Set Of Model Constants Research Articles

A Detailed PAH and Soot Model for Complex Fuels in CFD Applications

A model to predict soot evolution during the combustion of complex fuels is presented. On one hand, gas phase, hbox {polycyclic aromatic hydrocarbon (PAH)} and soot chemistry are kept large enough to cover all relevant processes in aero engines. On the other hand, the mechanisms are reduced as far as possible, to enable complex computational fluid dynamics (CFD) combustion simulations. This is important because all species transport equations are solved directly in the hbox {CFD}. Moreover, emphasis is placed on the applicability of the model for a variety of fuels and operating conditions without adjusting it. A kinetic scheme is derived to describe the chemical breakdown of short- and long-chain hydrocarbon fuels and even blends of them. hbox {PAHs} are the primary soot precursors which are modeled by a sectional approach. The reversibility of the interaction between different hbox {PAH} classes is achieved by the introduction of hbox {PAH} radicals. Soot particles are captured by a detailed sectional approach too, which takes a non-spherical growth of particles into account. In this way the modeling of surface processes is improved. The applicability and validity of the gas phase, hbox {PAH}, and soot model is demonstrated by a large number of shock tube experiments, as well as in atmospheric laminar sooting flames. The presented model achieves excellent results for a wide range of operating conditions and fuels. One set of model constants is used for all simulations and no case-dependent optimization is required.

Three-equation model for the self-similar growth of Rayleigh-Taylor and Richtmyer-Meskov instabilities.

In the present work, the two-equation k-L model [G. Dimonte and R. Tipton, Phys. Fluids 18, 085101 (2006)] is extended by the addition of a third equation for the mass-flux velocity. A set of model constants is derived to satisfy an ansatz of self-similarity in the low Atwood number limit. The model is then applied to the simulation of canonical Rayleigh-Taylor and Richtmyer-Meshkov test problems in one dimension and is demonstrated to reproduce analytical self-similar growth and to recover growth rates used to constrain the model.

Bounding Surface Plasticity Model Incorporating the State Pressure Index for Rockfill Materials

AbstractThe critical state line of Tacheng rockfill material (TRM) was described by a pressure-power function. A simple bounding-surface plasticity model, combining the dilatancy stress ratio and peak-failure stress ratio pertaining to the state pressure index, was developed for TRM within the framework of critical-state soil mechanics and the bounding surface plasticity. Detailed investigations of influences of design parameters on the model predictions were carried out. The developed model, using a set of model constants, captured well behaviors such as strain hardening and volumetric contraction for TRM in a loose state (e.g., at a large initial void ratio or a high initial confining pressure) and strain softening and volumetric expansion in a dense state (e.g., at a small initial void ratio or a low initial confining pressure).

A family of explicit algebraic models for Reynolds stresses and passive scalar fluxes

A family of explicit algebraic models for Reynolds stresses and passive scalar fluxes is proposed and a few variants are discussed, according to the selected set of model constants. Validation is attempted against direct numerical simulation in a steady closed-channel flow for two values of the Reynolds number, indicating the ability of the model to reasonably predict the anisotropy tensor and the scalar flux components when considering wall functions to account for near-wall effects. The production of turbulent kinetic energy is well predicted, while the neutral turbulent Prandtl number has values in agreement with the literature. The proposed model is then compared with an experiment on a heated cylinder with slight temperature variations, with satisfactory results. The proposed model can be applied to various 2D simply-sheared flows involving a passive scalar, i.e. if stratification effects are negligible.

Modeling shape memory effect in uncrosslinked amorphous biodegradable polymer

Shape memory effect (SME) is critical for minimally invasive surgical procedures in medicine. In this paper, the shape memory behavior of amorphous biodegradable polymer, poly( d, l-lactide-co-glycolide), is experimentally investigated. Based on the experimental observations and the understanding of the underlying mechanism of SME, a one-dimensional constitutive model is derived to describe the shape memory behavior in the context of (1) the stress-strain behavior in deformation, (2) the isothermal recovery and (3) the recovery at constant heating rate, by using a set of model constants. By fine tuning the model constants, a good agreement between the experimental results and computer predictions was achievable.

Parallel FEM LES with one-equation subgrid-scale model for incompressible flows

This article develops a parallel large-eddy simulation (LES) with a one-equation subgrid-scale (SGS) model based on the Galerkin finite element method and three-dimensional (3D) brick elements. The governing filtered Navier–Stokes equations were solved by a second-order accurate fractional-step method, which decomposed the implicit velocity–pressure coupling in incompressible flow and segregated the solution to the advection and diffusion terms. The transport equation for the SGS turbulent kinetic energy was solved to calculate the SGS processes. This FEM LES model was applied to study the turbulence of the benchmark open channel flow at a Reynolds number Reτ = 180 (based on the friction velocity and channel height) using different model constants and grid resolutions. By comparing the turbulence statistics calculated by the current model with those obtained from direct numerical simulation (DNS) and experiments in literature, an optimum set of model constants for the current FEM LES model was established. The budgets of turbulent kinetic energy and vertical Reynolds stress were then analysed for the open channel flow. Finally, the flow structures were visualised to further reveal some important characteristics. It was demonstrated that the current model with the optimum model constants can predict well the organised structure near the wall and free surface, and can be further applied to other fundamental and engineering applications.

A critical state subloading surface model of sands with shear hardening

Based on the framework of critical state soil mechanics, a subloading surface plastic model for sand, being applicable to cyclic loading, was proposed. The model can be used to describe strain softening behaviour of sand under monotonic loading when the similarity-ratio equals to unity. The characteristics of the model are as follows: 1) A reverse bullet-shaped yield surface is adopted to ensure accurate prediction of the behavior of sand, instead of bullet-shaped or elliptical yield surface in Cam-Clay model. 2) No unique relationship between void ratio and the mean normal stress for sand prevents the direct coupling of yield surface size to void ratio, so incremental deviatoric strain hardening rule is used. 3) The model combines the concept of state-dependent dilatancy by incorporating state parameter in Rowe’s stress dilatancy equation, which accounts for the dependence of dilatancy on the stress state and the material internal state. A single set of model constants, which is calibrated, can simulate stress—strain response under different initial void ratios and different confine pressures. The model is validated true by comparing predicted results with experimental results under monotonic and cyclic loading conditions.

A Modified k-ε Closure for Turbulent Self-preserving Jets

This paper presents a modified k-e model for turbulent self-preserving jets. It is well known that the standard k-ε model yields reasonable solutions for both the plane and round jets, however, with different sets of coefficients. It is believed that such behavior might be due to the inadequacy of the modeling of the production/distruction terms in the ε-equation,where cε1 and cε2 are assumed to be constants. Based on the analysis of the k- and ε-equations at the state of equilibrium near the outer edge of the turbulent jet, a set of model constants and a variable form for the production coefficient in the dissapation equation, cε1, h ve b en logically derived. This modification significantly improved the agreement of the calculations with experimet for both the plane and the round jets equally well with no need to tune any of the model coefficients. The calculated mean and turbulent profiles, the jet spreading rate and the mean centerline velocity decay rate are compared with the most recent experimental data. The numerical solutions show that the modified turbulence model employed in the present study predicts both plane and round jets with the same set of constants and parameters equally well.

Modeling of Richtmyer–Meshkov instability-induced turbulent mixing in shock-tube experiments

A turbulence transport model for the analysis of shock interface interaction-induced turbulent mixing is presented. Results given by the one-dimensional (1-D) version of this model are compared with data obtained in shock-tube experiments. Calibrations are made from an air/He interface destabilized by a 1.3 incident shock wave Mach number, taking into account the successive interactions with the different reshocks on the shock tube end wall. Then, using the same set of model constants, different gas pairs with both various Atwood and incident shock wave Mach numbers are considered, in order to point out the influence of these main parameters on the results. Mixing zone thickness time evolutions and 1-D density profiles are presented and directly compared with experimental results. Profiles of other variables such as the space integral of the turbulent kinetic energy ∫k dx, translation energy ∫ρux2 dx, and the ratio ∫ρk dx/∫ρux2 dx, given by the computations, are also shown. Using two different initializations, in particular, to better describe the first phases of the phenomenon under study (i.e., taking into account the initial membrane in a horizontal shock tube configuration), we have found good agreement between calculations and experiments.

Two-Equation Turbulence Closure Model for Wall Bounded and Free Shear Flows

The κ-ζ model can be used for both wall bounded and free shear flows using one set of model constants

Combustion in a Divided Chamber, Stratified Charge, Reciprocating Engine: Inititial Comparisons of Calculated and Measured Flame Propagation

Abstract A divided-chamber, stratified-charge engine was operated at different speeds and loads, and with two prechamber nozzle sizes. Pressure was recorded and flame propagation was filmed through a transparent head. For each condition, several sets of data were taken and the experimental scatter identified. Pressure and flame propagation were then computed with a two-dimensional model and compared with the measured ones. Several quantities which influence the computed results were not directly measured in the experiment and assumptions had to be made about them in the model. They include amount and state of cylinder air and residual products, turbulence, and wall heat transfer. A turbulent diffusivity, an overall reaction rate, and a wall heat loss proportional to the heat released thus were adopted in the model. The same set of model constants was used for all cases and, through successive iterations, adequate agreement of computed and measured results was obtained. For this specific engine design and ...

Improved form of the low Reynolds number k−ε turbulence model

The behavior of the k−ε turbulence model at a turbulent/nonturbulent interface is examined to determine the turbulent diffusion coefficients σk and σε. The analysis leads to a dilemma caused by the modeling of the generation term in the k and ε equations. Based on limiting values of σk and σε derived from this analysis, a revised form of the low−Reynolds−number incompressible k−ε turbulence model is presented which incorporates a new set of model constants, a linear form of the near wall dissipation term in the k equation, and deletion of a nonphysical term in the ε equation. The revised model is applied to fully developed flow in a channel and found to give better agreement with experiment than the original form of the model developed by Jones and Launder.