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1. A Regime Diagram for Internal Lee Waves in Coastal Plain Estuaries

   https://doi.org/10.1175/JPO-D-21-0261.1  

   Li, Renjian; Li, Ming (December 2022, Journal of Physical Oceanography)

   Abstract Using an idealized channel representative of a coastal plain estuary, we conducted numerical simulations to investigate the generation of internal lee waves by lateral circulation. It is shown that the lee waves can be generated across all salinity regimes in an estuary. Since the lateral currents are usually subcritical with respect to the lowest mode, mode-2 lee waves are most prevalent but a hydraulic jump may develop during the transition to subcritical flows in the deep channel, producing high energy dissipation and strong mixing. Unlike flows over a sill, stratified water in the deep channel may become stagnant such that a mode-1 depression wave can form higher up in the water column. With the lee wave Froude number above 1 and the intrinsic wave frequency between the inertial and buoyancy frequency, the lee waves generated in coastal plain estuaries are nonlinear waves with the wave amplitude Δ h scaling approximately with , where V is the maximum lateral flow velocity and is the buoyancy frequency. The model results are summarized using the estuarine classification diagram based on the freshwater Froude number Fr f and the mixing parameter M . The Δ h decreases with increasing Fr f as stronger stratification suppresses waves, and no internal waves are generated at large Fr f . The Δ h initially increases with increasing M as the lateral flows become stronger with stronger tidal currents, but decreases or saturates to a certain amplitude as M further increases. This modeling study suggests that lee waves can be generated over a wide range of estuarine conditions. 

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2. Elastic Metasurfaces for Full Wavefront Control and Low-Frequency Energy Harvesting

   https://doi.org/10.1115/1.4050275  

   Lin, Zhenkun; Tol, Serife (December 2021, Journal of Vibration and Acoustics)

   null (Ed.)

   Abstract Controlling and manipulating elastic/acoustic waves via artificially structured metamaterials, phononic crystals, and metasurfaces have gained an increasing research interest in the last decades. Unlike others, a metasurface is a single layer in the host medium with an array of subwavelength-scaled patterns introducing an abrupt phase shift in the wave propagation path. In this study, an elastic metasurface composed of an array of slender beam resonators is proposed to control the elastic wavefront of low-frequency flexural waves. The phase gradient based on Snell’s law is achieved by tailoring the thickness of thin beam resonators connecting two elastic host media. Through analytical and numerical models, the phase-modulated metasurfaces are designed and verified to accomplish three dynamic wave functions, namely, deflection, non-paraxial propagation, and focusing. An oblique incident wave is also demonstrated to show the versatility of the proposed design for focusing of wave energy incident from multiple directions. Experimentally measured focusing metasurface has nearly three times wave amplification at the designed focal point which validates the design and theoretical models. Furthermore, the focusing metasurface is exploited for low-frequency energy harvesting and the piezoelectric harvester is improved by almost nine times in terms of the harvested power output as compared to the baseline harvester on the pure plate without metasurface. 

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3. Teleseismic earthquake wavefields observed on the Ross Ice Shelf

   https://doi.org/10.1017/jog.2020.83  

   Baker, Michael G.; Aster, Richard C.; Wiens, Douglas A.; Nyblade, Andrew; Bromirski, Peter D.; Gerstoft, Peter; Stephen, Ralph A. (February 2021, Journal of Glaciology)

   null (Ed.)

   Abstract Observations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data. 

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4. Tsunami versus Infragravity Surge: Comparison of the Physical Character of Extreme Runup

   https://doi.org/10.1029/2018GL080594  

   Montoya, Luis; Lynett, Patrick (December 2018, Geophysical Research Letters)

   Abstract Recent observations of energetic infragravity (IG) flooding events, such as those in the Philippines during Typhoon Haiyan, suggest that IG surges may approach the coast as breaking bores with periods of minutes: a very tsunami‐like characteristic. Energetic IG waves have been observed in various locations around the world and have led to loss of lives and damages to property. In this study, a comparison of overland flow characteristics between tsunamis and energetic IG wave events is presented. In general, whenever the tsunamis and energetic IG waves have similar runup, tsunamis tend to generate greater flow depths and longer flood durations than IG. However, flow velocities and Froude number are larger for IG primarily due to bore‐bore capture. This study provides a statistical and physical discriminant between tsunami and IG, such that in areas exposed to both, a proper interpretation of overland transport, deposition, and damage is possible. 

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5. Non-Hermitian planar elastic metasurface for unidirectional focusing of flexural waves

   https://doi.org/10.1063/5.0097177  

   Stojanoska, Katerina; Shen, Chen (June 2022, Applied Physics Letters)

   Metasurfaces exhibiting spatially asymmetric inner structures have been shown to host unidirectional scattering effects, benefiting areas where directional control of waves is desired. In this work, we propose a non-Hermitian planar elastic metasurface to achieve unidirectional focusing of flexural waves. The unit cells are constructed by piezoelectric disks and metallic blocks that are asymmetrically loaded. A tunable material loss is then introduced by negative capacitance shunting. By suitably engineering the induced loss profile, a series of unit cells are designed, which can individually access the exceptional points manifested by unidirectional zero reflection. We then construct a planar metasurface by tuning the reflected phase to ensure constructive interference at one side of the metasurface. Unidirectional focusing of the incident waves is demonstrated, where the reflected wave energy is focused from one direction, and zero reflection is observed in the other direction. The proposed metasurface enriches the flexibility in asymmetric elastic wave manipulation as the loss and the reflected phase can be tailored independently in each unit cell. 

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