stable and efficient hovering. However, not all phases are appropriate for maximum efficiency. Phase has to be tuned to maximize wake-capturing mechanisms and therefore flying efficiency. Finally, vorticity removal mechanisms are depicted to maintain a clean and uniform background flow that optimizes hovering efficiency. ... Read more

Many wind turbines experience leading-edge erosion on their blades due to rain and hail impacting at speeds of up to 100 m/s. The impact speed is driven predominantly by the blade tip-speed, which is expected to grow in future turbine generations as they become larger. Erosion can remove substantial amounts of material from the blades. Eventually, the damage can reach deep into the structural layers of the blade, where it then starts to jeopardize its structural integrity. The associated roughening of the blade is accompanied by losses in the annual energy production (AEP). These are estimated to be up to several percent, depending on the severity of the erosion damage. While some leading-edge protection systems have been developed, no satisfying solution has been found, and the mechanisms that lead to erosion have yet to be fully understood. The aim of this thesis is to enhance the understanding of the physical mechanisms that promote erosion, understand which site conditions contribute to erosion and apply the gained insight in the erosion-safe mode.

This thesis starts in Chapter 2 by analyzing the impact of erosion on the AEP loss by using reduced-order modeling and subsequently compares it with the erosion-safe mode (ESM). The ESM is an alternative operational erosion mitigation strategy that aims to mitigate erosion by reducing the tip-speed of the turbine during precipitation events. It is shown that, depending on the mean wind speed and frequency of damaging rain at the site, the erosion-safe mode can lead to a lower AEP loss in comparison to a mildly eroded blade or a blade that was fitted with a leading-edge protection solution that leads to similar flow disturbance. However, it still needs to be sufficiently understood what rain is damaging and what other site conditions might promote erosion.

A step toward resolving this knowledge gap is taken in Chapter 3 by investigating the behavior of rain droplets before impact with the blade. Contrary to prior state-of-the-art, it is shown that droplets deform and break up near an incoming wind turbine blade. This finding contradicts the current approach in erosion research of modeling rain droplets as circular. It is shown that deformation reduces the impact velocity of rain droplets with the blade. This effect depends on the diameter of the rain droplets and can be in the order of 10 m/s. Small droplets experience significantly more slowdown than larger rain droplets. This reduction highly influences the formation of erosion damage since the main driver for erosion is the impact velocity. As droplet deformation and slowdown depend on the rain droplet diameter, the described effect can be termed drop-size-dependent effect.

Chapter 4 continues the investigation of drop-size-dependent effects in leading-edge erosion. An advanced erosion damage model is built that includes several drop-size-dependent effects. It is shown that the significant drop-size-effects all suggest that the erosiveness of rain droplets increases with increasing droplet diameter. This is found to be true on a per-drop basis but also when normalizing for droplet size. Therefore, selecting an appropriate droplet diameter for experiments and numerical studies is essential since not all droplet diameters contribute equally toward forming erosion damage. Drop-size effects have substantial implications for the ESM, as increasing rain intensities shift the composition of precipitation from primarily small droplets to a composition dominated by larger ones. For an equal rain column, high-intensity precipitation events are, hence, more erosive. It is found that, for a coastal site in the Netherlands, 50 % of the erosion damage is produced by the 10 % highest-rain intensity events. Thus, in ESM operation, it is advantageous to reduce the tip-speed mainly during high-intensity precipitation events to maximize lifetime and minimize AEP loss. However, a precise relation between precipitation intensity and tip-speed that optimizes this objective is not yet known in leading-edge erosion research. A novel semi-analytical approach is devised to bridge this gap, taking into account site conditions, turbine type, and drop-size effects. With this approach, it is possible to extend the erosion lifetime of a contemporary blade by a factor of 13 for a moderate AEP loss of 1 %.

A critical component for the successful utilization of the ESM is the accurate forecasting of precipitation events minutes to hours ahead. However, the best approach for obtaining this information is still debated. For the first time, Chapter 5 benchmarks a state-of-the-art weather-radar-based probabilistic rainfall nowcast product by the Royal Netherlands Meteorological Institute (KNMI). The performance of the nowcast is assessed for various lead times for three sample sites in the Netherlands and for two distinct ESM strategies. The results show that the quality of the nowcast degrades with increasing lead times. The 5- and 15-minute lead times exhibit sufficiently good accuracy and response time for the successful utilization of the ESM. Across the sites, for a large 15 MW turbine, a lifetime extension of factor five can be achieved for an AEP loss of about 1 %.

To summarize, this thesis introduced the highly significant effect of droplet slowdown and deformation occurring in the vicinity of wind turbine blades. It investigated drop-size-dependent effects and established their significance for ESM operation. It provided new theoretical insights into the ESM and used these to devise a method for finding optimal ESM strategies that exploit drop-size effects. Finally, it benchmarked the devised strategies using a state-of-the-art (operational) nowcasting product and showed that the ESM could already be a viable erosion-mitigation strategy. ... Read more

This thesis, divided into two parts, explores shock interaction phenomena occurring in high-speed platforms. part one focuses on shock-shock interactions (ssis), which manifest as a bi-stable system with either the regular interaction (ri) or the Mach interaction (mi) as outcomes. Through numerical simulations, both interaction types are examined in the presence of perturbations to provide insights into their stability characteristics and the riÕmi transition process, which exhibits hysteresis effects. Subsequently, attempts are made to replicate these hysteresis effects in the transonic–supersonic blow-down wind tunnel (tst-27) at tu Delft by continuously varying the free-stream Mach number during a run. The evolving shock system is continually tracked using a systematic post-processing methodology that integrates schlieren visualizations, synchronous pressure readings, and insights gained from a variable focal plane study conducted with a focusing schlieren system... ... Read more

Wall-modeled large eddy simulations (WMLES) are becoming an increasingly viable tool to study complex unsteady turbulent flows. Conventional wall models applied in these simulations are however not applicable to laminar boundary layers. While these encompass only a tiny fraction of the total surface area, erroneous predictions in this region of the flow can greatly impact the downstream flow field. In the present study, a new wall model is proposed by combining the laminar wall model and turbulent wall model with the use of a transition model marking the laminar and turbulent regions. The proposed wall model is applied to the laminar flat plate, wedge and laminar NACA 0012 flow. Results show that errors incurred at the unresolved leading edge, where the similarity solution used by the laminar wall model is invalid, accumulate in the velocity profiles for the flat plate and wedge flow cases. In underresolved regions near the leading of the NACA 0012 or near the tip of the wedge, good approximations to reference data have been found. The proposed wall model is also applied to a high Reynolds number flow involving an airfoil near stall. The proposed wall model shows promising results with good agreement for the skin friction distribution, especially in capturing the laminar skin friction peak if the transition location is known. However, the transition sensor considered for switching between the laminar and turbulent mode of the wall-stress model performs unsatisfactory. Other discrepancies in the results, such as capturing the laminar separation bubble and trailing edge separation are attributed to the relatively coarse meshes used. Last but not least, the computational cost incurred by the new wall model is marginal. ... Read more

Hydrofoils can under certain circumstances cause a phase change from liquid water to water vapor. This phenomenon is called cavitation and is caused by the low pressure over the hydrofoil when the vessel exceeds a certain velocity though the water. Cavitation can take different forms depending the angle of attack α and the flow conditions which are characterized with the cavitation number σ . The different types of cavitation can have different effects on flow and loads.
Computational Fluid Dynamics (CFD) is widely used in aero and hydrodynamic design, with (U)RANS most commonly used for CFD in the industry due to its relatively low computational cost while providing sufficiently accurate results. Cavitation models in URANS simulations need a multiphase framework in order to model the liquid/vapor interface of the cavitation bubbles. The Schnerr-Sauer cavitation model uses simplified bubble dynamics equations for relatively fast calculation while providing accurate results. In current project, a Volume of Fluid method is used the Schnerr-Sauer cavitation model is used with URANS CFD simulations to improve and enhance the behaviour and performance prediction of hydrofoils sections under cavitating conditions. Given the industrial context of this project, the simulations are conducted using constrained computational resources.
Validation is performed for a non-cavitating test case using a NACA-0012 section, followed by validation for a cavitating test case using a modified NACA-66 section. Mesh convergence studies have been performed, turbulence models have been compared and the turbulent viscosity has been modified. The final set-up uses the SST turbulence model with a modified turbulent viscosity exponent n = 2.3.
To assess the flow behavior and hydrofoil section performance under cavitating conditions, a comparison is made in CFD using cavitation models, relative to the current practice. This study shows that the lift and drag results for a simulation without cavitation model are underestimated compared to the simulation with cavitation model in conditions of stable cavitation. For conditions with unstable cavitation, strong unsteady disturbed flow and loads are found that are not captured by the simulation without cavitation model. The transition from stable to unstable cavitation is studied by investigating cavitation bubble length and its
corresponding fluctuation as a function of the stability parameter
ps = σ/2(α−α0) . The conditions found for the transition from stable to unstable cavitation are consistent with reference value at about ps = 4. The inception of stable cavitation is found at about ps = 7 which is considered to be more optimized to delay the formation of cavitation compared to the NACA-0012 at ps = 8.5.
The lift, drag and performance polars are studied for several values of σ . The lift and drag polars for lowerσ , i.e. higher cavitation rate, show a stronger increase in both lift and drag for stable cavitation cases. The performance (or Lift over Drag) is slightly increased at α = 4◦ for σ = 1.2 and 1. For higher angles of attack, the increase in drag surpasses the increase in lift and the performance decreases. These finding only hold the stable cavitation cases (α < 8◦ for all tested σ , and α = 6◦ for σ = 1) since the unstable cavitation results are
inconclusive.
The main limitation of the set-up developed in the current project is that the predictions show significant discrepancies in capturing the unstable bubble shedding characteristics, with respect to the reference data. As a result, the cloud cavitation shedding frequency is not accurately captured, resulting in an inadequate representation of vibrations and loading due to cloud cavitation.

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Maintaining laminar flow on large swept surfaces of subsonic transport aircraft, i.e. the wings and the stabilisers, is currently posing a considerable challenge for aerodynamic design. Improving the efficiency of aircraft by delaying or removing the laminar-to-turbulent transition process over the wing and tail parts can substantially reduce contaminant emissions. The dominant flow instability causing laminar-turbulent transition of swept-wing flow is the so-called crossflow instability (CFI). Ongoing research at TU Delft has shown potential to delay transition by use of passive mechanisms. As such, a framework has been designed to numerically compute crossflow development and transition to turbulence on swept wings. Through the use of experimental data acquired in wind-tunnel measurements at TU Delft, the CFI development and transition process on swept wings has been modelled numerically by means of Direct Numerical Simulation (DNS). Based on a DNS laminar flow field generated from the pressure distribution along the model surface, a numerical primary CFI mode in good agreement with the experiment was obtained through Non-linear Parabolized Stability Equations (NPSE). Following this steady flow field analysis, the simulation was made unsteady by the implementation of numerical free-stream turbulence. This novel method resulted in unprecedented modelling of the receptivity mechanisms of transition in three-dimensional crossflow cases, overcoming ad-hoc treatments. Both experimental and numerical flow fields indicated a Type-I dominant secondary CFI (i.e. KH-type response in the laterally inclined shear layer of the stationary crossflow vortex), which consequently carries the formation of near-wall hairpins and ultimately turbulence. Crossflow vortex frequency content also agrees well in the low-frequency band (450 Hz ≤ f ≤ 3000 Hz), whilst the numerical high-frequency content (3500 Hz ≤ f ≤ 9000 Hz) does show a distinct delay in amplitude growth throughout the majority of the transition region. Contradicting the promising qualitative analysis of the free-stream turbulence methodology, this discrepancy in the frequency spectrum indicates a major shortcoming in the numerical setup, which was shown to be biased towards introducing more low-frequency disturbances at the inflow boundary. ... Read more