Colloquium: Phononic crystals for passive hypersonic flow control

Hypersonic flight is governed by strongly coupled aerothermodynamic, acoustic, and structural phenomena. Among the central challenges is the onset of boundary-layer transition, which can substantially increase skin-friction drag, wall heat flux, and unsteady loading. In cold-wall hypersonic boundary layers, transition is often dominated by Mack’s second mode, a high-frequency instability whose physical character is primarily acoustic, or thermoacoustic, in nature. Because this mode may be interpreted as an acoustic disturbance trapped within the boundary layer, its amplification is inherently sensitive to the dynamic response and effective impedance of the wall. This observation motivates the use of phononic subsurfaces as passive, structurally integrated mechanisms for frequency-selective flow control.

In this colloquium, I will present our recent work on bandgap formation in two-dimensional solid–void phononic crystals with four-fold rotational symmetry. The discussion will focus on experimentally realizable architectures belonging to the p4, p4mm, and p4gm plane groups. Using Bloch–Floquet band-structure analysis, evanescent-mode calculations, finite-size transmission-loss simulations, and ultrasonic experimental validation, I will show how Bragg scattering and local resonance can become strongly coupled to produce exceptionally wide complete bandgaps. Attention will be given to the roles of resonator size, ligament slenderness, and crystallographic symmetry in controlling attenuation of both longitudinal and transverse elastic waves.

I will then discuss how these bandgap mechanisms can be translated toward passive hypersonic flow control. The proposed concept is to embed single-phase metallic or ceramic phononic subsurfaces beneath an aerodynamically smooth wall, thereby targeting the ultrasonic frequencies associated with Mack’s second mode without introducing surface roughness or requiring external actuation. This approach is intended to combine the spectral selectivity of phononic cystals with the thermal robustness and mechanical integrity required for hypersonic environments. The talk will conclude by outlining a research path toward compact phononic subsurface inserts (less than 2 cm!) with bandgaps in the approximate 50–200 kHz range, suitable for future cone–flare experiments and for the broader development of passive transition-delay strategies in hypersonic flight.