| dc.description.abstract | Five experimental investigations concerning metasurfaces and phononic crystal like structures are presented in this thesis. This work covers a broad range of topics, from radiative effects of interference between the symmetric and anti-symmetric Lamb modes in submerged plates, to hybrid acoustic-elastic modes in an airborne structure. The overarching aim is to find potential routes to mitigate unwanted near- and far-field noise, primarily the stochastic near-field noise generated by turbulent fluid flow.
The first chapter investigates the radiative behaviour of a stack of soft elastic plates submerged in water, building on existing work in the literature the study explores the broadband, angular-dependent reflection caused by interference of the symmetric and anti-symmetric Lamb modes supported in a plate, and how this effect can be enhanced by building a structure consisting of multiple plates of differing thickness. A submerged graded stack of five acrylonitrile-butadiene-styrene plates (thicknesses 6, 5, 4, 3, 2 mm) was constructed, with neighbouring plates separated by a 1 mm thick water-filled cavity and a maximum reflection bandwidth of 250 kHz was found for 35.75 degree incidence.
The second chapter develops from the first to consider non-radiative surface acoustic waves supported by a multi-plate stack. In this case the stack consists of 2 or 3 acrylic plates of 3 mm thickness, separated by water filled cavities of either 1.5 or 0.75 mm thickness. This study investigates the behaviour of the Krauklis-like modes supported by the structure; these modes are the result of pairs of Scholte-modes coupling between not only the two elastic surfaces of each plate, but also across each of the water filled cavities. This gives rise to several highly dispersive modes. The dispersion of these modes is predicted numerically and characterised experimentally.
The third experimental chapter concerns a metasurface consisting of an array of cavities, each closed at one end, which is coupled with a turbulent air flow via an additional cavity with open ends. The surface modes supported by the array are investigated numerically and their excitation by the turbulent flow demonstrated experimentally. This is followed by a numerical investigation into the efficiency of the coupling between the open cavity and the array and a `first look' proof-of-concept model to determine the viability of re-introducing the turbulence-excited surface wave back into the flow as a method of exerting additional control over the flow. Whilst the excitation of the surface mode by flow is readily demonstrated with modelling predicting a coupling efficiency of 90\% between the open cavity and array, the proof-of-concept modelling on re-introduction shows that this is unlikely to prove a fruitful avenue to develop, with pressure fields on the `flow side' having amplitudes more than an order of magnitude smaller than those presented to the array.
In light of the findings of the third chapter, the fourth concerns initial investigations into alternative uses of the array structure. It details the `in-house' manufacture of a modular variant of the slot array structure. It is found that the thin, elastic walls of the cavity are sufficiently compliant that the structure supports several hybrid acoustic-elastic modes, in contradiction with the typical rigid wall assumption of airborne physics. These hybrid modes are identified experimentally, with numerical modelling used to calculate the effective elastic properties of the 3d printed structure with these effective values confirmed via a bending beam experiment. As one of these hybrid modes exhibits negative dispersion further modelling was performed to investigate the distribution of power within the structure as this mode propagates and it was found that the forward and backward power channels within the structure interact and share power in a thin region localised near the mouths of the cavities.
The fifth and final experimental chapter adapts the array structure considered previously to include periodic open cavities, effectively constructing a diffraction grating coupled to a metasurface. This structure scatters coherent excitation into predictable wavenumber ranges dependent on their frequency, allowing for the determination of the incident angle through measurement of the frequency-wavenumber spectra beneath the array. The angle detection capabilities are confirmed in a quiescent environment, with angles of 0, 5, 10, 15, 20, 25, and 50 $\pm1.5^{\circ}$ successfully resolved to within error. In addition, the performance of the system in the presence of stochastic noise is investigated via numerical modelling. The structure is found to allow accurate measurement of the incident angle of a coherent far-field signal in low signal to noise (SNR) environments with SNR ratios as low as 0.2. | en_GB |
