The spatio-temporal evolution of a turbulent swirling jet undergoing vortex breakdown has been investigated. Experiments suggest the existence of a self-excited global mode having a single dominant frequency. This oscillatory mode is shown to be absolutely unstable and leads to a rotating counter-winding helical structure that is located at the periphery of the recirculation zone. The resulting time-periodic 3D velocity field is predicted theoretically as being the most unstable mode determined by parabolized stability analysis employing the mean flow data from experiments. The 3D oscillatory flow is constructed from uncorrelated 2D snapshots of particle image velocimetry data, using proper orthogonal decomposition, a phase-averaging technique and an azimuthal symmetry associated with helical structures. Stability-derived modes and empirically derived modes correspond remarkably well, yielding prototypical coherent structures that dominate the investigated flow region. The proposed method of constructing 3D time-periodic velocity fields from uncorrelated 2D data is applicable to a large class of turbulent shear flows.

The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic sta...

The development of the next generation of large multimegawatt wind turbines presents exceptional challenges to the applied aerodynamic design tools. Because their operation is often outside the validated range of current state-of-the-art momentum balance models, there is a demand for more sophisticated, but still computationally efficient simulation methods. In contrast to the blade element momentum method (BEM), the lifting line theory (LLT) models the wake explicitly by a shedding of vortex rings. The wake model of freely convecting vortex rings induces a time-accurate velocity field, as opposed to the annular-averaged induction that is computed from the momentum balance, with computational costs being magnitudes smaller than those of a full computational fluid dynamics (CFD) simulation. The open source code qblade, developed at the Berlin Institute of Technology, was recently extended with a lifting line-free vortex wake algorithm. The main motivation for the implementation of an LLT algorithm into qblade is to replace the unsteady BEM code aerodyn in the coupling to fast to achieve a more accurate representation of the unsteady aerodynamics and to gain more information on the evolving rotor wake and flow-field structure. Therefore, optimization for computational efficiency was a priority during the integration and the provisions that were taken will be presented in short. The implemented LLT algorithm is thoroughly validated against other benchmark BEM, LLT, and panel method codes and experimental data from the MEXICO and National Renewable Energy Laboratory (NREL) Phase VI tests campaigns. By integration of a validated LLT code within qblade and its database, the setup and simulation of LLT simulations are greatly facilitated. Simulations can be run from already existing rotor models without any additional input. Example use cases envisaged for the LLT code include: providing an estimate of the error margin of lower fidelity codes, i.e., unsteady BEM, or providing a baseline solution to check the soundness of higher fidelity CFD simulations or experimental results.