Metasurfaces and Phononic Crystals for Manipulating Fluid Flow and Acoustics

The studentship is part of the UK’s Centre of Doctoral Training in Metamaterials (XM2) based in the Departments of Physics and Engineering on the Streatham Campus in Exeter. Its aim is to undertake world-leading research, while training scientists and engineers with the relevant research skills and knowledge, and professional attributes for industry and academia.

The interaction between a fluid and a solid surface in relative motion represents a dynamical process that is central to the problem of laminar-to-turbulent transition (and consequent drag increase) for air, sea and land vehicles, as well as long-range pipelines. We will extend our existing metamaterial concepts into this regime by building on our previous research involving the acoustic transport of energy across and through structured surfaces [1], investigations into the interaction of flow with, and across patterned surfaces [2] and resonant structures [3], and the study of topological modes [4].

Sound is a weak by-product of a subsonic turbulent flow. The main convective elements of the turbulence are silent and it is only spectral components with supersonic phase speeds that couple to the far-field sound. This is relatively well understood [e.g., 5] however the effect of materials and boundary conditions on the development of turbulence and flow separation is an area of active research. Recent papers have indicated that phononic crystals can control the development of flow instabilities in a waveguide, patterning can provide enhancement of heat transfer in flow channels, and hydrodynamic cloaks can influence the development of wakes [6,7,8].

The project will involve investigating the effect of patterned surfaces (‘metasurfaces’) and phononic crystal system [9] on flow instabilities. Systems will be modelled using analytical and numerical (e.g. COMSOL or OpenFOAM) methods to understand the influence of the patterning on the spectrum of flow instabilities. In this way, it will be possible to replace the substrate with an impedance boundary condition, with flow- and wavevector-dependent dispersion characteristics. We will also consider reconfigurable metasurfaces, or those offer stabilisation of flow patterns using time-dependent forcing methods [10].

In order to pursue the optimum experimental methods, we will seek to build on existing collaborations with our academic partners around the UK and beyond. It is our expectation that a suitable experimental apparatus will be established at Exeter in the medium term.

[1] Starkey el al., “Thin structured rigid body for acoustic absorption”, Appl. Phys. Lett. 110, 041902 (2017).
[2] S. Shelley et al., “Fluid mobility over corrugated surfaces in the Stokes regime”, Physics of Fluids 28, 083101 (2016).
[3] S. Shelley et al., “Flow Control Behind Bluff Bodies through the Interaction of an Attached Resonant Flexible Tail”, APS Division of Fluid Dynamics (Fall) 2016, abstract #H18.003.
[4] T. Atherton et al., “Topological Modes in One Dimensional Solids and Photonic Crystals”, Phys. Rev. B 93, 125106 (2016).
[5] A. P. Dowling, T. P. Hynes, “Sound generation by turbulence”, Eur. J. Mech. B Fluids 23, 491 (2004).