arXiv subjects

Armin Kianfar

Publications and source records attributed to Armin Kianfar.

4 recordsLinked to original sources

arXiv2025

Super resonance: Breaking the bandwidth limit of resonant modes and its application to flow control

We report the discovery of super resonance--a new regime of resonant behavior in which a mode's out-of-phase response persists far beyond its classical bandwidth. This effect emerges from a coiled phononic structure composed of a locally resonant elastic metamaterial and architected to support multiple internal energy pathways. These pathways converge at a single structural location, enabling extended modal dominance and significantly broadening the frequency range over which a resonant phase is sustained. We demonstrate by direct numerical simulations the implications of this mechanism in the context of flow instability control, where current approaches are inherently constrained by the characteristically narrow spectral bandwidth of conventional resonances. Using a super-resonant phononic subsurface structure interfacing with a channel flow, we show passive simultaneous suppression of four unstable flow perturbations across a frequency range more than five times wider than that is achievable with a standard resonance in an equivalent uncoiled structure. By enabling broadband, passive control of flow instabilities, super resonance overcomes a longstanding limitation in laminar flow control strategies. More broadly, it introduces a powerful new tool for phase-engineered wave-matter interaction. The ability to preserve out-of-phase modal response across wide spectral ranges establishes a fundamental advance in the physics of resonance, with far-reaching implications for suppressing fully developed turbulent flows and beyond.

physics.flu-dyn↗

arXiv2025

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.

physics.flu-dyn↗

arXiv2023

Local flow control by phononic subsurfaces over extended spatial domains

Local phonon motion underneath a surface interacting with a flow may cause the flow to passively stabilize, or destabilize, as desired within the region adjacent to the subsurface motion. This mechanism has been extensively analyzed over only a spatial region on the order of the instability wavelength along the fluid-structure interface. Here we uncover fundamental relations between the behavior of flow instabilities and the frequency response characteristics of the phononic subsurface structure admitting the elastic motion. These relations are then utilized to demonstrate the possibility of extensive spatial expansion of the control regime along the downstream direction with minimal loss of performance--potentially covering the entire surface exposed to the flow.

physics.flu-dyn↗

arXiv2023

Phononic-subsurface flow stabilization by subwavelength locally resonant metamaterials

The interactions between a solid surface and a fluid flow underlie dynamical processes relevant to air, sea, and land vehicle performance and numerous other technologies. Key among these processes are unstable flow disturbances that contribute to fundamental transformations in the flow field. Precise control of these disturbances is possible by introducing a phononic subsurface (PSub). This comprises locally attaching a finite phononic structure perpendicular to an elastic surface exposed to the flowing fluid. This structure experiences ongoing excitation by an unstable flow mode traveling in conjunction with the mean flow. The excitation generates small deformations at the surface that trigger elastic wave propagation within the structure. By targeted tuning of the unit-cell and finite-structure characteristics of the PSub, the returning waves may be devised to resonate and reenter the flow out of phase, leading to significant destructive interference of the incoming flow waves near the surface and subsequently to their attenuation over the spatial extent of the control region. This control mechanism is passive, responsive, and engineered offline without needing coupled fluid-structure simulations; only the flow instability's frequency, wavelength, and overall modal characteristics must be known. We present a PSub in the form of a locally resonant elastic metamaterial, designed to operate in the elastic subwavelength regime and hence being significantly shorter in length compared to a phononic-crystal-based PSub. Using direct numerical simulations (DNS) of channel flows, both types of PSubs are investigated, and their controlled spatial and energetic influence on the wall-bounded flow behavior is demonstrated and analyzed. We show that the PSub's effect is spatially localized as intended, with a rapidly diminishing streamwise influence away from its location in the subsurface.

physics.flu-dyn↗