Published online by Cambridge University Press: 26 April 2006
Using the parallel-flow approximation the prechaotic bifurcation behaviour of Blasius boundary-layer flow is studied at finite Reynolds numbers. The objective of this investigation is to search for qualitative solution changes which might be linked to the rapid breakdown at transition. As a first step this requires the computation of the two-dimensional primary equilibrium surface of nonlinear Tollmien-Schlichting waves. This two-dimensional neutral surface exhibits a period-halving bifurcation which is qualitatively different from the situation for plane Poiseuille flow. At the same time the numerically computed equilibrium solution offers the possibility of assessing the range of convergence of weakly nonlinear results.
In a second step the stability of this nonlinear equilibrium solution is investigated with respect to three-dimensional disturbances. Of particular importance is the existence of a modal degeneracy between amplified secondary instability modes, implying locally algebraic growth. On decreasing the Reynolds number, the amplification rate of this direct resonance point switches from being amplified to being damped. Interestingly, the Reynolds number corresponding to this zero-amplification point seems to be in the vicinity of the experimentally observed transition Reynolds number for Blasius flow.