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 is chosen to represent the compliant wall. In previous two-dimensional studies, coatings were found that provided significant transition delays. The present study allows for threedimensional 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 and obliquity are determined as a function of Reynolds and frequency. The propagation or group velocity direction is found to be nearly axial. 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 deg. Other modes of instability that arise as a result of the compliance are discussed.