Direct Numerical Simulations of Turbulence Subjected to a Straining and De-Straining Cycle

In many turbulent flows, significant interactions between fluctuations and mean velocity gradients occur in nonequilibrium conditions, i.e., the turbulence does not have sufficient time to adjust to changes in the velocity gradients applied by the large scales. The simplest flow that retains such physics is the time dependent homogeneous strain flow. A detailed experimental study of initially isotropic turbulence subjected to a straining and destraining cycle was reported by Chen et al. [“Scale interactions of turbulence subjected to a straining-relaxation-destraining cycle,” J. Fluid Mech. 562, 123 (2006)] . Direct numerical simulation (DNS) of the experiment of Chen et al. [“Scale interactions of turbulence subjected to a straining-relaxation-destraining cycle,” J. Fluid Mech. 562, 123 (2006)] is undertaken, applying the measured straining and destraining cycle in the DNS. By necessity, the Reynolds number in the DNS is lower. The DNS study provides a complement to the experimental one including time evolution of small-scale gradients and pressure terms that could not be measured in the experiments. The turbulence response is characterized in terms of velocity variances, and similarities and differences between the experimental data and the DNS results are discussed. Most of the differences can be attributed to the response of the largest eddies, which, even if are subjected to the same straining cycle, evolve under different conditions in the simulations and experiment. To explore this issue, the time evolution of different initial conditions parametrized in terms of the integral scale is analyzed in computational domains with different aspect ratios. This systematic analysis is necessary to minimize artifacts due to unphysical confinement effects of the flow. The evolution of turbulent kinetic energy production predicted by DNS, in agreement with experimental data, provides a significant backscatter of kinetic energy during the destraining phase. This behavior is explained in terms of Reynolds stress anisotropy and nonequilibrium conditions. From the DNS, a substantial persistency of anisotropy is observed up to small scales, i.e., at the level of velocity gradients. Due to the time dependent deformation, we find that the major contribution in the Reynolds stresses budget is provided by the production term and by the pressure/strain correlation, resulting in large time variation of velocity intensities. The DNS data are compared with predictions from the classical Launder–Reece–Rodi isoptropic production [ B. E. Launder et al., “Progress in the development of a Reynolds stress turbulence closure,” J. Fluid Mech. 68, 537 (1975) ] Reynolds stress model, showing good agreement with some differences for the redistribution term

Computational study of the transport mechanisms of molecules and ions in solid materials

Transport of ions and molecules in solids is a very important process in many technological applications, for example, in drug delivery, separation processes, and in power sources such as ion diffusion in electrodes or in solid electrolytes. Progress in the understanding of the ionic and molecular transport mechanisms in solids can be used to substantially increase the performance of devices. In this dissertation we use ab initio calculations and molecular dynamics simulations to investigate the mechamisn of transport in solid. We first analyze molecular transport and storage of H2. Different lightweight carbon materials have been of great interest for H2 storage. However, pure carbon materials have low H2 storage capacity at ambient conditions and cannot satisfy current required storage capacities. Modification of carbon materials that enhance the interaction between H2 and absorbents and thus improve the physisorption of H2, is needed for hydrogen storage. In this dissertation, corannulene and alkali metal-doped corannulene are investigated as candidate materials for hydrogen storage. Molecularalso investigated. Using computational chemistry, we predict enhanced H2 adsorption on molecular systems with modification and hydrogen uptake can reach DOE target of 6.5wt% at at 294 bar at 273 K, and 309 bar at 300 K. In the second part of this dissertation, we study the lithium ion transport from a solid electrolyte phase to a solid electrode phase. Improvement of ionic transport in solid electrolytes is a key element in the development of the solid lithium ion batteries. One promising material is dilithium phthalocyanine (Li2Pc), which upon self-assembly may form conducting channels for fast ion transport. Computational chemistry is employed to investigate such phenomena: (1) to analyze the crystalline structure of Li2Pc and formation of conducting channels; (2) to understand the transport of Li ions inside channels driven by an electric field; (3) to study the continuity of the conducting channels through interface. The study shows Li2Pc has higher conductivity than PEO as electrolyte

System analysis of a piston steam engine employing the uniflow principle, a study in optimized performance

Results are reported which were obtained from a mathematical model of a generalized piston steam engine configuration employing the uniflow principal. The model accounted for the effects of clearance volume, compression work, and release volume. A simple solution is presented which characterizes optimum performance of the steam engine, based on miles per gallon. Development of the mathematical model is presented. The relationship between efficiency and miles per gallon is developed. An approach to steam car analysis and design is presented which has purpose rather than lucky hopefulness. A practical engine design is proposed which correlates to the definition of the type engine used. This engine integrates several system components into the engine structure. All conclusions relate to the classical Rankine Cycle

Advanced Technology Spark-Ignition Aircraft Piston Engine Design Study

The advanced technology, spark ignition, aircraft piston engine design study was conducted to determine the improvements that could be made by taking advantage of technology that could reasonably be expected to be made available for an engine intended for production by January 1, 1990. Two engines were proposed to account for levels of technology considered to be moderate risk and high risk. The moderate risk technology engine is a homogeneous charge engine operating on avgas and offers a 40% improvement in transportation efficiency over present designs. The high risk technology engine, with a stratified charge combustion system using kerosene-based jet fuel, projects a 65% improvement in transportation efficiency. Technology enablement program plans are proposed herein to set a timetable for the successful integration of each item of required advanced technology into the engine design

Seals design guide - Study of dynamic and static seals for liquid rocket engines Final report, Jul. 1968 - Sep. 1969

Design guide for seals in liquid rocket engines with emphasis on rotating seal

Aeronautical Engineering. A continuing bibliography, supplement 112

This bibliography lists 424 reports, articles, and other documents introduced into the NASA scientific and technical information system in July 1979

Aeronautical engineering: A special bibliography with indexes, supplement 80

This bibliography lists 277 reports, articles, and other documents introduced into the NASA scientific and technical information system in January 1977