SEARCH · Search NASA

Results for “constriction”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4

All sources NASA NTRS DOE OSTI

NASA NTRS2003

Neural Control of the Cardiovascular System in Space

During the acute transition from lying supine to standing upright, a large volume of blood suddenly moves from the chest into the legs. To prevent fainting, the blood pressure control system senses this change immediately, and rapidly adjusts flow (by increasing heart rate) and resistance to flow (by constricting the blood vessels) to restore blood pressure and maintain brain blood flow. If this system is inadequate, the brain has a backup plan. Blood vessels in the brain can adjust their diameter to keep blood flow constant. If blood pressure drops, the brain blood vessels dilate; if blood pressure increases, the brain blood vessels constrict. This process, which is called autoregulation, allows the brain to maintain a steady stream of oxygen, even when blood pressure changes. We examined what changes in the blood pressure control system or cerebral autoregulation contribute to the blood pressure control problems seen after spaceflight. We asked: (1) does the adaptation to spaceflight cause an adaptation in the blood pressure control system that impairs the ability of the system to constrict blood vessels on return to Earth?; (2) if such a defect exists, could we pinpoint the neural pathways involved?; and (3) does cerebral autoregulation become abnormal during spaceflight, impairing the body s ability to maintain constant brain blood flow when standing upright on Earth? We stressed the blood pressure control system using lower body negative pressure, upright tilt, handgrip exercise, and cold stimulation of the hand. Standard cardiovascular parameters were measured along with sympathetic nerve activity (the nerve activity causing blood vessels to constrict) and brain blood flow. We confirmed that the primary cardiovascular effect of spaceflight was a postflight reduction in upright stroke volume (the amount of blood the heart pumps per beat). Heart rate increased appropriately for the reduction in stroke volume, thereby showing that changes in heart rate regulation alone cannot be responsible for orthostatic hypotension after spaceflight. All of the astronauts in our study had an increase in sympathetic nerve activity during upright tilting on Earth postflight. This increase was well calibrated for the reduction in stroke volume induced by the upright posture. The results obtained from stimulating the sympathetic nervous system using handgrip exercise or cold stress were also entirely normal during and after spaceflight. No astronaut had reduced cerebral blood flow during upright tilt, and cerebral autoregulation was normal or even enhanced inflight. These experiments show that the cardiovascular adaptation to spaceflight does not lead to a defect in the regulation of blood vessel constriction via sympathetic nerve activity. In addition, cerebral autoregulation is well-maintained. It is possible that despite the increased sympathetic nerve activity, blood vessels did not respond with a greater degree of constriction than occurred preflight, possibly uncovering a limit of vasoconstrictor reserve.

Levine, Benjamin D.↗

NASA NTRS1991

Energy balance in coronal funnels

The energy balance in magnetic flux tubes is examined semianalytically for the case in which thermal conduction balances radiation or in which enthalpy transport occurs. Different values are considered for areal constriction, shape, length, and maximum temperature. The overall energy budget of the solar corona is not significantly affected by magnetic constriction. A bowl-shaped funnel with a constriction factor of 4 describes the empirical differential-emission measure for log-T values between approximately 5.3 and 6.0. Loop-scaling relationships are derived for the full range of models to illustrate the dependence of the constant of proportionality on the properties of the magnetic constriction. Constriction can reduce the total energy requirement of the funnel by a factor of 5 and not affect the differential emission in flow-dominated models.

Rabin, Douglas↗

NASA NTRS2016

Flow Energy Piezoelectric Bimorph Nozzle Harvester

A flow energy harvesting device having a harvester pipe includes a flow inlet that receives flow from a primary pipe, a flow outlet that returns the flow into the primary pipe, and a flow diverter within the harvester pipe having an inlet section coupled to the flow inlet, a flow constriction section coupled to the inlet section and positioned at a midpoint of the harvester pipe and having a spline shape with a substantially reduced flow opening size at a constriction point along the spline shape, and an outlet section coupled to the constriction section. The harvester pipe may further include a piezoelectric structure extending from the inlet section through the constriction section and point such that the fluid flow past the constriction point results in oscillatory pressure amplitude inducing vibrations in the piezoelectric structure sufficient to cause a direct piezoelectric effect and to generate electrical power for harvesting.

Sherrit, Stewart↗

NASA NTRS1987

Effect of structure on current and potential distributions in porous electrode

Porous electrodes generally contain constricted macropores and localized micropores. The effects of the macropore constrictions on the resistance of a capillary were studied and an analytical model was developed for predicting the current distribution in a constricted macropore which directly includes constriction effects and does not require an empirical tortuosity parameter. The current and concentration distributions in localized micropores were also investigated and it was shown that the microporous area is fully accessible to charge and mass transfer processes. From these analyses it was concluded that the micropores primarily affect the kinetics of the interfacial processes by contributing to the interfacial area, while the macropores impose ohmic and mass transport limitations through the volume of the porous electrode.

Lanzi, Oscar↗

NASA NTRS1972

Role of distal reabsorption and peritubular environment in glomerulotubular balance.

Total kidney glomerulotubular balance was examined during aortic constriction and release in saline-loaded dogs and in dogs undergoing water diuresis. Aortic constriction lowered the glomerular filtration rate by 45% in both groups, and glomerulotubular balance, as judged by changes in absolute sodium reabsorption, was also comparable. During water diuresis, a linear relationship was observed between free water clearance and urine flow during all maneuvers, suggesting that distal sodium reabsorption is related primarily to distal delivery. The results suggest that if alterations in the peritubular environment are responsible for the changes in tubular sodium reabsorption during aortic constriction in the saline- or water-loaded dog, then a change in renal plasma flow, and presumably delivery rate of oncotic force, may be the most likely mediator.

Schrier, R. W.↗

NASA NTRS1972

Thermal conductance in a bolted joint.

The thermal contact resistance of a bolted joint is composed of two components: large scale constriction which is influenced by the extent of the interfacial pressure distribution and small scale constriction which is influenced by the magnitude of the local pressure. It is analytically shown how the roughness asperities at the contact surfaces of the joint affect these two constrictions but in opposite senses. The sum of the two effects, the total resistance, may increase or decrease with changing resistance and examples are given where an optimum roughness will minimize total joint resistance.

Roca, R. T.↗

NASA NTRS1995

Arcjet anode

There is disclosed an anode for an arcjet thruster which resists erosion during start-up on constriction during steady-state operation. The anode includes a converging upstream portion, a diverging downstream portion and a constricted portion disposed therebetween. In one embodiment of the invention, rails formed in the constricted portion accelerate the passage of an arc during start-up reducing erosion. In a second embodiment, a higher strength material resists bulging as a result of the thermal gradient within the nozzle.

Lichon, Paul G.↗

NASA NTRS1987

Electrode support for gas arc welding torch having coaxial vision

An improved electrode mounting structure for a gas tungsten arc welding torch having a coaxial imaging system. The electrode mounting structure includes a support having a central hub and a plurality of spokes which extend from the hub generally radially with respect to the axis of the torch into supporting engagement with the interior walls of the torch. The spaces between the spokes are optical passages for transmission of light to form the image. A tubular collet holder is threadedly engaged at its upper end to the hub and extends downwardly toward the open end of the torch. The collet holder has an inwardly tapering constriction near its lower end. An electrode-retaining, tubular collet is mounted within the collet holder and has a longitudinally split and tapered end seating against the tapered constriction. A spring seats against the upper end of the collet and forces the split end against the tapered constriction to wedge the split end radially inwardly to grip the electrode within the collet.

Richardson, Richard W.↗

NASA NTRS1999

A fluid--structure interaction finite element analysis of pulsatile blood flow through a compliant stenotic artery

A new model is used to analyze the fully coupled problem of pulsatile blood flow through a compliant, axisymmetric stenotic artery using the finite element method. The model uses large displacement and large strain theory for the solid, and the full Navier-Stokes equations for the fluid. The effect of increasing area reduction on fluid dynamic and structural stresses is presented. Results show that pressure drop, peak wall shear stress, and maximum principal stress in the lesion all increase dramatically as the area reduction in the stenosis is increased from 51 to 89 percent. Further reductions in stenosis cross-sectional area, however, produce relatively little additional change in these parameters due to a concomitant reduction in flow rate caused by the losses in the constriction. Inner wall hoop stretch amplitude just distal to the stenosis also increases with increasing stenosis severity, as downstream pressures are reduced to a physiological minimum. The contraction of the artery distal to the stenosis generates a significant compressive stress on the downstream shoulder of the lesion. Dynamic narrowing of the stenosis is also seen, further augmenting area constriction at times of peak flow. Pressure drop results are found to compare well to an experimentally based theoretical curve, despite the assumption of laminar flow.

Non-NASA Center↗

NASA NTRS1973

Preliminary study of a wall stabilized constructed arc

An iterative, implicit, finite-difference numerical technique is described which is suitable for obtaining solutions to the governing equations for a gas flowing in an axially symmetric constricted-arc heater. The method is shown to provide adequate solutions for three cases of simple pipe flows found in the literature, and for flow in a constricted-arc heater by direct comparison with experimental data. The comparison with arc-heater data includes static pressure, arc voltage, and wall heat flux, all as a function of axial location, and a radial temperature profile at one axial station. The arc-heater data were taken with air as the test gas at a heater inlet pressure approximately 0.40 atm and two flow rates of 2.2 and 4.8 g/sec. The arc currents investigated were between 377 and 584 amperes.

Graves, R. A.↗

NASA NTRS1980

Effect of gravitational overloads, hypokinesia and hypodynamia on the vessels of the pulmonary blood circuit

Vessels of the pulmonary circuit are studied under normal conditions, in exposure to single stress or continuous threshold endurable chestspine gravitational stresses, and one to eight weak hypokinesia and hypodynamic effects followed by stress. Examination methods include rentgenography and microrentgenography, clearing, and histology. In exposure to gravitational stress the distal portions of the arterial vessels of the 3 and 4 orders constrict, while all veins dilate. Sinuosity of all vessels is noted. The volume of the capillary bed increases and signs of perivascular edema occur. Due to hypokinesia and hypodynamia the arteries constricted and the arterial bed becomes poor. The veins of all orders dilate and the volume of the capillary bed increases. The changes grew greater the longer the terms of hypodyamic effects. Successive combination of hypokinesia and hypodynamia and gravitational stresses cause more pronounced changes than separate effects of these two factors and result in great deformity of the vascular walls, including their rupture and penetration of formed elements beyond the limits of the vascular bed.

Kasimtsev, A. A.↗

NASA NTRS1980

Effects of gravitational stress, hypokinesia and hypodynamia on the structure of the vascular bed of the spleen

The effects of two extreme factors, hypokinesia and hypodynamia, on spleen vascular beds were studied on 180 male and female albino rats. Vessels were studied by roentgenography, microroentgenography, clearing of sections and histology. Gravity stress yielded construction of all links of arterial bed and of order 5-7 veins. Large intraorganic vein diameters changed significantly but erratically. Hypokinesia in early phases produced pronounced spleen size reduction. Veins and arteries constricted along entire length. Later hypokinetic stages showed arteries still constricted; veins began to dilate from week 4 of hypokinesia. Sinuosity, uneven contours and varicose dilations of walls in large arteries and veins occurred. Abrupt changes in parenchyma, e.g., atrophy of folliculi, narrowing of lumen of central arteries from thickening of muscular wall. After exposure to hypokinesia followed by gravitational stress, pronounced lesions such as deformation of vascular wall, including rupture, in all vessels of the spleen vascular bed.

Nesterenko, N. T.↗

NASA NTRS1981

A groundwater convection model for Rio Grande rift geothermal resources

It has been proposed that forced convection, driven by normal groundwater flow through the interconnected basins of the Rio Grande rift is the primary source mechanism for the numerous geothermal anomalies along the rift. A test of this concept using an analytical model indicates that significant forced convection must occur in the basins even if permeabilities are as low as 50-200 millidarcies at a depth of 2 km. Where groundwater flow is constricted at the discharge areas of the basins forced convection can locally increase the gradient to a level where free convection also occurs, generating surface heat flow anomalies 5-15 times background. A compilation of groundwater data for the rift basins shows a strong correlation between constrictions in groundwater flow and hot springs and geothermal anomalies, giving strong circumstantial support to the convection model.

Morgan, P.↗

NASA NTRS1992

Numerical Modeling of a Vortex Stabilized Arcjet

Arcjet thrusters are being actively considered for use in Earth orbit maneuvering applications. Experimental studies are currently the chief means of determining an optimal thruster configuration. Earlier numerical studies have failed to include all of the effects found in typical arcjets including complex geometries, viscosity, and swirling flow. Arcjet geometries are large area ratio converging nozzles with centerbodies in the subsonic portion of the nozzle. The nozzle walls serve as the anode while the centerbody functions as the cathode. Viscous effects are important because the Reynolds number, based on the throat radius, is typically less than 1,000. Experimental studies have shown that a swirl or circumferential velocity component stabilizes a constricted arc. This dissertation describes the equations governing flow through a constricted arcjet thruster. An assumption that the flowfield is in local thermodynamic equilibrium leads to a single fluid plasma temperature model. An order of magnitude analysis reveals the governing fluid mechanics equations are uncoupled from the electromagnetic field equations. A numerical method is developed to solve the governing fluid mechanics equations, the Thin Layer Navier-Stokes equations. A coordinate transformation is employed in deriving the governing equations to simplify the application of boundary conditions in complex geometries. An axisymmetric formulation is employed to include the swirl velocity component as well as the axial and radial velocity components. The numerical method is an implicit finite-volume technique and allows for large time steps to reach a converged steady-state solution. The inviscid fluxes are flux-split, and Gauss-Seidel line relaxation is used to accelerate convergence. Converging-diverging nozzles with exit-to-throat area ratios up to 100:1 and annular nozzles were examined. Quantities examined included Mach number and static wall pressure distributions, and oblique shock structures. As the level of swirl and viscosity in the flowfield increased the mass flow rate and thrust decreased. The technique was used to predict the flow through a typical arcjet thruster geometry. Results indicate swirl and viscosity play an important role in the complex geometry of an arcjet.

Pawlas, Gary E.↗

NASA NTRS1993

Effect of ground and ceiling planes on shape of energized wakes

Energized wakes expelled by devices like thrust augmentors, propellers and rotors are studied theoretically in order to explore how their shape changes as they interact with ground and ceiling planes. It is found that when the airstream is stationary and the vehicle in hover, the presence of a ceiling plane causes an energized wake to constrict even more than when in an unbounded medium. The presence of a ground plane is found to cause the wake to constrict less than in free space. The computations also show that the vortex sheets first move inward before moving downward. Along their trajectory, the vortex sheets that separate the energized fluid from the ambient fluid have a nearly constant strength and velocity which indicates that the velocity just inside the wake is approximately constant. When a velocity is given to the wind tunnel airstream, the forward edge of the wake first rises and then descends so that it spends more time in the vicinity of the actuator disk. Implications of these results on measurements obtained in wind tunnels are discussed.

Rossow, Vernon J.↗

NASA NTRS1997

An Optical Study of the Circumstellar Environment Around the Crab Nebula

Long-slit spectra of two peripheral regions around the Crab Nebula show no H(alpha) emission down to a flux level of 1.5 x 10(exp -7)erg/sq cm s sr (0.63 Rayleigh), corresponding to an emission measure limit of 4.2 cm(sup - 6) pc (3(sigma)) assuming A(sub V)= 1.6(sup m) and T(sub e)=7000 K. This is below the flux levels reported by Murdin & Clark (Nature, 294, 543 (198 1)) for an H(alpha) halo around the Crab. Narrow H(beta) emission as described by Murdin (MNRAS, 269, 89 (1994)) is detected but appears to be Galactic emission unassociated with the remnant. A review of prior searches indicates no convincing observational evidence to support either a high- or low-velocity envelope around the remnant. Spectral scans confirm a well-organized, N-S expansion asymmetry of the filaments with a approx. 500 km/s central velocity constriction as described by MacAlpine et al. (ApJ, 342, 364 (1989)) and Lawrence et (it. (AJ, 109, 2635 (1995)] but questioned by Hester et al. (ApJ, 448, 240 (1995)). The velocity pinching appears to coincide with an cast-west chain of bright [O III] and helium-rich filaments. This expansion asymmetry might be the result of ejecta interaction with a disk of circumstellar matter, but such a model may be inconsistent with H and He filament abundances in the velocity constriction zone. A re-analysis of the remnant's total mass suggests that the filaments contain 4.6 +/- 1.8 M(solar) in ionized and neutral cas, about twice that of earlier estimates. For a 10M(solar) progenitor, this suggests that approx.equals 4M(solar) remains to be detected in an extended halo or wind.

Fesen, Robert A.↗

NASA NTRS2002

Understanding How Space Travel Affects Blood Vessels: Arterial Remodeling and Functional Adaptations Induced by Microgravity

Ever rise quickly from the couch to get something from the kitchen and suddenly feel dizzy? With a low heart rate and relaxed muscles, the cardiovascular system does not immediately provide the resistance necessary to keep enough blood going to your head. Gravity wins, at least for a short time, before your heart and blood vessels can respond to the sudden change in position and correct the situation. Actually, the human cardiovascular system is quite well adapted to the constant gravitational force of the Earth. When standing, vessels in the legs constrict to prevent blood from collecting in the lower extremities. In the space environment, the usual head-to-foot blood pressure and tissue fluid gradients that exist during the upright posture on Earth are removed. The subsequent shift in fluids from the lower to the upper portions of the body triggers adaptations within the cardiovascular system to accommodate the new pressure and fluid gradients. In animal models that simulate microgravity, the vessels in the head become more robust while those in the lower limbs become thin and lax. Similar changes may also occur in humans during spaceflight and while these adaptations are appropriate for a microgravity environment, they can cause problems when the astronauts return to Earth or perhaps another planet. Astronauts often develop orthostatic intolerance which means they become dizzy or faint when standing upright. This dizziness can persist for a number of days making routine activities difficult. In an effort to understand the physiological details of these cardiovascular adaptations, Dr. Michael Delp at Texas A&M University, uses the rat as a model for his studies. For the experiment flown on STS-107, he will test the hypothesis that blood vessels in the rats' hindlimbs become thinner, weaker, and constrict less in response to pressure changes and to chemical signals when exposed to microgravity. In addition, he will test the hypothesis that arteries in the brain become thicker as a result of microgravity-induced fluid shifts toward the head.

Delp, Michael↗

NASA NTRS2003

Performance Enhancement of Tunable Bandpass Filters Using Selective Etched Ferroelectric Thin Films

The inclusion of voltage-tunable barium strontium titanate (BSTO) thin films into planar band pass filters offers tremendous potential to increase their versatility. The ability to tune the passband so as to correct for minor deviations in manufacturing tolerances, or to completely reconfigure the operating frequencies of a microwave communication system, are highly sought-after goals. However, use of ferroelectric films in these devices results in higher dielectric losses, which in turn increase the insertion loss and decrease the quality factors of the filters. This study explores the use of patterned ferroelectric layers to minimize dielectric losses without degrading tunability. Patterning the ferroelectric layers enables us to constrict the width of the ferroelectric layers between the coupled microstrip lines, and minimize losses due to ferroelectric layers. Coupled one-pole microstrip bandpass filters with fundamental resonances at approx. 7.2 GHz and well defined harmonic resonances at approx. 14.4 and approx. 21.6 GHz, were designed, simulated and tested. For one of the filters, experimental results verified that its center frequency was tunable by 528 MHz at a center frequency of 21.957 GHz, with insertion losses varying from 4.3 to 2.5 dB, at 0 and 3.5 V/micron, respectively. These data demonstrate that the tuning-to-loss figure of merit of tunable microstrip filters can be greatly improved using patterned ferroelectric thin films as the tuning element, and tuning can be controlled by engineering the ferroelectric constriction in the coupled sections.

Miranda, Felix A.↗