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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Theoretical flow regime diagrams for the AGCE

The major criterion for the design of the Atmospheric General Circulation Experiment is that it be possible to realize strong baroclinic instability in the apparatus. A spherical annulus configuration which allows only steady basic state flows was chosen for the first set of stability analyses. Baroclinic instability was found for this configuration and few results suggest a regime diagram very different from the cylindrical annulus regime diagram.

Fowlis, W. W.↗
NASA NTRS1999

Thermoconvective Instability in a Rotating Magnetic Field

The effect of a rotating magnetic field (RMF) on the stability of a fluid contained in a cylindrical column and heated from below is investigated. The RMF increases the critical Rayleigh number for asymmetric flow modes but does not affect the onset of instability for axisymmetric modes. The critical Rayleigh number is dependent upon the relative penetration of the magnetic field into the cylinder and the Prandtl number of the fluid. Instability first develops in the form of a single asymmetric meridional roll rotating around the axis of the cylinder, driven by the azimuthal component of the magnetic field.

Volz, M. P.↗
NASA NTRS1978

On the fine structure of cometary plasma tails

It is argued that the small-scale structures observed close to the plasma tail axis of comets may be a consequence of well-known finite-resistivity instabilities in the cometary cross-tail current sheet. While the 'tearing' mode instability, which causes a breakup of the layer along the current flow lines, may be responsible for the observed small condensation or 'knots' near the axis, the 'ripple' mode instability may give rise to the small-scale wavy structures.

Morrison, P. J.↗
NASA NTRS2008

Characterization and Simulation of the Thermoacoustic Instability Behavior of an Advanced, Low Emissions Combustor Prototype

Extensive research is being done toward the development of ultra-low-emissions combustors for aircraft gas turbine engines. However, these combustors have an increased susceptibility to thermoacoustic instabilities. This type of instability was recently observed in an advanced, low emissions combustor prototype installed in a NASA Glenn Research Center test stand. The instability produces pressure oscillations that grow with increasing fuel/air ratio, preventing full power operation. The instability behavior makes the combustor a potentially useful test bed for research into active control methods for combustion instability suppression. The instability behavior was characterized by operating the combustor at various pressures, temperatures, and fuel and air flows representative of operation within an aircraft gas turbine engine. Trends in instability behavior versus operating condition have been identified and documented, and possible explanations for the trends provided. A simulation developed at NASA Glenn captures the observed instability behavior. The physics-based simulation includes the relevant physical features of the combustor and test rig, employs a Sectored 1-D approach, includes simplified reaction equations, and provides time-accurate results. A computationally efficient method is used for area transitions, which decreases run times and allows the simulation to be used for parametric studies, including control method investigations. Simulation results show that the simulation exhibits a self-starting, self-sustained combustion instability and also replicates the experimentally observed instability trends versus operating condition. Future plans are to use the simulation to investigate active control strategies to suppress combustion instabilities and then to experimentally demonstrate active instability suppression with the low emissions combustor prototype, enabling full power, stable operation.

DeLaat, John C.↗
NASA NTRS2008

Characterization and Simulation of Thermoacoustic Instability in a Low Emissions Combustor Prototype

Extensive research is being done toward the development of ultra-low-emissions combustors for aircraft gas turbine engines. However, these combustors have an increased susceptibility to thermoacoustic instabilities. This type of instability was recently observed in an advanced, low emissions combustor prototype installed in a NASA Glenn Research Center test stand. The instability produces pressure oscillations that grow with increasing fuel/air ratio, preventing full power operation. The instability behavior makes the combustor a potentially useful test bed for research into active control methods for combustion instability suppression. The instability behavior was characterized by operating the combustor at various pressures, temperatures, and fuel and air flows representative of operation within an aircraft gas turbine engine. Trends in instability behavior vs. operating condition have been identified and documented. A simulation developed at NASA Glenn captures the observed instability behavior. The physics-based simulation includes the relevant physical features of the combustor and test rig, employs a Sectored 1-D approach, includes simplified reaction equations, and provides time-accurate results. A computationally efficient method is used for area transitions, which decreases run times and allows the simulation to be used for parametric studies, including control method investigations. Simulation results show that the simulation exhibits a self-starting, self-sustained combustion instability and also replicates the experimentally observed instability trends vs. operating condition. Future plans are to use the simulation to investigate active control strategies to suppress combustion instabilities and then to experimentally demonstrate active instability suppression with the low emissions combustor prototype, enabling full power, stable operation.

DeLaat, John C.↗
NASA NTRS1972

Subsonic, transonic, and supersonic nozzle flow by the inverse technique.

The inverse technique is used to obtain a mathematically and physically consistent solution of the flowfield in a nozzle from the mass generation surface through the supersonic region. The inverse method employs an assumed centerline function which is of the Cauchy type in that the values and the derivatives of the function are known. Since the Cauchy boundary conditions can give rise to numerical instabilities, the governing gasdynamic equations for rotational steady flow were transformed into a form which puts the geometry into a rectangular shape, and which spaces the network of interior points more finely in regions of the greatest gradients of the dependent variables. For arbitrarily specified centerline data, the solution of the governing flow equations may not exist, and if it does it may not depend continuously on the data.

Norton, D. J.↗
NASA NTRS1991

AIAA/IKI Microgravity Science Symposium, 1st, Moscow, USSR, May 13-17, 1991, Proceedings

The state of the art of microgravity science is examined in reviews and reports. Consideration is given to fluid physics; modeling and numerical analysis; crystal growth of electronic materials, proteins, metals, alloys, glasses, and ceramics; biotechnology, combustion science, microgravity hardware, and fundamental phenomena; and education. Particular attention is given to containerless science and technology, a low-gravity two-phase flow with heat transfer, numerical investigation on Benard instability in a finite liquid layer, synthesis and crystallization of refractory compounds from solutions in metallic melts under microgravitation conditions, the long duration protein crystallization experiment aboard the Mir Space Station, directional solidification in immiscible systems, formation of periodic layer structure in oxide melts under different gravity values, opposed flow flame spread in normal, enhanced, and reduced gravity, automatic equipment for semiconductor production in space, and consort and joust sounding rocket missions.

Source record↗
DOE OSTI2021

The Effect of the Degree of Premixedness on Self-Excited Combustion Instability

The use of lean, premixed fuel and air mixtures is a common strategy to reduce NO x emissions in gas turbine combustors. However, this strategy causes an increased susceptibility to self-excited instability, which manifests as fluctuations in heat release rate, flow velocity, and combustor acoustics that oscillate in-phase in a feedback loop. Here, this study considers the effect of the level of premixedness on the self-excited instability in a single-nozzle combustor. In this system, the fuel can be injected inside the nozzle to create a partially-premixed mixture or far upstream to create a fully-premixed mixture, varying the level of premixedness of the fuel and air entering the combustor. When global equivalence ratio is held constant, the cases with higher levels of premixing have higher instability amplitudes. High-speed CH* chemiluminescence imaging shows that the flame for these cases is more compact and the distribution of the heat release rate oscillations is more concentrated near the corner of the combustor in the outer recirculation zones. Rayleigh index images, which are a metric for the relative phase of pressure and heat release rate oscillations, suggest that vortex rollup in the corner region is primarily responsible for determining instability characteristics when premixedness is varied. This finding is further supported through analysis of local flame edge dynamics.

42 ENGINEERING↗
NASA NTRS2021

Transition Analysis for the CRM-NLF Wind Tunnel Configuration

This paper presents the results of an ongoing study into the linear stability characteristics of the boundary layer flow over the common research model with natural laminar flow (CRMNLF) aircraft configuration. The flow conditions match selected test conditions from a recent wind tunnel experiment in the National Transonic Facility at the NASA Langley Research Center. Previous work involving parallel stability computations of a boundary layer flow based on the conical flow approximation has shown that the measured onset of laminar-turbulent transition during the experiments can be correlated with the linear amplification of Tollmien- Schlichting (TS) and stationary crossflow (CF) instabilities in the swept wing boundary layer. Here, we examine the effects of the simplifying approximations in both basic state computation and the stability analysis, with the goal of quantifying the resulting changes in the N-factor correlations. Specifically, the basic states are computed by using full Navier-Stokes equations and the stability analysis is performed by using a nonorthogonal coordinate system that allows a clear distinction between planar TS and CF instabilities. Furthermore, the effects of curvature and nonparallel mean flow have been included in the stability computations based on the parabolized stability equations (PSE). The fully turbulent Reynolds-Averaged-Navier-Stokes (RANS) mean flow solutions show good agreement with the measured wall pressure distribution. Viscous-inviscid interactive effects are observed to be important because the shock fronts along the suction surface are influenced by the imposed transition front. The stability results confirm the previous findings related to TS amplification within the inboard region of the wing and the dominance of stationary CF modes in the outboard region. However, given the close proximity of the measured transition front and the dual shock system within the outer part of the wing, the onset of transition may well be shock limited within the outboard region. In general, the transition criterion based on the dual N-factor method with N TS = N CF = 6 is reasonably successful at correlating with the measured transition fronts at Re MAC = 15 million and AoA = 1.5, 2 degrees; however, the low values of the correlating N-factors at Re MAC = 17.5 million support the hypothesis that the measured transition at the higher Reynolds number may have been strongly influenced by the merging of turbulent wedges that originate from surface imperfections near the leading edge.

Boundary layer transition↗
NASA NTRS1997

Stability of Thin Liquid Sheet Flows

A two-dimensional, linear stability analysis of a thin nonplanar liquid sheet flow in vacuum is carried out. A sheet flow created by a narrow slit of W and tau attains a nonplanar cross section as a consequence of cylinders forming on the sheet edge under the influence of surface tension forces. The region where these edge cylinders join the sheet is one of high curvature, and this is found to be the location where instability is most likely to occur. The sheet flow is found to be unstable, but with low growth rates for symmetric wave disturbances and high growth rates for antisymmetric disturbances. By combining the symmetric and antisymmetric disturbance modes, a wide range of stability characteristics is obtained. The product of unstable growth rate and flow time is proportional to the width-to-thickness ratio of the sift generating the sheet Three-dimensional effects can alter these results, particularly when the sheet length-to-width ratio is not much greater than unity.

McConley, Marc W.↗
DOE OSTI2020

Shocks propagate in a 2D dusty plasma with less attenuation than due to gas friction alone

In a dusty plasma, an impulsively generated shock, i.e., blast wave, was observed to decay less than would be expected due to gas friction alone. In the experiment, a single layer of microparticles was levitated in a radio frequency glow-discharge plasma. In this layer, the microparticles were self-organized as a 2D solid-like strongly coupled plasma, which was perturbed by the piston-like mechanical movement of a wire. To excite a blast wave, the wire's motion was abruptly stopped, so that the input of mechanical energy ceased at a known time. It was seen that, as it propagated across the layer, the blast wave's amplitude persisted with little decay. This result extends similar findings, in previous experiments with 3D microparticle clouds, to the case of 2D clouds. In our cloud, out-of-plane displacements were observed, lending support to the possibility that an instability, driven by wakes in the ion flow, provides energy that sustains the blast wave's amplitude despite the presence of gas damping.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

ELM-free enhanced D α H-mode with near zero NBI torque injection in DIII-D tokamak

Enhanced D α H-mode (EDA H-mode), an ELM-free H-mode regime, and the concomitant quasi-coherent mode (QCM) are explored in neutral beam heated, lower single null plasmas with near zero external torque injection. This regime exhibits good energy confinement (H 98y2 ~ 1) with β N ~ 2, high density, regime access at low input power, and no ELMs. New time–resolved measurements of electron and ion density, temperature, plasma rotation, and radial electric field as well as turbulence, flows, thermal and particle transport, and linear instability calculations examining the dynamics of the QCM are presented. Measurements using multiple fluctuation diagnostics reveal the QCM to be a separatrix spanning mode, peaking just inside the separatrix, existing in a wide range of k ⊥ ρ s ~ 0.1–1.2 with multiple harmonics, and propagating with a very small phase velocity in the plasma frame, where k ⊥ is the binormal wavenumber and ρ s is the ion sound radius. Linear gyrokinetic simulations of this EDA H-mode discharge with CGYRO indicate that the trapped electron mode (TEM) and electron temperature gradient mode are the dominant instabilities in the radial region where QCM is also unstable. Qualitative analysis indicates that the properties of TEM are consistent with the experimentally observed characteristics of the QCM. These similarities suggest that the QCM is consistent with a TEM instability existing in the edge region of the EDA H-mode plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Collaborative Research: Vlasov-Maxwell Simulations to Resolve Electron Heating and Dissipation, in Quasi-Perpendicular Shocks

The major goals of the project are to study three fundamental questions related to collisionless, non-relativistic, quasi-perpendicular shocks by combining continuum Vlasov-Maxwell simulations (the Gkeyll framework) and high resolution, in situ heliospheric observations from the MMS spacecraft. The specific questions are: 1. How and where are electrons energized and heated in a quasi-perpendicular shock, including the ultimate source of entropy production? 2. What role do waves, turbulence, and secondary instabilities play in the conversion of directed flow energy to electron energy? 3. How do the parameters of the shock determine the energization of electrons?

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1978

Formation of a rotational accretion column

We consider the problem of the flow produced by the accretion of gas from a rotating cloud onto a point gravitational source. It is shown that, for a cloud initially in uniform rotation, the effect of accretion is to produce a Taylor column in the cloud parallel to the rotation axis. Only gas within the column is accreted; the gas outside the column undergoes oscillatory flow, which is probably subject to shear instability. The column forms in approximately one-half rotation period from the time at which accretion begins. Its initial radius is determined by the accretion rate and the angular velocity of the cloud. Application of the results to the formation of Jupiter by accretion is discussed.

Cassen, P.↗
NASA NTRS1979

The electric field and global electrodynamics of the magnetosphere

The conception of the electrodynamics of the quiet-time magnetosphere obtained during the last four years of magnetospheric study is presented. Current understandings of the open magnetosphere, convective plasma flows in the plasma sheet, the shielding of the inner magnetosphere from the convective magnetospheric electric field, the space charge produced when injected electrons drift towards dawn and injected ions drift towards dusk, the disruption of the flow of the Birkeland current by plasma instabilities and the shielding of the convective electric field by the dayside magnetopause are discussed. Attention is also given to changes of magnetic field line topology magnetic storms and substorms. Unresolved questions and new tools which may play a role in the further understanding of magnetospheric electrodynamics and the role of the magnetospheric electric field are presented.

Stern, D. P.↗
NASA NTRS1982

Nonlocal stability analysis of the MHD Kelvin-Helmholtz instability in a compressible plasma

A general stability analysis is given of the Kevin-Helmholtz instability, for the case of sheared MHD flow of finite thickness in a compressible plasma which allows for the arbitrary orientation of the magnetic field, velocity flow, and wave vector in the plane perpendicular to the velocity gradient. The stability problem is reduced to the solution of a single second-order differential equation including a gravitational term to represent the coupling between the Kelvin-Helmholtz mode and the interchange mode. Compressibility and a magnetic field component parallel to the flow are found to be stabilizing effects, with destabilization of only the fast magnetosonic mode in the transverse case, and the presence of both Alfven and slow magnetosonic components in the parallel case. Analysis results are used in a discussion of the stability of sheared plasma flow at the magnetopause boundary and in the solar wind.

Miura, A.↗
NASA NTRS1982

Accretion instability models for Dwarf Novae and X-ray transients

Steady state and time dependent calculations of a model for accretion instability are presented, in which matter accumulates in a cold torus and then undergoes a thermal instability causing the matter to heat and flow down onto the central star. The model is compared to the observations of dwarf novae and certain examples of both hard and soft X-ray transients.

Cannizzo, J. K.↗