The present licentiate thesis addresses the use of well-resolved simulations to simulate turbulent boundary layers (TBL) subjected to adverse pressure gradients. Within the thesis a wide variety of analyses are performed, and a method to improve the performance of the simulations is presented. The first aim of the thesis is to assess the effect of adverse pressure gradients and flow history on the development and fundamental characteristics of turbulent boundary layers. With this in mind, well-resolved large-eddy simulations (LES) of the turbulent boundary layers over two wing sections are performed using the spectral-element-method (SEM) code Nek5000. In order to assess the effects of the adverse pressure gradient on turbulent boundary layers, turbulence statistics are computed and time series are collected from the simulations. The turbulence statistics show a significant effect of the adverse pressure gradient on the mean velocity profiles, turbulent fluctuations and turbulent kinetic energy budgets. In addition, the time series are used to compute the power-spectral densities of the turbulent boundary layers and to analyse the effect of the adverse pressure gradient on the turbulent scales across the boundary layer. After having compared both wings at moderate Reynolds number Re_c=400,000, the next goal is to perform high-resolution simulations of wings at higher Reynolds numbers in order to study conditions closer to those in reality, and to evaluate the effect of adverse pressure gradient with increasing Reynolds numbers. To achieve this, better and more efficient computational methods are required. In this thesis, the performance of the adaptive mesh refinement method recently implemented in Nek5000 is assessed for the first time on wing simulations. The obtained results show a large potential of this new method (which includes the use of non-conformal meshes) with respect to the previous simulations carried out with conformal meshes. Lastly, we performed a modal decomposition of the TBLs developing around both wing sections. To this end, we consider spectral proper orthogonal decomposition (SPOD), which can be used to identify the most energetic structures of the turbulent boundary layer.

The present doctoral thesis investigates the turbulent flow developing around wing sections, focusing on the impact of adverse-pressure-gradient (APG) conditions on turbulent boundary layers (TBLs) and the physics of flow separation. Both experimental and numerical methods are employed to generate high-fidelity data sets and provide an in-depth analysis of the flow.

The first objective of this thesis is the development of a comprehensive database for the flow around a NACA 4412 wing profile. For this purpose, adaptive mesh refinement (AMR) is used together with the spectral-element method code Nek5000. With AMR, high-resolution Large Eddy Simulations (LES) are conducted at various Reynolds numbers (Re_c = 2×10⁵, 4×10⁵ and 1×10⁶) and angles of attack (AoA=5°, 8°, 11°, 14°), which were previously unattainable. The effect that strong APGs have on TBLs developing around a wing section is assessed through the collection of statistics and time series. The results demonstrate the influence of APG conditions on both the mean and variance profiles of velocity, and on the distribution and production of turbulence energy within the TBL. Additionally, the connection of APG TBLs with flow separation is explored through the development of an in-situ identification and tracking algorithm, tightly integrated into Nek5000. Our findings show that, in contrast to canonical flows, backflow events in TBLs under strong APGs extensively merge to form larger structures that grow exponentially in size, eventually leading to significant flow separation near the wing’s trailing edge.

Furthermore, a wind-tunnel experimental campaign is conducted to validate and extend the numerical results. Pressure, wall-shear stress and velocity measurements were carried out in the MTL wind tunnel at KTH Royal Institute of Technology. The study also scrutinizes measurement methodologies for APG TBLs, examining uncertainties in skin-friction determination and the impact of hot-wire probe lengths on velocity variance profiles.

Finally, a study based on Reynolds-averaged Navier–Stokes (RANS) simulations, utilizing high-fidelity data for validation, is performed to assess the optimization of flow-control schemes based on blowing and suction. This study, later extended to a transonic airfoil, showcases Bayesian optimization (BO) as an efficient method for computational fluid dynamics (CFD)-based optimization problems.

Denna doktorsavhandling undersöker det turbulenta flödet runt vingsektioner, med fokus på effekten av negativa tryckgradienter (APG) på turbulenta gränsskikt (TBL) och fysiken bakom avlösning. Både experimentella och numeriska metoder används för att generera data och genomföra en noggrant analys av flödet.

Det första målet med avhandlingen är att utveckla en omfattande databas för flödet runt en NACA 4412 vingprofil. För detta ändamål används adaptiv nätförfining (AMR) i det spektralelementbaserade programmet Nek5000. Med AMR genomförs väggupplösta large-eddy simuleringar (LES) vid olika Reynoldstal (Re_c = 2×10⁵, 4×10⁵ och 1×10⁶) och anfallsvinklar (AoA=5°, 8°, 11°, 14°), vilka tidigare varit ouppnåeliga. Effekten av den starka negativa tryckgradienten på TBL som utvecklas runt vingsektionen bedöms genom insamling av statistik och tidsserier. Resultaten visar på APG:s påverkan på både medel- och variansprofiler för hastighet, samt på fördelning och produktion av turbulenta energien inom TBL. Dessutom utforskas sambandet mellan APG TBL och avlösning genom utveckling av en in-situ identifierings- och spårningsalgoritm, integrerad i Nek5000. Våra resultat visar att negativa hastigheter händelser i TBL under starka APG interagerar betydligt med varandra, sammanflätar och bildar större strukturer som ökar exponentiellt i storlek, och så småningom leder till betydande avlösning nära vingens bakkant. 

Dessutom genomförs en experimentkampanj i vindtunnel för att validera och utöka de numeriska resultaten. Mätningar av tryck, väggskjuvspänning och hastighet utförs i MTL vindtunneln vid KTH Kungliga Tekniska Högskolan. Studien granskar också mätmetoder för APG TBL, undersöker osäkerheter i bestämningen av väggskjuvspänning samt effekterna av längden på varmtrådprober på mätprofiler för hastighetsvarians.

Slutligen utförs en RANS-studie, där högupplösta data används för validering, för att bedöma optimering av flödeskontrollmetoder baserade på blåsning och sugning. Denna studie, som senare utvidgas till ett transoniskt vingprofil, visar på Bayesiansk optimering som en effektiv metod för CFD (computational fluid dynamics)-baserade optimeringsproblem.