Experimental and numerical investigations of vortex decay in a rotating system

• Applicants: Dr. Manuel Keßler; Professor Dr.-Ing. Markus Raffel
• Subject Area: Fluid Mechanics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 490853673

The project focusses on the decay process of blade tip vortices, which has a decisive influence on the performance of rotors - for example helicopter rotors or wind turbine rotors. An understanding of the underlying mechanisms and a precise numerical prediction capability is therefore an important key to the aerodynamic design of future rotor systems. This follow-up research proposal is based on the findings of the previous project on the investigation of secondary vortices, which surround the primary blade tip vortices on S-shaped curves with increasing age. The second funding period will pay particular attention to the ageing and decay of the primary tip vortices. While the diffusion of a purely laminar vortex is fully understood, the occurrence of turbulence continues to result in uncertainties. In semi-empirical models, turbulence is modelled using a Reynolds number-dependent eddy viscosity. However, the literature shows a large uncertainty in the quantification of this viscosity, which can vary by several orders of magnitude depending on the source. The combined experimental (DLR Göttingen) and numerical (IAG Stuttgart) approach is continued in order to better understand the mechanisms of vortex ageing based on the three pillars described below. The first pillar is based on a precise isolation of the scale effect. For this purpose, a model rotor is integrated into the high-pressure wind tunnel Göttingen (HDG), which enables a variation of the stagnation pressure and, thus, the Reynolds number by a factor of 100. By using a constant rotor, uncertainties resulting from the usual geometric scaling are reduced (e.g. comparison of model rotors with free-flying helicopters). The second pillar is based on consideration of the secondary structures, which can be varied via the blade passing frequency and independently of the Reynolds number. If the secondary vortices are now understood as fluid transport in the radial direction of the primary vortex and, thus, as vortex ageing, it can be hypothesized that this little-noticed mechanism explains part of the previous modeling uncertainties. Thirdly, the experiments in the HDG are carried out not only with a driving rotor, but also with a rotor driven by the flow in "windmill-like" operation. This causes a widening of the rotor wake and, hence, an axial stretching of the blade tip vortices, in contrast to the vortex compression observed with driving rotors. This effect delays vortex ageing, which has not been sufficiently considered in previous models. A precise recreation of the experimental flow conditions and geometries in the numerical simulations is not only expected to provide a high level of new insight into vortex-laden flows, but also in extended "best practices" for conducting fluid mechanical investigations.

• DFG Programme: Research Grants
• Co-Investigator: Privatdozent Dr.-Ing. Claus Christian Wolf