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At least 217 records · Page 12

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NASA NTRS2001

Cutaneous Microvascular Flow In the Foot During Simulated Variable Gravities

Our objective was to elucidate how varying gravitational fields affect blood perfusion in the sole of the foot. Human subjects underwent whole-body tilting at four angles: upright (1 G(sub z), 22 deg (.38 G(sub z)), 10 deg (.17 G(sub z)), and supine (0 G(sub z)), simulating the gravitational fields of Earth, Mars, Moon, and microgravity, respectively. Cutaneous capillary blood flow was monitored on the plantar surface of the heel by laser Doppler flowmetry while weight bearing load was measured beneath the same foot with a calibrated scale. Foot mean arterial pressures (MAP) were calculated by adding estimated hydrostatic pressures to continuously recorded heart-level blood pressures for each subject. At each tilt angle, subjects increased weight bearing on one foot in graded load increments of one kilogram beginning with zero. The weight bearing at which null flow first occurred was determined as the closing load (CL). Subsequently, the weight bearing was reduced in reverse steps until blood flow returned (opening load, OL). CL and OL were normalized to each subject's body weight and expressed as percent of body weight. Mean CLs (SD in parentheses) for simulated Earth, Mars, Moon, and micro-gravities were 14 (5), 8 (2), 6 (2), and 5 (2) percent of body weight, respectively. OLs were 12 (3), 6 (1), 5 (2), and 4 (1), respectively. Calculated foot MAPs for each simulated gravitational field were 192 (19), 127 (12), 106 (10), and 87 (9) mm Hg, respectively. CL and OL were significantly correlated with foot MAP (r = 0.70, 0.72, respectively). Overall, CL and OL were significantly different (p is less than 0.001). The data suggest that decreased local arterial pressure in the foot lowers tolerance to external compression. Consequently, the human foot sole may be more prone to cutaneous ischernia during loading in microgravity than on Earth.

Chang, David S.↗

NASA NTRS1958

Three-Dimensional Orbits of Earth Satellites, Including Effects of Earth Oblateness and Atmospheric Rotation

The principal purpose of the present paper is to present sets of equations which may be used for calculating complete trajectories of earth satellites from outer space to the ground under the influence of air drag and gravity, including oblateness effects, and to apply these to several examples of entry trajectories starting from a circular orbit. Equations of motion, based on an "instantaneous ellipse" technique, with polar angle as independent variable, were found suitable for automatic computation of orbits in which the trajectory consists of a number of revolutions. This method is suitable as long as the trajectory does not become nearly vertical. In the terminal phase of the trajectories, which are nearly vertical, equations of motion in spherical polar coordinates with time as the independent variable were found to be more suitable. In the first illustrative example the effects of the oblateness component of the earth's gravitational field and of atmospheric rotation were studied for equatorial orbits. The satellites were launched into circular orbits at a height of 120 miles, an altitude sufficiently high that a number of revolutions could be studied. The importance of the oblateness component of the earth's gravitational field is shown by the fact that a satellite launched at circular orbital speed, neglecting oblateness, has a perigee some 67,000 feet lower when oblateness forces are included in the equations of motion than when they are not included. Also, the loss in altitude per revolution is double that of a satellite following an orbit not subject to oblateness. The effect of atmospheric rotation on the loss of altitude per revolution was small. As might be surmised, the regression of the line of nodes as predicted by celestial mechanics is unchanged when drag is included. It is clear that the inclination of the orbital plane to the equator will be relatively unaffected by drag for no atmospheric rotation since the drag lies in the orbital plane in this case. With the inclusion of atmospheric rotation it was found that the inclination of the plane changed about one-millionth of a radian per revolution. Thus the prediction of the position of the orbital plane of an earth satellite is not complicated by the introduction of drag. The line of apsides, which without drag but with oblateness moves slowly in space, tends to move with the satellite when drag is included in the calculations. As a results, the usual linearized solutions based on oblateness alone must be basically altered when drag is included to take into account the rapid movement of the line of apsides. In the second illustrative example the final revolution was calculated to impact for a number of trajectories in an orbital plane inclined at 650 to the equator. Of particular interest is the large effect the oblateness gravitational field and atmospheric rotation can have on the impact point. For a value of CDA/m of unity, and for an initial downward angle at 80 miles altitude of 0.01 radian, such as might be utilized for manned re-entry, oblateness had an influence of about 300 miles in the impact point, and atmospheric rotation had about a 150-mile influence.

Nielsen, Jack N.↗

NASA NTRS1996

Gravitational Redshift in a Local Freely Falling Frame: A Proposed New Null Test of the Equivalence Principle

We consider the gravitational redshift effect measured by an observer in a local freely failing frame (LFFF) in the gravitational field of a massive body. For purely metric theories of gravity, the metric in a LFFF is expected to differ from that of flat spacetime by only "tidal" terms of order (GM/c(exp 2)R)(r'/R )(exp 2), where R is the distance of the observer from the massive body, and r' is the coordinate separation relative to the origin of the LFFF. A simple derivation shows that a violation of the equivalence principle for certain types of "clocks" could lead to a larger apparent redshift effect of order (1 - alpha)(G M/c(exp 2)R)(r'/R), where alpha parametrizes the violation (alpha = 1 for purely metric theories, such as general relativity). Therefore, redshift experiments in a LFFF with separated clocks can provide a new null test of the equivalence principle. With presently available technology, it is possible to reach an accuracy of 0.01% in the gravitational field of the Sun using an atomic clock orbiting the Earth. A 1% test in the gravitational field of the galaxy would be possible if an atomic frequency standard were flown on a space mission to the outer solar system.

Krisher, Timothy P.↗

NASA NTRS2000

Cavendish Balance Automation

This is the final report for a project carried out to modify a manual commercial Cavendish Balance for automated use in cryostat. The scope of this project was to modify an off-the-shelf manually operated Cavendish Balance to allow for automated operation for periods of hours or days in cryostat. The purpose of this modification was to allow the balance to be used in the study of effects of superconducting materials on the local gravitational field strength to determine if the strength of gravitational fields can be reduced. A Cavendish Balance was chosen because it is a fairly simple piece of equipment for measuring gravity, one the least accurately known and least understood physical constants. The principle activities that occurred under this purchase order were: (1) All the components necessary to hold and automate the Cavendish Balance in a cryostat were designed. Engineering drawings were made of custom parts to be fabricated, other off-the-shelf parts were procured; (2) Software was written in LabView to control the automation process via a stepper motor controller and stepper motor, and to collect data from the balance during testing; (3)Software was written to take the data collected from the Cavendish Balance and reduce it to give a value for the gravitational constant; (4) The components of the system were assembled and fitted to a cryostat. Also the LabView hardware including the control computer, stepper motor driver, data collection boards, and necessary cabling were assembled; and (5) The system was operated for a number of periods, data collected, and reduced to give an average value for the gravitational constant.

Thompson, Bryan↗

NASA NTRS1998

Lineage Analysis of Axis Formation Under Novel Gravity

Recent intriguing work by Cooke ('86) and Neff, et al. ('93) suggests that there are subtle developmental changes in the Xenopus laevis embryos subjected to novel gravitational fields. These changes include the position of the third cleavage plane, the dorsal lip of the blastopore, and also the size of the head and eyes. However, compensation occurred later in development, so that by the tadpole stages there is no apparent difference between experimental and control embryos. How these early morphological changes are corrected is not clear. Through this project, we plan to determine whether the distribution of cytoplasmic morphogenetic determinants, and thus the developmental fate of blastomeres, is altered by novel gravitational fields by either tilting them or rotating them in a horizontal clinostat. We then plan to compare the control and experimental embryos with respect to blastomere fate (by lineage tracing with fluorescent dextrans), blastomere commitment and autonomous differentiation potential (by transplantation and culture), and distribution of cytoplasmic morphogens (by in situ hybridization). These three approaches, when applied in tandem, will provide a definitive test of the hypothesis that the distribution of cytoplasmic morphogenetic determinants and thus the developmental fate of blastomeres can be altered by novel gravitational fields.

Huang, Sen↗

DOE OSTI2026

Feynman diagrams for matter wave interferometry

We introduce a new theoretical framework based on Feynman diagrams to compute phase shifts in matter wave interferometry. The method allows for analytic computation of higher order quantum corrections, beyond the traditional semi-classical approximation. These additional terms depend on the finite size of the initial matter wavefunction and/or have higher order dependence on ℏ. We apply the method to compute the response of matter wave interferometers to power law potentials and potentials with an arbitrary spatial dependence. The analytic expressions are validated by comparing to numerical simulations, and estimates are provided for the scale of the quantum corrections to the phase shift response to the gravitational field of the earth, anharmonic trapping potentials, and gravitational fields from local proof masses. We also find that for certain experimentally feasible parameters, these corrections are large enough to be measured and could lead to systematic errors if they are not mitigated. We find that to first order in a spatially dependent potential, quantum corrections vanish when the initial matter wavepacket has spherical symmetry and the potential satisfies Laplace's equation. We anticipate these quantum corrections will be especially important for trapped matter wave interferometers and for free-space matter wave interferometers in the presence of proof masses. These interferometers are becoming increasingly sensitive tools for mobile inertial sensing, gravity surveying, tests of gravity and its interplay with quantum mechanics, and searches for dark energy.

Glick, Jonah [Northwestern U.; Fermilab] (ORCID:00↗

NASA NTRS1988

Symmetries in tetrad theories

The isometry conditions for gravitational fields are given directly at the tetrad level, rather than in terms of the metric. As an illustration, an analysis of the curvature collineations and Killing fields for a twisting type-N vacuum gravitational field is made.

Chinea, F. J.↗

NASA NTRS1983

Large numbers hypothesis. IV - The cosmological constant and quantum physics

In standard physics quantum field theory is based on a flat vacuum space-time. This quantum field theory predicts a nonzero cosmological constant. Hence the gravitational field equations do not admit a flat vacuum space-time. This dilemma is resolved using the units covariant gravitational field equations. This paper shows that the field equations admit a flat vacuum space-time with nonzero cosmological constant if and only if the canonical LNH is valid. This allows an interpretation of the LNH phenomena in terms of a time-dependent vacuum state. If this is correct then the cosmological constant must be positive.

Adams, P. J.↗

DOE OSTI2021

Equivalence principle in Reissner–Nordström geometry

The Equivalence Principle is a key element in the development of General Relativity. In one of its formulations, the Equivalence Principle states that a reference frame at rest in a uniform gravitational field is equivalent to a reference frame in uniformly accelerated motion in the absence of any gravitation field. We analyze the spacetime surrounding a non-rotating spherically symmetric charged body, known as Reissner–Nordström geometry, and exhibit a coordinate transformation, which makes explicit its compatibility with the Equivalence Principle. We revisit the Schwarzschild case, previously analyzed in the literature. We also consider second order terms of the relevant expansion parameters in the approximate metric, which is needed for the computed curvature quantities to be correct at zeroth order.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1992

Interstellar cloud shapes - A minimum-hypothesis account involving a purely gravitational effect

The effect produced by the anisotropic gravitational field on the shape of a nonspherical, self-gravitating interstellar cloud is examined. The projected axial ratios observed for molecular clouds can be accounted for if clouds are not generally in a state of strict dynamical equilibrium. For clouds having spectral line widths dominated by supersonic turbulent motions, this purely gravitational effect predicts a mean true axial ratio q about 0.3 for both oblate and prolate cloud geometries. Clouds having p less than about 0.3 are likely to be prolate and quite elongated. The observational data on molecular cloud cores, which have essentially thermal line widths, can be fitted equally well to either oblate or prolate spheroids having q between 0.1 and 0.4 with q about 0.2 providing the best match: highly elongated cores are again more likely to be prolate. While other processes certainly operate throughout the Galaxy to influence cloud shape, it appears plausible that at least some clouds may be shaped principally by their anisotropic gravitational field.

Fleck, Robert C., Jr.↗

NASA NTRS2012

Lightweight, Miniature Inertial Measurement System

A miniature, lighter-weight, and highly accurate inertial navigation system (INS) is coupled with GPS receivers to provide stable and highly accurate positioning, attitude, and inertial measurements while being subjected to highly dynamic maneuvers. In contrast to conventional methods that use extensive, groundbased, real-time tracking and control units that are expensive, large, and require excessive amounts of power to operate, this method focuses on the development of an estimator that makes use of a low-cost, miniature accelerometer array fused with traditional measurement systems and GPS. Through the use of a position tracking estimation algorithm, onboard accelerometers are numerically integrated and transformed using attitude information to obtain an estimate of position in the inertial frame. Position and velocity estimates are subject to drift due to accelerometer sensor bias and high vibration over time, and so require the integration with GPS information using a Kalman filter to provide highly accurate and reliable inertial tracking estimations. The method implemented here uses the local gravitational field vector. Upon determining the location of the local gravitational field vector relative to two consecutive sensors, the orientation of the device may then be estimated, and the attitude determined. Improved attitude estimates further enhance the inertial position estimates. The device can be powered either by batteries, or by the power source onboard its target platforms. A DB9 port provides the I/O to external systems, and the device is designed to be mounted in a waterproof case for all-weather conditions.

Tang, Liang↗

NASA NTRS1986

Fugacity and concentration gradients in a gravity field

Equations are reviewed which show that at equilibrium fugacity and concentration gradients can exist in gravitational fields. At equilibrium, the logarithm of the ratio of the fugacities of a species at two different locations in a gravitational field is proportional to the difference in the heights of the two locations and the molecular weight of the species. An analogous relation holds for the concentration ratios in a multicomponent system. The ratio is calculated for a variety of examples. The kinetics for the general process are derived, and the time required to approach equilibrium is calculated for several systems. The following special topics are discussed: ionic solutions, polymers, multiphase systems, hydrostatic pressure, osmotic pressure, and solubility gradients in a gravity field.

May, C. E.↗

NASA NTRS1972

The mass-zero spin-two field and gravitational theory.

Demonstration that the conventional theory of the mass-zero spin-two field with sources introduces extraneous nonspin-two field components in source regions and fails to be covariant under the full or restricted conformal group. A modified theory is given, expressed in terms of the physical components of mass-zero spin-two field rather than in terms of 'potentials,' which has no extraneous components inside or outside sources, and which is covariant under the full conformal group. For a proper choice of source term, this modified theory has the correct Newtonian limit and automatically implies that a symmetric second-rank source tensor has zero divergence. It is shown that possibly a generally covariant form of the spin-two theory derived here can be constructed to agree with general relativity in all currently accessible experimental situations.

Coulter, C. A.↗

NASA NTRS2001

Filtration Combustion in Smoldering and SHS

Smolder waves and SHS (self-propagating high-temperature synthesis) waves are both examples of filtration combustion waves propagating in porous media. Smoldering combustion is important for the study of fire safety. Smoldering itself can cause damage, its products are toxic and it can also lead to the more dangerous gas phase combustion which corresponds to faster propagation at higher temperatures. In SHS , a porous solid sample, consisting of a finely ground powder mixture of reactants, is ignited at one end. A high temperature thermal wave, having a frontal structure, then propagates through the sample converting reactants to products. The SHS technology appears to enjoy a number of advantages over the conventional technology, in which the sample is placed in a furnace and "baked" until it is "well done". The advantages include shorter synthesis times, greater economy, in that the internal energy of the reactions is employed rather than the costly external energy of the furnace, purer products, simpler equipment and no intrinsic limitation on the size of the sample to be synthesized as exists in the conventional technology. When delivery of reactants through the pores to the reaction site is an important aspect of the combustion process, it is referred to as filtration combustion. The two types of filtration combustion have a similar mathematical formulation, describing the ignition, propagation and extinction of combustion waves in porous media. The goal in each case, however, is different. In smoldering the desired goal is to prevent propagation, whereas in SHS the goal is to ensure propagation of the combustion wave, leading to the synthesis of desired products. In addition, the scales in the two areas of application differ. Smoldering generally occurs at lower temperatures and propagation velocities than in SHS nevertheless, the two applications have much in common so that what is learned fit make application can be used to advantage in the other. In porous media, melting often occurs ahead of the propagating combustion wave. In certain cases there is so much melting that the porous solid structure is destroyed, e.g., by melting and a suspension arises, consisting of a liquid bath containing solid particles and/or gas bubbles. The resulting combustion wave is referred to as a liquid flame. We have considered a number of problems involving filtration combustion. Here, we describe four such studies: (A) rapid buoyant filtration combustion waves; (B) diffusion driven combustion waves; (C) rapidly propagating liquid flames in gravitational fields; and (D) gas-phase influence on liquid flames in gravitational fields.

Matkowsky, Bernard J.↗

NASA NTRS1993

Mars Gravity Field From Dual Satellite Observations

Two strategies are examined in this paper to improve the spherical harmonic representations of the Martian gravitational field. Continous observation over a week-long data arc and a 30th degree gravitational field are assumed for analysis.

Mars↗

NASA NTRS1972

Blast waves in rotating media.

The model investigated involves a cylindrically symmetric blast wave generated by an infinitely long line explosion in a cold and homogeneous gas rotating rigidly in its self-gravitational field. It is found that within the context of rotation in a gravitational field a blast wave will not adopt the one-zone form familiar from similarity solutions but, rather, a two-zone form. The inner compression zone arises as a response to the presence of the restoring force, which drives a rarefaction wave into the outer compression zone.

Rossner, L. F.↗

NASA NTRS1988

Isothermal magnetostatic atmospheres. III - Similarity solutions with current proportional to the magnetic potential squared

A family of isothermal magnetostatic atmospheres with one ignorable coordinate corresponding to a uniform gravitational field in a plane geometry is considered. It is assumed that the current (J) is proportional to the square of the magnetostatic potential and falls off exponentially with distance. Results are presented for the contributions of the anisotropic J x B force (where B is the magnetic field induction), the gravitational force, and the gas pressure gradient to the force balance.

Webb, G. M.↗

NASA NTRS1991

Computed stress fields in GaAs during vertical Bridgman growth

The thermoelastic stress field of GaAs crystals grown by the vertical Bridgman method has been approximated using a linear elastic, axisymmetric stress model. The model included the effects due to the thermal field and the gravitational field, as well as the interaction with the ampoule, but did not fully account for the elastic anisotropy of the crystal. Realistic thermal fields for different furnace temperatures and insulation zone thicknesses were computed with a similar model. The difference between the Von Mises stress and the critical resolved shear stress (CRSS) was used as a measure of the number of dislocations present. Different sets of growth parameters were examined to determine which produced the lowest amount of stress in the crystal. Results show that the crystal-ampoule interaction is the most important parameter in dislocation generation. In addition, for the same sticking boundary conditions, concave interfaces have lower stress than convex interfaces.

Rosch, William↗