Experimental and numerical investigations of vortex decay in a rotating system.

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The complex flow field around helicopters is dominated by vortices and their interaction. Of particular importance here are the tip vortices that develop at the blades and are convected with the flow. On the one hand, these vortices generate noise (BVI) in certain flight situations through interaction with following blades; on the other hand, this rotor wake near the ground can also stir up loose particles such as sand or snow. The resulting loss of vision poses a significant safety risk. In this respect, the formation, conservation and decay of vortices as well as their motion define an important topic of flight physics.In addition, for many years the investigation of these vortices with the help of simulations was considerably impaired by too large numerical dissipation, so that the physically correct preservation of the vortices was not guaranteed. Recently, however, considerable progress could be achieved by improved methods of higher order as well as refined spatial resolution, enabling the vortices to be preserved sufficiently long. At the same time, however, new decay processes and secondary structures could be observed, whose physical mechanisms are currently unclear, since so far no sufficiently high-quality experimental results are available to rule out numerical errors reliably. This is where the proposed project comes in. On the experimental side, detailed investigations of the actually relevant decay processes, in particular of the tip vortices in hovering flight, are to be carried out using current high-precision measurement technology. The focus is on the question at which vortex age which physical mechanisms, such as instabilities or vortex pairing, contribute to the decay of the primary structures, and which disturbance parameters play a role. At the same time, this will set the standard for where and how a comparable decay should occur in the numerical simulation. It is to be explored which numerical variations of procedures and models influence these decay processes and to derive appropriate guidelines from this, so that the simulated vortex decay resembles the actual one in location and mechanism as close as possible. In this context, the experimentally identified dependence on operating parameters is also to be taken into account, so that the correct process is represented.Various mechanisms of short- and long-wave instabilities and vortex pairing are known from fundamental investigations on individual vortices. Overall, the current project is intended to clarify the extent to which these can be transferred to the rotating system on the helicopter rotor, and how they can best be reproduced in numerical simulations. Building on these findings, further developments could then contribute to more efficient and safer helicopter rotors.