Numerical Study of Instabilities in Separated-Reattached Flows

Transition process in separated–reattached flows plays a key role in many practical engineering applications. Hence, accurately predicting transition is crucial since the transition location has a significant impact on the aerodynamic performance and a fundamental understanding of the instability mechanisms involved in transition process is required in order to make significant advances in engineering design and transition control, for example, to delay the turbulent phase where laminar flow characteristics are desirable (low friction drag) or to accelerate it where high mixing of turbulent flow are of interest (in a combustor). The current understanding of instabilities involved in the transition process in separated–reattached flows is far from complete and it is usually very difficult to theoretically and experimentally study the transition process since theoretical studies suffer from the limitation imposed by nonlinearity of the transition process at later stages and experimental studies are limited by temporal and spatial resolution; hence, a thorough description of the transition process is lacking. Nevertheless, significant progress has been made with the simulation tools, such as large eddy simulation (LES), which has shown improved predictive capabilities and can predict transition process accurately. This paper will first briefly present LES formalism followed by its applications to study the transition process in separated–reattached flows, reviewing our current understanding of several important phenomena associated with the transition process and focusing on the instabilities in particular.