Static and dynamic instability of nanowire-fabricated nanoelectromechanical systems: effects of flow damping, van de Waals force, surface energy and microstructure

Surface energy and microstructure-dependent size phenomena can play significant roles in physical performance of nanoelectromechanical systems (NEMS). Herein, the static and dynamic pull-in instability of cantilever and double-clamped NEMS fabricated from conductive cylindrical nanowires with circular cross section is studied. The Gurtin–Murdoch surface elasticity in combination with the couple stress continuum theory is employed to incorporate the coupled effects of surface energy and microstructure-dependent size phenomenon. Using Green–Lagrange strain, the higher order surface stress components are incorporated into the governing equation. The effect of gas damping is considered in the model as well as structural damping. The nonlinear governing equation is solved using analytical reduced order method. The effects of various parameters on the static and dynamic pull-in parameters, phase plans, and stability threshold of the nanowire-based structures are demonstrated.

Static and dynamic instability characteristics of curved laminates with internal damage subjected to follower loading

This article deals with the study of vibration, buckling, and dynamic instability characteristics in damaged cross-ply and angle-ply curved laminates under uniform, uniaxial follower loading, using finite element approach. First-order shear deformation theory is used to model the doubly curved panels and is formulated according to Sandars' first approximation. Damage is modelled using an anisotropic damage formulation. Analysis is carried out on plate and three types of curved panels to obtain vibration, buckling, and dynamic instability (flutter) behaviour. The effect of damage on natural frequency, critical buckling load, flutter load, and flutter frequency is studied. The results show that the introduction of damage influences the flutter characteristics of panels more profoundly than the free-vibration or buckling characteristics. The results also indicate that, compared to undamaged panels, heavily damaged panels show steeper deviations in stability characteristics than mildly damaged ones.

The Primary Nonlinear Dynamics of Modal and Nonmodal Perturbations of Monochromatic Inertia–Gravity Waves

The breaking of an inertia–gravity wave (IGW), initiated by its leading normal modes (NMs) or singular vectors (SVs), and the resulting small-scale eddies are investigated by means of direct numerical simulations of a Boussinesq fluid characterizing the upper mesosphere. The focus is on the primary nonlinear dynamics, neglecting the effect of secondary instabilities. It is found that the structures with the strongest impact on the IGW and also the largest turbulence amplitudes are the NM (for a statically unstable IGW) or short-term SV (statically and dynamically stable IGW) propagating horizontally transversely with respect to the IGW, possibly in agreement with observations of airglow ripples in conjunction with statically unstable IGWs. In both cases these leading structures reduce the IGW amplitude well below the static and dynamic instability thresholds. The resulting turbulent dissipation rates are within the range of available estimates from rocket soundings, even for IGWs at amplitudes low enough precluding NM instabilities. Thus SVs can help explain turbulence occurring under conditions not amenable for the classic interpretation via static and dynamic instability. Because of the important role of the statically enhanced roll mechanism in the energy exchange between IGW and eddies, the turbulent velocity fields are often conspicuously anisotropic. The spatial turbulence distribution is determined to a large degree by the elliptically polarized horizontal velocity field of the IGW.

Modeling of Two-Phase Flow Instabilities in Microchannels

This paper addresses a non-dimensional analytical stability model aimed at predicting the occurrence of flow instabilities at micro-scale. In this context, linear stability model using homogenous flow was considered. Towards that, a linear stability model was developed using perturbation method. A characteristic equation (the response of pressure drop to a hypothetical perturbation in inlet velocity) obtained in this analysis, is shown to be a function of sub-cooling number, Zuber number, Froude number, friction number and inlet and outlet restriction coefficients. Then, a neutral dynamic stability curve is obtained using D-Partition approach. Similarly, static or excursive stability curve is also obtained from the characteristic equation. The derived analytical form for static and dynamic instability threshold is represented in the form of simplified correlations. The experimental data reported by other researchers agree well with these correlations. From the results, it is amply clear that for all practical purposes, two-phase cooling will be unstable. The question to be answered in future is, therefore, whether the oscillations that accompany can be tolerated from the application viewpoint.