Instability of the optimal edge trajectory in the Blasius boundary layer

In the context of linear stability analysis, considering unsteady base flows is notoriously difficult. A generalisation modal allowing for arbitrarily over a finite time, therefore required. The recently developed optimally time-dependent (OTD) modes form projection basis tangent space. They capture leading amplification directions in state space under constraint that they an orthonormal at all times. present numerical study illustrates possibility to describe complex flow case using OTD modes. investigation Blasius boundary layer, featuring streamwise streaks length and relevant bypass transition. It corresponds trajectory initiated by minimal seed; such unsteady, free from any spatial symmetry shadows laminar–turbulent separatrix time only. finite-time instability this investigated 8 analysis includes computation Lyapunov exponents as well instantaneous eigenvalues, associated structures. reconstructed eigenmodes are outer type. map unambiguously regions largest growth. Other structures, previously reported secondary, identified with method streak switching vortical ejections. dynamics inside features both non-modal amplification. Non-normality within reduced subspace, quantified abscissa, emerges only unsteadiness reduced.