The role of three-dimensional instabilities in compliant wall boundary layer transition

The use of passive devices to obtain drag and noise reductions or transition delays in boundary layers is highly desirable. One such device that shows promise for hydrodynamic applications is the compliant coating. In the present study, a mechanical model i3 chosen to represent the compliant wall. In previous two-dimensional studies, coatings were found that provided significant transition delays. The present study allows for three-dimensional waves. These instabilities are found to dominate transition over compliant walls. However, transition delays are still obtained, compared with transition predictions for rigid walls. The angles of wave propagation are plotted with Reynolds number and frequency, Low frequency waves are found to be highly three-dimensional. Waves of all frequencies approach two-dimensional dominance as they propagate downstream. Calculations at fixed Reynolds numbers are also presented. These results indicate that the dominant mode for the coatings considered occurs for an oblique angle of approximately 40-60 degrees. Other modes of instability that arise as a result of the compliance are shown to remain marginally stable for oblique waves.