Experiments on single oblique laminar-instability waves in a boundary layer: Introduction, growth, and transition
The laminar-turbulent transition in an incompressible flat-plate boundary layer was studied experimentally by using a spanwise array of computer-controlled surface heating elements to generate small disturbances. Oblique Tollmien-Schlichting waves were successfully introduced, and their downstream development into the intermittent region was studied using flush-mounted hot-film wall-shear sensors and dye flow visualization. Comparative studies of the development of single oblique waves were made for various wave angles, frequencies, and amplitudes. As these single oblique waves grew and began to break down, higher harmonics and subharmonics appeared in the wall shear. The amplitude of the subharmonic component decreased rapidly with increasing oblique-wave angle, so that a 10 degrees oblique wave had a subharmonic amplitude an order of magnitude below that for a two-dimensional (2-D) wave. Thus, the nonlinear mechanism that produces the subharmonic is affected by the symmetry of the primary wave. Intermittency measurements, carried out farther downstream, show that a 2-D wave is most effective in moving the transition point upstream, for a given power input.
Copyright © 1994 American Institute of Physics. Received 11 November; accepted 10 September 1993. This work was carried out at the Graduate Aeronautical Laboratories, California Institute of Technology. The first author gratefully acknowledges the advice and support of his thesis advisors, H. W. Liepmann and D. Coles, and the GALCIT community. Dr. James Kendall of the Jet Propulsion Laboratory provided useful comments on several drafts of the manuscript. The work was funded by the Office of Naval Research, under Contract Nos. NOO014-85-K-0205 and NOO014-87-K-0100.