Earlier Simulation Work Research Articles

Surface Free Energy of a Hard-Disk Fluid at Curved Hard Walls: Theory and Simulation.

In this work, we examine the surface thermodynamics of a hard-disk fluid at curved hard walls using Monte Carlo (MC) simulation and a generalized scaled particle theory (gSPT). The curved walls are modeled as hard disks of varying radii, R. The surface free energy, γ, and excess surface volume, vex, for this system are calculated as functions of both the fluid packing fraction and the wall radius. The simulation results are used to test, for this system, the assumptions of morphometric thermodynamics (MT), which predicts that both γ and vex are linear functions of the surface curvature, 1/R, for a two-dimensional system. In addition, we compare the simulation results to the gSPT developed in this work, as well as with virial expansions derived from the known virial coefficients of the binary hard-sphere fluid. At low to intermediate packing fractions, the non-MT terms (terms of higher order than 1/R in a expansion of γ and vex) of γ are zero within the simulation error; however, at the highest densities, deviations from MT become significant, similar to what was seen in our earlier simulation work on the three-dimensional hard-sphere/hard-wall system. In addition, the new gSPT gives improved results for both γ and vex over standard scaled particle theory (SPT) but underestimates the deviations from MT at high density.

Experimental study of occupancy-based control of HVAC zones

We present experimental evaluation of two occupancy-based control strategies for HVAC (heating, ventilation, and air-conditioning) systems in commercial buildings that were proposed in our earlier simulation work. We implement these strategies in a test-zone of Pugh Hall at the University of Florida campus. By comparing their performance against a conventional baseline controller (that does not use real-time occupancy measurements) on days when exogenous inputs—such as weather—are similar, we establish the energy savings potential for each of these strategies. The two control strategies are of vastly different complexity: one is a rule-based feedback controller while the other is based on MPC (model predictive control) that requires real-time optimization based on dynamic models. The results of the evaluation are consistent with those of our prior simulation work, that (i) both occupancy based controllers yield substantial energy savings over the baseline controller without sacrificing thermal comfort and indoor air quality, and (ii) the much higher complexity MPC controller yields negligible benefit over the simple rule-based feedback controller. The experimental evaluation provides further confidence that high degree of energy savings is possible with simple control algorithms that use real-time occupancy measurements.

Optimal policies of non-cross-resistant chemotherapy on Goldie and Coldman’s cancer model

Mathematical models can be used to study the chemotherapy on tumor cells. Especially, in 1979, Goldie and Coldman proposed the first mathematical model to relate the drug sensitivity of tumors to their mutation rates. Many scientists have since referred to this pioneering work because of its simplicity and elegance. Its original idea has also been extended and further investigated in massive follow-up studies of cancer modeling and optimal treatment.Goldie and Coldman, together with Guaduskas, later used their model to explain why an alternating non-cross-resistant chemotherapy is optimal with a simulation approach. Subsequently in 1983, Goldie and Coldman proposed an extended stochastic based model and provided a rigorous mathematical proof to their earlier simulation work when the extended model is approximated by its quasi-approximation. However, Goldie and Coldman’s analytic study of optimal treatments majorly focused on a process with symmetrical parameter settings, and presented few theoretical results for asymmetrical settings. In this paper, we recast and restate Goldie, Coldman, and Guaduskas’ model as a multi-stage optimization problem. Under an asymmetrical assumption, the conditions under which a treatment policy can be optimal are derived. The proposed framework enables us to consider some optimal policies on the model analytically. In addition, Goldie, Coldman and Guaduskas’ work with symmetrical settings can be treated as a special case of our framework. Based on the derived conditions, this study provides an alternative proof to Goldie and Coldman’s work. In addition to the theoretical derivation, numerical results are included to justify the correctness of our work.

Insertion of nanoparticles into polymer brush under variable solvent conditions

In this work, two-dimensional lattice-based self-consistent field theory is used to study the free energy cost associated with the insertion of a nanoparticle into a polymer brush. The nanoparticle is modeled as a cylinder and the self-consistent field equations are formulated on a cylindrical lattice. The use of two-dimensional formalism makes it possible to take into account the distortion of the brush density profile due to the embedded nanoinclusion. The insertion free energy penalty is analyzed as a function of the particle size, the brush grafting density, and the solvent quality. In agreement with the earlier simulation work, we find that the insertion free energy cost increases both with the particle size and the brush grafting density and decreases with deteriorating solvent quality. For nanoparticles located deep inside the brush, the insertion free energy is shown to scale with either squared (good solvent) or cubed (poor solvent) monomer volume fraction profile, in agreement with the earlier theoretical results. For shallow nanoinclusions located close to the brush surface, the insertion free energy is shown to scale linearly with the monomer volume fraction profile under both good and theta solvent conditions, again in agreement with the earlier work.

ON THE FREQUENCY OF JUPITER ANALOGS

The Anglo-Australian Planet Search has now accumulated 12 years of radial-velocity data with long-term instrumental precision better than 3 m s−1. In this paper, we expand on earlier simulation work, to probe the frequency of near-circular, long-period gas-giant planets residing at orbital distances of 3–6 AU—the so-called Jupiter analogs. We present the first comprehensive analysis of the frequency of these objects based on radial-velocity data. We find that 3.3% of stars in our sample host Jupiter analogs; detailed, star-by-star simulations show that no more than 37% of stars host a giant planet between 3 and 6 AU.

THE SIGNIFICANCE OF FLOW-MODIFIED PERMITTIVITY: A NEW MODEL AND COMPUTER SIMULATION OF ELECTRORHEOLOGY

We propose a model that takes into account the effect of flow-modified permittivity on electrorheology (ER). Due to dielectric relaxation, a shear flow causes the induced particle dipole moments in an ER fluid to tilt in a direction away from the direction of the applied DC electric field. Results from our computer simulation indicate that at high shear rates this misalignment (tilt angle) between the particle dipole moments and the applied electric field plays a crucial role in producing ER effects. By choosing particle-fluid dielectric and conductive mismatches to optimize the tilt angle, our simulation produces ER effects at much higher shear rates than those in earlier simulation work, even though there is no chain structure at these high shear rates. The increase in shear stress due to the applied electric field in our simulation is nearly constant over the wide range of shear rates examined, in qualitative agreement with experimental results. In addition, our model generates results that agree with earlier simulation work at low shear rates, where the particle dipole moments are essetially aligned with the field and the chain model is adequate.

Does mortality select size or growth potential in young sockeye salmon (Oncorhynchus nerka)? A simulation based on otolith - fish size relationships

There is conflicting evidence on the role of size-selective mortality in the demography of populations of young sockeye salmon (Oncorhynchus nerka). A previous field study suggested that increases in mean fork length and otolith size at emergence observed in a cohort of salmon between fry and smolt stages were due to size-selective mortality, such that fry with small fork lengths underwent higher mortality than larger individuals. However, a subsequent study used a simulation to show that such increases could not have been achieved by size-selective mortality without levels of survivorship of the cohort far lower than those that had been observed in the field. To account for field observations, the simulation study proposed that individuals with high metabolic rates had better survivorship, a process that was termed "selection for growth potential." Here, we use a simulation approach to show that size-selective mortality may result in shifts in mean fork length and otolith size at emergence comparable with those observed in the field at total mortalities within the range of estimates of natural values. The contrasting outcomes of earlier simulation work and the present study are probably due to the assumption by the former that otolith and fish size in young salmon were weakly correlated and the use of an inappropriate model of size-selective mortality. We conclude that size-selective mortality can explain the results of previous field studies and that little empirical evidence exists to support the hypothesis of selection for growth potential in cohorts of young salmon.

Moderated Reinforcement Learning of Active and Semi-Active Vehicle Suspension Control Laws

This paper is concerned with the application of reinforcement learning to the dynamic ride control of an active vehicle suspension system. The study makes key extensions to earlier simulation work to enable on-line implementation of the learning automaton methodology using an actual vehicle. Extensions to the methodology allow safe and continuous learning to take place on the road, using a limited instrumentation set. An important new feature is the use of a moderator to set physical limits on the vehicle states. It is shown that the addition of the moderator has little direct effect on the system's ability to learn, and allows learning to take place continuously even when there are unstable controllers present. The study concludes with the results of an experimental trial using vehicle hardware, where the successful synthesis of a semi-active ride controller is demonstrated.

Analysis of Computed Trajectories of Penetrant Micromolecules in a Simulated Polymeric Material

Molecular dynamics trajectories of oxygen-like spherical molecules within a medium of eicosane-like chains at 303.5 K confined to a density very close to that of amorphous polyethylene are analyzed geometrically to elucidate mechanistic aspects of penetrant motion. To enable this analysis, reduced trajectories, i.e., sets of equidistant positions along the paths of penetrant molecules, are constructed with various step lengths λ. The degree and prevailing direction of local orientation in the chain liquid are quantified through diagonalization of an orientation tensor defined in the vicinity of individual nodes or steps of the reduced trajectories. As in earlier simulation work, an anomalous (subdiffusive) regime is detected, which extends up to times of ∼150 ps or penetrant displacements of ∼1.5 nm. Directional correlations between steps of the reduced trajectory reveal that the penetrant is temporarily confined within elongated “cavities”. The prevalent direction of chain orientation “seen” by the penetrant is found to persist over length scales of ∼2 nm. The degree of local anisotropy is lower along the penetrant trajectories than in the chain melt on average, because dense, strongly oriented regions can only form in the absence of penetrant. A weak but significant directional correlation is detected between penetrant displacements and the prevalent orientation of the surrounding polymer; the range of this correlation indicates that it is intimately related to anomalous diffusion. Over length scales of up to ∼1.5 nm, penetrant displacement is most facile along the backbones of chains. This is reminiscent of the idealized mechanistic model invoked in the Brandt and Pace and Datyner theories, although the degree of orientation is, of course, not nearly as large as suggested by that model.