Computational analysis of receptivity and growth mechanisms in laminar boundary layers

本文研究了自由流涡扰动与边界层扰动之间的关系。例如，在通过涡轮机的流动中，外部扰动的存在引起快速分解过程，通常称为旁路转变，其将有序层流转变为混沌湍流状态。湍流与粘性阻力水平的增加有关，并且通常导致效率降低。为了优化效率，翼型的设计过程因此通常旨在尽可能地延迟向湍流的转变。在其他情况下，例如热交换器，寻求有效地获得湍流状态，同时保持粘性损失最小。通过优化方法，本研究确定的第一次最危险的外部扰动，这是最有效的触发放大的扰动在前过渡边界层。这种关于由于外部扰动引起的过渡的流动物理学的改进的知识可以有助于在广泛的应用中提高效率和减少排放。此外，在这种情况下开发的计算框架将提供基础的软件工具，可以直接集成到工业设计过程中，并预测扰动增长的潜力，由于在各种流动配置的外部干扰。

The relationship between free-stream vortical disturbances and the resulting perturbations in boundary layers is investigated. For example in the flow through turbines, the presence of external disturbances induces a rapid breakdown process, commonly known as bypass transition, which transforms the orderly laminar flow into a chaotic turbulent state. Turbulent flows are associated with increased levels of viscous drag and often entail lower efficiency. In order to optimize efficiency, the design process of airfoils thus often aims to delay transition to turbulence as far as possible. In other cases, such as heat exchangers, it is sought to efficiently obtain a turbulent state while keeping viscous losses at a minimum. By means of an optimization approach, the present study identifies for the first time the most dangerous external disturbances, which are most effective in triggering the amplification of perturbations in pre-transitional boundary layers. This improved knowledge about the flow physics of transition due to external disturbances can help to improve efficiency and reduce emissions in a wide range of applications. Furthermore, the computational framework developed in this context will provide the basis for software tools which can be directly integrated into the industrial design process and which predict the potential for perturbation growth due to external disturbances in various flow configurations.