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Results for “EFFECTIVE MASS”

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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Bounds on dark matter in solar orbit

The possibility is considered that a spherical distribution of dark matter (DM), matter not visible with current instruments, is trapped in the sun's gravitational field. Bounds are placed from the motion of Uranus and Neptune, on the amount of DM that could be so trapped within the radius of those planets' orbits, as follows: from the Voyager 2, Uranus-flyby data new, more accurate ephemeris values are generated. Trapped DM mass is bounded by noting that such a distribution would increase the effective mass of the sun as seen by the outer planets and by using the new ephemeris values to bound such an increase.

Anderson, John D.↗
NASA NTRS1983

Theory of silicon superlattices - Electronic structure and enhanced mobility

A realistic tight-binding band-structure model of silicon superlattices is formulated and used to study systems of potential applied interest, including periodic layered Si-Si(1-x)Ge(x) heterostructures. The results suggest a possible new mechanism for achieving enhanced transverse carrier mobility in such structures: reduced transverse conductivity effective masses associated with the superlattice band structure. For electrons in 100-line-oriented superlattices, a reduced conductivity mass arises intrinsically from the lower symmetry of the superlattice and its unique effect on the indirect bulk silicon band gap. An order of magnitude estimate of the range of mobility enhancement expected from this mechanism appears to be consistent with preliminary experimental results on Si-Si(1-x)Ge(x) superlattices.

Moriarty, J. A.↗
NASA NTRS2022

Vibration mitigation using an inerter-based isolation system

Extreme dynamic environments can result in the failure of structures and equipment. To mitigate these effects, isolation systems can be used. In recent years, inerters have been studied as a part of different structural control methods. The inerter is attractive for many structural control methods because it can produce large mass effects despite having a small physical mass through the transformation of translational motion to the rotational motion of a flywheel. While inerters have been considered in structural control, experimental studies of isolation systems featuring inerters has been limited. This project seeks to study the ability of the addition of an inerter to improve the dynamic performance of isolation systems when considering a random vibration environment. The results of this study demonstrate that the inerter can have a large impact on the dynamic properties of an isolated system, including shifting the isolation mode frequency lower while reducing its amplitude. High frequency noise did result from the inclusion of the inerter but may not be of significant concern depending on the frequency range of interest. The strong performance of the inerter in a simplified isolation system observed during this work motivates future studies of inerters in isolation systems.

S.S. Abolghasemi↗
NASA NTRS1993

On the origin of planetary spins

We examine the rate of accretion of mass and spin angular momentum by a spherical solid body ('planet') immersed in a differentially rotating disk of particles ('planetesimals'). If the planet travels on a circular orbit, the accretion process is described by two dimensionless parameters r and s, which measure the ratio of the planet's radius to its Hill radius and the ratio of the rms radial velocity of the planetesimals to the shear across the Hill radius, respectively. Using a combination of analytic arguments and numerical simulations, we derive the mass and angular momentum accretion rates and their scaling with r and s. By introducing an additional parameter, the effective mass of the planetesimals relative to the planet, we can derive the obliquities and spins arising from the stochastic nature of the accretion process. Our results are consistent with those of previous calculations by Lissauer and Kary (1991) wherever there is overlap in parameter space. In particular, we conclude that ordered accretion from a uniform or slowly varying disk of small bodies cannot result in rotation as rapid as that of earth or Mars for any value of the disk velocity dispersion s. The spin rates of these planets are most naturally explained as arising from one or a few 'giant' impacts by planetesimals.

Dones, Luke↗
NASA NTRS1991

Evaluation of on-board hydrogen storage methods f or high-speed aircraft

Hydrogen is the fuel of choice for hypersonic vehicles. Its main disadvantage is its low liquid and solid density. This increases the vehicle volume and hence the drag losses during atmospheric flight. In addition, the dry mass of the vehicle is larger due to larger vehicle structure and fuel tankage. Therefore it is very desirable to find a fuel system with smaller fuel storage requirements without deteriorating the vehicle performance substantially. To evaluate various candidate fuel systems, they were first screened thermodynamically with respect to their energy content and cooling capacities. To evaluate the vehicle performance with different fuel systems, a simple computer model is developed to compute the vehicle parameters such as the vehicle volume, dry mass, effective specific impulse, and payload capacity. The results indicate that if the payload capacity (or the gross lift-off mass) is the most important criterion, only slush hydrogen and liquid hydrogen - liquid methane gel shows better performance than the liquid hydrogen vehicle. If all the advantages of a smaller vehicle are considered and a more accurate mass analysis can be performed, other systems using endothermic fuels such as cyclohexane, and some boranes may prove to be worthy of further consideration.

Akyurtlu, Ates↗
NASA NTRS1993

An expanded set of brown dwarf and very low mass star models

We present in this paper updated and improved theoretical models of brown dwarfs and late M dwarfs. The evolution and characteristics of objects between 0.01 and 0.2 solar mass are exhaustively investigated and special emphasis is placed on their properties at early ages. The dependence on the helium fraction, deuterium fraction, and metallicity of the masses, effective temperature and luminosities at the edge of the hydrogen main sequence are calculated. We derive luminosity functions for representative mass functions and compare our predictions to recent cluster data. We show that there are distinctive features in the theoretical luminosity functions that can serve as diagnostics of brown dwarf physics. A zero-metallicity model is presented as a bound to or approximation of a putative extreme halo population.

Burrows, A.↗
NASA NTRS1984

Wall mass transfer and pressure gradient effects on turbulent skin friction

The effects of mass injection and pressure gradients on the drag of surfaces were studied theoretically with the aid of boundary-layer and Navier-Stokes codes. The present investigation is concerned with the effects of spatially varying the injection in the case of flat-plate drag. Effects of suction and injection on wavy wall surfaces are also explored. Calculations were performed for 1.2 m long surfaces, one flat and the other sinusoidal with a wavelength of 30.5 cm. Attention is given to the study of the effect of various spatial blowing variations on flat-plate skin friction reduction, local skin friction coefficient calculated by finite difference boundary-layer code and Navier-Stokes code, and the effect of phase-shifting sinusoidal mass transfer on the drag of a sinusoidal surface.

Watson, R. D.↗
NASA NTRS1985

Tether Fundamentals

Forces on tethered satellites, tether tension as a function of the effective mass, and parameters of candidate tether materials are examined. Illustrations show: stabilization control law; tethered satellite deployment and retrieval; angular momentum; electric motor effect reboost; a very long tether phenomenon; shuttle tethered satellite effects on the orbiter; artificial gravity; tethered satellite rendezvous docking relative velocity; and scaling up from the shuttle/TSS. Limitations of the tethered satellite system and of tether properties as well as cost/benefits trades for future applications are considered.

Rupp, C.↗
NASA NTRS1993

Magnetic activity in low-mass stars

The manifestations of magnetic activity in low-mass stars, particularly M dwarfs, can be used as a tool to study their evolution, the operation of the interior dynamo with changing interior conditions, and the structure of their outer atmospheres. Extensive background material on the current understanding of low-mass stellar activity is presented. Two new surveys are described which will greatly increase the number of active low-mass stars known in the field and in nearby open clusters. These surveys will define the characteristics of the activity in low-mass stars, and how the activity changes with a number of parameters of interest, including mass, effective temperature, and age. The data will also allow a rigorous determination of a possible age-activity relation among the low-mass M dwarfs. Theoretical models of M dwarf atmospheres, and their connection to the understanding of the observations, are also discussed.

Hawley, Suzanne L.↗
NASA NTRS1996

Noninvasive Determination of Bone Mechanical Properties Using Vibration Response: A Refined Model and Validation in vivo

Accurate non-invasive mechanical measurement of long bones is made difficult by the masking effect of surrounding soft tissues. Mechanical response tissue analysis (MRTA) offers a method for separating the effects of the soft tissue and bone; however, a direct validation has been lacking. A theoretical analysis of wave propagation through the compressed tissue revealed a strong mass effect dependent on the relative accelerations of the probe and bone. The previous mathematical model of the bone and overlying tissue system was reconfigured to incorporate the theoretical finding. This newer model (six-parameter) was used to interpret results using MRTA to determine bone cross-sectional bending stiffness, EI(sub MRTA). The relationship between EI(sub MRTA) and theoretical EI values for padded aluminum rods was R(sup 2) = 0.999. A biological validation followed using monkey tibias. Each bone was tested in vivo with the MRTA instrument. Postmortem, the same tibias were excised and tested to failure in three-point bending to determine EI(sub 3-PT) and maximum load. Diaphyseal bone mineral density (BMD) measurements were also made. The relationship between EI(sub 3-PT) and in vivo EI(sub MRTA) using the six-parameter model is strong (R(sup 2) = 0.947) and better than that using the older model (R(sup 2) = 0.645). EI(sub MRTA) and BMD are also highly correlated (R(sup 2) = 0.853). MRTA measurements in vivo and BMD ex vivo are both good predictors of scaled maximum strength (R(sup 2) = 0.915 and R(sup 2) = 0.894, respectively). This is the first biological validation of a non- invasive mechanical measurement of bone by comparison to actual values. The MRTA technique has potential clinical value for assessing long-bone mechanical properties.

Roberts, S. G.↗
NASA NTRS1996

Noninvasive Determination of Bone Mechanical Properties using Vibration Response: A Refined Model and Validation in vivo

Accurate non-invasive mechanical measurement of long bones is made difficult by the masking effect of surrounding soft tissues. Mechanical Response Tissue Analysis (MRTA) offers a method for separating the effects of the soft tissue and bone; however, a direct validation has been lacking. A theoretical analysis of wave propagation through the compressed tissue revealed a strong mass effect dependent on the relative accelerations of the probe and bone. The previous mathematical model of the bone and overlying tissue system was reconfigured to incorporate the theoretical finding. This newer model (six-parameter) was used to interpret results using MRTA to determine bone cross-sectional bending stiffness, EI(sub MRTA). The relationship between EI(MRTA) and theoretical EI values for padded aluminum rods was R(exp 2) = 0.999. A biological validation followed using monkey tibias. Each bone was tested in vivo with the MRTA instrument. Postmortem, the same tibias were excised and tested to failure in three-point bending to determine EI(sub 3-PT) and maximum load. Diaphyseal Bone Mineral Density (BMD) measurements were also made. The relationship between E(sub 3-PT) and in vivo EI(sub MRTA) using the six-parameter model is strong (R(exp 2) = 0.947) and better than that using the older model (R(exp 2) = 0.645). EI(MRTA) and BMD are also highly correlated (R(exp 2) = 0.853). MRTA measurements in vivo and BMD ex vivo are both good predictors of scaled maximum strength (R(exp 2) = 0.915 and R(exp 2) = 0.894, respectively). This is the first biological validation of a non-invasive mechanical measurement of bone by comparison to actual values. The MRTA technique has potential clinical value for assessing long-bone mechanical properties.

Roberts, S. G.↗
NASA NTRS1973

Transmission of sound from a nozzle carrying a compressible subsonic flow

The effect of an aperture on an impinging plane wave in a duct is extended to the case where the duct carries a compressible mean flow. The effect of the mean flow is to induce the closing mechanical impedance to have a significant resistive component in addition to the reactive component due to the 'attached mass' effect. The effect of the reactive load on the aperture is shown to be especially important for large contraction ratios. Of special interest is the acoustic particle velocity at the exit plane of the aperture. This is the 'piston velocity' that drives the spherical acoustic waves in the far field. The effect of aperture geometry on the piston velocity is established, and a criterion for minimum acoustic radiation is presented. An experimental arrangement for measuring the piston velocity with a hot-wire anemometer is described.

Davis, S. S.↗
NASA NTRS1988

Mass killings and detection of impacts

Highly energetic bolide impacts occur and their flux is known. For larger bodies the energy release is greater than for any other short-term global phenomenon. Such impacts produce or release a large variety of shock induced changes including major atmospheric, sedimentologic, seismic and volcanic events. These events must necessarily leave a variety of records in the stratigraphic column, including mass killings resulting in major changes in population density and reduction or extinction of many taxonomic groups, followed by characteristic patterns of faunal and flora replacement. Of these effects, mass killings, marked by large-scale loss of biomass, are the most easily detected evidence in the field but must be manifest on a near-global scale. Such mass killings that appear to be approximately synchronous and involve disappearance of biomass at a bedding plane in many sedimentologically independent sections globally suggest a common cause and probable synchroneity. Mass killings identify an horizon which may be examined for evidence of cause. Geochemical markers may be ephemeral and absence may not be significant. There appears to be no reason why ongoing phenomena such as climate and sea-level changes are primary causes of anomolous episodic events.

Mclaren, Digby J.↗
NASA NTRS1989

An integrating matrix formulation for buckling of rotating beams including the effects of concentrated masses

The integrating matrix technique of computational mechanics is extended to include the effects of concentrated masses. The stability of a flexible rotating beam with discrete masses is analyzed to determine the critical rotational speeds for buckling in the inplane and out-of-plane directions. In this problem, the beam is subjected to compressive centrifugal forces arising from steady rotation about an axis which does not pass through the clamped end of the beam. To determine the eigenvalues from which stability is assessed, the differential equations of motion are solved numerically by combining the extended integrating matrix method with an eigenanalysis. Stability boundaries for a discrete mass representation of a uniform beam are shown to asymptotically approach the stability boundaries for the corresponding continuous mass beam as the number of concentrated masses is increased. An error in the literature is also noted for the discrete mass problem concerning the behavior of the critical rotational speed for inplane buckling as the radius of rotation of the clamped end of the beam is reduced.

Lakin, William D.↗
NASA NTRS2008

Structural Dynamic Analysis of a Spacecraft Multi-DOF Shaker Table

Finite element enforced response analysis was performed on a three axis expander head shaker table to aid in the design of the table structure and vibration control system. The payload for this shaker system is a generic spacecraft with a multitude of flexible modes across a broad frequency band. A Craig-Bampton representation of the spacecraft was used to expedite analysis of multiple shaker table designs. The analysis examines the required forces in the actuators for a constant amplitude base acceleration sine sweep test, the resulting forces in the spacecraft and table attachment restraints, and the resulting accelerations on the spacecraft structure. The results show the spacecraft response is very high at the spacecraft center of gravity (CG) due to the high CG offset and cantilever effect of the low frequency spacecraft bending modes. The high response can be addressed by "notching" the input vibration levels to avoid over-testing the spacecraft. At frequencies above 25 Hz, the spacecraft modal effective masses are very small, and the response of the shaker table dominates the response. Anti-resonances of the shaker table in the frequency range of interest reduce the acceleration output and require much higher actuator forces to achieve the acceleration specification. These effects may require stiffening the shaker structure to move the modes out of the test frequency range or increasing the shaker table damping.

Pray, Carl↗
NASA NTRS2008

Structural Dynamic Analysis of a Spacecraft Multi-DOF Shaker Table

Finite element enforced response analysis was performed on a multiple degree of freedom expander head shaker table to aid in the design of the table structure and vibration control system. The payload for this shaker system is a spacecraft with a multitude of flexible modes across a broad frequency band. A Craig-Bampton representation of the spacecraft was used to expedite analysis of multiple shaker table designs. The analysis examines the required forces in the actuators for a constant amplitude base acceleration sine sweep test, the resulting forces in the spacecraft and table attachment restraints, and the resulting accelerations on the spacecraft structure. The results show the spacecraft response at low frequencies is very high near the tip due to the low order spacecraft bending and axial modes. The high response can be addressed by notching the input vibration levels to avoid overtesting the spacecraft. At frequencies above 25 Hz, the spacecraft modal effective masses are very small, and the response of the shaker table dominates the response. Anti-resonances of the shaker table in the frequency range of interest reduce the acceleration output and require much higher actuator forces to achieve the acceleration specification. These effects may require stiffening the shaker structure to move the modes out of the test frequency range or increasing the shaker table damping.

Pray, Carl↗
NASA NTRS2009

Ground Vibration Test of the Aerostructure Test Wing 2

The Aerostructures Test Wing (ATW) was developed to test unique concepts for flutter prediction and control synthesis. A follow-on to the successful ATW, denoted ATW2, was fabricated as a test bed to validate a variety of instrumentation in flight and to collect data for development of advanced signal processing algorithms for flutter prediction and aviation safety. As a means to estimate flutter speed, a ground vibration test (GVT) was performed. The results of a GVT are typically utilized to update structural dynamics finite element (FE) models used for flutter analysis. In this study, two GVT methodologies were explored to determine which nodes provide the best sensor locations: (i) effective independence and (ii) kinetic energy sorting algorithms. For measurement, ten and twenty sensors were used for three and 10 target test modes. A total of six accelerometer configurations measured frequencies and mode shapes. This included locations used in the original ATW GVT. Moreover, an optical measurement system was used to acquire data without mass effects added by conventional sensors. A considerable frequency shift was observed in comparing the data from the accelerometers to the optical data. The optical data provided robust data for use of the ATW2 finite element model update.

Herrera, Claudia↗
NASA NTRS1977

Vibration analysis of rotor blades with an attached concentrated mass

The effect of an attached concentrated mass on the dynamics of helicopter rotor blades is determined. The point transmission matrix method was used to define, through three completely automated computer programs, the natural vibrational characteristics (natural frequencies and mode shapes) of rotor blades. The problems of coupled flapwise bending, chordwise bending, and torsional vibration of a twisted nonuniform blade and its special subcase pure torsional vibration are discussed. The orthogonality relations that exist between the natural modes of rotor blades with an attached concentrated mass are derived. The effect of pitch, rotation, and point mass parameters on the collective, cyclic, scissor, and pure torsional modes of a seesaw rotor blade is determined.

Murthy, V. R.↗