Adaptive multiresolution computations applied to detonations

A space-time adaptive method is presented for the reactive Euler equations describing chemically reacting gas flow where a two species model is used for the chemistry. The governing equations are discretized with a finite volume method and dynamic space adaptivity is introduced using multiresolution analysis. A time splitting method of Strang is applied to be able to consider stiff problems while keeping the method explicit. For time adaptivity an improved Runge--Kutta--Fehlberg scheme is used. Applications deal with detonation problems in one and two space dimensions. A comparison of the adaptive scheme with reference computations on a regular grid allow to assess the accuracy and the computational efficiency, in terms of CPU time and memory requirements.Comment: Zeitschrift f\"ur Physicalische Chemie, accepte

Computational experience with a three-dimensional rotary engine combustion model

A new computer code was developed to analyze the chemically reactive flow and spray combustion processes occurring inside a stratified-charge rotary engine. Mathematical and numerical details of the new code were recently described by the present authors. The results are presented of limited, initial computational trials as a first step in a long-term assessment/validation process. The engine configuration studied was chosen to approximate existing rotary engine flow visualization and hot firing test rigs. Typical results include: (1) pressure and temperature histories, (2) torque generated by the nonuniform pressure distribution within the chamber, (3) energy release rates, and (4) various flow-related phenomena. These are discussed and compared with other predictions reported in the literature. The adequacy or need for improvement in the spray/combustion models and the need for incorporating an appropriate turbulence model are also discussed

Reactive infiltration instability amplifies the difference between geometric and reactive surface areas in natural porous materials

Reactive infiltration instability (RII) drives the development of many natural and engineered flow systems. These are encountered e.g. in hydraulic fracturing, geologic carbon storage and well stimulation in enhanced oil recovery. The surface area of the rocks changes as the pore structure evolves. We combined a reactor network model with grey scale tomography to seek the morphological interpretation for differences among geometric, reactive and apparent surface areas of dissolving natural porous materials. The approach allowed us to delineate the experimentally convoluted variables and study independently the effects of initial geometry and macroscopic flowrate. Simulations based on North Sea chalk microstructure showed that geometric surface not only serves as the interface for water-rock interactions but also represents the regional transport heterogeneities that can be amplified indefinitely by dissolutive percolation. Hence, RII leads to channelization of the solid matrix, which results in fluid focusing and an increase in geometric surface area. Fluid focusing reduces the reactive surface area and the residence time of reactants, both of which amplify the differences in question, i.e. they are self-supporting. Our results also suggested that the growing and merging of microchannels near the fluid entrance leads to the macroscopic "fast initial dissolution" of chemically homogeneous materials.Comment: 37 pages, 12 figure

Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets

The concept of local isotropy in a chemically reacting turbulent wall-jet flow is addressed using direct numerical simulation (DNS) data. Different DNS databases with isothermal and exothermic reactions are examined. The chemical reaction and heat release effects on the turbulent velocity, passive scalar and reactive species fields are studied using their probability density functions (PDF) and higher order moments for velocities and scalar fields, as well as their gradients. With the aid of the anisotropy invariant maps for the Reynolds stress tensor the heat release effects on the anisotropy level at different wall-normal locations are evaluated and found to be most accentuated in the near-wall region. It is observed that the small-scale anisotropies are persistent both in the near-wall region and inside the jet flame. Two exothermic cases with different Damkohler number are examined and the comparison revealed that the Damkohler number effects are most dominant in the near-wall region, where the wall cooling effects are influential. In addition, with the aid of PDFs conditioned on the mixture fraction, the significance of the reactive scalar characteristics in the reaction zone is illustrated. We argue that the combined effects of strong intermittency and strong persistency of anisotropy at the small scales in the entire domain can affect mixing and ultimately the combustion characteristics of the reacting flow

Chemo-mechanical modeling of artificially and naturally bonded soils

Chemo-mechanical effects are known to be significant in a number of applications in modern geomechanics, ranging from slope stability assessment to soil improvement and CO2 sequestration. This work focuses on coupled chemo-mechanical modeling of bonded geomaterials undergoing either mechanical strengthening, due to increased cementation, or weakening, due to cement dissolution. A constitutive model is developed that accounts for the multi-scale nature of the chemo-mechanical problem, introducing some cross-scale functions establishing a relationship between the evolution of microscopic variables and the macroscopic material behavior, realistically following the evolution of the reactive surface area, cross-sectional area and the number of bonds along with dissolution/deposition. The model presented here builds up on a previously introduced framework. However, at variance with existing works, it is specialized on materials with only reactive bonds, such as carbonate cemented sandstone or microbially cemented silica sand. Model validation is provided upon reproducing different types of chemo-mechanical experimental datasets, on different naturally and artificially cemented materials, to establish the reliability of the proposed framework

Progress of simulations for reacting shear layers

An attempt was made to develop a high speed, chemically reactive shear layer test rig. The purpose of the experiment was to study the mixing of oxidizer and fuel streams in reacting shear layers for various density, velocity, and Mach number. The primary goal was to understand the effects of the compressibility upon mixing and combustion in a fundamental way. Therefore, a two-dimensional shear layer is highly desirable for its simplicity to quantify the compressibility effects. The RPLUS 2D code is used to calculate the flow fields of different sections of the test rig. The emphasis was on the supersonic nozzle design, the vitiation process for the hot air stream and the overall thermodynamic conditions of the test matrix. The k-epsilon turbulence model with wall function was successfully implemented in the RPLUS code. The k and epsilon equations are solved simultaneously and the LU scheme is used to make it compatible with the flow solver

Growth Techniques for Bulk ZnO and Related Compounds

ZnO bulk crystals can be grown by several methods. 1) From the gas phase, usually by chemical vapor transport. Such CVT crystals may have high chemical purity, as the growth is performed without contact to foreign material. The crystallographic quality is often very high (free growth). 2) From melt fluxes such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts (PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild conditions (<1000 deg. C, atmospheric pressure), but the crystals always contain traces of solvent. The limited purity is a severe drawback, especially for electronic applications. 3) From hydrothermal fluxes, usually alkaline (KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric character of ZnO, the supercritical bases can dissolve it up to several per cent of mass. The technical requirements for this growth technology are generally hard, but this did not hinder its development as the basic technique for the growth of {\alpha}-quartz, and meanwhile also of zinc oxide, during the last decades. 4) From pure melts, which is the preferred technology for numerous substances applied whenever possible, e.g. for the growth of silicon, gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high growth rate and (ii) the absence of solvent related impurities. In the case of ZnO, however, it is difficult to find container materials that are compatible from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen partial pressure) point of view. Either cold crucible (skull melting) or Bridgman (with reactive atmosphere) techniques were shown to overcome the problems that are inherent to melt growth. Reactive atmospheres allow to grow not only bulk ZnO single crystals, but also other TCOs such as {\beta}-Ga2O3 and In2O3.Comment: 10 pages, 7 figures, talk on MRS Fall 2011 Bosto