Catastrophic events in separated flow theory.

研究固体表面的流动分离，以及确定由于分离而产生的流场的整体变化，是流体动力学中最基本和最困难的问题之一。分离不仅改变了流体颗粒在物体周围的轨迹形式，而且使作用在物体上的气动力的大小发生了显著变化，分离流动的行为相当复杂。例如，当薄翼型处于亚音速流动中，攻角逐渐增大时，首先在翼型前缘观察到分离。它通过在前缘附近形成一个反向流动的小气泡来表现出来。当攻角增大或几何形状改变时，气泡突然破裂，并被覆盖几乎整个翼型的巨大分离区所取代，就会发生灾难性的分离。这会引起升力和阻力等气动特性的剧烈变化。拟议的研究旨在加深我们对分离流动的理解。重点将放在分离流动的“爆破”现象和灾后行为上。我们想要证明这些现象与分离流的非线性不稳定性之间存在联系。

The study of flow separation from the surface of a solid body, and thedetermination of global changes in the flow field that develop as a result of the separation, are among the most fundamental and difficult problems of fluid dynamics. Separation not only alters the form of trajectories of fluid particles around the body, but also leads to a significant change in the values of the aerodynamic forces acting on the body.The behaviour of separated flow is quite complicated. For example, when a thin aerofoil is placed in subsonic flow, and the angle of attack is gradually increased, the separation is first observed at the leading edge of the aerofoil. It manifests itself through the formation of a small bubble of reversed flow near the leading edge. Catastrophic separation arises when for increased angles of attack, or changes in geometry, the bubble suddenly bursts and is replaced by a massive separation region covering almost the whole aerofoil. This causes drastic changes to aerodynamic properties such as lift and drag.The proposed research is aimed at furthering our understanding of separated flows. The emphasis will be on ``blow-up'' phenomena and post-catastrophic behaviour of separated flows. We intend to show that there is a relationship between these phenomena and nonlinear instability of separated flows.