On the experimental receptivity in a pre-transitional laminar boundary layer

This work is devoted to the study of the influence of the freestream turbulence intensity (FST) on a pre-transitional boundary layer formed on a flat plate. The FST was generated using a passive turbulence-generating grid, in a low-medium speed wind tunnel, and the data were recorded using a hot-wire system. The receptivity is investigated by means of various mathematical tools, which comprehend statistical analysis as well as spectral decomposition. An analytical approach has also been proposed in order to address the influence of the fluctuating components on the skin friction coefficient and the integral quantities. Furthermore, statistics from the pre-transitional boundary layer are analysed and reveal self-similar behaviour. The main characteristic is represented by the so-called Klebanoff modes, in the form of elongated positive and negative streaks and a method to estimate the length of the streaks is given. It is also demonstrated that, when normalised, the length of the positive streaks grows with the same rate of the negative streaks. In the frequency domain, this study has highlighted the importance of some frequencies and their different evolution. The strong dissipation found in proximity of the leading edge is responsible for destabilising the shear layer, while the downstream decay of the FST appears to have no major impact on the evolving disturbances and no significant dissipation is noticeable at the boundary layer edge, as an interaction of FST and negative streaks: this may support the idea that the transition process is initiated within the boundary layer. A frequency window, where the energy content shows no appreciable variations of the energy content, has been found and this seems to be appropriate to allow a feeding mechanism of the internal disturbances.