Computational fluid dynamics investigation on the irrigation of a real root canal with a side-vented needle.

BACKGROUND: Root canal therapy is one of the main treatments for root canal diseases, and effective irrigation is the key to successful treatment. Side-vented needle is one of the commonly used needle types in clinic. In the real root canal, due to the influence of the curvature of the root canal, the irrigation flow field in different needle directions shows obvious differences. At the same time, changes in root canal curvature and working depth will lead to changes in irrigation efficiency and the flow field. Both the mainstream of the irrigation flow and the shear stress near the wall changes significant. Consequently, either the replacement in the root canal or the removal efficiency of the smear layers is apparently modified. MATERIALS AND METHODS: In this paper, the permanent root canal of the maxillary first molar prepared until 15/04 were scanned by micro-CT, and then imported into the software for 3D reconstruction. The key parameters of flushing efficiency of 30G side needle at different working depths of 4.75 mm, 5 mm, 5.25 mm and 5.5 mm were compared. Meanwhile, the simulated models with different curvatures of 0°, 5°, 10°, 20° and 30° based on the real root canal were reconstructed to investigate the curvature effect on the irrigation efficiency. RESULTS: The results show that moderate working depth (such as 4.75 mm and 5.25 mm in present paper) helps to improve the replacement capacity of irrigation flow. At the same time, the apical pressure decreased as the working depth increased. The curvature of the root canal seriously affects the removal depth of the smear layers of the root canal. A root canal with a large curvature (especially 20° and 30°) can significantly improve the difficulty of irrigation. CONCLUSIONS: (1) Moderate working depth helps to improve the displacement capacity, the ERD of the irrigation flow is generally improved at the working depths of 4.75 mm and 5.25 mm, and the apical pressure will decrease with the increase of working depth. (2) The large curvature of the root canal can significantly improve the difficulty of irrigation. The curvature of the root canal can severely influence the removal depth of the smear layer on the wall. It can be found both the span and the depth of the ESS for little curvatures (5° and 10°) root canals are higher than those for large curvatures (20° and 30°).

Coupling at the molecular scale between the graphene nanosheet and water and its effect on the thermal conductivity of the nanofluid.

Graphene nanofluid is a promising way to improve heat transfer in many situations. As a two-dimensional material, graphene's anisotropic thermal conductivity influences the heat transfer of nanofluids. In the present study, a nonequilibrium molecular dynamics (MD) simulation is adopted to study the interaction between graphene nanosheets (GNSs) and liquid water in water-based graphene nanofluids. Consequently, the coupling interaction between the orientation and length of GNSs and the thermal conductivity of nanofluids is then investigated. We discover that the molecular thermal coupling between GNSs and water can effectively influence the orientation angle of the GNSs. A preferential orientation angle of the GNSs inside the nanofluid is then observed during heat transfer. The preferential orientation angle decreases with the GNS length and has no apparent relation with the size of heat flux in this study. The overall thermal conductivity of the nanofluid decreases as the orientation angle of the GNS rises. Increasing the GNS length not only reduces the preferential orientation angle but also improves the thermal conductivity along the graphene length direction. The thermal conductivity of the nanofluid along the graphene length direction increases from 0.414 to 4.085 W m K-1 as the length increases from 103 to 3274 A. Our results provide the fundamental knowledge of the heat transfer performance of graphene nanofluids.

The atom sampling method for mesoscale molecular dynamics and its application to graphene assemblies.

Several coarse-grained (CG) molecular dynamics models have been developed to predict the mechanical behaviors of mesoscale graphene structures because of the enormous promise of graphene in various carbon-based nanostructures and the spatiotemporal limitations of experimental testing and atomistic modeling. Although the CG models reduced the number of degrees of freedom and speeded up simulations, the increased mass of the beads caused a significant mismatch in the system's total kinetic and potential energy. In this paper, we propose an atom sampling method for mesoscale molecular dynamics simulations, which could effectively mitigate the consequences arising from the increased mass of the beads in the CG models while still maintaining the computational efficiency of CG models. Then, the Tersoff sampling model of graphene using the atom sampling method is developed to coarsen graphene in planar directions. The mechanical responses of monolayer graphene and multilayer graphene (MLG) assemblies are simulated by the Tersoff sampling model, and simulation results obtained with the all-atom (AA) Tersoff model and CG models are compared. The results show that the Tersoff sampling model can reduce the misfit of kinetic and potential energy due to the fact that temperature changes more accurately reproduce the mechanical behaviors of graphene in the elastic and fracture zones compared to the CG models. At the same time, it further validates the atom sampling method for mesoscale simulations. The atom sampling method can also be easily applied to isotropic fluids and ordered crystal structures (including established coarse-grained models of these substances).

Pyrolysis Mechanism of Wheat Straw Based on ReaxFF Molecular Dynamics Simulations.

Biomass has played an increasingly important role in the consumption of energy worldwide because of its renewability and carbon-neutral property. In this work, the pyrolysis mechanism of wheat straw is explored using reactive force field molecular dynamics simulations. A large-scale wheat straw model composed of cellulose, hemicellulose, and lignin is built. After model validation, the temporal evolutions of the main pyrolysis products under different temperatures are analyzed. As the temperature rises, the gas production increases and the tar yield can decrease after peaking. Relatively high temperatures accelerate the generation rates of the main gas and tar species. CO and CO2 molecules mainly come from the cleavage of CHO2 radicals, and numerous H2O molecules are generated on account of dehydration. Moreover, the evolution of six functional groups and pyran and phenyl rings as well as three types of bonds is also presented. It is observed that the phenyl rings reflect improved thermostability. Finally, the pyrolytic kinetics analysis is conducted, and the estimated activation energy of wheat straw pyrolysis is found to be 56.19 kJ/mol. All these observations can help deeply understand the pyrolytic mechanism of wheat straw biomass.

Influence of needle working length and root canal curvature on irrigation: a computational fluid dynamics analysis based on a real tooth.

BACKGROUNDS: To compare the irrigation efficiency with different needle working length and different root canal curvature based on a real unshaped root canal using computational fluid dynamics (CFD) method. METHODS: Images of the root canal of the maxillary first molar after being prepared to .04/15 were scanned using micro-CT, and then imported into the software for three-dimensional reconstruction. A palatal root canal with a curvature of 23.4° was selected as the experiment canal. The needle working length of the 30-G flat needle was 4.75 mm, 5 mm, 5.25 mm and 5.5 mm short of apical foramen respectively, the flow pattern, irrigation velocity, shear stress were compared. The modified curved canals with a curvature of 0°, 5°, 10°, 20° and 30°were reconstructed via software. The flat needle was replaced at the optical inserted depth, and key parameters of irrigation efficiency were analyzed. RESULTS: Decreased needle working length had a positive impact on irrigation efficiency. With the optimal needle working length, the replacement of the apical irrigation fluid, the effective velocity, and wall shear stress were significantly improved in more severely curved root canals. With the same needle working depth and analogous canal curvature, irrigation efficiency is higher in real canal than that of modified canal. CONCLUSIONS: Short needle working depth, large curvature and the anomalous inner wall of canals help to improve irrigation efficiency.

Large eddy simulation of the gas-particle turbulent wake flow.

To find out the detailed characteristics of the coherent structures and associated particle dispersion in free shear flow, large eddy simulation method was adopted to investigate a two-dimensional particle-laden wake flow. The well-known Sub-grid Scale mode introduced by Smagorinsky was employed to simulate the gas flow field and Lagrangian approach was used to trace the particles. The results showed that the typical large-scale vortex structures exhibit a stable counter rotating arrangement of opposite sign, and alternately form from the near wall region, shed and move towards the downstream positions of the wake with the development of the flow. For particle dispersion, the Stokes number of particles is a key parameter. At the Stokes numbers of 1.4 and 3.8 the particles concentrate highly in the outer boundary regions. While the particles congregate densely in the vortex core regions at the Stokes number of 0.15, and the particles at Stokes number of 15 assemble in the vortex braid regions and the rib regions between the adjoining vortex structures.

Modulation on coherent vortex structures by dispersed solid particles in a three-dimensional mixing layer.

Large-scale vortex structures and their effects on the dispersion of particles in turbulent free shear flows are very important in many industrial applications, such as combustion, pollution control, and materials processing. In order to understand large-scale vortex structures and particle dispersion in depth, as well as their interaction effects, a two-way-coupled three-dimensional mixing layer laden with particles at a Stokes number of 5 initially located in the upper half region is studied numerically. A pseudospectral method was used to directly simulate the flow fluid, and the Lagrangian approach was used to trace particles. The concept of computational particles is introduced to vary the mass loading of particles. The momentum coupling effect introduced by a particle approximates to a point force. The simulation results show that coherent structures are still dominant in the mixing layer, but the flow dynamics and particle dispersion are modulated. The length of large-scale vortex structures is shortened and the pairing is delayed. Higher mass loading results in lower energy of the fluid in the phase of Kelvin-Helmholtz rolling up, while in the pairing process of large-scale vortex structures, the energy of the fluid increases as the mass loading increases. Higher mass loading also leads to larger mixed fluid thickness and Reynolds stresses of the flow. In addition, the particle dispersion along the transverse direction differs from that along the spanwise direction, which indicates that the effects of the addition of a particle on the spanwise large-scale vortex structures are different from those on the streamwise large-scale vortex structures.

Large eddy simulation of a particle-laden turbulent plane jet.

Gas-solid two-phase turbulent plane jet is applied to many natural situations and in engineering systems. To predict the particle dispersion in the gas jet is of great importance in industrial applications and in the designing of engineering systems. A large eddy simulation of the two-phase plane jet was conducted to investigate the particle dispersion patterns. The particles with Stokes numbers equal to 0.0028, 0.3, 2.5, 28 (corresponding to particle diameter 1 microm, 10 microm, 30 microm, 100 microm, respectively) in Re = 11 300 gas flow were studied. The simulation results of gas phase motion agreed well with previous experimental results. And the simulation results of the solid particles motion showed that particles with different Stokes number have different spatial dispersion; and that particles with intermediate Stokes number have the largest dispersion ratio.