Numerical Study of Gas Flow in Super Nanoporous Materials Using the Direct Simulation Monte-Carlo Method.

The direct simulation Monte Carlo (DSMC) method, which is a probabilistic particle-based gas kinetic simulation approach, is employed in the present work to describe the physics of rarefied gas flow in super nanoporous materials (also known as mesoporous). The simulations are performed for different material porosities (0.5≤ϕ≤0.9), Knudsen numbers (0.05≤Kn≤1.0), and thermal boundary conditions (constant wall temperature and constant wall heat flux) at an inlet-to-outlet pressure ratio of 2. The present computational model captures the structure of heat and fluid flow in porous materials with various pore morphologies under rarefied gas flow regime and is applied to evaluate hydraulic tortuosity, permeability, and skin friction factor of gas (argon) flow in super nanoporous materials. The skin friction factors and permeabilities obtained from the present DSMC simulations are compared with the theoretical and numerical models available in the literature. The results show that the ratio of apparent to intrinsic permeability, hydraulic tortuosity, and skin friction factor increase with decreasing the material porosity. The hydraulic tortuosity and skin friction factor decrease with increasing the Knudsen number, leading to an increase in the apparent permeability. The results also show that the skin friction factor and apparent permeability increase with increasing the wall heat flux at a specific Knudsen number.

Direct Simulation Monte Carlo investigation of fluid characteristics and gas transport in porous microchannels.

The impetus of the current research is to use the direct simulation Monte Carlo (DSMC) algorithm to investigate fluid behaviour and gas transport in porous microchannels. Here, we demonstrate DSMC's capability to simulate porous media up to 40% porosity. In this study, the porous geometry is generated by a random distribution of circular obstacles through the microchannel with no interpenetration between the obstacles. The influence of the morphology along with rarefaction and gas type on the apparent permeability is investigated. Moreover, the effects of porosity, solid particle's diameter and specific surface area are considered. Our results demonstrate that although decreasing porosity intensifies tortuosity in the flow field, the tortuosity reduces at higher Knudsen numbers due to slip flow at solid boundaries. In addition, our study on two different gas species showed that the gas type affects slippage and apparent gas permeability. Finally, comparing different apparent permeability models showed that Beskok and Karniadakis model is valid only up to the early transition regime and at higher Knudsen numbers, the current data matches those models that take Knudsen diffusion into account as well.

Ballistic and Collisional Flow Contributions to Anti-Fourier Heat Transfer in Rarefied Cavity Flow.

This paper investigates anti-Fourier heat transfer phenomenon in a rarefied gas confined within a lid-driven cavity using a novel flow decomposition technique in the direct simulation Monte Carlo (DSMC) method proposed by Stefanov and co-workers. An isothermal cavity with different degrees of flow rarefaction from near continuum to mid transition regimes was considered to investigate cold-to-hot heat transfer from ballistic/collision flow decomposition viewpoint. A new cold-to-hot heat transfer indicator in the form of a scalar product of normalized heat flow vector and normalized temperature gradient vector has been introduced for the overall, ballistic and collision parts of these vectors. Using the new indicator, contributions of ballistic and collision flow parts to temperature and heat flux components was investigated with a specific emphasis on the cold-to-hot heat transfer phenomenon. We demonstrated that both ballistic and collision flow parts contribute to the occurrence of cold-to-hot heat transfer. However, it was found out that considered separately both ballistic and collision parts of heat transfer, when related to corresponding ballistic and collision temperature fields, they are ever hot-to-cold for all degrees of flow rarefaction. Thus, cold-to-hot heat transfer is a result of a subtle interplay between ballistic and collision parts in the slip and transition Knudsen regimes.

Thermally induced gas flows in ratchet channels with diffuse and specular boundaries.

A net gas flow can be induced in the gap between periodically structured surfaces held at fixed but different temperatures when the reflection symmetry along the channel axis is broken. Such a situation arises when one surface features a ratchet structure and can be augmented by altering the boundary conditions on different parts of this surface, with some regions reflecting specularly and others diffusely. In order to investigate the physical mechanisms inducing the flow in this configuration at various Knudsen numbers and geometric configurations, direct simulation Monte Carlo (DSMC) simulations are employed using transient adaptive subcells for collision partner selection. At large Knudsen numbers the results compare favorably with analytical expressions, while for small Knudsen numbers a qualitative explanation for the flow in the strong temperature inhomogeneity at the tips of the ratchet is provided. A detailed investigation of the performance for various ratchet geometries suggests optimum working conditions for a Knudsen pump based on this mechanism.

Some opinions on the review process of research papers destined for publication.

The current paper discusses the peer review process in journals that publish research papers purveying new science and understandings (scientific journals). Different aspects of peer review including the selection of reviewers, the review process and the decision policy of editor are discussed in details. Here, the pros and cons of different conventional methods of review processes are mentioned. Finally, a suggestion is presented for the review process of scientific papers.

Thermal and second-law analysis of a micro- or nanocavity using direct-simulation Monte Carlo.

In this study the direct-simulation Monte Carlo (DSMC) method is utilized to investigate thermal characteristics of micro- or nanocavity flow. The rarefied cavity flow shows unconventional behaviors which cannot be predicted by the Fourier law, the constitutive relation for the continuum heat transfer. Our analysis in this study confirms some recent observations and shows that the gaseous flow near the top-left corner of the cavity is in a strong nonequilibrium state even within the early slip regime, Kn=0.005. As we obtained slip velocity and temperature jump on the driven lid of the cavity, we reported meaningful discrepancies between the direct and macroscopic sampling of rarefied flow properties in the DSMC method due to existence of nonequilibrium effects in the corners of cavity. The existence of unconventional nonequilibrium heat transfer mechanisms in the middle of slip regime, Kn=0.05, results in the appearance of cold-to-hot heat transfer in the microcavity. In the current study we demonstrate that existence of such unconventional heat transfer is strongly dependent on the Reynolds number and it vanishes in the large values of the lid velocity. As we compared DSMC solution with the results of regularized 13 moments (R13) equations, we showed that the thermal characteristic of the microcavity obtained by the R13 method coincides with the DSMC prediction. Our investigation also includes the analysis of molecular entropy in the microcavity to explain the heat transfer mechanism with the aid of the second law of thermodynamics. To this aim, we obtained the two-dimensional velocity distribution functions to report the molecular-based entropy distribution, and show that the cold-to-hot heat transfer in the cavity is well in accordance with the second law of thermodynamics and takes place in the direction of increasing entropy. At the end we introduce the entropy density for the rarefied flow and show that it can accurately illustrate departure from the equilibrium state.

Sonographic and functional characteristics of thyroid nodules in a population of adult people in Isfahan.

INTRODUCTION: The aim of this study was to investigate the current status of sonographic characteristics of thyroid nodules in Isfahan, a previously iodine deficient area in central Iran. MATERIAL AND METHODS: In a cross-sectional study conducted in 2006, 2523 adult people (age > 20 years) were selected by a multistage clustering sampling method. Of these people, 263 volunteered persons were underwent sonographic evaluation. Thyroid examination was done by two expert sonographers. Serum T(3), T(3), T3RU, TSH, TPO Ab and Tg Ab, and urinary iodine were measured. RESULTS: Forty-six per cent of the 263 people were women. Their mean age was 35.5 years with a range of 20-64 years. Median urinary iodine was 19.4 microg/dL. The prevalence of thyroid nodules on sonography was 22.4% in the whole group; 30% in women and 16.3% in men (OR = 2.2, P = 0.01). The prevalence of thyroid nodules increased with age (P = 0.006). The prevalence of thyroid nodules was higher in hypothyroid people than in euthyroid people (35.1% v. 20.5%, OR = 2.1, P = 0.04). Neither urinary iodine nor autoantibody concentrations correlated with the prevalence of thyroid nodules in sonography. CONCLUSIONS: The prevalence of thyroid nodule by sonography is still high despite relatively normal urinary iodine in this population.

Thyroid incidentaloma in Isfahan, Iran - a population-based study.

INTRODUCTION: Thyroid nodules not detected in palpation but diagnosed following a radiological procedure or during surgery are called thyroid incidentalomas. We designed this study to investigate the prevalence of sonographic thyroid incidentaloma in Isfahan, Iran, for the first time. MATERIAL AND METHODS: By a multistage cluster sampling method, 2523 adults were selected randomly. From this group, 2045 had normal thyroid examination. 234 out of the 2045 were selected randomly for thyroid sonography. Thyroid stimulating hormone (TSH), urinary iodine concentrations (UIC) and antithyroid antibodies were measured. RESULTS: The prevalence of thyroid incidentaloma was 13.2% [Confidence interval (CI) 95% = 8-18]. The average age of subjects with and without incidentaloma was 46.1 (12.4) and 38.4 (12.1), respectively (P = 0.002). It was more prevalent in females than in males (19% vs. 10%) [Odds ratio (OR) = 2.59, CI 95% = 1.17-5.76, P value (P) = 0.01]. Median UIC in the incidentaloma group (14 microg/dl) was significantly lower than in the group of subjects without nodules (20 microg/dl) (OR = 0.9, CI 95% = 0.91-0.99, P = 0.02). There was no difference in the levels of TSH and antithyroid antibodies between the two groups (P > 0.05). Mean diameter of nodules was 8.14 (3.43) mm. CONCLUSIONS: The prevalence of thyroid incidentaloma was 13.2% in Isfahan. It was higher in females than males and increased with age.