Manipulation of Laminar-to-Turbulent Transition Using Phononic Subsurfaces

This paper describes the interaction of Tollmien-Schlichting (T-S) waves with a phononic subsurface that is flush-mounted under a cutout in a flat plate. The phononic subsurface is designed to have a bandgap from 176 to 898 Hz, and a truncation resonance at 250 Hz. Thus, the PSub interaction surface should respond strongly when excited by a passing T-S wave. Amplified T-S waves, at a frequency of 250 Hz, are generated by passing current periodically through a metal ribbon that spans the flat plate. Boundary layer profiles at numerous locations along the plate are collected using a boundary layer hot wire system, and the stability of the T-S wave is monitored using particle image velocimetry (PIV). The streamwise and wall-normal velocity fluctuations are measured at two locations on the flat plate. In all experiments, the wall-normal velocity fluctuations are consistently amplified when compared to baseline measurements without a PSub. An accelerometer measures the PSub interaction surface vibration showing a dominant 250 Hz component with a displacement of approximately 0.5 μm (with a maximum of 1.2 μm). There is concern that the amplification of the wall normal velocity fluctuations may not be solely due to the PSub interaction surface motion. It is possible that the air volume containing the PSub may act as an unintended Helmholtz resonator, which could have a separate contribution. To test this possibility, the interactive effect between a T-S wave and a rigid PSub with equivalent enclosing volume as the original design is also tested. It is determined there are significant acoustic contributions, and it is surmised that there is a coupled acoustoelastic response from the original flexible PSub design. For both PSub samples, a spectral magnitude map is presented, the standard deviation of T-S wave velocity fluctuations are computed, and the development of the T-S wave is plotted along the plate. This paper demonstrates that PSubs can passively alter T-S wave development, but acoustic and elastic contributions must be carefully considered.