Reynolds-number-independent instability of the boundary layer over a flat surface

A three-dimensional mode of spatial instability, related to the temporal algebraic growth that determines lift-up in parallel flow, is found to occur in the two-dimensional boundary layer growing over a flat surface. This unstable perturbation can be framed within the limits of Prandtl's standard boundary-layer approximation, and therefore develops at any Reynolds number for which the boundary layer exists, in sharp contrast to all previously known flow instabilities which only occur beyond a sharply defined Reynolds-number threshold. It is thus a good candidate for the initial linear amplification mechanism that leads to bypass transition.