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1. On the Nonlinear Mode-Coupling in Ultra Precision Manufacturing Machines: Experimental and Analytical Analyses

   https://doi.org/10.1115/DETC2020-22398  

   Gupta, Sunit K.; Bukhari, Mohammad A.; Barry, Oumar R.; Okwudire, Chinedum E. (August 2020, Proceedings of the ASME 2020 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference)

   null (Ed.)

   Abstract Recent studies in passively-isolated systems have shown that mode coupling is desirable for best vibration suppression, thus refuting the long-standing rule of mode decoupling. However, these studies have ignored the non-linearities in the isolators. In this work, we consider stiffness nonlinearity from pneumatic isolators and study the nonlinear free undamped vibrations of a passively-isolated ultra-precision manufacturing (UPM) machine. Experimental analysis is conducted to guide the mathematical formulation. The system comprises linearly and nonlinearly coupled in-plane horizontal and rotational motion of the UPM machine with quadratic nonlinear stiffness from the isolators. We present closed-form expressions using the method of multiple scales for two cases viz. the non-resonant case and the bounded internal resonance case. We validate our theoretical findings through direct numerical simulations. For the non-resonant case, we show that the system behaves similar to a linear system. However, for the nearly internal resonance case, we demonstrate strong energy exchange between the modes stemming from nonlinear mode coupling. We further study the effect of nonlinear mode coupling on the vibration isolation performance and demonstrate that mode coupling is not always desirable. 

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2. Internal‐Wave Dissipation Mechanisms and Vertical Structure in a High‐Resolution Regional Ocean Model

   https://doi.org/10.1029/2023GL108039  

   Skitka, Joseph; Arbic, Brian K; Ma, Yuchen; Momeni, Kayhan; Pan, Yulin; Peltier, William R; Menemenlis, Dimitris; Thakur, Ritabrata (September 2024, Geophysical Research Letters)

   Abstract Motivated by the importance of mixing arising from dissipating internal waves (IWs), vertical profiles of internal‐wave dissipation from a high‐resolution regional ocean model are compared with finestructure estimates made from observations. A horizontal viscosity scheme restricted to only act on horizontally rotational modes (such as eddies) is introduced and tested. At lower resolutions with horizontal grid spacings of 2 km, the modeled IW dissipation from numerical model agrees reasonably well with observations in some cases when the restricted form of horizontal viscosity is used. This suggests the possibility that if restricted forms of horizontal viscosity are adopted by global models with similar resolutions, they could be used to diagnose and map IW dissipation distributions. At higher resolutions with horizontal grid spacings of ∼250 m, the dissipation from vertical shear and horizontal viscosity act much more strongly resulting in dissipation overestimates; however, the vertical‐shear dissipation itself is found to agree well with observations. 

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3. Low‐Latitude Plasma Drifts From the Horizontal Neutral Wind Model and a Coupled Ionosphere‐Electric Field Model

   https://doi.org/10.1029/2020JA029056  

   Eccles, James Vincent; Valladares, Cesar Enrique (July 2021, Journal of Geophysical Research: Space Physics)

   Abstract Results from a dynamo electric field model are presented to examine the consistency of the widely used empirical models of low‐latitude plasma drifts and thermospheric neutral winds. The sector defined by the Jicamarca Radar measured plasma drifts is used due to the greater certainty of the empirical vertical plasma drifts. The plasma drifts produced by the Horizontal Wind Model (HWM) in a coupled ionosphere‐electric field model for geomagnetically quiet and moderate solar conditions are compared against empirical models of equatorial plasma drifts for the Peruvian sector. The HWM generates reasonable sunset prereversal enhancement of the vertical drift in all but May, June, July, and August when no prereversal enhancement exists in the empirical results. The daytime vertical drifts are deficient during all seasons. A solar diurnal and semi‐diurnal tidal forcing are required in the E region (100–150 km) to bring the HWM into better agreement as a dynamo driver for the daytime electric fields associated with the broad Solar Quiet current system. 

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4. On the Nature of Intense Sub‐Relativistic Electron Precipitation

   https://doi.org/10.1029/2022JA030571  

   Artemyev, A. V.; Zhang, X. ‐J.; Zou, Y.; Mourenas, D.; Angelopoulos, V.; Vainchtein, D.; Tsai, E.; Wilkins, C. (June 2022, Journal of Geophysical Research: Space Physics)

   Abstract Energetic electron precipitation into Earth's atmosphere is an important process for radiation belt dynamics and magnetosphere‐ionosphere coupling. The most intense form of such precipitation is microbursts—short‐lived bursts of precipitating fluxes detected on low‐altitude spacecraft. Due to the wide energy range of microbursts (from sub‐relativistic to relativistic energies) and their transient nature, they are thought to be predominantly associated with energetic electron scattering into the loss cone via cyclotron resonance with field‐aligned intense whistler‐mode chorus waves. In this study, we show that intense sub‐relativistic microbursts may be generated via electron nonlinear Landau resonance with very oblique whistler‐mode waves. We combine a theoretical model of nonlinear Landau resonance, equatorial observations of intense very oblique whistler‐mode waves, and conjugate low‐altitude observations of <200 keV electron precipitation. Based on model comparison with observed precipitation, we suggest that such sub‐relativistic microbursts occur by plasma sheet (0.1 − 10 keV) electron trapping in nonlinear Landau resonance, resulting in acceleration to ≲200 keV energies and simultaneous transport into the loss cone. The proposed scenario of intense sub‐relativistic (≲200 keV) microbursts demonstrates the importance of very oblique whistler‐mode waves for radiation belt dynamics. 

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5. Stability of internal gravity wave modes: from triad resonance to broadband instability

   https://doi.org/10.1017/jfm.2023.265  

   Akylas, T.R.; Kakoutas, Christos (April 2023, Journal of Fluid Mechanics)

   A theoretical study is made of the stability of propagating internal gravity wave modes along a horizontal stratified fluid layer bounded by rigid walls. The analysis is based on the Floquet eigenvalue problem for infinitesimal perturbations to a wave mode of small amplitude. The appropriate instability mechanism hinges on how the perturbation spatial scale relative to the basic-state wavelength, controlled by a parameter $$\mu$$ , compares to the basic-state amplitude parameter, $$\epsilon \ll 1$$ . For $$\mu ={O}(1)$$ , the onset of instability arises due to perturbations that form resonant triads with the underlying wave mode. For short-scale perturbations such that $$\mu \ll 1$$ but $$\alpha =\mu /\epsilon \gg 1$$ , this triad resonance instability reduces to the familiar parametric subharmonic instability (PSI), where triads comprise fine-scale perturbations with half the basic-wave frequency. However, as $$\mu$$ is further decreased holding $$\epsilon$$ fixed, higher-frequency perturbations than these two subharmonics come into play, and when $$\alpha ={O}(1)$$ Floquet modes feature broadband spectrum. This broadening phenomenon is a manifestation of the advection of small-scale perturbations by the basic-wave velocity field. By working with a set of ‘streamline coordinates’ in the frame of the basic wave, this advection can be ‘factored out’. Importantly, when $$\alpha ={O}(1)$$ PSI is replaced by a novel, multi-mode resonance mechanism which has a stabilising effect that provides an inviscid short-scale cut-off to PSI. The theoretical predictions are supported by numerical results from solving the Floquet eigenvalue problem for a mode-1 basic state. 

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