Numerical studies of separated boundary layer transition on a first plate with a blunt leading edge.

In this work the physics of transitional separated-reattched flow with and without free-stream turbulence on a blunt leading edge plate have been studied numerically employing the Large Eddy Simulation approach. One of the fundamental features of 'turbulent' separated-reattached flows is two basic modes of characteristic frequencies. The higher-frequency mode is associated with the usual large scale motions in the shear layer while the lower-frequency mode reflects overall separation bubble growth/decay dynamics or shear layer flapping as it is frequently called in the literature. It has been drawn from the current study that the low-frequency mode will not occur in low-Reynolds number transitional separated-reattached flows and the phenomena appears to be an integral feature of a fully turbulent separation. The numerical data have been comprehensively analysed to elucidate the entire transition process. Coherent structures have been visualised in the different stages of transition. In the case with no-free-stream turbulence, the 2D Kelvin - Holmholtz rolls are the dominant structures in the early stage of transition and the well known A-shaped vortices commonly associated with flat plate boundary layer transition are the common features in the late transition stages. Many experimental studies have indicated that the separated shear layer on a blunt plate is unstable owing to the Kelvin-Holmholtz instability. However, sufficient and detailed evidence has not been given in separated boundary layer transition studies to show that the instability mechanism at work is indeed the Kelvin-Holmholtz instability in this particular geometry. In the current study, it has been shown that the primary instability is indeed of the Kelvin-Holmholtz type. The results also strongly support the idea that 'helical-pairing' instability could be the secondary instability responsible for the breakdown to turbulence in the late stages of transition. The addition of free-stream turbulence result in the transition OCCllring earlier leading to a short mean reattachment bubble length. The coherent structures which are clearly observed in the no-free-stream turbulence case have been barely visible. The primary instability was found to be the same as in the no-free-stream turbulence case, i.e., Kelvin - Helmholtz instability.