EFFECTS OF STREAMWISE PRESSURE-GRADIENT ON TURBULENT SPOT DEVELOPMENT

A pressure distribution representative of a controlled diffusion compr essor blade suction surface is imposed on a flat plate. Boundary layer transition in this situation is investigated by triggering a wave pac ket, which evolves into a turbulent spot. The development from wave pa cket to turbulent spot is observed and the interactions of the turbule nt spot with the ongoing natural transition and the ensuing turbulent boundary layer are examined. Under this steeply diffusing pressure dis tribution, strong amplification of primary instabilities prevails. Bre akdown to turbulence is instigated near the centerline and propagates transversely along the wave packer until the turbulent region dominate s. An extensive calmed region is present behind the spot, which persis ts well into the surrounding turbulent layer. Celerities of spot lendi ng and trailing edges are presented, as is the spanwise spreading half -angle. Corresponding measurements for spots under a wide range of imp osed pressure gradients are compiled and the present results are compa red with those of other authors. Resulting correlations for spot propa gation parameters are provided for use in computational modeling of th e transition region under variable pressure gradients.