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

直升机周围复杂的流场是由旋涡及其相互作用控制的。这里特别重要的是在叶片处发展并与气流对流的叶尖涡。一方面，在特定的飞行情况下，这些涡通过与后续叶片的相互作用产生噪声（BVI）；另一方面，靠近地面的旋翼尾流也会搅动松散的颗粒，如沙子或雪。由此导致的视力丧失构成了重大的安全风险。在这方面，涡旋的形成、保持和衰减以及它们的运动定义了飞行物理学的一个重要课题。此外，多年来，由于数值耗散过大，对这些涡旋的模拟研究受到了很大的影响，因此无法保证涡旋在物理上的正确保存。然而，最近，通过改进的高阶方法和精确的空间分辨率，可以取得相当大的进展，使涡旋能够保持足够长的时间。然而，与此同时，可以观察到新的衰变过程和二级结构，其物理机制目前尚不清楚，因为到目前为止还没有足够高质量的实验结果来可靠地排除数值误差。这就是拟议项目的由来。在实验方面，需要利用现有的高精度测量技术，对实际相关的衰减过程，特别是悬停飞行中的叶尖涡进行详细的研究。研究的重点是在哪个涡旋年龄、哪些物理机制（如不稳定性或涡旋配对）导致了初级结构的衰变以及哪些扰动参数起了作用。同时，这将为数值模拟中发生可比衰减的位置和方式设定标准。研究过程和模型的数值变化对这些衰减过程的影响，并从中得出适当的指导方针，使模拟的涡衰减在位置和机制上与实际涡衰减尽可能接近。在这种情况下，也要考虑到实验确定的对操作参数的依赖，以便正确地表示过程。从对单个涡旋的基础研究中，我们知道了长短波不稳定性和涡旋配对的各种机制。总的来说，目前的项目旨在澄清这些可以转移到直升机旋翼上的旋转系统的程度，以及如何在数值模拟中最好地再现它们。在这些发现的基础上，进一步的发展将有助于更高效、更安全的直升机旋翼。

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.