Effect Of Local Properties Research Articles

Modeling wall effects in a micro-scale shock tube using hybrid MD–DSMC algorithm

Wall effects in a micro-scale shock tube are investigated using the Direct Simulation Monte Carlo method as well as a hybrid Molecular Dynamics–Direct Simulation Monte Carlo algorithm. In the Direct Simulation Monte Carlo simulations, the Cercignani–Lampis–Lord model of gas–surface interactions is employed to incorporate the wall effects, and it is shown that the shock attenuation is significantly affected by the choice of the values of tangential momentum accommodation coefficient. A loosely coupled Molecular Dynamics–Direct Simulation Monte Carlo approach is then employed to demonstrate incomplete accommodation in micro-scale shock tube flows. This approach uses fixed values of the accommodation coefficients in the gas–surface interaction model, with their values determined from a separate dynamically similar Molecular Dynamics simulation. Finally, a completely coupled Molecular Dynamics–Direct Simulation Monte Carlo algorithm is used, wherein the bulk of the flow is modeled using Direct Simulation Monte Carlo, while the interaction of gas molecules with the shock tube walls is modeled using Molecular Dynamics. The two regions are separate and coupled both ways using buffer zones and a bootstrap coupling algorithm that accounts for the mismatch of the number of molecules in both regions. It is shown that the hybrid method captures the effect of local properties that cannot be captured using a single value of accommodation coefficient for the entire domain.

Influence of Local Properties on the Perception of Global Object Orientation.

In previous studies, the effect on perception of individual features such as curvature and edges have been studied with specifically designed stimuli. However, the effect of local properties on the perception of the global object has so far received little attention. In this study, cylinders with an elliptical cross section and rectangular blocks were used to investigate the effect and relative importance of curvature, change in curvature and edges, as local properties, on the ability of subjects to determine the orientation of the stimuli, which is a global property. We found that when curvature was present the threshold to determine the orientation was 43 percent lower than when curvature was absent. When, in addition, the change in curvature could be felt, the threshold was 37 percent lower than when only curvature could be felt. Finally, when edges were felt during exploration, the threshold increased by 46 percent compared to when the subjects were instructed to avoid the edges in the blocks. We conclude that the perception of curvature and change in curvature improve the performance of humans in perception of the whole shape, whereas edges, when not directly contributing to the task, disrupt performance.