Turbulent spot evolution in the ASBL

Turbulent spots and their streamwise evolution play a major role in most transition scenarios. The appearance of turbulent spots in laminar boundary layers was first noted by Emmons,\footnote{Emmons, {\emph{J. Aero. Sci.}} {\bf{18}}, 490 (1951).} who proposed a spot probability appearance model, which recently\footnote{Fransson et al., {\emph{J. Fluid Mech.}} {\bf{527}}, 1 (2005).} has proven to work well for the free stream turbulence induced transition scenario. However, there are many fundamental questions still remaining unanswered, which are important in the striving after new transition prediction models. In this experimental study the effect of Reynolds number on turbulent spot evolution has been studied while keeping the boundary layer thickness constant. This type of study can only be performed in the asymptotic suction boundary layer where uniform suction through the wall is applied creating a boundary layer which does not develop in space. The velocity profile in the ASBL can readily be derived as, $ u(y) = U_\infty \{ 1 - \mathrm{e}^{yV_w/\nu} \}~, $ where $U_\infty$, $V_w$, and $\nu$ are the free stream velocity, the suction velocity ($<0$), and the kinematic viscosity, respectively. The Reynolds number based on the displacement thickness becomes $Re = -U_\infty / V_w$ allowing for $Re$-changes without necessarily changing the boundary layer thickness.