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A mechanism for bypass transition from localized disturbances.
Abstract
The linear, nonlinear and breakdown stages in the transition of localized disturbances in
plane Poiseuille flow is studied by direct numerical simulations and analysis of the
linearized Navier-Stokes equations. Three-dimensionality plays a key role and allows for
algebraic growth of the normal vorticity through the linear lift-up mechanism. This growth
primarily generates elongated structures in the streamwise direction since it is largest at
low streamwise wavenumbers. For finite-amplitude disturbances such structures will be
generated essentially independent of the details of the initial disturbance, since the
preferred nonlinear interactions transfer energy to low streamwise wavenumbers. The
nonlinear interactions also give a decrease in the span-wise scales. For the stronger initial
disturbances the streamwise vorticity associated with the slightly inclined streaks was
found to roll up into distinct streamwise vortices in the vicinity of which breakdown
occurred.
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