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.
... Read more

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.
... 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

With climate change posing increasing risks, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) aims to reduce CO2 emissions by 75% and NOx emissions by 90% per passenger kilometer by 2050, compared to a baseline aircraft from 2000. The ARENA project addresses this by developing a waste heat recovery system using aircraft engine exhaust gases, improving fuel efficiency. This thesis focuses on integrating the condenser of such a system into the propulsion unit, aiming to minimize drag from the ram air cooling duct by evaluating different heat exchanger topologies and duct designs.

The study uses the IMOTHEP Distributed fans Research Aircraft with electric Generators by ONERA (DRAGON) concept, a hybrid electric aircraft with two tail-mounted turbogenerators. The research proceeds in several stages. Initially, a multipass-condenser's potential to reduce pressure drop was examined by adjusting the heat exchanger blockage factor per pass, but results showed no reduction in pressure drop.

Next, a lumped parameter model was developed to analyze drag, pressure drop, temperature increase, and ram air duct length. This model evaluated the sensitivity of duct geometrical parameters on the drag recovery factor—a dimensionless number indicating net thrust. Findings revealed that inclining the heat exchanger efficiently increases the drag recovery factor, while the diffuser area ratio has a similar effect but is less space-efficient. The mass flow rate ratio showed less sensitivity, and fin height or pitch had the smallest effect on drag recovery.

Using the lumped parameter model, an optimal preliminary ram air duct design was identified for different spatial constraints. The study found that inline plain tube bundle and flat tube offset strip fin heat exchangers provided more compact solutions with higher drag recovery factors. For large diffuser-blocked area fractions, optimal duct geometry remained independent of heat exchanger type, characterized by a 70-degree maximum inclination angle, a 0.7 mass flow rate ratio, and maximized diffuser area ratio within spatial constraints.

A verification study of the lumped parameter model was conducted using a two-dimensional Reynolds Averaging Navier Stokes (RANS) computational fluid dynamics (CFD) analysis with k-ω SST turbulence and a porous zone to mimic the heat exchanger. The CFD analysis confirmed the lumped parameter model's predictions, with a 0.8% difference in drag recovery factor for optimal duct geometry. Across various geometries, the mean absolute difference in drag recovery factor between models was 1.082%, with a standard deviation of 0.584%.

In conclusion, integrating the condenser in the propulsion unit within spatial constraints results in positive thrust, thus supporting Meredith's 1935[2] claim. This integration reduces net drag and improves overall efficiency, contributing to significant emission reductions in line with ACARE's targets. ... Read more

This thesis presents an in-depth investigation of the consequences of adding a second facesheet on the aerodynamic performance of acoustic liners. This is done by performing pore-resolved Direct Numerical Simulations (DNS) of a channel flow at Reτ = 500. Simulations are conducted for various dual facesheet configurations, exploring different facesheet layouts and relative positions of the second facesheet. Design considerations are constrained by preserving the attenuation properties of the multiple facesheet liner, achieved by maintaining adequate spacing between the facsheet. The study examines how the presence and staggering of the second facesheet affect the pressure drop and the wall-normal velocity fluctuations, both of which correlate with the added liner drag. The simulations reveal a relationship between the staggering distance of dual facesheet liners and added drag, demonstrating that increased shifting distance of regular facesheet configurations leads to reduced added drag compared to lesser shifting distances. Furthermore, the staggering diminishes wall-normal velocity fluctuations between the facesheets, aligning with observed trends in added drag. However, the study finds that while dual facesheet configurations offer some reduction in added drag compared to conventional single facesheet designs, the extent of drag reduction is limited. The study shows that the addition of a second facesheet to the liner causes additional wall parallel permeabilities not present in a single facesheet liner influencing the flow below the wall. ... Read more

While the assumption of incompressible flow has been the prevailing standard for numerical simulations of wind turbine aerodynamics, the limit of this assumption is approached as the industry progresses towards increasingly higher operational tip speeds. This thesis explores the implications of compressibility and transonic flow phenomena, particularly in relation to a new rotor design that integrates the concept of rotor tilt.

The study is split into two phases. The first phase involves two-dimensional Computational Fluid Dynamics (CFD) simulations, employing an Unsteady Reynolds Averaged Navier-Stokes (URANS) framework, specifically for the thick NACA 63(4)-421 airfoil. Attention is directed at the effects of high subsonic and transonic Mach numbers, and the complex flow phenomena emerging from shock wave-boundary layer interactions (SBLI). The second phase extends the findings from the first phase by simulating a 3MW concept rotor, utilizing a lifting line method. Here, particular emphasis lies on runaway conditions; the operating point of an unloaded rotor where rotational velocity maximizes. Since airfoil drag significantly increases through the onset of “shock-stall”, it might provide a passive mechanism for overspeed protection.

The URANS study provides the first exploration of transonic buffet for a thick non-symmetrical airfoil; the phenomenon characterized by periodic shock movement, leading to large-scale load oscillations. The findings indicate that these fluctuations are significant and could pose a threat to the structural integrity of a full-scale rotor. Additionally, regarding the subcritical compressible regime, the study found that the von Karman-Tsien correction is the most appropriate method for the prediction of airfoil loads prior to reaching the critical boundary.

In the second phase, the rotor is simulated using the averaged values of the compressible aerodynamic coefficients, obtained in the first phase. Two flow scenarios are considered: Storm conditions typical for the North Sea, and subsequently, higher wind speeds often encountered during tropical typhoons in the Western Pacific. The results show that for the first scenario, shock-stall does not prevent the rotor from overspeeding. Instead, using rotor tilt is suggested as a more effective strategy. For the second scenario, a distinctive maximum rotational velocity has been found for an incoming wind speed of $57.5$ m/s, leading to the conclusion that shock-stall could effectively prevent the rotor from overspeeding.

Since the averaged values of the aerodynamic coefficients are used to assess the adequateness of shock-stall as an overspeed protection mechanism, the load fluctuations attributed to transonic buffet are initially neglected. A subsequent preliminary assessment of these fluctuations revealed that operating in the high transonic regime potentially poses severe risks regarding rotor safety. ... Read more

Aerodynamics has been an important aspect of the automotive industry for decades with the wheels being a notable contributing factor. They are responsible for up to 25% of the drag in the case of a general passenger car and up to 30%-50% for an open-wheeled race car.
In this research, the aerodynamic characteristics of an isolated rotating wheel in contact with the ground will be investigated using RANS simulations. Open-source software OpenFOAM is used for the simulations and the mesh is generated using cfMesh. The wheel geometry used in this work is the "Fackrell A2" and the contact region is modelled using the step size approach.
Firstly, the sensitivity of the step size, mesh fineness and domain size is assessed for an unyawed wheel and the k −ω SST, Realizable k − ε and Spalart-Allmaras are tested. Among these models, the Realizable k −ε model is chosen to investigate the effect of yaw and Reynolds number. The yaw is investigated up to 10° in increments of 2° and the Reynolds number effect is investigated for the Reynolds number range of 10 000 — 1 000 000.
The results show that yawing the wheel yields a fairly linear increase in the drag coefficient and the side force coefficient. Furthermore, only a significant increase of the lift coefficient is observed when going from a yaw of 4° to 6°. Moreover, the wake becomes asymmetric with increasing yaw. The vortex at the leeside on the ground becomes bigger while the vortex at the windward side becomes smaller. Additionally, a new vortex in the upper part of the wake further downstream is formed and the wake becomes shorter.
Increasing the Reynolds number, the value of the drag coefficient decreases and of the lift coefficient stays approximately the same. Moreover, the Reynolds number seems to affect the pressure peaks upstream and downstream of the contact patch. A lower value results in larger magnitude peaks. Finally, in the investigated Reynolds number range the wake structures are the same. However, the wake is bigger when the Reynolds number is smaller and asymmetry was observed in the wake for ReD = 1 000 000, which can be caused by the asymmetry of the wheel.
... Read more

With the growing global utilisation of wind energy, lowering of its levelised cost of energy is pursued. This effort is hindered by wind turbine wakes and their detrimental effects on wind farm profitability. Most currently researched wake mitigation methods are based on wind farm system-level interventions, and there is a research gap on individual wind turbine design wake alleviation measures.

This thesis investigates the wake-diffusion rotor concept, a wind turbine design with an outboard shifted thrust distribution along the blade, and its effects on wake mitigation and power generation in wind farms.
Three wind turbine rotors with equivalent thrust coefficient are modelled as actuator disks in a RANS CFD software PyWakeEllipSys, corresponding to an inboard shifted thrust distribution, a conventional thrust distribution, and an outboard shifted thrust distribution representing the wake-diffusion rotor concept. Investigated scenarios include a single wind turbine and rows of three and ten wind turbines aligned to the inflow.
Compared to the baseline case, outboard thrust distribution produces lower wake velocity deficits, increased momentum transfer and increased turbulence generation. The wake-diffusion rotor concept increases the wind farm power generation by 3.8 % and 2.5 % in the three and ten wind turbine row scenarios respectively, with the largest relative gain in wind turbine power of 13.9 % for the second wind turbine in the row. Consecutive wind turbines in the row experience diminishing gains. The inboard shifted thrust distribution had opposite effects for all of these aspects.
The benefits in wake mitigation and power generation in wind farms from using the wake-diffusion rotor concept are concluded to come from higher turbulence mixing caused by the outboard shifted thrust distribution. Recommendations on its use for maximum effectiveness are given. Future research could focus on the design implementation of this concept, investigate combinations with other wake mitigation methods and perform parametric wind farm AEP studies. ... Read more

In the aviation, maritime and oil&gas sector, the potential benefit in yearly savings deriving from the reduction of turbulent skin-friction drag through active flow control are estimated in the millions of U.S. Dollars and million tonnes of CO2 emissions. Research in this topic has brought forward numerous techniques, offering enticing drag reduction performance. However, no technique has yet proven to be suitable for large scale application in the aviation sector. A small research stream has been initiated in the field of rotating disc actuators for skin-friction drag reduction. Currently, research on this topic is limited to low Reynolds number numerical simulations, that are not representative of flight conditions.This thesis project aims at investigating the disc actuator performance in terms of drag reduction and power balance at friction Reynolds numbers higher than current literature on disc actuators. This has been achieved through direct numerical simulations, using an incompressible channel flow solver adapted for the implementation of the disc actuators. In total two disc configurations have been tested at friction Reynolds number Reτ=180, 550 and Reτ =1000. The disc diameter and tip velocity scaled in viscous units are kept constant across the Reτ range. The simulation results confirm the positive drag reduction performance observed in previous literature at Reτ =180. With increasing Reτ, a decrease in drag reduction performance is observed, with skin-friction drag reduction going from 21.6% at Reτ =180 to 16.01% Reτ =1000. The net power saving of the disc actuators show small variation with increasing Reτ, with an optimal net power saving at Reτ =1000 at -3.2%. Visualizations of local velocity profiles and local time averaged velocity have highlighted the disc influence to be limited in the region below y+< 400. Thus, the performance of disc actuators at friction Reynolds number higher than Reτ =1000 should be less influenced by the Reynolds number. Comparison with experimental testing of disc actuators from a parallel thesis study have shown good qualitative agreement between experimental and computational results. Lastly, the consistency in the drag reduction, net power balance result and disc velocity distribution within the boundary layer seem to reinforce the hypothesis that the disc performance scales with the disc diameter and tip velocity expressed in wall units.The outlook of this research shows that the performance of disc actuators at friction Reynolds numbers more representative for flight applications puts disc actuators on par with other passive skin-friction drag reduction techniques such as riblets in terms of net power saving. However, the reinforcement of the hypothesis that disc performance scales in wall units may result in too small actuators for flight conditions, resulting in more difficult full scale implementation. ... Read more

 Distributed surface roughness elements characterise Thermal Protection Systems (TPS) typical of supersonic and hypersonic flows. The presence of these distributed roughness elements cause an increase in drag and heat transfer.  As opposed to incompressible flow over roughness elements, there are very few experimental and numerical studies on supersonic flow over roughness. The most fundamental computational technique wherein, all scales of turbulence is resolved is Direct Numerical Simulation (DNS). The cost of performing DNS of fully resolved roughness is even higher than canonical DNS because of the refined mesh needed to solve the roughness elements.  To overcome this limitation, the current thesis aims at exploring low cost alternatives to DNS of fully resolved roughness for studying the effect of drag and heat transfer in supersonic flow over rough walls. DNS results from fully resolved full channel cube roughness for Mach 2 and Mach 4 at friction Reynolds number Re_τ = 500,1000  are analyzed. The results from the low cost models are compared against the fully resolved roughness simulated using full channels. Three lower-cost alternative, namely DNS of minimal channel flow of fully resolved roughness, DNS of modelled roughness and resolved RANS are considered. As for the DNS of minimal channel flow, it is found that the velocity shift ΔU⁺ is predicted accurately and therefore the added drag. However, it cannot be used to predict the temperature field because of lack of outer layer similarity for the thermodynamic statistics. As for the modeled roughness, an extension of the model by Busse and Sandham originally developed for incompressible flows is considered. In this case the roughness geometry is substituted by the additional drag and heat transfer that it induces on the flow, which take the form of source terms in the momentum and energy equations. We perform 17 DNS simulations with modeled roughness and compare the results to the fully resolved simulation. We find that the parametric forcing method is able to predict the velocity shift with good accuracy, although recovering the equivalent roughness height from the model parameters can only be done a posteriori. The model is able to qualitatively reproduce the temperature field, but thermodynamic statistics are inaccurate when compared to DNS of the fully resolved geometry. The final computational technique is RANS. In real case applications, RANS require the use of wall functions, and in the case of rough walls knowledge of the equivalent roughness height k_s⁺ is necessary. We attempt to see if RANS of fully resolved roughness can be used to estimate the velocity shift ΔU⁺ and therefore  k_s⁺ by limiting ourself to the  linear Spalart-Allmaras (SA) model. It is found to be inaccurate in computing the mean velocity profile at  k_s⁺≈ 40 with improvements in accuracy observed for k_s⁺≈ 80 when compared with the results from DNS for cube roughness element. However, the accuracy is still low to be used for estimating k_s⁺.  ... Read more