EXPERIMENTS IN A BOUNDARY-LAYER SUBJECTED TO FREE-STREAM TURBULENCE .1. BOUNDARY-LAYER STRUCTURE AND RECEPTIVITY

The modification of the mean and fluctuating characteristics of a flat-plate boundary layer subjected to nearly isotropic free stream turbulence (FST) is studied experimentally using hot-wire anemometry. The study is focused on the region upstream of the transition onset, where the fluctuations inside the boundary layer are dominated by elongated flow structures which grow downstream both in amplitude and length. Their downstream development and scaling are investigated, and the results are compared with those obtained by previous authors. This allows some conclusions about the parameters which are relevant for the modelling of the transition process. The mechanisms underlying the transition process and the relative importance of the Tollmien-Schlichting wave instability in this flow are treated in an accompanying paper (part 2 of the present report).