Affiliation:
1. Faculty of Aerospace Engineering, Technion-IITHaifa 32000, Israel
2. Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign1206 West Green Street, Urbana, IL 61801, USA
Abstract
The failure of normal-mode linear stability analysis to predict a transition Reynolds number (
Re
tr
) in pipe flow and subcritical transition in plane Poiseuille flow (PPF) has led to the search of other scenarios to explain transition to turbulence in both flows. In this work, various results associated with linear and nonlinear mechanisms of both flows are presented. The results that combine analytical and experimental approaches indicate the strong link between the mechanisms governing the transition of both flows. It is demonstrated that the linear transient growth mechanism is based on the existence of a pair of least stable nearly parallel modes (having opposite phases and almost identical amplitude distributions). The analysis that has been applied previously to pipe flow is extended here to a fully developed channel flow predicting the shape of the optimized initial disturbance (a pair of counter-rotating vortices, CVP), time for maximum energy amplification and the dependence of the latter on
Re
. The results agree with previous predictions based on many modes. Furthermore, the shape of the optimized initial disturbance is similar in both flows and has been visualized experimentally. The analysis reveals that in pipe flow, the transient growth is a consequence of two
opposite running modes
decaying with an
equal decay
rate whereas in PPF it is due to two
stationary modes
decaying with
different decay
rates. In the first nonlinear scenario, the breakdown of the CVPs (produced by the linear transient growth mechanism) into hairpin vortices is followed experimentally. The associated scaling laws, relating the minimal disturbance amplitude required for the initiation of hairpins and the
Re
, are found experimentally for both PPF and pipe flow. The scaling law associated with PPF agrees well with the previous predictions of Chapman, whereas the scaling of the pipe flow is the same as the one previously obtained by Hof
et al
., indicating transition to a turbulent state. In the second nonlinear scenario, the base flow of pipe when it is mildly deviated from the Poiseuille profile by an axisymmetric distortion is examined. The nonlinear features reveal a
Re
tr
of approximately 2000 associated with the bifurcation between two deviation solutions.
Subject
General Physics and Astronomy,General Engineering,General Mathematics
Cited by
10 articles.
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