![]() ![]() ![]() The results obtained from the pitch-up maneuvers indicate that the evolution of the dynamic stall vortex is primarily free-stream dependent and independent of the reduced frequency beyond a certain threshold. The first parametric study considers individual pitch-up and pitch-down motions in order to isolate the processes of dynamic stall and boundary layer reattachment. For this, three parametric studies were conducted in which a broad range of Reynolds numbers and reduced frequencies were investigated in addition to geometrical modifications of the pitching motion in the form of mean angle and amplitude variations, respectively. The principal part of the thesis is concerned with dynamic stall evoked by a variety of transient pitching maneuvers and motion profiles. Initial static airfoil tests revealed a strong Reynolds number dependency of the static stall angle, where higher Reynolds numbers delayed stall to higher angles of attack and increased load coefficients significantly. The present work circumvents this challenge by employing a high-pressure flow facility to conduct static as well as a broad variety of unsteady airfoils tests at full dynamic and kinematic similarity. Since two of the governing parameters of dynamic stall, the Reynolds number and the reduced frequency are inversely proportional with respect to the free-stream velocity, it is challenging to experimentally investigate dynamic stall at high Reynolds numbers in a conventional wind tunnel. This transient event describes the formation of a vortical flow structure resulting from a sudden increase in angle of attack, which can momentarily increase static loads many times over. With regard to large scale modern-day wind turbines, the unsteady phenomenon of dynamic stall at high Reynolds numbers is of particular interest. While nature has perfected the exploitation of unsteady flow for locomotion, for example in the flight of birds or the swimming of fish, unsteady flow is often undesirable in technical applications due to the difficult prediction of the resulting loads. Unsteady flows can be encountered in nature and technical applications alike. ![]()
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