Theory and simulations of bypass transition in two-fluid boundary layers

Bypass transition in boundary layers is sensitive to the flow configuration, and small changes in the mean-flow profile near the wall can have a dramatic impact on transition location. This change can be effected by introducing a thin wall film of different viscosity, which alters the mean-velocity profile and the boundary-layer response to free-stream vortical forcing. A judicious choice of the film properties can therefore stabilize the pre-transitional region and delay the onset of turbulence far downstream of its location in the single-fluid case. Linear theory provides a framework for making this choice and avoiding conditions that can be detrimental to stability. The theory explains how the film can weaken the lift-up mechanism that causes the amplification of streaks. Direct numerical simulations confirm this trend which is ultimately responsible for delaying the onset of secondary instability and reducing the frequency of formation of turbulence spots. As a result, the entire transition process is delayed relative to the single-fluid boundary layer.