Title:
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Localization in transitional shear flows
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In turbulent flows at low Reynolds numbers localized structures are observed
which can grow or spontaneously decay. Viewed as a dynamical
system, a turbulent evolution forms a path through a phase space built
upon exact invariant solutions of the Navier-Stokes equations. The leading
stable and unstable manifolds of these solutions organise the local
dynamics. In small periodic domains many of these solutions are known.
However, to understand the full spatial-temporal nature of turbulence requires
localized solutions which are unstable and live in a very high dimensional
system.
In the first half of this thesis we consider two problems in small, periodic
domains where turbulence is global. We consider the geometry of
the edge of chaos, a manifold which divides phase space and how such a
manifold can be understood in the context of turbulent decay. We demonstrate
that the edge is not separate from the turbulent dynamics but is
wrapped up into these dynamics. Next we consider how the dynamics
on the edge in short pipes are affected by Reynolds number and find new
high Reynolds number solutions.
In this second half we attack the problem of finding and understanding
the origins of localized solutions. These solutions hold the key to expanding
the theory towards physically realisable systems. Building upon the
short pipe research we find the origin of the first localized pipe flow solution
in a bifurcation from a downstream-periodic solution. Moving to a
model for plane Couette flow, we attempt to find evidence of homoclinic-snaking
as a route to spanwise localization. Instead we find a different
route which matches recent work in duct flow. Finally, motivated by questions of how localized structures interact, we introduce a new flow, "localized
Couette flow", and investigate the stability of this flow.
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