Dynamics and instabilities of helical vortex pairs

• Applicant: Professor Dr.-Ing. Eike Stumpf
• Subject Area: Fluid Mechanics
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 391677260

The aim of this project is to gain new fundamental knowledge on the dynamics and instabilities of vortex pairs having a large-scale helical geometry, which have relevance to applications involving flows around rotors. These applications include the wake generated by a helicopter and the flow behind a horizontal-axis wind turbine. In both cases, important fluid mechanical issues exist, related to the safety, comfort/nuisance or efficiency of operation. The strength of the BVI or a possible vortex-structure interaction is strongly influenced by certain vortex parameters, such as vortex core radius and peak tangential velocities. Modification of these quantities (widening of the vortex core, decreasing the peak velocities) to reduce the hazardous effect of the vortices has been the objective of a variety of approaches, involving active and passive systems. The first project phase of the DFG funded project TWIN-HELIX focused on the study of a novel generic configuration involving a closely-spaced pair of helical vortices. One or several regularly spaced helices have been used in the past to model rotor wakes, and the study of their dynamics and evolution has recently made significant progress. In the TWIN-HELIX project, the single vortex is replaced by a closely spaced pair of counter- or co-rotating vortices by means of an additional fin of the blade. While propagating downstream, the helical vortex pair undergoes an interaction process, involving phenomena as pairing, merging and mutual instabilities. After the interaction process, the decisive vortex parameters of the eventually merged vortex regarding BVI should change for the better (widened core radius, decreased peak tangential velocity) and consequently diminish the negative effects. The French-german cooperation consisting of the Institut de Recherche sur les Phénomènes Hors Equilibre (IRPHE) and the Institute of Aerospace Systems (ILR) collaboratively addressed these research topics in the first project phase. Combining experimental, numerical and theoretical methods, a detailed analysis of this specific configuration was performed during the first project phase. The approach taken in this fundamental study consisted in simplifying the problem as much as possible, keeping only the essential elements, in order to focus on the identification of physical (fluid mechanical) mechanisms and the parameters that govern them. The experimental analysis of the designed configuration revealed the presence of centrifugal instabilities which are causing rapid growth rates of three-dimensional perturbations. In consequence, the merging procedure is strongly accelerated. During the second project phase, the origin of these instability form shall be analyzed in detail, focusing on additional possibilities to enhance or suppress its occurrence. In addition, the influence of the proposed modification on the aerodynamic performance of the blade is investigated.

• DFG Programme: Research Grants
• International Connection: France
• Co-Investigator: Dr.-Ing. Ralf Hörnschemeyer
• Cooperation Partner: Dr. Thomas Leweke