The stability of boundary layer flow has been of interest to fluid dynamicists for a number of years. Even in its simplest guise, the incompressible flow over a flat plate with zero pressure gradient, it is an extremely complex problem. Predictions of the initial behaviour of a small disturbance in the boundary layer on a flat plate have been successful, However, full theoretical or experimental knowledge has yet to be attained of the behaviour of a disturbance of finite size. It is hoped that this thesis illuminates at least some aspects of the transition to turbulence of the boundary layer on a flat plate. Experimental work was first carried out in an 18 inch open -circuit tunnel, and it was from this tunnel that the work on the generation of harmonics, a phenomenon associated with a disturbance of finite size, was derived. Work was then transferred to the 4 foot closed -circuit tunnel, The flow in the tunnel had first to be investigated, and in fact, the measurements of the free stream turbulence are of as much interest from the point of view of tunnel behaviour as they are in relation to natural transition of a boundary layer. Finally, an experimental study was made of the behaviour of small disturbances in the boundary layer.
Results have been given which show a Reynolds Number, for natural transition of the boundary layer on a flat plate, 250/o greater than the previous maximum transition Reynolds Number reported by Schubauer and Skramstad. It is estimated that this Reynolds Number lies close to the maximum permissible RT suggested by Betchov and Szewczyk. It must be emphasised that the value of RT depends on the frequency spectrum of the disturbances in the boundary layer. Although the relationship between the disturbances in the boundary layer and in the free stream is not known exactly, it has been assumed that a discussion of RT as a function of the properties of the free stream turbulence is valid.