Localized Instabilities of Klebanoff Streaks and the Influence of Time-Harmonic Wall Forcing on Bypass Transition

This dissertation addresses two central aspects of bypass transition to turbulence. In the first part, streak instabilities, often referred to as harbingers of breakdown to turbulence, are investigated by means of direct stability analysis. The base flow is computed in direct simulations of bypass transition. The random nature of the free-stream perturbations causes the formation of a spectrum of streaks inside the boundary layer, with breakdown to turbulence preceded by the amplification of localized instabilities of individual streaks. Detailed analyses of two common types of instabilities are performed. The capability of the instability analysis to quantitatively capture the properties of the instabilities observed in the DNS and to identify the particular streaks that break down to turbulence farther downstream is established. Finally, the influence of pressure gradients on the growth of the instabilities is investigated. The second part of the work establishes a novel mechanism for the suppression of bypass breakdown by means of time-harmonic wall forcing. DNS studies show that at the optimal forcing parameters, a substantial stabilization of the laminar flow regime is achieved. The Reynolds number at the onset of fully-turbulent flow increases by a factor of more than three compared to an unforced reference case. Transition delay is attributed to a material weakening of boundary layer streaks. The underlying flow physics are explained through linear analyses. Studies of free-stream disturbances show that the shielding of the boundary layer from external vortical perturbations is significantly increased in presence of the forcing. Furthermore, optimal growth computations demonstrate a substantial reduction of the achievable energy gain through the nonmodal growth mechanism associated with the formation of boundary layer streaks. Stability analyses nonetheless show that forcing with high amplitudes gives rise to an inviscid instability that swiftly undermines the stabilizing effect on the transition process.Open Acces