In a flat plate boundary layer subjected to isotropic free stream turbulence of few percents turbulence intensity, the streaks, which longitudinally elongated regions of high and low streamwise velocity, appear in the boundary layer and then break down to turbulent spots. Experiments and DNS have revealed that the profile of the streamwise fluctuation energy has a peak at the middle of boundary layer and that the disturbance grows in proportion to the streamwise distance from the leading edge. These results were in good agreement with the non-modal theory. Though the theory suggested that in lower free stream turbulent case modal disturbance has chance to develop and breakdown to turbulence, experimental investigations have not clarified the maximum intensity of the free stream turbulence at which the modal disturbance triggers transition. It is know that there are other processes of the boundary transition that start with a short streak breakdown. In this study various types of the free stream turbulence including anisotropic turbulence are scrutinized using turbulence grids and their relation to the disturbance growth in the boundary layer is investigated. The result with hot-wire measurements shows that the spanwise spectra of the free stream turbulence are essential factor for the non-modal growth. This would be a major step for developing prediction method of boundary layer transition.

This content is only available via PDF.
You do not currently have access to this content.