Boundary layer transition by interaction of streaks and Tollmien–Schlichting waves

Abstract

AbstractTransition to turbulence in zero-pressure-gradient boundary layers is studied using direct numerical simulations (DNS). Both discrete and discrete-continuous mode transition are considered by prescribing particular eigenmodes of the OrrSommerfeld (OS) equation at the inlet plane of our computational domain. The downstream evolution of the modes, secondary instability, and non-linear breakdown are computed using DNS.The natural, or orderly, route is simulated starting from the discrete, TollmienSchlichting (TS) waves. Introducing streaks, which are forced by low-frequency continuous modes, promotes breakdown of the boundary layer. Transition location is controlled by two competing effects: In the presence of a streak distortion of the base flow, the growth rate of the primary TS wave is reduced. However, their secondary instability is enhanced. The first effect dominates for narrow streak distortions, and transition can be delayed. For wide streaks, direct resonance between the three-dimensional streaks and the secondary instability mode promotes transition.Simulations with a fully turbulent free-stream and TS waves are also carried out. For low turbulence intensity, transition via secondary instability of the TS waves is promoted, and proceeds in the same manner as the canonical simulations of continuous-discrete mode interactions.KeywordsDirect Numerical SimulationSecondary InstabilityFreestream TurbulenceBypass TransitionSchlichting WaveThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.