Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The differential quadrature method is a numerical solution technique for initial and/or boundary problems. It was developed by the late Richard Bellman and his associates in the early 70s and, since then, the technique has been successfully employed in a variety of problems in engineering and physical sciences. The method has been projected by its proponents as a potential alternative to the conventional numerical solution techniques such as the finite difference and finite element methods. This paper presents a state-of-the-art review of the differential quadrature method, which should be of general interest to the computational mechanics community.

Differential Quadrature Method in Computational Mechanics: A Review

Experiments in hearing

The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electromechanical systems. The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener’s well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.

Thermoelastic Damping in Micro- and Nano-Mechanical Systems.

Strong Formulation Finite Element Method Based on Differential Quadrature: A Survey

This study focuses on the prediction of complex eigenpairs of modified asymmetric systems. A method, which combines the reduced basis and the projection technique, is presented to predict the eigenpairs of modified asymmetric systems. The reduced basis obtained using the homotopy perturbation technique only requires the eigensolution of interest such that the modal truncation problem can be avoided. More importantly, the method can be carried out without using the left eigenvectors. The accuracy of the reduced basis approximation can be improved by introducing the projection technique. A rotor dynamic system is used to illustrate the capacity of predicting the changes of eigensolutions.

Modal Modification of Damped Asymmetric Systems without Using the Left Eigenvectors

Low quality (Q) factor is often the limiting factor for high performance carbon nanotube (CNT) resonators. The most commonly used approach to enhance the Q factor of CNTs is to reduce/eliminate the intrinsic defects. Herein, we show surprisingly that hole defects of suitable size and position are able to enhance the Q factor of CNT, which strongly contradicts to the common notion that the presence of defects promote intrinsic dissipation via defects dissipation. By analyzing the strain distribution, we find that such abnormal enhancement in Q factor of defected CNT originates from a coupling competition mechanism between the atomic mismatch around defected atoms and the thermoelastic damping. Although the presence of holes will introduce an additional defect dissipation source, suitable holes are capable of reducing the energy dissipation arisen from the thermoelastic damping, through changing the spatial strain field of defected CNT. This coupling competition mechanism provides a new route for designing high performance CNT resonators via defects engineering.

Abnormal enhancement to the quality factors of carbon nanotube via defects engineering

The stress corrosion cracking (SCC) susceptibility of 316L austenitic stainless steel milled with cryogenic cooling and conventional cutting fluid cooling is investigated and compared. The milled subsurface properties are characterized in terms of microstructure, residual stress and microhardness. The SCC susceptibility of the milled surfaces is evaluated in boiling MgCl2 by examining the density and depth of the SCC microcracks. Results show that the cryogenic milling increases the SCC susceptibility of 316L austenitic stainless steel. The increase of SCC susceptibility is attributed to the higher residual tensile stress level and the larger volume fraction of slip band produced in the cryogenic milling process in comparison with the conventional cooling milling.

Effects of Cryogenic Milling on Stress Corrosion Cracking Resistance of AISI 316L Austenitic Stainless Steel

Spatiotemporal Damping of Dissipative Metamaterial

Abstract The design of thermoelastic damping (TED) affected by the phase-lagging non-Fourier heat conduction effects becomes significant but challenging for enlarging the quality factor of widely-used microresonators operating in extreme situations, including ultra-high excitation frequency and ultra-low working temperature. However, there does not exist a rational method for designing the TED in the framework of non-Fourier heat conduction law. This work, therefore, proposes a design framework to achieve low thermoelastic dissipation of microresonators governed by the phase-lagging heat conduction law. The equation of motion and the heat conduction equation for phase-lagging TED microresonators are derived first, and then the non-Fourier TED design problem is proposed. A topology optimization-based rational design method is used to resolve the design problem. What is more, a two-dimensional (2D) plain-strain-based finite element method (FEM) is developed as a solver for the topology optimization process. Based on the suggested rational design technique, numerical instances with various phase lags are investigated. The results show that the proposed design method can remarkably reduce the dissipation of microresonators by tailoring their substructures. 

/pdf/rational-design-of-thermoelastic-damping-in-microresonators-2n7c2ihi.pdf

Rational design of thermoelastic damping in microresonators with phase-lagging heat conduction law

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