Boundary layer instability over compliant walls: Comparison between theory and experiment

Theoretical studies have shown that compliant walls are able to attenuate the Tollmien–Schlichting waves that lead to conventional two-dimensional boundary-layer transition. This phenomenon was demonstrated in towing-tank tests conducted by Gaster et al. The results of these experiments also featured a different and very dramatic form of boundary-layer breakdown. We contend that this type of breakdown was due to a hydroelastic mode of instability, namely traveling-wave flutter. In this paper we model the two-layer viscoelastic compliant wall of Gaster et al. and its interaction with the boundary-layer flow using the asymptotic theory of Carpenter and Gajjar; en-type calculations are carried out for the traveling-wave flutter. Excellent agreement is found between the stability characteristics of the TWF mode and the measurements of the new form of breakdown found in the experiments; thus a complete understanding of the physical features found in the experiments is now available. Such understanding is essential for progress to be made in the technological development of compliant panels for transition delay.