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
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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.