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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 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Asymptotic Behavior of Ensemble-Averaged Linear Disturbances in Homogeneous Shear Flow

In order to expand the predictive capability of single-point turbulence closure models too account for the early-stage transition regime, a methodology for the formulation and calibration of model equations for the ensemble-averaged disturbance kinetic energy and energy dissipation rate is presented. The calibration is based on homogeneous shear flow where disturbances can be described by rapid distort,ion theory (RDT). The relationship between RDT and linear stability theory is exploit,c d in order to obtain a closed set, of modeled equations. The linear disturbance equations are solved directly so that, the numerical simulation yields a database from which the closure coefficient,s in the ensemble-averaged disturbance equations can he determined.

Thacker, W. D.↗
NASA NTRS2004

Comparison of Rolling Moment Characteristics During Roll Oscillations for a Low and a High Aspect Ratio Configuration

Improvements in testing and modeling of nonlinear and unsteady aerodynamic effects for flight dynamics predictions of vehicle performance is critical to enable the design and implementation of new, innovative vehicle concepts. Any configuration which exhibits significant flow separation, nonlinear aerodynamics, control interactions or attempts maneuvering through one or more conditions such as these is, at present, a challenge to test, model or predict flight dynamic responses prior to flight. Even in flight test experiments, adequate models are not available to study and characterize the complex nonlinear and time-dependent flow effects occurring during portions of the maneuvering envelope. Traditionally, airplane designs have been conducted to avoid these areas of the flight envelope. Better understanding and characterization of these flight regimes may not only reduce risk and cost of flight test development programs, but also may pave the way for exploitation of those characteristics that increase airplane capabilities. One of the hurdles is that the nonlinear/unsteady effects appear to be configuration dependent. This paper compares some of the dynamic aerodynamic stability characteristics of two very different configurations - representative of a fighter and a transport airplane - during dynamic body-axis roll wind tunnel tests. The fighter model shows significant effects of oscillation frequency which are not as apparent for the transport configuration.

Brandon, Jay M.↗
NASA NTRS1993

Modulational stability of periodic solutions of the Kuramoto-Sivaskinsky equation

We study the long-wave, modulational, stability of steady periodic solutions of the Kuramoto-Sivashinsky equation. The analysis is fully nonlinear at first, and can in principle be carried out to all orders in the small parameter, which is the ratio of the spatial period to a characteristic length of the envelope perturbations. In the linearized regime, we recover a high-order version of the results of Frisch, She, and Thual, which shows that the periodic waves are much more stable than previously expected.

Papageorgiou, Demetrios T.↗
NASA NTRS1995

Aerodynamics model for a generic ASTOVL lift-fan aircraft

This report describes the aerodynamics model used in a simulation model of an advanced short takeoff and vertical landing (ASTOVL) lift-fan fighter aircraft. The simulation model was developed for use in piloted evaluations of transition and hover flight regimes, so that only low speed (M approximately 0.2) aerodynamics are included in the mathematical model. The aerodynamic model includes the power-off aerodynamic forces and moments and the propulsion system induced aerodynamic effects, including ground effects. The power-off aerodynamics data were generated using the U.S. Air Force Stability and Control Digital DATCOM program and a NASA Ames in-house graphics program called VORVIEW which allows the user to easily analyze arbitrary conceptual aircraft configurations using the VORLAX program. The jet-induced data were generated using the prediction methods of R. E. Kuhn et al., as referenced in this report.

Birckelbaw, Lourdes G.↗
DOE OSTI2020

Sign reversal in magnetic field amplification by relativistic laser-driven microtube implosions

In this work, we demonstrate and explain the surprising phenomenon of sign reversal in magnetic field amplification by the laser-driven implosion of a structured target. Relativistically intense laser pulses incident on the outer surface of a microtube target consisting of a thin opaque shell surrounding a μm-scale cylindrical void drive an initial ion implosion and later explosion capable of generating and subsequently amplifying strong magnetic fields. While the magnetic field generation is enhanced and spatially smoothed by the application of a kilotesla-level seed field, the sign of the generated field does not always follow the sign of the seed field. One unexpected consequence of the amplification process is a reversal in the sign of the amplified magnetic field when, for example, the target outer cross section is changed from square to circular. Using 2D particle-in-cell simulations, we demonstrate that sign reversal is linked to the stability of the surface magnetic field of opposite sign from the seed, which arises at the target inner surface during laser irradiation. The stability of the surface magnetic field and, consequently, the sign of the final amplified field depend sensitively on the target, laser, and seed magnetic field conditions, which could be leveraged to make laser-driven microtube implosions an attractive platform for the study of magnetic fields in high energy density plasma in regimes where sign reversal either is or is not desired.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Kinetics and selectivity of methane oxidation on an IrO 2 (110) film

Undercoordinated, bridging O-atoms (O br ) are highly active as H-acceptors in alkane dehydrogenation on IrO 2 (110) surfaces but transform to HO br groups that are inactive toward hydrocarbons. The low C–H activity and high stability of the HO br groups cause the kinetics and product selectivity during CH 4 oxidation on IrO 2 (110) to depend sensitively on the availability of O br atoms prior to the onset of product desorption. From temperature programmed reaction spectroscopy (TPRS) and kinetic simulations, we identified two O br -coverage regimes that distinguish the kinetics and product formation during CH 4 oxidation on IrO 2 (110). Under excess O br conditions, when the initial O br coverage is greater than that needed to oxidize all the CH 4 to CO 2 and HO br groups, complete CH 4 oxidation is dominant and produces CO 2 in a single TPRS peak between 450 and 500 K. However, under O br -limited conditions, nearly all the initial O br atoms are deactivated by conversion to HO br or abstracted after only a fraction of the initially adsorbed CH 4 oxidizes to CO 2 and CO below 500 K. Thereafter, some of the excess CH x groups abstract H and desorb as CH 4 above ~500 K while the remainder oxidize to CO 2 and CO at a rate that is controlled by the rate at which Obr atoms are regenerated from HObr during the formation of CH 4 and H 2 O products. We also show that chemisorbed O-atoms ('on-top O') on IrO 2 (110) enhance CO 2 production below 500 K by efficiently abstracting H from Obr atoms and thereby increasing the coverage of O br atoms available to completely oxidize CH x groups at low temperature. Furthermore, our results provide new insights for understanding factors which govern the kinetics and selectivity during CH 4 oxidation on IrO 2 (110) surfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1996

A quiet tunnel investigation of hypersonic boundary-layer stability over a cooled, flared cone

A flared-cone model under adiabatic and cooled-wall conditions was placed in a calibrated, low-disturbance Mach 6 flow and the stability of the boundary layer was investigated using a prototype constant-voltage anemometer. The results were compared with linear-stability theory predictions and good agreement was found in the prediction of second-mode frequencies and growth. In addition, the same 'N = 10' criterion used to predict boundary-layer transition in subsonic, transonic, and supersonic flows under low freestream noise conditions was found to be applicable for the hypersonic flow regime as well. Under cooled-wall conditions, a unique set of spectral data was acquired that documents the linear, nonlinear, and breakdown regions associated with the transition of hypersonic flow under low-noise conditions.

Blanchard, Alan E.↗
NASA NTRS2008

An Integrated Approach to Damage Accommodation in Flight Control

In this paper we present an integrated approach to in-flight damage accommodation in flight control. The approach is based on Multiple Models, Switching and Tuning (MMST), and consists of three steps: In the first step the main objective is to acquire a realistic aircraft damage model. Modeling of in-flight damage is a highly complex problem since there is a large number of issues that need to be addressed. One of the most important one is that there is strong coupling between structural dynamics, aerodynamics, and flight control. These effects cannot be studied separately due to this coupling. Once a realistic damage model is available, in the second step a large number of models corresponding to different damage cases are generated. One possibility is to generate many linear models and interpolate between them to cover a large portion of the flight envelope. Once these models have been generated, we will implement a recently developed-Model Set Reduction (MSR) technique. The technique is based on parameterizing damage in terms of uncertain parameters, and uses concepts from robust control theory to arrive at a small number of "centered" models such that the controllers corresponding to these models assure desired stability and robustness properties over a subset in the parametric space. By devising a suitable model placement strategy, the entire parametric set is covered with a relatively small number of models and controllers. The third step consists of designing a Multiple Models, Switching and Tuning (MMST) strategy for estimating the current operating regime (damage case) of the aircraft, and switching to the corresponding controller to achieve effective damage accommodation and the desired performance. In the paper present a comprehensive approach to damage accommodation using Model Set Design,MMST, and Variable Structure compensation for coupling nonlinearities. The approach was evaluated on a model of F/A-18 aircraft dynamics under control effector damage, augmented by nonlinear cross-coupling terms and a structural dynamics model. The proposed approach achieved excellent performance under severe damage effects.

Boskovic, Jovan D.↗
NASA NTRS1989

Stability of Long's vortex at large flow force

Long's self-similar vortex is known to have two solutions for each supercritical value of the flow force. Each of those solutions is shown to have a double structure if the flow force is large. The inertial instabilities of one of those large-flow-force limit solutions are investigated, showing that they are related to the instabilities of the Bickley jet in one regime. However, the swirl in the vortex becomes important for long waves, very strongly modifying the sinuous and varicose, Bickley modes. The asymptotic results obtained agree well with the numerical solutions for the sinuous mode, but not for the varicose mode, the difficulty in the latter case being apparently due to mode jumping.

Foster, M. R.↗
DOE OSTI2022

Destabilization of binary mixing layer in supercritical conditions

Compressible mixing layer instabilities are of importance to a wide range of environmental and industrial applications. Past studies have focused on either ideal-gas or real-fluid thermodynamic regimes of single-species mixing layers. However, mixing layers of binary mixtures at supercritical conditions, commonly encountered in fuel injection systems, introduce additional complexities due to the added compositional degree-of-freedom. Moreover, the effect of strong variations in thermodynamic response functions across the Widom line on the binary mixing layer stability remains poorly understood. Thus, the objective of this study is to examine the coupling between the hydrodynamic instability and the real-fluid thermodynamics across the Widom line and its effects on the overall binary mixing layer dynamics. To this end, we develop a linear stability analysis of the full binary-species compressible transport equations coupled with the PC-SAFT equation of state. Analysis shows the existence of a novel instability mechanism that arises from juxtapositioning of the Widom-line transition and the hydrodynamic inflection point. This novel thermodynamically induced instability mechanism has the net effect of destabilizing the binary mixing layer at lowering supercritical conditions towards the critical pressure point. This is in contrast to previous stability analyses of supercritical single-species mixing layers, where increasing pressure destabilizes the flow due to its effect on reducing the density stratification. The discovered thermodynamically induced instability mechanism of binary mixing flows highlights the need for an extension of classical instability criteria to incorporate the effect of strong variations in the thermodynamic response functions across the Widom line on mixing layer instability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Microstructural and preliminary optical and microwave characterization of erbium-doped CaMoO 4 thin films

This work explores erbium-doped calcium molybdate (Er:CaMoO4) thin films grown on silicon and yttria stabilized zirconia (YSZ) substrates, as a potential solid state system for C-band (utilizing the ∼1.5 μm Er 3+ 4f–4f transition) quantum emitters for quantum network applications. Through molecular beam epitaxial growth experiments and electron microscopy, X-ray diffraction, and reflection electron diffraction studies, we identify an incorporation limited deposition regime that enables a 1:1 Ca:Mo ratio in the growing film leading to single phase CaMoO 4 formation that can be in situ doped with Er (typically 2–100 ppm). We further show that growth on silicon substrates is single phase but polycrystalline in morphology, while growth on YSZ substrates leads to high-quality epitaxial single crystalline CaMoO 4 films. We perform preliminary optical and microwave characterization on the suspected Y 1 –Z 1 transition of 2 ppm, 200 nm epitaxial Er:CaMoO 4 annealed thin films and extract an optical inhomogeneous linewidth of 9.1(1) GHz, an optical excited state lifetime of 6.7(2) ms, a spectral diffusion-limited homogeneous linewidth of 6.7(4) MHz, and an EPR linewidth of 1.10(2) GHz.

Masiulionis, Ignas [Univ. of Chicago, IL (United S↗
DOE OSTI2026

Persistent structural distortion for anticipated improper ferroelectricity in ultrathin h-Lu 1− x Ca x MnO 3 films

Improper ferroelectricity in hexagonal rare-earth manganites (h-RMnO 3 , R = Ho–Lu, Y, Sc) arises from a geometric distortion as the primary order parameter, resilient to depolarizing fields and promising for ultrathin ferroelectric devices. However, the substrate-induced interface clamping effect, which suppresses the geometric distortion in the sub-nanometer regime, has thus far hindered the realization of two-dimensional improper ferroelectrics. This study demonstrates that doping with calcium can enhance ferroelectric structural distortion in h-LuMnO 3 thin films. Compressively strained h-Lu 1−x Ca x MnO 3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) epitaxial thin films were stabilized on sapphire substrates using an h-ScFeO 3 buffer layer. We have found that the interface clamping effect is entirely overcome when the doping concentration reaches x ⩾ 0.2, establishing a potential quasi-2D ferroelectric system with a remarkably high estimated structural transition temperature of larger than 1200 K inferred indirectly from temperature-resolved reflection high-energy electron diffraction. This finding suggests a general strain engineering strategy to enhance improper ferroelectricity in hexagonal manganites.

hexagonal manganites↗
DOE OSTI2022

The role of plasma instabilities in relativistic radiation-mediated shocks: stability analysis and particle-in-cell simulations

ABSTRACT Relativistic radiation-mediated shocks are likely formed in prodigious cosmic explosions. The structure and emission of such shocks are regulated by copious production of electron–positron pairs inside the shock-transition layer. It has been pointed out recently that substantial abundance of positrons inside the shock leads to a velocity separation of the different plasma constituents, which is expected to induce a rapid growth of plasma instabilities. In this paper, we study the hierarchy of plasma microinstabilities growing in an electron-ion plasma loaded with pairs and subject to a radiation force. Linear stability analysis indicates that such a system is unstable to the growth of various plasma modes which ultimately become dominated by a current filamentation instability driven by the relative drift between the ions and the pairs. These results are validated by particle-in-cell simulations that further probe the non-linear regime of the instabilities, and the pair-ion coupling in the microturbulent electromagnetic field. Based on this analysis, we derive a reduced-transport equation for the particles via pitch-angle scattering in the microturbulence and demonstrate that it can couple the different species and lead to non-adiabatic compression via a Joule-like heating. The heating of the pairs and, conceivably, the formation of non-thermal distributions, arising from the microturbulence, can affect the observed shock-breakout signal in ways unaccounted for by current single-fluid models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Entanglement and correlations in fast collective neutrino flavor oscillations

Collective neutrino oscillations play a crucial role in transporting lepton flavor in astrophysical settings like supernovae and neutron star binary merger remnants, which are characterized by large neutrino densities. In these settings, simulations in the mean-field approximation show that neutrino-neutrino interactions can overtake vacuum oscillations and give rise to fast collective flavor evolution on time-scales t ∝ μ –1 , with μ proportional to the local neutrino density. In this work, we study the full out-of-equilibrium flavor dynamics in simple multi-angle geometries displaying fast oscillations in the mean field linear stability analysis. Focusing on simple initial conditions, we analyze the production of pair correlations and entanglement in the complete many-body-dynamics as a function of the number N of neutrinos in the system, for up to thousands of neutrinos. Similarly to simpler geometries with only two neutrino beams, we identify three regimes: stable configurations with vanishing flavor oscillations, marginally unstable configurations with evolution occurring on long time scales τ ≈ µ –1 $\sqrt{N}$, and unstable configurations showing flavor evolution on short time scales τ ≈ μ –1 log(N). Here, we present evidence that these fast collective modes are generated by the same dynamical phase transition which leads to the slow bipolar oscillations, establishing a connection between these two phenomena and explaining the difference in their time scales. We conclude by discussing a semi-classical approximation which reproduces the entanglement entropy at short to medium time scales and can be potentially useful in situations with more complicated geometries where classical simulation methods starts to become inefficient.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

QCD moat regime and its real-time properties

Dense quantum chromodynamics (QCD) matter may exhibit crystalline phases. Their existence is reflected in a moat regime, where mesonic correlations feature spatial modulations. We study the real-time properties of pions at finite temperature and density in QCD in order to elucidate the nature of this regime. We show that the moat regime arises from particle-hole-like fluctuations near the Fermi surface. This gives rise to a characteristic peak in the spectral function of the pion at nonzero spacelike momentum. This peak can be interpreted as a new quasi particle, the moaton. In addition, our framework also allows us to directly test the stability of the homogeneous chiral phase against the formation of an inhomogeneous condensate in QCD. We find that an inhomogeneous instability is highly unlikely for baryon chemical potentials μ B ≤ 630 MeV . Published by the American Physical Society 2025

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2026

FFTSF: Revisiting Sub-Seasonal Streamflow Forecasting with Simple Feedforward Network

Accurate short-to-subseasonal streamflow forecasts are vital for water management, including flood preparedness, drought mitigation, hydropower scheduling, and ecosystem protection. However, extending a forecast beyond a few days remains challenging due to complexity of hydrological processes. While recent self-attention based transformer architectures such as iTransformer have gained traction in time-series forecasting, these models suffer from several critical limitations: (1) significant computational overhead that scales quadratically with sequence length, (2) vulnerability to overfitting on limited hydrological datasets, (3) degraded performance on long-horizon forecasts due to attention decay, and (4) excessive architectural complexity that hampers interpretability and operational deployment. In this study, we propose a simple Feedforward Time Series Forecasting (FFTSF) network that directly addresses these limitations through its lightweight architecture and long-range forecasting capabilities. We evaluate FFTSF across 178 USGS stream gauges spanning diverse climate regimes by forecasting lead times of 1-, 7-, 14-, and 30-days. Our results demonstrate that FFTSF achieves competitive performance at short lead times (NSE of 0.778 for 1-day forecasts) while substantially outperforming complex baselines at longer forecast period, achieving the highest NSE (0.271) at 30-day forecasts with greater robustness and stability. For 30-day forecasts, FFTSF achieves a 71% improvement over NLinear, 57% improvement over DLinear and 12% improvement over the computationally intensive iTransformer while requiring fewer computational resources. Our findings reveal that architectural complexity is not necessary for hydrological forecasting, demonstrating that well-designed simple models can outperform attention mechanisms for subseasonal streamflow forecasting. The computational efficiency and consistent long-range performance of FFTSF make it suitable for water management applications where reliable extended forecasts are essential.

Krishnan Kutty Ambika, Anukesh [ORNL] (ORCID:00000↗
DOE OSTI2023

Hot Isostatic Pressing Control of Tungsten-Based Composites

Metal-oxide composites are commonly used in high temperature environments for their thermal stability and high melting points. Commonly employed with refractory oxides or carbides such as ZrC and HfC, these materials may be improved with the use of a low density, high melting point ceramic such as CeO2. In this work, the consolidation of W-CeO2 metal matrix composites in the high CeO2 concentration regime is explored. The CeO2 concentrations of 50, 33, and 25 wt.%, the CeO2 particle size from nanometer to micrometer, and various hot isostatic pressing temperatures are investigated. Decreasing the CeO2 concentration is observed to increase the composite density and increase the Vickers hardness. The CeO2 oxidation state is observed to be a combination of Ce3+ and Ce4+, which is hypothesized to contribute to the porosity of the composites. The hardness of the metal-oxide composite can be improved more than 2.5 times compared to pure W processed by the same route. This work offers processing guidelines for further consolation of oxide-doped W composites.

36 MATERIALS SCIENCE↗
NASA NTRS1982

The condensational instability in the solar transition region and corona

The stability of plasmas at temperatures and densities typical of the solar transition region and corona is investigated using both a linear analysis and nonlinear time-dependent numerical simulations. Growth rates, decay rates, and oscillation frequencies of the perturbations determined from the linear analysis are in excellent agreement with the simulations. The nonlinear regime is characterized by a bifurcation of the plasma into a cool dense condensation surrounded by a hot tenuous corona. The condensation may then be accelerated by forces in the plasma such as those arising from gravity or differential heating. Finally, the results of the detailed simulation show that the transition region is a dynamically stable structure which is the result of the nonlinear evolution of the condensational instability.

Oran, E. S.↗