EFFECTS OF FREE-STREAM TURBULENCE AND ADVERSE PRESSURE-GRADIENTS ON BOUNDARY-LAYER-TRANSITION

Boundary layer measurements are presented through transition for six d ifferent free-stream turbulence levels and a complete range of adverse pressure gradients for attached laminar flow. Measured intermittency distributions provide an excellent similarity basis for characterizing the transition process under all conditions tested when the Narasimha procedure for determining transition inception is used. This inceptio n location procedure brings consistency to the data. Velocity profiles and integral parameters are influenced by turbulence level and pressu re gradient and do not provide a consistent basis. Under strong advers e pressure gradients transition occurs rapidly and the velocity profil e has not fully responded before the completion of transition. The sta rting turbulent layer does not attain an equilibrium velocity profile. A change in pressure gradient from zero to even a modest adverse leve l is accompanied by a severe reduction in transition length. Under dif fusing conditions the physics of the transition process changes and th e spot formation rate increases rapidly; instead of the ''breakdown in sets'' regime experienced in the absence of a pressure gradient, tran sition under strong adverse pressure gradients is more related to the amplification and subsequent instability of the Tollmien-Schlichting w aves. Measurements reveal an exponential decrease in transition length with increasing adverse pressure gradient; a less severe exponential decrease is experienced with increasing turbulence level. Correlations of transition length are provided that facilitate its prediction in t he form of suitable length parameters including spot formation rate.