We perform simulations to investigate how the energy carried by a molecule transfers to others in an equilibrium gas model. For this purpose we consider a microcanonical ensemble of equilibrium gas systems, each of them contains a tagged molecule located at the same position initially. The ensuing transfer process of the energy initially carried by the tagged molecule is then exposed in terms of the ensemble-averaged energy density distribution. In both a 2D and a 3D gas model with Lennard-Jones interactions at room temperature, it is found that the energy carried by a molecule propagates in the gas ballistically, in clear contrast with the Gaussian diffusion widely assumed in previous studies. A possible scheme of experimental study of this issue is also proposed.PACS numbers: 05.60. Cd, 51.10.+y, 51.20.+d One important task of statistical mechanics is to understand various transport processes. A well-known successful example is the self-diffusion of gas molecules: Due to Einstein's 1905 work, it has been widely accepted that a particle in a gas undergoes the Brownian motion [1], which can be modeled essentially with the random walk [2] and the resulting probability distribution function (PDF) follows the diffusion equation [1,2]. Because of its fundamental importance for various scientific disciplines, the study of Einstein's theory has never ceased. Very recently, the direct experimental measurement of the instantaneous velocity of a Brownian particle in a gas has been realized, and the random walk picture was confirmed with high precision [3].Another question of fundamental importance is how the energy carried by a molecule transfers with time, which is a key step towards understanding the macroscopic energy transport. In general, the existing theories, taking for example the Helfand theory [4], approached this issue by simply extending the random walk picture of a Brownian particle, and predicted a Gaussian energy density distribution as well. However, it should be pointed out that whether random walk is the underlying mechanism of the energy dispersion has never been examined experimentally nor numerically in a gas or more generally in fluids.Unlike in the study of the self-diffusion (or mass diffusion) where the position of a particle can be traced accurately [5][6][7][8] (and now even its instantaneous velocity can be measured [3]), a key difficulty in the study of the energy transfer is that it is hard to trace the energy transferred from particle to particle. The mass diffusion can be explored by focusing on the trace of an individual particle, but by nature, the energy transfer is a collective behavior involving all the molecules related by the transferred energy.In this work we perform an equilibrium molecular dynamics investigation to explore how the energy of a molecule may transfer in a gas. We will restrict ourselves to a 2D gas model, but it has been verified that in its 3D counterpart the results remain qualitatively the same. We assume that the gas is composed of only one kind of molecule w...

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In this study, we introduce a new model to study the particle diffusion among 2D scatterers. Different from previous models, the potential between the particle and scatterers consists of an attractive interaction as well as a repulsive one. The geometric arrangement of the scatterers has important effects on the diffusion behavior. In the case of periodic scatterers, the low-energy particles may show superdiffusive motion while the high-energy ones diffuse normally. In the case of random scatterers, the global subdiffusive motion may be observed in an energy region slightly above the localization threshold. The subdiffusion phenomenon is explored for the first time in Hamiltonian systems with deterministic scatterers. The mechanism of the observed diffusion behavior is linked to the stickiness effect of chaotic Hamiltonian systems.

Air jet impingement is one of the effective cooling techniques employed in micro-electronic industry. To enhance the heat transfer performance, a cooling system with air jet impingement on a finned heat sink is evaluated via the computational fluid dynamics method. A 2-D confined slot air impinging jet on a finned flat plate is modeled. The numerical model is validated by comparison of the computed Nusselt number distribution on the impingement target with published experimental results. The flow characteristics and heat transfer performance of jet impingement on both of smooth and finned heat sinks are compared. It is observed that jet impingement over finned target plate improves the cooling performance significantly. A dimensionless heat transfer enhancement factor is introduced to quantify the effect of jet flow Reynolds number on the finned surface. The effect of rectangular fin dimensions on impingement heat transfer rate is discussed in order to optimize the cooling system. Also, the computed flow and thermal fields of the air impingement system are examined to explore the physical mechanisms for heat transfer enhancement.