Charles B. Watkins

Atomistic hybrid DSMC/NEMD method for nonequilibrium multiscale simulations

A multiscale hybrid method for coupling the direct simulation Monte Carlo (DSMC) method to the nonequilibrium molecular dynamics (NEMD) method is introduced. The method addresses Knudsen layer type gas flows within a few mean free paths of an interface or about an object with dimensions of the order of a few mean free paths. It employs the NEMD method to resolve nanoscale phenomena closest to the interface along with coupled DSMC simulation of the remainder of the Knudsen layer. The hybrid DSMC/NEMD method is a particle based algorithm without a buffer zone. It incorporates a new, modified generalized soft sphere (MGSS) molecular collision model to improve the poor computational efficiency of the traditional generalized soft sphere GSS model and to achieve DSMC compatibility with Lennard-Jones NEMD molecular interactions. An equilibrium gas, a Fourier thermal flow, and an oscillatory Couette flow, are simulated to validate the method. The method shows good agreement with Maxwell-Boltzmann theory for the equilibrium system, Chapman-Enskog theory for Fourier flow, and pure DSMC simulations for oscillatory Couette flow. Speedup in CPU time of the hybrid solver is benchmarked against a pure NEMD solver baseline for different system sizes and solver domain partitions. Finally, the hybrid method is applied to investigate interaction of argon gas with solid surface molecules in a parametric study of the influence of wetting effects and solid molecular mass on energy transfer and thermal accommodation coefficients. It is determined that wetting effect strength and solid molecular mass have a significant impact on the energy transfer between gas and solid phases and thermal accommodation coefficient.

Dynamics of microscale shock/vortex interaction

Molecular simulations in a dilute monatomic gas were carried out to characterize the mutual interactions of impinging planar shocks of up to Mach 3 with transverse microvortices having core sizes comparable to the thickness of the shock. Time dependent simulations were performed using the direct simulation Monte Carlo method and then analyzed by applying transport theory to the sampled molecular results. Several flow cases were computed for initially stationary, composite vortices. The results reveal the generic features of the interaction, the effect of vortex size, and the effects of shock strength. In all cases, the applied straining compression in the shock was of the same order as the time scale of the vortex rotation. Most of the features found in shock interaction with a macroscale vortex of the same type were also found at microscale. These include acoustic wave formation, shock diffraction and refraction, vortex deformation and displacement, and dilatational vorticity generation greater than baro...

Multiscale molecular simulations of argon vapor condensation onto a cooled substrate with bulk flow

A hybrid simulation method is employed to study the condensation of saturated argon vapor flowing tangentially across a stationary cooled substrate, at nanoscale resolution. The method combines a direct simulation Monte Carlo treatment of the bulk vapor phase with a nonequilibrium molecular dynamics treatment of the condensed liquid and interphase regions; it provides an efficient simulation procedure for a heterogeneous system with a large ratio of vapor to liquid length scales. Starting from a bare, crystalline solid wall, the condensation process evolves from a transient unsteady state to a quasisteady state, where interfacial properties and heat and mass transfer parameters are analyzed. The Knudsen layer structure from the hybrid simulation is compared with kinetic theory predictions from a modified moment method analysis and from pure DSMC simulation. The effects of condensation strength and a tangential flow velocity that is on the order of the condensation velocity are examined. A comparison is ma...

Biodynamics Model for Operator Head Injury in Stand-Up Lift Trucks

Biodynamics and injury potential of operators in stand-up rider lift truck accidents have been investigated with a special focus on head injury. An anthropomorphic test device (ATD) model was used as an operator surrogate in computer simulations of off-the-dock (OTD) and tip-over (TO) accidents. The biomechanical model representing the ATD was developed based on rigid body segments, and then combined with a rigid body truck model in the accident simulations. The operator compartment of the truck model was enclosed with a rear door. The computed kinematics are in agreement with the results of previous experimental testing. A 2D finite element model of the head was created to compute head impact decelerations in the sagittal plane. Values of the head injury criterion for the TO cases were computed from the model and shown to compare favourably with experimental values. The results advance the state of knowledge concerning injury potential in TO and OTD accidents and simulation models for such accidents.

Simulation of Stand-Up Lift Truck Accidents to Evaluate Their Design and Operator Training Implications

OSHA regulations mandate that operators of stand-up rider lift trucks be trained to jump clear of the truck in overturn accidents. Operator compartment doors have been considered as a design modification to protect operators against lower leg injuries in some other types of stand-up rider accidents. However, they restrict the ability of operators to rapidly jump clear and must be evaluated against the injuries that operators can incur by remaining in the truck. This paper describes computer simulation by a newly developed biodynamic model and testing with anthropomorphic test devices and actual trucks to advance the state of knowledge concerning injury potential in tip-over and off-the-dock accidents. Implications of the results for design as well as for OSHA training regulations are discussed.Copyright

Compressibility and Density Effects in Free Subsonic Jet Flows: Three Dimensional PIV Measurements of Turbulence

The behavior of compressible subsonic turbulent jets issuing in still air has been investigated at three different subsonic Mach numbers, 0.3, 0.6 and 0.9. Helium, nitrogen and krypton gases were used to generate the jet flows and investigate the effects of density on the structure of turbulence. Stereo Particle Image Velocimetry was used in the present investigation. Helium jets were found to have the largest spreading rate among the three different gas jets used in the present investigation, while krypton had the lowest spreading rate. All jets attain self-similarity at downstream locations. Extremely large velocity fluctuations and correlations were measure in the case of helium jets.

Shock/Nonlinear Harmonic Wave Interaction

*† ‡ This study is a computational investigation of a normal shock wave interacting with strongly and weakly nonlinear, longitudinal harmonic waves created by an oscillating wall. The interaction is examined over a range of oscillating amplitudes and incident shock Mach numbers. The effects of the interaction on fluctuating kinetic energy, observed frequency of oscillation, and shock speed are presented. Departures from the observations of previous investigations based on linear oscillations are noted. Nomenclature I a = speed of sound at initial conditions (dimensional) f = non-dimensional fundamental frequency of fluctuating velocity S M = shock Mach number referenced to I a W M = amplitude Mach number of oscillating wall referenced to I a

Arachnoid Trabeculae and CSF Roles in Blunt Head Impacts

The blunt head impacts due to vehicular collisions, contact sports or falls cause relative motion between the brain and skull and an increase in contact and shear stresses in meningeal region. Several models have been developed to better understand brain injuries. In this study the mechanical role of the fibrous trabeculae and the Cerebrospinal Fluid (CSF) in Subarachnoid space (SAS) is investigated. Two-dimensional solid and fluid global models of the head and a local model of the SAS trabeculae were developed. The CSF pressure distribution and the trabeculae deformations were determined. It is concluded that the arachnoid trabeculae reduce the pressure in the CSF and both play a major role in damping the blunt head impact.Copyright

Biodynamics of Stand-Up Rider Lift Truck Accidents

This paper describes testing and simulation of operator and truck kinematics during stand-up rider forklift truck accidents. A Hybrid III, ATD was used as an operator surrogate and tested in an off-the-dock accident experiment. A rigid multibody, biomechanical model, representing the ATD, was developed and validated with deceleration sled data. The biomechanical model was then used with a truck model in a simulation of the test. The kinematic results are in agreement with the testing. Various physical and initial conditions of the truck and biomechanical model were studied.Copyright

Molecular Dynamics Simulations of Thermo-Mechanical Effects in Nanodrop Impact

Nanodrop impact onto a solid substrate is of interest for nano-scale liquid-impingement, phase-change cooling and for material deposition processes. In the present study, classical molecular dynamics (MD) simulation techniques were implemented to study the thermo-mechanical properties of the impact of nanometer scale liquid droplets upon an atomistic substrate at a temperature higher than that of the droplet. The droplets were comprised of approximately 50,000 Lennard-Jones atoms arranged in tetramer finitely extensible non-linear elastic (FENE) chain molecules forming a sphere of 8 nm radius. They were equilibrated and then projected towards a wall, where we observed the response upon collision by changes in shape, temperature, and density gradients, across a variety of impingement velocities, substrate temperatures, and wetting conditions. The baseline cases of equal substrate and nano-drop temperature were validated by comparison with previously reported results. A reaction spectrum ranging from full thermal vaporization of the drop, with respective substrate cooling, to complete kinetic disintegration upon impact and surface heating are analyzed. The variation of thermal and kinetic effects across the parametric environment is used to identify those regimes that optimize heat transfer from the surface, as well as those that best facilitate material deposition processes.© 2014 ASME