Majdalani J.,University of Tennessee Space Institute
Fluid Dynamics Research | Year: 2012
In this work, two families of helical motions are investigated as prospective candidates for describing the bidirectional vortex field in a right-cylindrical chamber. These basic solutions are relevant to cyclone separators and to idealized representations of vortex-fired liquid and hybrid rocket engines in which bidirectional vortex motion is established. To begin, the bulk fluid motion is taken to be isentropic along streamlines, with no concern for reactions, heat transfer, viscosity, compressibility or unsteadiness. Then using the Bragg-Hawthorne equation for steady, inviscid, axisymmetric motion, two families of Euler solutions are derived. Among the characteristics of the newly developed solutions one may note the axial dependence of the swirl velocity, the Trkalian and Beltramian types of the helical motions, the sensitivity of the solutions to the outlet radius, the alternate locations of the mantle, and the increased axial and radial velocity magnitudes, including the rate of mass transfer across the mantle, for which explicit approximations are obtained. Our results are compared to an existing, complex lamellar model of the bidirectional vortex in which the swirl velocity reduces to a free vortex. In this vein, we find the strictly Beltramian flows to share virtually identical pressure variations and radial pressure gradients with those associated with the complex lamellar motion. Furthermore, both families warrant an asymptotic treatment to overcome their endpoint limitations caused by their omission of viscous stresses. From a broader perspective, the work delineates a logical framework through which self-similar, axisymmetric solutions to bidirectional and multidirectional vortex motions may be pursued. It also illustrates the manner through which different formulations may be arrived at depending on the types of wall boundary conditions. For example, both the slip condition at the sidewall and the inlet flow pattern at the headwall may be enforced or relaxed. © 2012 The Japan Society of Fluid Mechanics and IOP Publishing Ltd.
Chedevergne F.,ONERA |
Casalis G.,ONERA |
Majdalani J.,University of Tennessee Space Institute
Journal of Fluid Mechanics | Year: 2012
In this article, a biglobal stability approach is used in conjunction with direct numerical simulation (DNS) to identify the instability mode coupling that may be responsible for triggering large thrust oscillations in segmented solid rocket motors (SRMs). These motors are idealized as long porous cylinders in which a Taylor-Culick type of motion may be engendered. In addition to the analytically available steady-state solution, a computed mean flow is obtained that is capable of securing all of the boundary conditions in this problem, most notably, the no-slip requirement at the chamber headwall. Two sets of unsteady simulations are performed, static and dynamic, in which the injection velocity at the chamber sidewall is either held fixed or permitted to vary with time. In these runs, both DNS and biglobal stability solutions converge in predicting the same modal dependence on the size of the domain. We find that increasing the chamber length gives rise to less stable eigenmodes. We also realize that introducing an eigenmode whose frequency is sufficiently spaced from the acoustic modes leads to a conventional linear evolution of disturbances that can be accurately predicted by the biglobal stability framework. While undergoing spatial amplification in the streamwise direction, these disturbances will tend to decay as time elapses so long as their temporal growth rate remains negative. By seeding the computations with the real part of a specific eigenfunction, the DNS outcome reproduces not only the imaginary part of the disturbance, but also the circular frequency and temporal growth rate associated with its eigenmode. For radial fluctuations in which the vorticoacoustic wave contribution is negligible in relation to the hydrodynamic stability part, excellent agreement between DNS and biglobal stability predictions is ubiquitously achieved. For axial fluctuations, however, the DNS velocity will match the corresponding stability eigenfunction only when properly augmented by the vorticoacoustic solution for axially travelling waves associated with the Taylor-Culick profile. This analytical approximation of the vorticoacoustic mode is found to be quite accurate, especially when modified using a viscous dissipation function that captures the decaying envelope of the inviscid acoustic wave amplitude. In contrast, pursuant to both static and dynamic test cases, we find that when the frequency of the introduced eigenmode falls close to (or crosses over) an acoustic mode, a nonlinear mechanism is triggered that leads to the emergence of a secondary eigenmode. Unlike the original eigenmode, the latter materializes naturally in the computed flow without being artificially seeded. This natural occurrence may be ascribed to a nonlinear modal interplay in the form of internal, eigenmode-to-eigenmode coupling instead of an external, eigenmode pairing with acoustic modes. As a result of these interactions, large amplitude oscillations are induced. © 2012 Cambridge University Press.
Agency: NSF | Branch: Standard Grant | Program: | Phase: CERAMICS | Award Amount: 319.84K | Year: 2016
Non-technical Description: This research creates special coatings for solar cells that increase the amount of the suns energy that the cells can use, making them more efficient. The coatings also help reduce heating of the solar cells, which wastes energy. These coatings can be used with currently-available solar cell materials, enabling more attractive viability as a commercial product. The coatings may also be applied to light emitting diodes, helping control the color of light over a wide temperature range. Although not the focus of this investigation, the coatings have additional applications in medical X-ray imaging, non-destructive evaluation and homeland security. These research efforts are integrated with educational activities exposing graduate students to real world problems of energy saving as well as the full academic experience of information dissemination in the form of writing papers and presenting research, travel, grant writing and teaching. As part of the outreach activities, a summer internship is available for high school and undergraduate students, which gives a small number of students, each year, the opportunity to learn more about scientific research, perform experiments, give presentations, and participate in many other aspects of a scientific career.
Technical Description: The aim of this activity is to develop novel designer glass ceramics based on a modified fluorozirconate glass composition, and to explore their luminescence behavior. The ultimate goal is to gain insights leading to optimization of these designer nanocomposites for applications as wavelength shifters pertaining to up- and down-converters in solar cells and light emitting diodes. Pulsed laser deposition is used to synthesize layered nanocomposite materials so that very fine control of the distribution of the optically-active dopant and the nanocrystalline structure responsible for the optical behavior is achieved, leading to the development of a glass ceramic with enhanced light output. Systematic studies of optical behavior, as a function of parameters such as the designed distribution of the optically-active component layers within the glass matrix and the relative concentrations of optical dopants and nanocrystals, enable an understanding of how energy transfer between the luminescent dopant atoms and the nanocrystals (and thus the optical efficiency) can be controlled. The combination of in situ¬ transmission electron microscopy, ellipsometry and X-ray diffraction enables the structure-property relationships to be visualized and linked together. For example, critical control over optimum dopant position can be achieved by studying the diffusion paths of the dopants during in situ heating.
Agency: NSF | Branch: Standard Grant | Program: | Phase: | Award Amount: 299.16K | Year: 2010
The goal of this project is to determine the microstructural and chemical origins of the optical properties of Europium-doped fluorozirconate glasses, additionally doped with chlorine nanocomposite materials. The glass ceramics have potential applications as an x-ray imaging plate, for example in a digital mammography system. A further goal of the research is to provide insights into ways in which the material could be optimized for this application. Europium-doped fluorozirconate glasses, additionally doped with chlorine, can be heat-treated in such a way that it forms a novel nanocomposite material containing barium chloride nanocrystals, with the ability to convert ionizing radiation (usually x-rays) into stable electronhole pairs. These can be read out afterwards with a scanning laser beam in a so-called photostimulated luminescence (PSL) process. Optical studies have shown that the nanocomposite glasses gives out five times more light than the equivalent volume of the single crystal. The reason for this increased light output is not understood but the answer lies in the interface between the nanoparticles and its host glass matrix, as a result of the formation of the barium chloride nanoparticles. The ideal technique to analyze the structure and composition across nanoscale interfaces is by transmission electron microscopy (TEM). Ex situ TEM analysis has previously been carried out on samples that were annealed at various temperatures but this only provides a snapshot of the available science. The planned TEM studies include high resolution imaging of the atomic-scale structure of the nanocrystals and their interfaces with the glass matrix, in conjunction with energy-filtered TEM (EFTEM) composition mapping of the chemical distribution in and around the nanocrystals. Further, samples, which have previously been heat-treated in a furnace or irradiated by a laser with different energies and pulse length, in order to induce nanocrystal nucleation, will be examined and the three nucleation techniques, ex situ thermal, laser, and in situ thermal will be compared.
The University of Tennessee Space Institute regularly runs summer science camps for K-12 and employs summer interns. The entire UTSI staff participates in the outreach efforts. Dr. Johnson has developed a detailed seven week summer research experience program for a combination of high-school and undergraduate students. The students receive a glass-ceramic sample and go through a program of characterization, oral presentation, report preparation, career day and mock grant writing workshop. The program culminates in the preparation of a journal article. The PI regularly acts as a mentor for the Introduce a Girl to Engineering day, which is a program at Argonne for Middle School female students.
Agency: NSF | Branch: Standard Grant | Program: | Phase: EXP PROG TO STIM COMP RES | Award Amount: 264.13K | Year: 2015
There is an increasing need to predict materials response and failure behavior at macroscopic scale from its microstructural composition. In brittle and quasi-brittle materials, such as glass, concrete, rocks, and ceramics, failure is particularly sensitive to the microstructure leading to a large scatter in failure loads. Most existing fracture models fail to reliably predict this scatter. This award supports fundamental research in developing theoretical and computational tools for fracture of brittle and quasi-brittle materials that directly link their microstructure to failure loads and the scatter observed. Brittle and quasi-brittle fracture mechanics finds applications in a variety of material and structural designs and plays a central role in many other fields. For example, ceramics are used with metals to develop high-strength and light-weight materials for armor designs and aerospace industry. Rock fracture, whether occurring naturally as in earthquake or manmade for enhanced oil recovery and CO2 sequestration is another example. Finally, obtaining more accurate probabilities of fracture reduces uncertainties in current design practices and can aid in the assessment of the structural integrity of existing infrastructure systems. Educational goals focus on development of short course toolkits on random models and computational tools to attract high school students to STEM fields, and software modules that will be shared with scientific community.
The field of stochastic partial differential equations provides systematic approaches for the propagation of randomness in an analysis in general. However, there is currently no means to relate material microstructures to initial random field description needed for these stochastic models. This research fills the knowledge gap by deriving continuum models that directly translate microstructure distribution to the initial material field description. Unlike common homogenization schemes, stochastic representative volume elements still preserve the spatial variability and randomness of material. This enables realistic modeling of brittle and quasi-brittle fracture. To ensure accurate rendering of this theoretical model an advanced finite element model is formulated that can efficiently capture complicated fracture patterns by incorporating both bulk and interfacial failure mechanisms. Moreover, a novel adaptive computational scheme eliminates the sensitivity of the failure load on initial mesh discretization and guarantees the estimation of probability of failure within the user-specified error bounds. The microstructure-based probabilistic fracture model approach aims to explain a variety of phenomena that are not well captured with commonly used deterministic models. Some examples are size effect in brittle and quasi-brittle materials, scatter in failure load, and formation of complex fracture patterns even under uniform loads.