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Turbulent spots in channel flows.
Abstract
A number of investigations into the formation and development of turbulent spots in plane
Poiseuille, plane Couette and watertable flows are reviewed. Three main observations are
drawn from this work. Firstly, the initial development is associated with the transient
growth due to the three-dimensional lift-up effect. Secondly, the spreading and
propagation velocities of the different spots are quite similar. Thirdly, the velocity field
inside the spots show essentially all the characteristics of fully developed turbulent flow,
albeit at low Reynolds numbers. The mechanism behind the rapid spreading of the spots,
however, is not yet fully understood, although observations of wave activity, modifications
of the surrounding flow field and stability calculations point in the direction of growth by
destabilization.
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