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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Aeroacoustic Flow Phenomena Accurately Captured by New Computational Fluid Dynamics Method

One of the challenges in the computational fluid dynamics area is the accurate calculation of aeroacoustic phenomena, especially in the presence of shock waves. One such phenomenon is "transonic resonance," where an unsteady shock wave at the throat of a convergent-divergent nozzle results in the emission of acoustic tones. The space-time Conservation-Element and Solution-Element (CE/SE) method developed at the NASA Glenn Research Center can faithfully capture the shock waves, their unsteady motion, and the generated acoustic tones. The CE/SE method is a revolutionary new approach to the numerical modeling of physical phenomena where features with steep gradients (e.g., shock waves, phase transition, etc.) must coexist with those having weaker variations. The CE/SE method does not require the complex interpolation procedures (that allow for the possibility of a shock between grid cells) used by many other methods to transfer information between grid cells. These interpolation procedures can add too much numerical dissipation to the solution process. Thus, while shocks are resolved, weaker waves, such as acoustic waves, are washed out.

Blech, Richard A.↗
NASA NTRS2007

A Synopsis of Interfacial Phenomena in Lithium-Based Polymer Electrolyte Electrochemical Cells

The interfacial regions between electrode materials, electrolytes and other cell components play key roles in the overall performance of lithium-based batteries. For cell chemistries employing lithium metal, lithium alloy or carbonaceous materials (i.e., lithium-ion cells) as anode materials, a "solid electrolyte interphase" (SEI) layer forms at the anode/electrolyte interface, and the properties of this "passivating" layer significantly affect the practical cell/battery quality and performance. A thin, ionically-conducting SEI on the electrode surface can beneficially reduce or eliminate undesirable side reactions between the electrode and the electrolyte, which can result in a degradation in cell performance. The properties and phenomena attributable to the interfacial regions existing at both anode and cathode surfaces can be characterized to a large extent by electrochemical impedance spectroscopy (EIS) and related techniques. The intention of the review herewith is to support the future development of lithium-based polymer electrolytes by providing a synopsis of interfacial phenomena that is associated with cell chemistries employing either lithium metal or carbonaceous "composite" electrode structures which are interfaced with polymer electrolytes (i.e., "solvent-free" as well as "plasticized" polymer-binary salt complexes and single ion-conducting polyelectrolytes). Potential approaches to overcoming poor cell performance attributable to interfacial effects are discussed.

Baldwin, Richard S.↗
NASA NTRS2007

Critical Phenomena in Microgravity: Past, Present, and Future

This review provides an overview of the progress in using the low-gravity environment of space to explore critical phenomena and test modern theoretical predictions. Gravity-induced variations in the hydrostatic pressure and the resulting density gradients adversely affect ground-based measurements near fluid critical points. Performing measurements in a low-gravity environment can significantly reduce these difficulties. A number of significant experiments have been performed in low-Earth orbit. Experiments near the lambda transition in liquid helium explored the regime of large correlation lengths and tested the theoretical predictions to a level of precision that could not be obtained on Earth. Other studies have validated theoretical predictions for the divergence in the viscosity as well as the unexpected critical speeding up of the thermal equilibrium process in pure fluids near the liquid-gas critical point. We describe the scientific content of previously flown low-gravity investigations of critical phenomena as well as those in the development stage, and associated ground-based work.

renormalization-group theory↗
NASA NTRS2012

EUV Dimmings as a Diagnostic of CMEs and Related Phenomena

Large-scale coronal EUV dimmings, developing on timescaJes of minutes to hours in association with a flare or filament eruption, are known to exhibit a high correlation with coronal mass ejections. While most observations indicate that the decrease in emission in a dimming is due, at least in part, to a density decrease, a complete understanding requires us to examine at least four mechanisms that have been observed to cause darkened regions in the corona: 1) mass loss, 2) cooling, 3) heating, and 4) absorption/obscuration. Recent advances in automatic detection, observations with improved cadence and resolution, multi-viewpoint imaging, and spectroscopic studies have continued to shed light on dimming formation, evolution, and recovery. However, there are still some outstanding questions, including 1) Why do some CMEs show dimming and some do not? 2) What determines the location of a dimming? 3) What determines the temporal evolution of a dimming? 4) How does the post-eruption dimming connect to the ICME? 5) What is the relationship between dimmings and other CME-associated phenomena? The talk will emphasize the different formation mechanisms of dimmings and their relationship to CMEs and CME-associated phenomena.

Thompson, Barbara J.↗
NASA NTRS2013

Influence of Tailored Applied Magnetic Fields on High-Power MPD Thruster Current Transport and Onset-Related Phenomena

This work investigated the effects of tailored, externally-applied magnetic fields on current transport and near-anode processes in the plasma discharge of a magnetoplasmadynamic thruster (MPDT). Electrical and plasma diagnostics were used to investigate how localized applied magnetic fields could mitigate the effects of the "onset" phenomena, including large-amplitude terminal voltage fluctuations and high anode fall voltages associated with unstable operation and anode erosion. An MPDT with a multi-channel hollow cathode was developed and tested with quasi-steady pulses of 1 millisecond duration at power levels of 36 kilowatts (20 volts, 1800 amperes) to 3.3 milliwatts (255 volts, 13.1 kiloamperes) with argon propellant in three different magnetic configurations: self-field, applied B field tangential to the anode lip near the exit plane, and applied cusp B field. The current pattern and current densities redistributed to follow the applied poloidal magnetic field lines, which created increased conduction paths to the anode. Also, the anode fall voltage was substantially reduced with both applied B field topologies over a large range of currents. For example, at 10.7 kiloamperes, the cusp applied magnetic field decreased anode fall voltages from 45-83 volts down to 15 volts or lower along much of the anode. The amplitude and frequency of the voltage fluctuations were also reduced over a broad range of currents with the applied fields. E.g., the standard deviations of the fluctuations were lowered by 37-49 percent at 8-9 kiloamperes. In addition, decreases in the mean terminal voltages as large as 31 percent were measured with the applied magnetic fields. These effects are shown to be associated with the increased current conduction along the applied magnetic field lines in the near-anode region. These results also suggest a reduction in frequency and intensity of current-concentrating filaments and anode spots, which contribute to erosion. Overall, both applied magnetic field configurations enabled significant reductions in onset-related behaviors relative to self-field operation. These improvements should lead to reduced anode erosion, i.e., improved thruster lifetime, and increased thruster efficiency with the applied fields. The applied fields used in this study differ from both the topologies and relative field strengths typically used in the vast majority of conventional, so-called "applied-field MPD thrusters" (AF-MPDTs). These results suggest a distinctive and more effective approach to influencing the near-anode phenomena and mitigating the deleterious effects of onset with appropriately designed applied magnetic fields.

delta-V↗
NASA NTRS2022

Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena in Heliophysics and Beyond

Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. New research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this white paper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations among the heliophysics, astrophysics, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events.

H. Ji↗