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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Simultaneous cerebrovascular and cardiovascular responses during presyncope

BACKGROUND AND PURPOSE: Presyncope, characterized by symptoms and signs indicative of imminent syncope, can be aborted in many situations before loss of consciousness occurs. The plasticity of cerebral autoregulation in healthy humans and its behavior during this syncopal prodrome are unclear, although systemic hemodynamic instability has been suggested as a key factor in the precipitation of syncope. Using lower body negative pressure (LBNP) to simulate central hypovolemia, we previously observed falling mean flow velocities (MFVs) with maintained mean arterial blood pressure (MABP). These findings, and recent reports suggesting increased vascular tone within the cerebral vasculature at presyncope, cannot be explained by the classic static cerebral autoregulation curve; neither can they be totally explained by a recent suggestion of a rightward shift in this curve. METHODS: Four male and five female healthy volunteers were exposed to presyncopal LBNP to evaluate their cerebrovascular and cardiovascular responses by use of continuous acquisition of MFV from the right middle cerebral artery with transcranial Doppler sonography, MABP (Finapres), and heart rate (ECG). RESULTS: At presyncope, MFV dropped on average by 27.3 +/- 14% of its baseline value (P < .05), while MABP remained at 2.0 +/- 27% above its baseline level. Estimated cerebrovascular resistance increased during LBNP. The percentage change from baseline to presyncope in MFV and MABP revealed consistent decreases in MFV before MABP. CONCLUSIONS: Increased estimated cerebrovascular resistance, falling MFV, and constant MABP are evidence of an increase in cerebral vascular tone with falling flow, suggesting a downward shift in the cerebral autoregulation curve. Cerebral vessels may have a differential sensitivity to sympathetic drive or more than one type of sympathetic innervation. Future work to induce dynamic changes in MABP during LBNP may help in assessing the plasticity of the cerebral autoregulation mechanism.

Non-NASA Center↗
NASA NTRS1998

Transfer function analysis of dynamic cerebral autoregulation in humans

To test the hypothesis that spontaneous changes in cerebral blood flow are primarily induced by changes in arterial pressure and that cerebral autoregulation is a frequency-dependent phenomenon, we measured mean arterial pressure in the finger and mean blood flow velocity in the middle cerebral artery (VMCA) during supine rest and acute hypotension induced by thigh cuff deflation in 10 healthy subjects. Transfer function gain, phase, and coherence function between changes in arterial pressure and VMCA were estimated using the Welch method. The impulse response function, calculated as the inverse Fourier transform of this transfer function, enabled the calculation of transient changes in VMCA during acute hypotension, which was compared with the directly measured change in VMCA during thigh cuff deflation. Beat-to-beat changes in VMCA occurred simultaneously with changes in arterial pressure, and the autospectrum of VMCA showed characteristics similar to arterial pressure. Transfer gain increased substantially with increasing frequency from 0.07 to 0.20 Hz in association with a gradual decrease in phase. The coherence function was > 0.5 in the frequency range of 0.07-0.30 Hz and < 0.5 at < 0.07 Hz. Furthermore, the predicted change in VMCA was similar to the measured VMCA during thigh cuff deflation. These data suggest that spontaneous changes in VMCA that occur at the frequency range of 0.07-0.30 Hz are related strongly to changes in arterial pressure and, furthermore, that short-term regulation of cerebral blood flow in response to changes in arterial pressure can be modeled by a transfer function with the quality of a high-pass filter in the frequency range of 0.07-0.30 Hz.

NASA Discipline Cardiopulmonary↗
DOE OSTI2020

Long-range Doppler lidar measurements of wind turbine wakes and their interaction with turbulent atmospheric boundary-layer flow at Perdigao 2017

As part of the Perdigão 2017 campaign, vertical RHI (range-height indicator) scans with long-range pulsed Doppler wind lidars were performed aligned with the main wind direction and a wind turbine (WT) located on a mountain ridge. The measurements are used to not only retrieve flow velocities, but also their variance and - by using the turbulent broadening of the Doppler spectrum - also turbulent kinetic energy (TKE) dissipation rate. The study shows that turbulence in the WT wake is dependent on the turbulence of the inflow, but also on atmospheric stability. In stable atmospheric conditions, wakes could be analyzed up to five rotor diameters downstream (D) and showed the maximum turbulence in the wake at 2-3 D, whereas in unstable conditions, the maximum was found at 2 D and the wake could not be detected further than 3 D. A clear dependency of wake turbulence enhancement on inflow turbulence intensity is found, which levels out to no further enhancement at turbulence intensities of 30%.

17 WIND ENERGY↗
NASA NTRS1982

Plasma injection and transport in the mid-altitude polar cusp

DE-1 hot plasma observations in the mid-altitude polar cusp have shown evidence of a significant velocity filtering phenomenon which is consistent with a latitudinally narrow region of plasma injection located at a geocentric distance of about eight earth radii (in a dipole approximation). Plasma convection from the injection region into the polar cap results in a V-shaped log E vs. observed pitch angle relation at geocentric distances near 4 earth radii. This velocity filtering effect allows the measurement of much smaller flow velocities (about 10 km/sec) than have heretofore been possible with hot plasma measurements. The flows thus determined are consistent with ionospheric flows measured nearly simultaneously by the DE-2 spacecraft, although the magnitudes of the higher altitude flows are higher by a factor of 2 or more than an approximately r to the 3/2 power dipole-field mapping would predict.

Burch, J. L.↗
NASA NTRS1991

High enthalpy, hypervelocity flows of air and argon in an expansion tube

An expansion tube with a free piston driver has been used to generate quasi-steady hypersonic flows in argon and air at flow velocities in excess of 9 km/s. Irregular test flow unsteadiness has limited the performance of previous expansion tubes, and it has been found that this can be avoided by attention to the interaction between the test gas accelerating expansion and the contact surface in the primary shock tube. Test section measurements of pitot pressure, static pressure and flat plate heat transfer are reported. An approximate analytical theory has been developed for predicting the velocities achieved in the unsteady expansion of the ionizing or dissociating test gas.

Neely, A. J↗
NASA NTRS1985

Configuration of the high-latitude thermosphere neutral circulation for IMF B(y) negative and positive

Asymmetries in the Northern Hemisphere neutral circulation that are dependent on the sign of the east-west 'B(y)' component of the IMF are presently illustrated by measurements of the neutral wind in the polar F-region which were conducted by the DE-2 spacecraft. The data thus obtained are categorized according to the sign of the hourly averaged IMF B(y) component measured by ISEE-3 for the hour preceding the DE-2 measurements. It is noted that there is (1) an asymmetry in the polar cap neutral flow velocity, with the region of most rapid antisunward flow shifting from the dawn side to the dusk side of the polar cap as B(y) changes from positive to negative; (2) a shift occurs in the magnetic local time of the region of entry of neutral gas into the polar cap, from a location on the dawn side of the moon-midnight meridian for B(y) positive to one more biased, for B(y) negative; and (3) there is an enhancement of velocities associated with the dawn anticlockwise neutral vortex for B(y) negative relative to those observed for B(y) positive.

Mccormac, F. G.↗
NASA NTRS1986

Ion neutral coupling in the high latitude thermosphere, part 1

Measurements of the neutral wind in the polar F region from Dynamics Explorer-2 (DE-2) were used to illustrate asymmetries in the neutral circulation that are dependent on the sign of the B sub y component of the interplanetary magnetic field (IMF). Individual DE-2 orbits and averaged data sets from different Universal times are presented. The data are categorized according to the sign of the hourly averaged IMF B sub y component measured by ISEE-3 for the hour preceding the DE-2 measurement. The major features observed are: (1) an asymmetry in the polar cap neutral flow velocity with the region of most rapid antisunward flow shifting from the dawn side to the dusk side of the polar cap as B sub y changes from positive to negative; (2) a shift in magnetic local time of the region of entry of neutral gas into the polar cap from a location on the dawn side of the noon-midnight meridian for B sub y positive to one more biased towards the dusk side for B sub y negative; (3) an enhancement in the velocities associated with the dawn, anti-clockwise neutral vortex B suby y negative relative to those observed for B sub y positive. The B sub y neutral wind asymmetries can be explained by similar asymmetries, previously observed, in the polar ion convection pattern. They imply a direct causal relationship between solar wind/magnetosphere coupling and neutral thermospheric dynamics.

Killeen, T. L.↗
NASA NTRS1982

The energetic molecular outflow near AFGL 961 - Millimeter-wave and infrared observations

Sensitive millimeter-wave C-12O line observations and near-infrared hydrogen recombination line observations of AFGL 961 are reported and analyzed. The observations show that the high-velocity flow has a maximum extent of at least six arcmin or 1.9 pc at the distance of AFGL 961. The flow is anisotropic and the emission profiles are asymmetric in velocity. The mass of the gas involved in the flow is determined to be approximately 19 solar masses, and the kinetic energy of this gas is estimated to be about 8 x 10 to the 46th ergs. These observations are interpreted as evidence that an energetic bipolar outflow of molecular gas is occurring near AFGL 961. Comparison of infrared recombination line data, radio continuum, and CO observations shows that a strong, ionized, stellar wind would not be a sufficient force to drive the outflow. The effects of the outflow on the surrounding molecular cloud was also investigated.

Lada, C. J.↗
NASA NTRS1974

A modified Newton-Raphson analysis of flight measurements of the trailing vortices of a heavy jet transport

A modified Newton-Raphson method has been used to determine the parameters in equations describing the vortex flow to obtain a best match with flight measurements of the flow behind the C-5A airplane. The flight measurements were made using a specially instrumented T-33 airplane which passed as closely as possible to the centers of the trailing vortices at several distances behind the C-5A airplane. The flight measurements were transformed to flow velocity relative to an inertial frame of reference. The assumed form of the flow consisted of the superposition of two counterrotating, finite core vortices. The positions of the vortex centers, their total circulation, the effective eddy viscosity and measurement bias were the parameters adjusted. The assumed form of vortex flow fit well the measured velocities for the numerous sets of data, both flaps up and down for the C-5A airplane. The resulting values of total circulation, however, were about two-thirds that expected of a wing with an elliptical loading. A partial explanation of the less than expected circulation is a dip in the spanwise lift distribution at the airplane's center line. The distance between the trailing vortices at the smallest times encountered is somewhat less than that expected for an elliptical wing loading.

Taylor, L. W., Jr.↗
NASA NTRS1995

Magnetic reconnection in the presence of sheared flow and density asymmetry: Applications to the Earth's magnetopause

Classical models of magnetic reconnection consist of a small diffusion region bounded by two symmetric slow shocks, across which the plasma is accelerated. Asymmetries often present in space plasmas are sheared plasma flow and dissimilar plasma densities on the two sides of current sheets. In this paper, we investigate magnetic reconnection in the presence of a shear flow and an asymmetric density across the current sheet using two-dimensional magnetohydrodynamic (MHD) simulations. The results demonstrate that magnetic reconnection can occur only for a plasma flow velocity (in the frame of the X line) which is below the Alfven speed in each inflow region. This limits the velocity of the X line to a certain range for a given flow shear and provides an upper limit to the total velocity shear at which reconnection ceases to operate. Depending on the direction of the flow in the adjacent inflow region, the effects from the sheared flow and from the density asymmetry will compete with or enhance each other in respect to the magnitude and location of the currents which bound the outflow regions. The results are applied to the dayside and flank regions of the magnetosphere. For the dayside region where the magnetosheath flow is slow, the magnetic field transition region is thin and the accelerated flow is earthward of the sharp current layer (magnetopause). At the flanks tailward of the X line, shear flow and density asymmetry effects compete making the magnetic field transition layer broad with the high-speed flow contained within the transition region which explains corresponding observations. At the flanks sunward of the X line, shear flow and density asymmetry effects enhance each other and lead to a strong current sheet on the magnetosheath side of the accelerated flow. The total volume affected by magnetic reconnection is much larger than the steady state region. A large bulge region precedes the steady state region. Qualitatively, the bulge and the steady state region have similar signatures and both can explain observations. We provide criteria in order to distinguish between the bulge and the steady state region in observations.

La Belle-Hamer, A. L.↗
DOE OSTI2021

Atomic oxygen density determination in the effluent of the COST reference source using in situ effective lifetime measurements in the presence of a liquid interface

Abstract Spatially resolved, absolute densities of atomic oxygen are measured for several helium-based admixtures in the effluent of the COST Reference Microplasma Jet using two-photon absorption laser induced fluorescence (TALIF). Admixtures investigated include a helium-only admixture, four helium/oxygen admixtures (0.1 % , 0.5 % , 0.6 % , and 1.0 % oxygen), and a helium/water admixture (2500 ppm water), chosen to coincide with previously published characterizations of plasma-treated liquid. The atomic oxygen TALIF signal is calibrated for density using the noble gas xenon, which possesses a very similar two-photon excitation and fluorescence scheme. Measurements are conducted for the jet operating in ambient air with both an open effluent and a liquid surface present, allowing for comparison with liquid phase measurements conducted under similar conditions. The presence of a water surface does not appear to alter the background chemistry in the effluent but reduces O densities close to the liquid interface when compared to a similar distance from the nozzle in an open effluent case. This may be the result of a reduction in flow velocity caused by the liquid obstructing the gas flow. Additionally, measurements near the liquid surface revealed a region of atomic oxygen well outside of where the core of the effluent impinges on the liquid. This is likely relevant for applications as it considerably expands the surface area subject to O absorption. Critically, in situ measurements of the effective lifetimes of the laser-excited 3p 3 P J state of atomic oxygen were recorded in the effluent by employing a picosecond (ps) laser and a nanosecond (ns) ICCD. By experimentally determining the contribution from collisional quenching via the in situ effective lifetime measurements, significant improvements in the accuracy of the atomic oxygen density calibration were made, with differences of approximately 30 % from existing methods of estimating quenching rates at atmospheric pressure. Finally, spatially resolved atomic oxygen densities allow for an investigation of O formation and extinction pathways in the effluent and a comparison between admixtures.

laser diagnostics↗
NASA NTRS1981

Hot relativistic winds and the Crab Nebula

Efforts to formulate a self-consistent model of pulsar magnetospheres which links the particle source near the pulsar to the outflowing relativistic wind and couples the wind to the surrounding nebula are reviewed. The use of a relativistic MHD wind is recommended to account for global photon emission and the invisibility of the method of plasma transport. Consideration of a magnetic monopole relativistic wind due to an axially symmetric aligned rotator is combined with calculations of the initial velocity of the wind to show that the flow velocity in such a model will never exceed Mach 1. Extending the solution to the case of a hot relativistic wind at supersonic speeds is noted to yield results consistent with observations of the Crab Nebula

Fujimura, F. S.↗
NASA NTRS1991

A determination of relativistic shock jump conditions using Monte Carlo techniques

Monte Carlo techniques are used, assuming isotropic elastic scattering of all particles, to calculate jump conditions in parallel relativistic collisionless shocks in the absence of Fermi acceleration. The shock velocity and compression ratios are shown for arbitrary flow velocities and for any upstream temperature. Both single-component electron-positron plasma and two-component proton-electron plasmas are considered. It is shown that protons and electrons must share energy, directly or through the mediation of plasma waves, in order to satisfy the basic conservation conditions, and the electron and proton temperatures are determined for a particular microscopic, kinetic-theory model, namely, that protons always scatter elastically. The results are directly applicable to shocks in which waves of scattering superthermal particles are absent.

Ellison, Donald C.↗
NASA NTRS1996

Calibration of Hot Wire Anemometers. (Latest Citations from the Aerospace Database)

The bibliography contains citations concerning calibration methods and measurement correction schemes for hot wire anemometers. Coverage includes static and dynamic calibration of sensors having single, multiple, cross, and ring wire configurations. Correction methods to account for yaw angle, low-velocity flow, microgravity, wall proximity, and highly fluctuating turbulence, velocity, or temperature are covered. Correction methods are also referenced for installations having multiple sensors. Hot film and laser anemometers, and the use of anemometers in specific industrial and aerospace applications are extensively covered in separate biblographies. (Contains 50-250 citations and includes a subject term index and title list.)

HOT WIRE ANEMOMETERS↗
NASA NTRS1989

3-D composite velocity solutions for subsonic/transonic flows

A composite velocity procedure for the three-dimensional reduced Navier-Stokes equations is developed. In the spirit of matched asymptotic expansions, the velocity components are written as a combination multiplicative and additive composite of viscouslike velocities and pseudopotential or inviscid velocities. The solution procedure is then consistent with both asymptotic inviscid flow and boundary layer theory. For transonic flow cases, the Enquist-Osher flux biasing scheme developed for the full potential equation is used. A quasi-conservation form of the governing equations is used in the shock region to capture the correct rotational shock with the standard nonconservation form of the equations used in nonshock regions. The consistent coupled strongly implicit procedure coupled with a plane relaxation procedure is used to solve the discretized equations.

Gordnier, R. E.↗
NASA NTRS1990

The effects of forcing on a single stream shear layer and its parent boundary layer

Forcing and its effect on fluid flows has become an accepted tool in the study and control of flow systems. It has been used both as a diagnostic tool, to explore the development and interaction of coherent structures, and as a method of controlling the behavior of the flow. A number of forcing methods have been used in order to provide a perturbation to the flow; among these are the use of an oscillating trailing edge, acoustically driven slots, external acoustic forcing, and mechanical piston methods. The effect of a planar mechanical piston forcing on a single stream shear layer is presented; it can be noted that this is one of the lesser studied free shear layers. The single stream shear layer can be characterized by its primary flow velocity scale and the thickness of the separating boundary layer. The velocity scale is constant over the length of the flow field; theta (x) can be used as a width scale to characterize the unforced shear layer. In the case of the forced shear layer the velocity field is a function of phase time and definition of a width measure becomes somewhat problematic.

Haw, Richard C.↗
NASA NTRS2009

Spatially and Temporally Resolved Measurements of Velocity in a H2-air Combustion-Heated Supersonic Jet

This paper presents simultaneous measurements at multiple points of two orthogonal components of flow velocity using a single-shot interferometric Rayleigh scattering (IRS) technique. The measurements are performed on a large-scale Mach 1.6 (Mach 5.5 enthalpy) H2-air combustion jet during the 2007 test campaign in the Direct Connect Supersonic Combustion Test facility at NASA Langley Research Center. The measurements are performed simultaneously with CARS (Coherent Anti-stokes Raman Spectroscopy) using a combined CARS-IRS instrument with a common path 9-nanosecond pulsed, injection-seeded, 532-nm Nd:YAG laser probe pulse. The paper summarizes the measurements of velocities along the core of the vitiated air flow as well as two radial profiles. The average velocity measurement near the centerline at the closest point from the nozzle exit compares favorably with the CFD calculations using the VULCAN code. Further downstream, the measured axial velocity shows overall higher values than predicted with a trend of convergence at further distances. Larger discrepancies are shown in the radial profiles.

Bivolaru, Daniel↗