EXPERIMENTS ON SINGLE OBLIQUE LAMINAR-INSTABILITY WAVES IN A BOUNDARY-LAYER - INTRODUCTION, GROWTH, AND TRANSITION

The laminar-turbulent transition in an incompressible flat-plate bound ary layer was studied experimentally by using a spanwise array of comp uter-controlled surface heating elements to generate small disturbance s. Oblique Tollmien-Schlichting waves were successfully introduced, an d their downstream development into the intermittent region was studie d using flush-mounted hot-film wall-shear sensors and dye flow visuali zation. Comparative studies of the development of single oblique waves were made for various wave angles, frequencies, and amplitudes. As th ese single oblique waves grew and began to break down, higher harmonic s and subharmonics appeared in the wall shear. The amplitude of the su bharmonic component decreased rapidly with increasing oblique-wave ang le, so that a 10 degrees oblique wave had a subharmonic amplitude an o rder of magnitude below that for a two-dimensional (2-D) wave. Thus, t he nonlinear mechanism that produces the subharmonic is affected by th e symmetry of the primary wave. Intermittency measurements, carried ou t farther downstream, show that a 2-D wave is most effective in moving the transition point upstream, for a given power input.