The aeroelastic stability of airplanes is one of the most important aspects of airplane design. Flutter or divergence instabilities arising out of the interaction of fluid forces and structural elasticity must be avoided by design or through the limitation of the flight envelope. Classical unsteady theories have been established since the 1930s however, recent investigations with laminar wings and in transitional flows have found the theories to be unreliable in these regimes. The current work investigates the flow around unsteady airfoils in these flow regimes. A linear framework for the stability analysis of fluid-structure-interaction (FSI) problems is derived and validated. The derived formulation is then used to investigate the changes in the structural sensitivity of an eigenvalue for an oscillating cylinder, which is found to change significantly when the fluid and structural systems are close to resonance. The linear stability analysis is then applied to investigate the aeroelastic stability of a NACA0012 airfoil with a free pitch-deegree-of-freedom at transitional Reynolds numbers. The stability results of the coupled FSI system are found to be in good agreement with previously performed experimental results and were able to predict the onset of aeroelastic pitch-oscillations. The boundary layer evolution for a natural laminar flow airfoil undergoing forced small-amplitude pitch-oscillations is investigated at Rec = 7.5×10⁵. Large changes in laminar-to-turbulent transition location are found throughout the pitch cycle which cause a non-linear aerodynamic force response. The origins of the non-linear unsteady aerodynamic response is explained on the basis of the phase-lagged quasi-steady evolution of the boundary layer. A simple empirical model is developed using the phase-lag concept to model the unsteady aerodynamic forces which fits the experimental data very well. On the other hand, the forced pitching investigation at Rec = 1.0×10⁵ for the same airfoil found abrupt changes in transition during the pitch-cycle. A local stability analysis in the reverse flow region indicates that the stability characteristics of the LSB change character from convective to absolute, and it is conjectured that this change in stability characteristics may be the cause of abrupt changes inboundary-layertransition.

This thesis is concerned with two distinct topics related to the study of wall-bounded turbulence: the connection between instantaneous three-dimensional coherent structures and mean-flow properties, and the development and analysis of pre-determined control techniques for adverse-pressure-gradient boundary layers.

We examined regions with intense velocity fluctuations in various flowcases. In turbulent ducts, we found that, on the one hand, the specific geometry of the domain has measurable effects on the shape and dimensions of these structures. On the other hand, however, their contribution to the mean secondary flow, which is the main distinguishing feature of turbulent ducts, is not particularly significant. Intense events contribute to the mean velocity in a similar way as in periodic channels, where the secondary flow is not present. Studying adverse- and zero-pressure-gradient turbulent boundary layers, we found that there are qualitative differences in how intense-fluctuation events affect the mean properties of these two flows. Our results suggest that coherent structures may help to explain history effects and development of the outer peak in wall-tangential fluctuations. An efficient algorithm for percolation analysis and an in-situ adaptor for the simulation code Nek5000 and the visualization software Paraview have also been developed as part of this effort.

We also created a new dataset including various combinations of uniform blowing and suction applied to a NACA4412 airfoil, employing high-fidelity numerical simulations and turbulence models. There are significant discrepancies between how the control interacts with turbulence under different pressure-gradient conditions, which illustrates the need of considering test cases as similar as possible to operative conditions in control studies. We also found that the most promising control configuration for a wide range of Reynolds numbers is uniform blowing applied to the airfoil pressure side. In particular, it reduces both pressure and skin-friction drag, resulting in higher aerodynamic efficiency and potential net-energy saving when the actuation cost is included.

Denna avhandling handlar om två olika ämnen relaterade till studien av väggbunden turbulens: sambandet mellan momentana tredimensionella koherenta strukturer och medelflödesegenskaper och utveckling och analys av förutbestämda styrtekniker för negativ tryckgradientgräns skikten. 

Vi undersökte regioner med intensiva hastighetsfluktuationer i olika studiefall. I turbulenta kanaler fann vi att å ena sidan har domänens specifika geometri mätbara effekter på formen och dimensionerna hos dessa strukturer. Å andra sidan är deras bidrag till det genomsnittliga sekundära flödet, som är huvudfunktionen i turbulenta kanaler, inte särskilt signifikant. Intensiva händelser bidrar till medelhastigheten på ett liknande sätt som i periodiska kanalströmmar, där sekundärflödet inte finns. När vi studerade ogynnsamma och nolltryckgradient turbulenta gränsskikt, fann vi att det finns kvalitativa skillnader i hur intensiva fluktuationshändelser påverkar medelegenskaperna för dessa två flöden. Våra resultat tyder på att sammanhängande strukturer kan hjälpa till att förklara historiska effekter och utveckling av den yttre toppen i väggtangentiella fluktuationer. En effektiv algoritm för perkoleringsanalys och en adapter på plats för simuleringskoden Nek5000 och visualiseringsmjukvaran Paraview har också utvecklats som en del av detta arbete.

Vi skapade en ny dataset inklusive olika kombinationer av enhetlig blåsning och sugning applicerad på en NACA4412-flygplatta, med användning av högkvalitativa numeriska simuleringar och turbulensmodeller. Det finns signifikanta skillnader mellan hur kontrollen interagerar med turbulens under olika tryckgradientförhållanden, vilket illustrerar behovet av att betrakta testfall som liknar operativa förhållanden i kontrollstudier. Vi fann också att den mest lovande kontrollkonfigurationen för ett brett spektrum av Reynolds-nummer är enhetlig blåsning som appliceras på flygplattans trycksida. I synnerhet minskar det både tryck- och hudfriktionsdragningar, vilket resulterar i högre aerodynamisk effektivitet och potentiell nettoenergibesparing när manöverkostnaden beräknas.