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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Reversal-bounded multipushdown machines

Several representations of the recursively enumerable (r.e.) sets are presented. The first states that every r.e. set is the homomorphic image of the intersection of two linear context-free languages. The second states that every r.e. set is accepted by an on-line Turing acceptor with two pushdown stores such that in every computation, each pushdown store can make at most one reversal (that is, one change from 'pushing' to 'popping'). It is shown that this automata theoretic representation cannot be strengthened by restricting the acceptors to be deterministic multitape, nondeterministic one-tape, or nondeterministic multicounter acceptors. This provides evidence that reversal bounds are not a natural measure of computational complexity for multitape Turing acceptors.

Baker, B. S.↗
NASA NTRS1975

Quiet, clean short-haul experimental engine (QCSEE) test results from a 14 cm inlet for a variable pitch fan thrust reverser

A 1/13th scale model of the fan discharge duct was tested with a matrix of flare exlets. Exlets are flared fan nozzles configured for improved reverse flow inlet performance for reverse pitch fan applications. Results showed that a flare type exlet is an acceptable design for QCSEE, as indicated by high pressure recovery and low distortion performance.

Vier, W. F.↗
NASA NTRS1976

Performance of 1.15-pressure-ratio fan stage at several rotor blade setting angles with reverse flow

A 51 cm diameter low pressure ratio fan stage was tested in reverse flow. Survey flow data were taken over the range of rotative speed from 50 percent to 100 percent design speed at several rotor blade setting angles through both flat and feather pitch. Normal flow design values of pressure ratio and weight flow were 1.15 and 29.9 kg/sec with a rotor tip speed of 243.8 m/sec. The maximum thrust in reverse flow was 52.5 percent of design thrust in normal flow.

Kovich, G.↗
NASA NTRS1976

Reverse Current in Solar Flares

The theory that impulsive X ray bursts are produced by high energy electrons streaming from the corona to the chromosphere is investigated. Currents associated with these streams are so high that either the streams do not exist or their current is neutralized by a reverse current. Analysis of a simple model indicates that the primary electron stream leads to the development of an electric field in the ambient corona which decelerates the primary beam and produces a neutralizing reverse current. It appears that, in some circumstances, this electric field could prevent the primary beam from reaching the chromosphere. In any case, the electric field acts as an energy exchange mechanism, extracting kinetic energy from the primary beam and using it to heat the ambient plasma. This heating is typically so rapid that it must be expected to have important dynamical consequences.

Knight, J. W.↗
NASA NTRS1977

Reverse pitch fan with divided splitter

A guide vane arrangement is described for improving gas turbine performance in the reverse thrust mode. This flow straightening method produces low losses in the fluid entering a core engine during reverse thrust operations and allows large supercharging of the core engine during forward thrust.

Smith, L. H., Jr.↗
NASA NTRS1977

Dynamics of high-bypass-engine thrust reversal using a variable-pitch fan

The test program demonstrated that successful and rapid forward-to reverse-thrust transients can be performed without any significant engine operational limitations for fan blade pitch changes through either feather pitch or flat pitch. For through-feather-pitch operation with a flight inlet, fan stall problems were encountered, and a fan blade overshoot technique was used to establish reverse thrust.

Schaefer, J. W.↗
NASA NTRS1978

Static performance of five twin-engine nonaxisymmetric nozzles with vectoring and reversing capability

Transonic tunnel test was performed to determine the static performance of five twin-engine nonaxisymmetric nozzles and a base-line axisymmetric nozzle at three nozzle power settings. Static thrust-vectoring and thrust-reversing performance were also determined. Nonaxisymmetric-nozzle concepts included two-dimensional convergent-divergent nozzles, wedge nozzles, and a nozzle with a single external-expansion ramp. All nonaxisymmetric nozzles had essentially the same statis performance as the axisymmetric nozzle. Effective thrust vectoring and reversing was also achieved.

Capone, F. J.↗
NASA NTRS1978

High rate of reversal in Nicd cells

Pressure response to reverse current, pressure decay after reversal, and discharge performance of nickel cadmium cells are presented.

Killian, H.↗
NASA NTRS1980

Additional Reversal Characteristics of Sealed Nickel Cadmium Cells

The reversal characteristics of nickel cadmium cells are discussed. The evolution of hydrogen from the positive electrode is described when the nickel cadmium cell is completely discharged. Results indicate that one ampere hour of overdischarge in reversal can generate enough hydrogen to increase the cell pressure of an average size cell, 20- to 50-ampere hour cell, by 120 psi.

Ritterman, P. F.↗
NASA NTRS1980

The relationships between high latitude convection reversals and the energetic particle morphology observed by the Atmosphere Explorer

Simultaneous measurements of the auroral zone particle precipitation and the ion convection velocity by Atmosphere Explorer show a consistent difference between the location of the poleward boundary of the auroral particle precipitation and the ion convection reversal. The difference of about 1.5 degrees of invariant latitude is such that some part of the antisunward convection lies wholly within the auroral particle precipitation region. The nature of the convection reversals within the precipitation region suggests that in this region the convection electric field is generated on closed field lines that connect in the magnetosphere to the low latitude boundary layer.

Heelis, R. A.↗
NASA NTRS1980

The relationships between high-latitude convection reversals and the energetic particle morphology observed by Atmosphere Explorer

A statistical investigation and a case study of simultaneous particle and ion convection velocity measurements from Atmosphere Explorer are presented. They show that at all local times between 1800 and 0600 hours the gross convection reversal boundary lies equatorward of the boundary between the discrete, highly structured boundary plasma sheet precipitation and the polar rain. This study indicates the existence of shear and rotational convection reversals on closed field lines; the observed morphology is consistent with a partially closed magnetosphere in which some portion of convection electric field is generated in the low-latitude boundary layer.

Heelis, R. A.↗
NASA NTRS1981

Magnetic field diffusion and dissipation in reversed-field plasmas

A diffusion equation is derived which describes the evolution of a magnetic field in a plasma of arbitrary beta and resistivity. The equation is valid for a one-dimensional slab geometry, assumes the plasma remains in quasi-equilibrium throughout its evolution and does not include thermal transport. Scaling laws governing the rate of change of the magnetic energy, particle drift energy, and magnetic flux are calculated. It is found that the magnetic free energy can be substantially larger than the particle drift energy and can be an important energy reservoir in driving plasma instabilities (e.g., the lower-hybrid-drift instability). In addition, the effect of a spatially varying resistivity on the evolution of a reversed-field plasma is studied. The resistivity model used is based upon the anomalous transport properties associated with the nonlocal mode structure of the lower-hybrid-drift instability. The relevance of this research to laboratory plasmas (e.g., theta pinches, reversed-field theta pinches) and space plasmas (e.g., the earth's magnetotail) is discussed.

Drake, J. F.↗
NASA NTRS1982

NiCd cell reversals during reconditioning

An accelerated battery test with 15 periods of cell reversals during reconditioning was performed. The nature and extent of hydrogen generation was examined, and the effects of reversals on battery performance were determined.

Hwang, W.↗
NASA NTRS1984

Preventing Motor Damage Due to Rapid Reversal

Reversal switch takes more time to operate, allows gentler deceleration before reversal. Switch requires both twisting and pulling motion, giving motor time to decelerate gradually.

Fetters, R.↗
NASA NTRS1983

What causes the buoyancy reversal in compressible convection?

The problem posed by the existence of a negative buoyancy work region at the top of cellular type convection in a deeply stratified superadiabatic layer (Massaguer and Zahn, 1980) is addressed. It is approached by studying two-dimensional cellular compressible convection with different physical parameters. The results suggest that a large viscosity, together with density stratification, is responsible for the buoyancy reversal. The numerical results obtained are analyzed. It is pointed out, however, that in an astrophysical situation a fluid involved in convection will generally have very small viscosity. It is therefore thought unlikely that buoyancy reversal occurs in this way.

Chan, K. L.↗
NASA NTRS1984

Solution of the Navier-Stokes equations for flow within a 2-D thrust reversing nozzle

Implicit numerical procedures have been developed to solve the Reynold's Averaged Navier-Stokes equations for the flowfield within a two-dimensional thrust reversing engine nozzle. The procedures have been tested on four different thrust reverser nozzle geometries with both fixed and moving walls. The results have compared well with available experimental data.

Maccormack, R. W.↗
NASA NTRS1985

Port geometry effects on thrust reverser static performance

An investigation has been conducted to provide some insight into the effect of several geometric parameters on the internal performance of thrust reverser ports for nonaxisymmetric nozzles using a single port test apparatus. This test apparatus simulates thrust reversal (conceptually) which would occur in the convergent section of the forward flight nozzle or in the constant area duct just upstream. The port opening had an aspect ratio of 6.1 (throat width/throat height) and had constant passage area from the geometric throat to the exit. The geometric parameters investigated were port angle, port corner radius, port location, and flow blocker angle. The apparatus had a single port so that thrust (vector) angle could be obtained from force balance measurements. The tests were conducted in the Static Test Stand of the Langley 16-Foot Transonic Tunnel at nozzle pressure ratios from 1.5 to 8.0 using high pressure air for propulsion simulation. Port angles (measured forward from a horizontal reference line) investigated were 75, 90, 120 and 135 deg. Sharp and rounded port corners were investigated for each port angle. Discharge coefficient, thrust ratio, and passage static pressures were measured.

Re, R. J.↗