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

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 a transitioning boundary-layer flow. The subsurface consists of a wall-parallel lattice of wall-normal frequency-dependent phononic structural units, each designed to respond to local flow perturbations in an out-of-phase manner, suppressing them at the point of interaction. Collectively, the lattice induces interference effects that cause the kinetic energy of flow instabilities to decay downstream, thereby delaying laminar-to-turbulent transition. To guide the design of the phononic subsurface lattice, a Bloch-wave unit-cell analysis is developed for the flow perturbations, and direct numerical simulations validate the concept. This work establishes scatterless interference as a distinct physical phenomenon and represents a paradigm shift in the design of aerodynamic and hydrodynamic surfaces--moving beyond streamlined shaping to leveraging subsurface phonon engineering for drag reduction and enhanced performance.