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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Crack Instability Predictions Using a Multi-Term Approach

Present crack instability analysis for fracture critical flight hardware is normally performed using a single parameter, K(sub C), fracture toughness value obtained from standard ASTM 2D geometry test specimens made from the appropriate material. These specimens do not sufficiently match the boundary conditions and the elastic‐plastic constraint characteristics of the hardware component, and also, the crack instability of most commonly used aircraft and aerospace structural materials have some amount of stable crack growth before fracture which makes the normal use of a K(sub C) single parameter toughness value highly approximate. In the past, extensive studies have been conducted to improve the single parameter (K or J controlled) approaches by introducing parameters accounting for the geometry or in‐plane constraint effects. Using 'J‐integral' and 'A' parameter as a measure of constraint is one of the most accurate elastic‐plastic crack solutions currently available. In this work the feasibility of the J‐A approach for prediction of the crack instability was investigated first by ignoring the effects of stable crack growth i.e. using a critical J and A and second by considering the effects of stable crack growth using the corrected J‐delta a using the 'A' parameter. A broad range of initial crack lengths and a wide range of specimen geometries including C(T), M(T), ESE(T), SE(T), Double Edge Crack (DEC), Three‐Hole‐Tension (THT) and NC (crack from a notch) manufactured from Al7075 were studied. Improvements in crack instability predictions were observed compared to the other methods available in the literature.

Zanganeh, Mohammad↗
NASA NTRS2015

Assessing Spontaneous Combustion Instability with Nonlinear Time Series Analysis

Considerable interest lies in the ability to characterize the onset of spontaneous instabilities within liquid propellant rocket engine (LPRE) combustion devices. Linear techniques, such as fast Fourier transforms, various correlation parameters, and critical damping parameters, have been used at great length for over fifty years. Recently, nonlinear time series methods have been applied to deduce information pertaining to instability incipiency hidden in seemingly stochastic combustion noise. A technique commonly used in biological sciences known as the Multifractal Detrended Fluctuation Analysis has been extended to the combustion dynamics field, and is introduced here as a data analysis approach complementary to linear ones. Advancing, a modified technique is leveraged to extract artifacts of impending combustion instability that present themselves a priori growth to limit cycle amplitudes. Analysis is demonstrated on data from J-2X gas generator testing during which a distinct spontaneous instability was observed. Comparisons are made to previous work wherein the data were characterized using linear approaches. Verification of the technique is performed by examining idealized signals and comparing two separate, independently developed tools.

Eberhart, C. J.↗
NASA NTRS2016

The Effect of Acoustic Forcing on Instabilities and Breakdown in Swept-Wing Flow over a Backward-Facing Step

Instability interaction and breakdown were experimentally investigated in the flow over a swept backward-facing step. Acoustic forcing was used to excite the Tollmien-Schlichting (TS) instability and to acquire phase-locked results. The phase-averaged results illustrate the complex nature of the interaction between the TS and stationary cross flow instabilities. The weak stationary cross flow disturbance causes a distortion of the TS wavefront. The breakdown process is characterized by large positive and negative spikes in velocity. The positive spikes occur near the same time and location as the positive part of the TS wave. Higher-order spectral analysis was used to further investigate the nonlinear interactions between the TS instability and the traveling cross flow disturbances. The results reveal that a likely cause for the generation of the spikes corresponds to nonlinear interactions between the TS, traveling cross flow, and stationary cross flow disturbances. The spikes begin at low amplitudes of the unsteady and steady disturbances (2-4% U (sub e) (i.e. boundary layer edge velocity)) but can achieve very large amplitudes (20-30 percent U (sub e) (i.e. boundary layer edge velocity)) that initiate an early, though highly intermittent, breakdown to turbulence.

Eppink, Jenna L.↗
NASA NTRS2016

Influence of Stationary Crossflow Modulation on Secondary Instability

A likely scenario for swept wing transition on subsonic aircraft with natural laminar flow involves the breakdown of stationary crossflow vortices via high frequency secondary instability. A majority of the prior research on this secondary instability has focused on crossflow vortices with a single dominant spanwise wavelength. This paper investigates the effects of the spanwise modulation of stationary crossflow vortices at a specified wavelength by a subharmonic stationary mode. Secondary instability of the modulated crossflow pattern is studied using planar, partial-differential-equation based eigenvalue analysis. Computations reveal that weak modulation by the first subharmonic of the input stationary mode leads to mode splitting that is particularly obvious for Y-type secondary modes that are driven by the wall-normal shear of the basic state. Thus, for each Y mode corresponding to the fundamental wavelength of results in unmodulated train of crossflow vortices, the modulated flow supports a pair of secondary modes with somewhat different amplification rates. The mode splitting phenomenon suggests that a more complex stationary modulation such as that induced by natural surface roughness would yield a considerably richer spectrum of secondary instability modes. Even modest levels of subharmonic modulation are shown to have a strong effect on the overall amplification of secondary disturbances, particularly the Z-modes driven by the spanwise shear of the basic state. Preliminary computations related to the nonlinear breakdown of these secondary disturbances provide interesting insights into the process of crossflow transition in the presence of the first subharmonic of the dominant stationary vortex.

Vortex structure↗
NASA NTRS2017

Time Resolved Stereo Particle Image Velocimetry Measurements of the Instabilities Downstream of a Backward-Facing Step in a Swept-Wing Boundary Layer

Time-resolved particle image velocimetry (TRPIV) measurements are performed down-stream of a swept backward-facing step, with a height of 49% of the boundary-layer thickness. The results agree well qualitatively with previously reported hotwire measurements, though the amplitudes of the fluctuating components measured using TRPIV are higher. Nonetheless, the low-amplitude instabilities in the flow are fairly well resolved using TR- PIV. Proper orthogonal decomposition is used to study the development of the traveling cross flow and Tollmien-Schlichting (TS) instabilities downstream of the step and to study how they interact to form the large velocity spikes that ultimately lead to transition. A secondary mode within the traveling cross flow frequency band develops with a wavelength close to that of the stationary cross flow instability, so that at a certain point in the phase, it causes an increase in the spanwise modulation initially caused by the stationary cross flow mode. This increased modulation leads to an increase in the amplitude of the TS mode, which, itself, is highly modulated through interactions with the stationary cross flow. When the traveling cross flow and TS modes align in time and space, the large velocity spikes occur. Thus, these three instabilities, which are individually of low amplitude when the spikes start to occur (U'rms/Ue <0.03), interact and combine to cause a large flow disturbance that eventually leads to transition.

Eppink, Jenna L.↗
NASA NTRS2018

Instability and Target Design in a Z-Pinch Driven Pulsed Fission / Fusion Engine

Advanced forms of propulsion are required to drastically improve upon the limits of existing technology and enable deep space exploration. Nuclear reactors, both fission and fusion, have great potential as systems with high energy density for spacecraft propulsion. There are a variety of nuclear propulsion concepts that have been proposed over the last several decades. Some of these rely on the confinement of plasmas by both magnetic and inertial means. When discussing the plasma confinement one must consider the stability of the system. The conditions of the plasma dictate the burn rate of the nuclear fuel; therefore, the plasma must maintain a minimum set of conditions in order to achieve an adequate burn. Instabilities limit plasma confinement through growing fluctuations in density and pressure. This results in turbulent mixing and loss of confinement. Thus it is desirable to find ways of managing the impact of instabilities that arise in order for a sufficient burn of the fuel such that surplus energy can be converted into a propulsive force. Note that the same instabilities plague the confinement of plasmas for terrestrial power generation. Over the last several years there has been research relating to the development of a Pulsed Fission-Fusion Engine (PuFF) conducted by the authors and others at their respective institutions. The propulsion system concept centers on the use of a z-pinch to compress a plasma in order to induce nuclear reactions. The plasma along with the additional energy released by the fission and fusion processes is expanded with a magnetic nozzle to produce thrust. The concept, if developed, is expected to have an improvement in specific impulse of several orders of magnitude in comparison with chemical propulsion systems. The z-pinch has been explored in the past as a method to confine a plasma via magnetic confinement fusion (MCF) for power generation. It suffers; however, from instabilities that so far have prevented break even conditions.

Taylor, Brian↗
NASA NTRS2016

Hall2De Simulations with a First-Principles Electron Transport Model Based on the Electron Cyclotron Drift Instability

Several years of work have combined plasma measurements in a laboratory Hall thruster and r-z numerical simulations with Hall2De to isolate the spatial variation of the anomalous collision frequency needed in Ohm’s law to produce the observed thruster behavior. This numerical solution is used here to test the validity of a first-principles model of the anomalous transport in these devices before such model is implemented self-consistently in r-z fluid codes like Hall2De. The first-principles model employs quasi-linear theory and is based on the hypothesis that the Electron Cyclotron Drift Instability (ECDI) excites ion acoustic turbulence that, in turn, enhances the effective collision frequency in these devices. We find that a model of the ECDI that assumes Maxwellian velocity distributions for electrons and singly-charged, main-beam, cold ions (Ti=0.07 eV) is insufficient to explain the expected variation of the anomalous collision frequency both in the interior and exterior of the acceleration channel. When warm ions (~0.5-3 eV) are accounted for, the ECDI model in the channel interior appears more promising but fails by orders of magnitude in the near plume region due to the much higher Landau damping of the ion acoustic waves there. This implies that either (a) some process allows the ECDI instability to remain uninhibited by classical Landau damping or, (b) that a different instability (or instabilities) altogether, also insusceptible to Landau damping, is/are active in this region. A previous hypothesis, that convection of wave energy generated by the ECDI in the channel plays a significant role in the near plume, is not supported by the results of the simulations.

Mikellides, Ioannis G.↗
NASA NTRS2020

Cross-field Streaming Instabilities in the Lower Hybrid Frequency Near the Front Pole Regions of a Magnetically Shielded Hall Thruster

The mechanism that led to the erosion of the inner front pole cover in wear tests of the Hall-effect Rocket with Magnetic Shielding (HERMeS) at 300 V and 12.5 kW has been elusive. Several classical mechanisms have been considered yet none has explained conclusively the source of the discrepancy between wear measurements and simulations. In this article we consider a new mechanism. The electrostatic dispersion relation for an unbounded, homogeneous plasma in a fixed magnetic field is solved numerically in the near-plume region of this thruster. The plasma conditions have been obtained from Hall2De (r-z) multifluid-PIC simulations. We find that the modified two-stream instability can be excited in most regions of the domain, with the fastest growth approaching the fluid-limit value of ωLH/2 near the magnetic poles where ωpe/ωce~1. In this region the relative drift between magnetized electrons and beam ions perpendicular to the magnetic field can exceed 3uTi, while that between counter-streaming beam and cathode ions can exceed 4uTi. The latter suggests that the ion-ion cross-field lower hybrid instability may also be active here. Unlike the classical ion-acoustic instability, lower hybrid instabilities can heat ions to temperatures that are comparable to the electron temperature. Such anomalous heating would enhance the average energy of these ions and, in turn, the erosion of the nearby surfaces they sputter. An idealized model that scales the anomalous ion heating with ωLH has been incorporated in Hall2De and the simulations now yield significantly improved agreement with the measured wear rates and IVDFs, as reported in our companion paper.

Ortega, Alejandro Lopez↗
NASA NTRS2021

Kozai–Lidov Oscillations Triggered by a Tilt Instability of Detached Circumplanetary Discs

Circumplanetary discs can be linearly unstable to the growth of disc tilt in the tidal potential of the star–planet system. We use3D hydrodynamical simulations to characterize the disc conditions needed for instability, together with its long-term evolution.Tilt growth occurs for disc aspect ratios, evaluated near the disc outer edge, ofH/r0.05, with a weak dependence on viscosityin the wave-like regime of warp propagation. Lower mass giant planets are more likely to have circumplanetary discs thatsatisfy the conditions for instability. We show that the tilt instability can excite the inclination to above the threshold where thecircumplanetary disc becomes unstable to Kozai–Lidov (KL) oscillations. Dissipation in the KL unstable regime caps furthertilt growth, but the disc experiences large oscillations in both inclination and eccentricity. Planetary accretion occurs in episodicaccretion events. We discuss implications of the joint tilt–KL instability for the detectability of circumplanetary discs, for theobliquity evolution of forming giant planets, and for the formation of satellite systems.

Rebecca G Martin↗
NASA NTRS2023

Oblique Propagation and Temperature Effects on the Resonant Right-Hand Ion Beam Instability

The resonant right-hand instability (RHI) is often the dominant mode driven by reflected ions upstream of Earth’s quasi-parallel bow shock. In the tradition of Peter Gary, this paper further explores the right-hand instability using numerical solutions of the plasma dispersion relation and non-linear kinetic simulations, with parameters inspired by observations from NASA’s Magnetospheric Multiscale (MMS) mission. Agreement is found between the ion distributions in the particle-in-cell simulations and Magnetospheric Multiscale spacecraft data, which show the gyrophase bunching characteristic of the instability. The non-linear structures created by right-hand instability tend to be stronger when the plasma beta is lower. These structures have sizes of around 100 to 200 ion inertial lengths perpendicular to the magnetic field, presenting planet-sized disturbances to the magnetosphere. 2d and 3D hybrid particle-in-cell simulations show that modes with a range of propagation angles oblique to the magnetic field are excited, providing a ground to understand previous statistical studies of observed foreshock waves.

Ari Le↗
NASA NTRS1943

Some Investigations of the General Instability of Stiffened Metal Cylinders VI : Stiffened Metal Cylinders Subjected to Combined Bending and Transverse Shear

This is the sixth of a series of reports covering an investigation of the general instability problem by the California Institute of Technology. The first five reports of this series cover investigations of the general instability problem under the loading conditions of pure bending and were prepared under the sponsorship of the Civil Aeronautics Administration. This report and the succeeding reports of this series cover the work done on other loading conditions under the sponsorship of the National Advisory Committee for Aeronautics. This report summarizes the work that has been carried on in the experimental investigation of the problem of general instability of stiffened metal cylinders subjected to combined bending and transverse shear at the C.I.T. This part of the investigation includes tests on 55 sheet-covered specimens.

TESTING MACHINES↗
NASA NTRS1943

Some investigations of the general instability of stiffened metal cylinders III : continuation of tests of wire-braced specimens and preliminary tests of sheet-covered specimens

This is the third of a series of reports covering an investigation of the general instability problem by the California Institute of Technology. The first five reports of this series cover investigations of the general instability problem under the loading conditions of pure bending and were prepared under the sponsorship of the Civil Aeronautics Administration. The succeeding reports of this series cover the work done on other loading conditions under the sponsorship of the National Advisory Committee for Aeronautics. This report is concerned primarily with the continuation of the tests of wire-braced specimens, and preliminary tests of sheet-covered specimens that had been made in the experimental investigation on the problem of the general instability of stiffened metal cylinders at the C.I.T.

ALUMINUM ALLOYS, SHEETS, ALCOA 17ST↗
NASA NTRS2018

The Interplay Between Radiation Pressure and the Photoelectric Instability in Optically Thin Disks of Gas and Dust

In optically thin disks, dust grains are photoelectrically stripped of electrons by starlight, heating nearby gas and possibly creating a dust clumping instability-the photoelectric instability (PeI)-that significantly alters global disk structure. In the current work, we use the Pencil Code to perform the first numerical models of the PeI that include stellar radiation pressure on dust grains in order to explore the parameter regime in which the instability operates. In some models with low gas and dust surface densities, we see a variety of dust structures, including sharp concentric rings. In the most gas- and dust-rich models, nonaxisymmetric clumps, arcs, and spiral arms emerge that represent dust surface density enhancements of factors of ∼5-20. In one high gas surface density model, we include a large, low-order gas viscosity and find that it observably smooths the structures that form in the gas and dust, suggesting that resolved images of a given disk may be useful for deriving constraints on the effective viscosity of its gas. Our models show that radiation pressure does not preclude the formation of complex structure from the PeI, but the qualitative manifestation of the PeI depends strongly on the parameters of the system. The PeI may provide an explanation for unusual disk morphologies, such as the moving blobs of the AU Mic disk, the asymmetric dust distribution of the 49Ceti disk, and the rings and arcs found in the HD 141569A disk.

photoelectric instability (PeI)↗
NASA NTRSDate not supplied

Magnetic Measurements of Arc Instabilities in Segmented Arc Heater Column

The Aerodynamic Heating Facility (AHF) at NASA Ames Research Center (ARC) generates high enthalpy, supersonic flow environments for ground testing of NASAheat shield materials. This arc jet heater operates by passing a DC arc (<3200A,<2500V) through a gas column (~10cm diameter, 2m length) and ejecting the heatedgas through a supersonic nozzle into a test chamber. The arc plasma is a partially ionized air mixture at low temperature (< 2eV) but needs to be stable for long test times (up to 30min) at high power (< 20MW). Currently, the operational envelope of the facility is limited by arc instabilities including the kink instability. Consequently, it is of interest to characterize these instabilities to better understand facility limits and inform methods to extend stable operational ranges. Due to the high enthalpy and long timescales (i.e., minutes) diagnostics must be non-invasive. This work will cover the development of two sets of magnetic sensors: differential B-dot sensors and tri-axial magnetic hall sensors and will describe initial measurements of arc motion usingthese sensors.

Kink instability↗
NASA NTRSDate not supplied

Magnetic Measurements of Arc Instabilities in Segmented Arc Heater Column

The Aerodynamic Heating Facility (AHF) at NASA Ames Research Center (ARC) generates high enthalpy, supersonic flow environments for ground testing of NASAheat shield materials. This arc jet heater operates by passing a DC arc (<3200 A,<2500 V) through a gas column (~6 cm diameter, 2 m length) and ejecting the heated gas through a supersonic nozzle into a test chamber. The arc plasma is a partially ionized air mixture at low temperature (< 2 eV) but needs to be stable for long test times (up to 30 min) at high power (< 20 MW). Currently, the operational envelope of the facility is limited by arc instabilities including the kink instability. Consequently, it is of interest to characterize these instabilities to better understand facility limits and inform methods to extend stable operational ranges. Due to the high enthalpy and long timescales (i.e., minutes) diagnostics must be non-invasive. This work will cover the development of two sets of magnetic sensors: differential B-dot sensors and tri-axial magnetic hall sensors and will describe initial measurements of arc motion using these sensors.

Kink instability↗
NASA NTRS1968

Secondary nondestructive instability in medium size liquid fuel rocket engines

Linearized mathematical models of feed-system-coupled combustion instability are summarized and discussed. Following the derivation of sufficient conditions for stability from the simplest bipropellant model, the complications that are imposed on the model by compressible fluids in the feed system, nonrigid feed system structures, and time-varying combustion processes are considered. A specific case of an instability associated with a nonrigid injector is described; it is shown that the instability was eliminated by the method predicted by the model to be most effective for control. A bibliography lists papers dealing with these kinds of stabilities.

R J Fontaine↗
NASA NTRS1971

Instabilities and turbulence in highly ionized plasmas in a magnetic field

Experimental and theoretical work has been extended on the physical mechanisms of plasma turbulence and coherent instabilities. Turbulence is described in terms of parametric effects and experimental and theoretical results show that energy is transferred in both directions of the frequency spectrum. Energy transfer to higher frequencies results in turbulence acting as a safety valve against the growth of coherent, low frequency instabilities. Energy transfer to lower frequencies results in anomalously high diffusion rates. A linear theory has been applied to the drift instability in the Rensselaer HCD and reasonable agreement between predictions and experimental results has been obtained.

Noon, J. H.↗
NASA NTRS1972

Additional research on instabilities in atmospheric flow systems associated with clear air turbulence

Analytical and experimental fluid mechanics studies were conducted to investigate instabilities in atmospheric flow systems associated with clear air turbulence. The experimental portion of the program was conducted using an open water channel which allows investigation of flows having wide ranges of shear and density stratification. The program was primarily directed toward studies of the stability of straight, stratified shear flows with particular emphasis on the effects of velocity profile on stability; on studies of three-dimensional effects on the breakdown region in shear layers; on the the interaction of shear flows with long-wave length internal waves; and on the stability of shear flows consisting of adjacent stable layers. The results of these studies were used to evaluate methods used in analyses of CAT encounters in the atmosphere involving wave-induced shear layer instabilities of the Kelvin-Helmholta type. A computer program was developed for predicting shear-layer instability and CAT induced by mountain waves. This technique predicts specific altitudes and locations where CAT would be expected.

Stoeffler, R. C.↗