High Reynolds number Airfoil Experiments

Unsteady flows can be encountered in nature and technical applications alike. 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.

With regard to large scale modern-day wind turbines, the unsteady phenomenon of dynamic stall at high Reynolds numbers is of particular interest. 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. 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. 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.

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 principal part of the thesis is concerned with dynamic stall evoked by a variety of transient pitching maneuvers and motion profiles. 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 first parametric study considers individual pitch-up and pitch-down motions in order to isolate the processes of dynamic stall and boundary layer reattachment. 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. Moreover, the Reynolds number dependent static stall angle was found to play a significant role even in the case of dynamic pitching maneuvers. The excess of the static stall angle has severe implications on the duration until the onset of stall, where a smaller overshoot results in a severely prolonged stall delay. The location of transient boundary layer reattachment during pitch-down maneuvers appeared to be governed by an angle dependent adverse pressure gradient. As such, the study depicts an unprecedented investigation of the isolated influence of the Reynolds number on the dynamic stall process.

Classical dynamic stall experiments in the form of periodic oscillations were performed in the second parametric study with nearly identical parameter variations. It was found that transient boundary layer modifications can evolve over multiple pitching cycles until a cyclic flow field equilibrium is attained. This behavior is evident in the first several pitching cycles for particular kinematic settings. The data sets of this and the two aforementioned studies are publicly available.

Large amplitude oscillations related to the prevailing blade aerodynamics of vertical axis wind turbines were considered in the third parametric study. Here, the variation of the reduced frequency resulted in substantial excursions from static values and a significant difference in stall angle.

In a separate chapter, three-dimensional flow phenomena induced by wing twist and vortex generators are investigated. The influence of wing twist on pressure distributions and flow topology manifests itself in a homogeneous and attached flow across the span at lower angles of attack, whereas the flow field becomes three-dimensional when the high-alpha side of the wing begins to stall. Pressure-sensitive paint experiments were performed in the wake of vortex generators, where low-pressure signatures from the emerging vortices were found in the surface-pressure field.