Scatterless interference: Delay of laminar-to-turbulent flow transition by a lattice of subsurface phonons

Abstract Wave interference has historically relied on scattering objects placed within the wave domain. Here, we introduce a fundamentally new mechanism: scatterless interference induced by a lattice of subsurface phonon motion beneath a smooth wall interfacing with an unstable laminar channel or boundary-layer flow. The subsurface consists of a wall-parallel lattice of wall-normal frequency-dependent phononic structural units, each designed to locally respond in an out-of-phase manner to a flow perturbation that is growing along the streamwise direction, dynamically influencing it at the point of interaction. Collectively, the lattice induces an interference effect that causes the kinetic energy (KE) of the flow instability to decay downstream, thereby delaying laminar-to-turbulent transition. To guide the design of the phononic subsurface (PSub) lattice, a Bloch-wave unit-cell analysis is developed for flow perturbations, and direct numerical simulations (DNS) validate the concept. This work establishes scatterless interference as a distinct physical phenomenon, marking a paradigm shift in the design philosophy for aerodynamic and hydrodynamic surfaces across aircraft, marine vessels, ground vehicles and other applications. This shift moves beyond streamlined shaping, leveraging subsurface phonon engineering for drag reduction and enhanced performance.