Design and development of a multi-functional bi-anisotropic metasurface with ultra-wide out of band transmission

This paper presents a multi-functional bi-anisotropic metasurface having ultra-wide out of band transmission characteristics. The proposed metasurface is comprised of 90° rotated T-shaped configuration yielding greater than or equal to 50% out-of-band transmission from above L- to X-band. Moreover, this metasurface achieves a maximum of 99% out-of-band transmission at lower frequency bands (i.e., L-band). The simultaneous absorptive and controlled reflection functionalities are achieved at 15.028 to 15.164 GHz along with polarization-insensitive and angular stable properties. The proposed metasurface yields state-of-the-art features compared to already published papers and has broader scope for Fabry Perot cavity, Radar cross-section (RCS) reduction, electromagnetic compatibility and interference (EMC/I) shielding, selective multi-frequency bolometers, ultrathin wave trapping filters, sensors and beam-splitters in the microwave domain.

Dynamic soliton–mean flow interaction with non-convex flux

The interaction of localised solitary waves with large-scale, time-varying dispersive mean flows subject to non-convex flux is studied in the framework of the modified Korteweg–de Vries (mKdV) equation, a canonical model for internal gravity wave propagation and potential vorticity fronts in stratified fluids. The effect of large amplitude, dynamically evolving mean flows on the propagation of localised waves – essentially ‘soliton steering’ by the mean flow – is considered. A recent theoretical and experimental study of this new type of dynamic soliton–mean flow interaction for convex flux has revealed two scenarios where the soliton either transmits through the varying mean flow or remains trapped inside it. In this paper, it is demonstrated that the presence of a non-convex cubic hydrodynamic flux introduces significant modifications to the scenarios for transmission and trapping. A reduced set of Whitham modulation equations is used to formulate a general mathematical framework for soliton–mean flow interaction with non-convex flux. Solitary wave trapping is stated in terms of crossing modulation characteristics. Non-convexity and positive dispersion – common for stratified fluids – imply the existence of localised, sharp transition fronts (kinks). Kinks play dual roles as a mean flow and a wave, imparting polarity reversal to solitons and dispersive mean flows, respectively. Numerical simulations of the mKdV equation agree with modulation theory predictions. The mathematical framework developed is general, not restricted to completely integrable equations like mKdV, enabling application beyond the mKdV setting to other fluid dynamic contexts subject to non-convex flux such as strongly nonlinear internal wave propagation that is prevalent in the ocean.

Realization of Ultrathin Waveguides by Elastic Metagratings

Guiding transports of classical waves has inspired a wealth of nontrivial physics and momentous applications in a wide range of fields. To date, a robust and compact way to guide energy flux travelling along an arbitrary, prescheduled trajectory in a uniform medium is still a fundamental challenge. Here we propose and experimentally realize a generic framework of ultrathin waveguides for full-angle wave trapping and routing. The metagrating-based waveguide can totally suppress all high-order parasitic diffractions to efficiently route guided elastic waves without leakage. Remarkably, the proposed waveguide protype works in a broad frequency range from 12 to 18 kHz and regardless of the incident angle. An analytical slab-waveguide model is further presented to predict and tailor the diffracted patterns in the metagrating-based waveguide. Compared with existing methods based on topological edge states or defected metamaterials, our meta-waveguide strategy exhibits absolute advantages in compact size, robust performance, and easy fabrication, which may provide a new design paradigm for vibration control in solids, wave steering in electromagnetics, acoustics and other waves.

Trapped Modes and Negative Refraction in a Locally Resonant Metamaterial: Transient Insights into Manufacturing Bounds for Ultrasonic Applications

The transient scattering of in-plane elastic waves from a finite-sized periodic structure, comprising a regular grid of Swiss-cross holes arranged according to a square lattice, is considered. The theoretical and numerical modelling focuses on the unexplored ultrasonic frequency regime, well beyond the first, wide, locally resonant band-gap of the structure. Dispersive properties of the periodic array, determined by Bloch–Floquet analysis, are used to identify candidates for high-fidelity GPU-accelerated transient scattering simulations. Several unusual wave phenomena are identified from the simulations, including negative refraction, focusing, partial cloaking, and wave trapping. The transient finite element modelling framework offers insights on the lifetimes of such phenomena for potential practical applications. In addition, nonideal counterparts with rough edges are modelled using characteristic statistical parameters commonly observed in additive manufacturing. The analysis shows that the identified wave effects appear likely to be robust with respect to potential manufacturing uncertainties in future studies.

Near-inertial wave critical layers over sloping bathymetry

This study describes a specific type of critical layer for near-inertial waves (NIWs) that forms when isopycnals run parallel to sloping bathymetry. Upon entering this slantwise critical layer, the group velocity of the waves decreases to zero and the NIWs become trapped and amplified, which can enhance mixing. A realistic simulation of anticyclonic eddies on the Texas-Louisiana shelf reveals that such critical layers can form where the eddies impinge onto the sloping bottom. Velocity shear bands in the simulation indicate that windforced NIWs are radiated downward from the surface in the eddies, bend upward near the bottom, and enter critical layers over the continental shelf, resulting in inertially-modulated enhanced mixing. Idealized simulations designed to capture this flow reproduce the wave propagation and enhanced mixing. The link between the enhanced mixing and wave trapping in the slantwise critical layer is made using ray-tracing and an analysis of the waves’ energetics in the idealized simulations. An ensemble of simulations is performed spanning the relevant parameter space that demonstrates that the strength of the mixing is correlated with the degree to which NIWs are trapped in the critical layers. While the application here is for a shallow coastal setting, the mechanisms could be active in the open ocean as well where isopycnals align with bathymetry.

The energy budget of the tropical band in future climate

<p>The atmospheric circulation is expected to change in response to anthropogenic CO<sub>2</sub> emissions. Both theory and model simulations of future climate suggest that the tropical overturning will weaken, with a weaker Hadley Circulation ascent, while the stratification of moist static energy (MSE) will strengthen. These two changes have opposite effects on the energy balance of the deep tropics. In the unperturbed system, the equatorward convergence of the mean flow in the lower troposphere (i.e. at low MSE) is compensated by a divergence in the upper troposphere (i.e. at high MSE), resulting in a net lateral export of MSE out of the region of ascent.</p><p>The weakening of the circulation in a future warmer climate would weaken the export of MSE while the strengthening of the stratification -- an increase of the MSE contrast between the upper and lower branches -- would reinforce it. However, previous studies suggest that these two effects do not exactly cancel out. A neglected element in this picture is the primary driver of these changes: due to the long-wave trapping by higher CO<sub>2 </sub>concentration, the tropical atmosphere will also receive more energy at the top and bottom (an increased Net Energy Input, NEI).</p><p>In this study, we attempt to reconcile changes in the circulation, stratification and NEI under climate change. Specifically, we investigate 1) to which extent the effects of circulation and stratification changes on the MSE budget compensate and 2) if inclusion of the NEI changes brings the MSE budget closer to equilibrium.</p><p>To address these questions, we compute the Gross Moist Stability in a series of simulations from the Coupled Model Intercomparison Project 5 archive. To test our understanding of the MSE budget, we consider both a future climate scenario (RCP8.5) and the mid-Holocene (6000 A.D). For the future climate, we show that, although there is a rough balance by the circulation and stratification effects, inclusion of the NEI term significantly improves the closure of the MSE budget in the deep tropics. The mid-Holocene case is, however, fundamentally different as both stratification and circulation weaken, reinforcing their effects on the MSE export. In this case, inclusion of the NEI term is critical to establish the MSE balance of the deep tropics.</p><p>Both cases underline that a three-term balance (between changes in circulation, MSE stratification and NEI) provides a robust description of the deep tropics MSE budget under climate change.</p>