Direct transition to turbulence in a 3D boundary layer

The transition to turbulence process in a rotating boundary layer is numerically investigated. In particular we will show that, when the Reynolds number based on the forced throughflow is increased above a threshold value, an impulsive perturbation gives birth to a self-sustained saturated wave which caracteristics identify as a global elephant mode as described by [1].This saturated wave is itself absolutely unstable with zero Floquet number, and this secondary instability leads to a very unorganized state, which an be labelled as incipient turbulence.This scenario relies on low incoming noise upstream of the primary front, and a sufficiently strong impulsive perturbation as the first global bifurcation is known to be subcritical [2]. For the first time it confirms the possibility of a direct transition to turbulence through theelephant cascade.