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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: 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.

Agency: National Aeronautics and Space Administration | Branch: | Program: STTR | Phase: Phase I | Award Amount: 124.93K | Year: 2014

GTL has been developing three transformational technologies that have the capability to disrupt the traditional launch vehicle paradigm. BHL composite cryotank technology provides a four times improvement over large Al-Li tanks, offering a 6 percentage point improvement in stage PMF. Superior Stability Engine is an innovative liquid rocket engine configured to maximize combustion stability margin while also maximizing engine performance. NORPS is a non-helium gas generator system that can be used to pressurize the propellant tanks for 1/3 the mass and 1/10 the volume of a comparable helium based system. The Advanced Cryogenic Expendable (ACE) Booster design uses these technologies to achieve high performance and low cost in a small vehicle. When implemented in an optimized design, the ACE technologies offer revolutionary performance. In the proposed Phase I effort, GTL will perform a conceptual design study to assess the impact of design constraints on the implementation of the ACE technologies. From this, an optimized design will be developed. A technology roadmap will be created to show how the capabilities can be achieved in the near term.

Agency: Department of Defense | Branch: Missile Defense Agency | Program: STTR | Phase: Phase II | Award Amount: 999.95K | Year: 2012

The overall objective of the Phase II effort is to enhance the capability of the UCDS process to simulate and predict the characteristics of combustion instability in propulsion devices, specifically focusing on the propulsion devices used in missile defense applications. GTL shall apply proven techniques to rigorously develop a reaction wave model that is consistent with the rest of the UCDS process. The effort shall also enhance the capability of the UCDS acoustic models to simulate the dynamics in chambers with a catalyst beds, multi-port nozzles, and pintle nozzles. The last part of the effort is focused on validating the UCDS reaction wave and acoustic models examined in this effort. This shall be accomplished by performing a series of experiments that are designed to target the specific physical phenomena addressed by the models. In each case, the UCDS models shall be used to simulate and analyze the configuration of the test system. These predictions shall then be compared to the results of the experiments using the test system. The proposed model enhancement and validation will provide a solid foundation for the application of UCDS technology to MDA systems.

Agency: National Aeronautics and Space Administration | Branch: | Program: STTR | Phase: Phase II | Award Amount: 750.00K | Year: 2015

GTL has been developing a suite of transformational technologies that have the capability to disrupt the traditional launch vehicle paradigm. BHL composite cryotank technology provides a four times improvement over large aluminum iso-grid tanks, offering a 6 percentage point improvement in small stage PMF. Superior Stability Engine is an innovative liquid rocket engine configured to maximize combustion stability margin while also maximizing engine performance. NORPS is a non-helium gas generator system that can be used to pressurize the propellant tanks for 1/3 the mass and 1/10 the volume of a comparable helium based system. Using these and other technologies, GTL has developed the conceptual design for the Advanced Cryogenic Expendable (ACE) nano-launch vehicle. The 7700 lb gross lift-off weight ACE vehicle is capable of delivering a 154 lb payload to 400 nmi circular orbit at 28.5 deg inclination. With a launch cost of less than $1M at low launch rate, ACE is directly competitive with existing large launch vehicles on a $/lb basis. This affordability is enabled by a combination of high performance, reduced stages and parts count, and simplified operations. The proposed Phase II effort will seek to reduce the ACE vehicle development risk by increasing the technology readiness level of critical technologies. Specifically, GTL will fabricate and test a prototype NORPS gas generator and pressurization system. Along with this, GTL shall fabricate a full-scale BHL composite cryotank for use in the system testing using modular manufacturing techniques. The integrated system shall be tested for operational capabilities to demonstrate the effectiveness of the technology and optimize the system design. The data from these tests will be used to refine and optimize the design of the ACE vehicle.

Agency: Department of Defense | Branch: Missile Defense Agency | Program: STTR | Phase: Phase I | Award Amount: 99.99K | Year: 2010

The UCDS process uses a rigorous theoretical model to understand the dynamics of combustion instability. In addition to predicting the amplitude of pressure oscillations, UCDS provides clear insight into why a propulsion device oscillates. To enhance and optimize UCDS for missile defense applications, GTL proposes to refine the UCDS unsteady combustion model to capture the special features of monopropellant and hypergolic combustion. To accomplish this, GTL intends to merge a new analytical chemistry model with a computational reactor model. The objective of this effort is to provide inputs to the UCDS distributed heat release model, which describes how the steady and unsteady heat release and temperature work together to feed energy into the wave motion of the chamber. With the capability to predict both the unsteady and steady heat release and temperature for complex reaction processes, it will be possible to begin to model the impact of multiphase effects, thermo-chemical processes and complex kinetics on the system dynamics. This will allow UCDS to begin to explain the available experimental data in order to reveal insight into how these processes really work in monopropellant and hypergolic propellant engines.

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: CHEMICAL OCEANOGRAPHY | Award Amount: 91.20K | Year: 2015

In this project, a group of investigators participating in the 2015 U.S. GEOTRACES Arctic expedition will measure concentrations of atmospherically-derived mercury in the Arctic Ocean. In common with other multinational initiatives in the International GEOTRACES Program, the goals of the U.S. Arctic expedition are to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions. Some trace elements are essential to life, others are known biological toxins, and still others are important because they can be used as tracers of a variety of physical, chemical, and biological processes in the sea. Mercury, primarily as methylmercury, is an element that substantially bioaccumulates through aquatic food webs and impacts neurological functions in humans and wildlife, and it is therefore critical to understand the inputs of mercury to the region. Educational activities as part of this study include training and mentoring of undergraduate and graduate students and a postdoctoral researcher. Researchers will also conduct public outreach activities about mercury impacts to local Arctic communities.

In the Arctic Ocean, subsistence local fishermen and several species of Arctic wildlife, such as beluga whales, seals and polar bears, commonly have elevated levels of methylmercury in their system. Atmospheric deposition is the major pathway of mercury input to the marine environment as both wet and dry (aerosol and gaseous ionic mercury) deposition. Therefore, measurements of mercury and a better understanding of its cycling in the Arctic Ocean are critical. This study will provide further understanding of the drivers of mercury speciation in air and surface waters, including snow/ice, melt ponds, and surface seawater and how these concentrations, and other physical and biological factors, impact deposition rates at the air-sea interface. The primary measurements to be made include a baseline of mercury measurements over the open water from the ship, and over sea-ice environments of the Arctic Ocean, which will be compared to simultaneous and historic coastal measurements, as well as model studies. Overall, results will provide the crucial data and information necessary to comprehend the role of human activity and climate change in exacerbating or ameliorating the exposure of humans and wildlife to methylmercury in the Arctic Ocean.

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