Analysis of the transition process around laminar separation bubbles (LSB‘s) in a towing tank using time-resolved 3D particle tracking techniques

在相对较低的雷诺数下，层流边界层分离对各种升力面的性能有很大影响。相关应用的例子从人造系统，如商用飞机上的涡扇发动机叶片和尾部表面，风力涡轮机叶片，以及自然的飞行物和游泳者。层流边界层分离通常发生在翼型的吸力侧，并导致形成分离的剪切层。然后，气流可以经历快速转变，并在平均意义上重新附着到翼型表面，形成层流分离气泡(LSB)。由于LSB改变了翼型的循环，从而改变了升力和阻力，因此整体流动发展取决于LSB的形成和动态，这是本提案的主要焦点。尽管迄今在理解LSB动态方面取得了重大研究进展，但在这两个方面的根本重要性的知识方面仍有一些关键差距需要解决，这是本提议的动机。由于自由流湍流度对LSB的形成和过渡过程有很大的影响，实验将在自由流湍流度在0~2%之间变化的拖曳通道中进行。此外，先进的三维时间分辨流量测量技术将被用来解析空间和时间上的所有流动过程，具有高空间分辨率和低测量不确定度。预计这些研究将显著改善目前的知识状况，并能够可靠地回答公开的研究问题。

The performance of various lifting surfaces at relatively low Reynolds numbers is known to be affected significantly by laminar boundary layer separation. The examples of the relevant application range from man-made systems, such as turbofan engine blades and tail surfaces on commercial aircraft, wind turbine blades, as well as to nature’s flyers and swimmers. The laminar boundary layer separation typically takes place on the suction side of the airfoil and leads to the formation of a separated shear layer. The flow can then undergo rapid transition and be reattached to the airfoil surface in the mean sense forming a Laminar Separation Bubble (LSB). As LSB’s alter the circulation of the airfoil and consequently the lift and drag, the overall flow development therefore depends on the formation and dynamics of the LSB, which are the main focal points of the present proposal. Despite a significant research progress made to date towards understanding LSB dynamics, a number of critical gaps in knowledge of both fundamental importance remain to be addressed, which serves as a motivation for the present proposal. As the free stream turbulence level has a significant effect on the formation of LSB and the transition process, the experiments will be performed in a towing channel whose free stream turbulence can be varied between zero and two percent. Furthermore, sophisticated three-dimensional time-resolved flow measurement techniques will be used to resolve all flow processes in space and time with high special resolution and low measurement uncertainty. It is expected that the studies will significantly improve the current state of knowledge and that open research questions can be reliably answered.