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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Visual field influence on manual roll and pitch stabilization

Human control performance in nulling perceived tilt angles was investigated for combinations of pseudo-random vestibular disturbances and different waveforms of low frequency wide visual field motions. For both roll and pitch axes, subjects tilted the trainer in which they were seated in the direction of field rotation. This visual bias was much stronger for pitch backwards with upward field rotation. Frequency response analysis showed the dominance of visual cues at low frequencies (below 0.06 Hz) and the reliance on vestibular information in the high frequency range for both axes. Models suggest that operator balancing responses at high frequencies are mainly processed by the semicircular canals rather than the otolith organs. The results also suggest that the subject tends to rely less on the otolith organs for pitch perception than for roll.

Huang, J.-K.↗
NASA NTRS1999

Sensitivity of Climate to Changes in NDVI

The sensitivity of global and regional climate to changes in vegetation density is investigated using a coupled biosphere-atmosphere model. The magnitude of the vegetation changes and their spatial distribution are based on natural decadal variability of the normalized difference vegetation index (ndvi). Different scenarios using maximum and minimum vegetation cover were derived from satellite records spanning the period 1982-1990. Albedo decreased in the northern latitudes and increased in the tropics with increased ndvi. The increase in vegetation density revealed that the vegetation's physiological response was constrained by the limits of the available water resources. The difference between the maximum and minimum vegetation scenarios resulted in a 46% increase in absorbed visible solar radiation and a similar increase in gross photosynthetic C02 uptake on a global annual basis. This caused the canopy transpiration and interception fluxes to increase, and reduced those from the soil. The redistribution of the surface energy fluxes substantially reduced the Bowen ratio during the growing season, resulting in cooler and moister near-surface climate, except when soil moisture was limiting. Important effects of increased vegetation on climate are : (1) A cooling of about 1.8 K in the northern latitudes during the growing season and a slight warming during the winter, which is primarily due to the masking of high albedo of snow by a denser canopy. and (2) A year round cooling of 0.8 K in the tropics. These results suggest that increases in vegetation density could partially compensate for parallel increases in greenhouse warming . Increasing vegetation density globally caused both evapotranspiration and precipitation to increase. Evapotranspiration, however increased more than precipitation resulting in a global soil-water deficit of about 15 %. A spectral analysis on the simulated results showed that changes in the state of vegetation could affect the low-frequency modes of the precipitation distribution and might reduce its low frequency variability in the tropics while increasing it in northern latitudes.

Bounoua, L.↗
NASA NTRS1999

Novel Electric Nucleation Technique for Growing Large Single Crystal in Space

We present, herein, an electrical model for growing crystals without a seed which might not be free of defects and thereby still hinder the growth of a perfect crystal in space. The system is designed to confine nucleation to a single site automatically in an under saturated solution to avoid multiple nucleation. The technique is based on the effect of electrostriction, which is the tendency of a material to become more compressed in the presence of an electric field. The system is designed to create an electrical potential well between two hyperboloid electrodes with applied voltage at low frequency. The induced potential well between the electrodes oscillates at low frequency and attracts the solute and condenses it into the region of maximum field intensity. The alternating voltage prevents molecules with intrinsic charge from being attracted to the electrodes. The continuous presence of the electric field during the duration of the experiment, provides a continuous migration of the molecules toward the trapping site. This will eliminate the creation of a depletion region around the nucleation center and will enhance the crystal growth rate. Aside from the above mentioned advantages, the system is compact, safe to operate, and inexpensive to build.

Abdeldayem, Hossin↗
DOE OSTI2020

Similarity law and frequency scaling in low-pressure capacitive radio frequency plasmas

We verify the similarity law (SL) and show a violation of frequency scaling (f-scaling) in low-pressure capacitive radio frequency (rf) plasmas via fully kinetic particle-in-cell simulations. The SL scaling relations for electron density and electron power absorption are first confirmed in similar rf discharges. Based on these results, with only the driving frequency varied, the f-scaling for electron density is also validated, showing almost the same trend as the SL scaling, across most of the frequency regime. However, violations of the f-scaling are observed at lower frequencies, which are found to be relevant to the electron heating mode transition from stochastic to Ohmic heating. Electron kinetic invariance is illustrated for the SL and f-scaling being valid, respectively, whereas the electron kinetic variation is observed when the f-scaling is violated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2003

Effects of broad frequency vibration on cultured osteoblasts

Bone is subjected in vivo to both high amplitude, low frequency strain, incurred by locomotion, and to low amplitude, broad frequency strain. The biological effects of low amplitude, broad frequency strain are poorly understood. To evaluate the effects of low amplitude strains ranging in frequency from 0 to 50 Hz on osteoblastic function, we seeded MC3T3-E1 cells into collagen gels and applied the following loading protocols for 3 min per day for either 3 or 7 days: (1) sinusoidal strain at 3 Hz, with 0-3000 microstrain peak-to-peak followed by 0.33 s resting time, (2) "broad frequency vibration" of low amplitude strain (standard deviation of 300 microstrain) including frequency components from 0 to 50 Hz, and (3) sinusoidal strain combined with broad frequency vibration (S + V). The cells were harvested on day 4 or 8. We found that the S + V stimulation significantly repressed cell proliferation by day 8. Osteocalcin mRNA was up-regulated 2.6-fold after 7 days of S + V stimulation, and MMP-9 mRNA was elevated 1.3-fold after 3 days of vibration alone. Sinusoidal stimulation alone did not affect the cell responses. No differences due to loading were observed in alkaline phosphatase activity and in mRNA levels of type I collagen, osteopontin, connexin 43, MMPs-1A, -3, -13. These results suggest that osteoblasts are more sensitive to low amplitude, broad frequency strain, and this kind of strain could sensitize osteoblasts to high amplitude, low frequency strain. This suggestion implies a potential contribution of stochastic resonance to the mechanical sensitivity of osteoblasts. Copyright 2002 Elsevier Science Ltd.

Non-NASA Center↗
NASA NTRS1966

A Vibration Isolation Mount

This paper describes a vibration isolation mount for low-frequency isolation, and presents some preliminary test results. The mount described can be designed for any allowable stroke length and can provide the system with a low natural frequency. However, it is limited to an atmospheric environment and a specified load.

VIBRATION↗
NASA NTRS2011

Observing Solar Radio Bursts from the Lunar Surface

Locating low frequency radio observatories on the lunar surface has a number of advantages, including fixes locations for the antennas and no terrestrial interference on the far side of the moon. Here, we describe the Radio Observatory for Lunar Sortie Science (ROLSS), a concept for a low frequency, radio imaging interferometric array designed to study particle acceleration in the corona and inner heliosphere. ROLSS would be deployed during an early lunar sortie or by a robotic rover as part of an unmanned landing. The prime science mission is to image type II and type III solar radio bursts with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Secondary science goals include constraining the density of the lunar ionosphere by searching for a low radio frequency cutoff of the solar radio emissions and constraining the low energy electron population in astrophysical sources. Furthermore, ROLSS serves a pathfinder function for larger lunar radio arrays designed for faint sources.

MacDowall, R. J.↗
NASA NTRS1978

Sound-suppressing structure with thermal relief

Sound-suppressing structure comprising stacked acoustic panels wherein the inner high frequency panel is mounted for thermal expansion with respect to the outer low frequency panel is discussed. Slip joints eliminate the potential for thermal stresses, and a thermal expansion gap between the panels provides for additional relative thermal growth while reducing heat convection into the low frequency panel.

Nash, D. O.↗
NASA NTRS1989

Potential uses of vacuum bubbles in noise and vibration control

Vacuum bubbles are new acoustic elements which are dynamically more compliant than the gas volume they replace, but which are statically robust. They are made of a thin metallic shell with vacuum in their cavity. Consequently, they pose no danger in terms of contamination or fire hazard. The potential of the vacuum bubble concept for noise and vibration control was assessed with special emphases on spacecraft and aircraft applications. The following potential uses were identified: (1) as a cladding, to reduce sound radiation of vibrating surfaces and the sound excitation of structures, (2) as a screen, to reflect or absorb an incident sound wave, and (3) as a liner, to increase low frequency sound transmission loss of double walls and to increase the low frequency sound attenuation of muffler baffles. It was found that geometric and material parameters must be controlled to a very high accuracy to obtain optimal performance and that performance is highly sensitive to variations in static pressure. Consequently, it was concluded that vacuum bubbles have more potential in spacecraft applications where static pressure is controlled more than in aircraft applications where large fluctuations in static pressure are common.

Ver, Istvan L.↗
NASA NTRS1990

Long-range sound propagation: A review of some experimental data

Three experimental studies of long range sound propagation carried out or sponsored in the past by NASA are briefly reviewed to provide a partial prospective for some of the analytical studies presented in this symposium. The three studies reviewed cover (1) a unique test of two large rocket engines conducted in such a way as to provide an indication of possible atmospheric scattering loss from a large low-frequency directive sound source, (2) a year-long measurement of low frequency sound propagation which clearly demonstrated the dominant influence of the vertical gradient in the vector sound velocity towards the receiver in defining excess sound attenuation due to refraction, and (3), a series of excess ground attenuation measurements over grass and asphalt surfaces replicated several times under very similar inversion weather conditions.

Sutherland, Louis C.↗
NASA NTRS2008

Re-active Passive (RAP) Devices for Control of Noise Transmission through a Panel

Re-Active Passive (RAP) devices have been developed to control low frequency (<1000 Hz) noise transmission through a panel. These devices use a combination of active, re-active, and passive technologies packaged into a single unit to control a broad frequency range utilizing the strength of each technology over its best suited frequency range. The RAP device uses passive constrained layer damping to cover the relatively high frequency range (>200 Hz), reactive distributed vibration absorber) to cover the medium frequency range (75 to 250 Hz), and active control for controlling low frequencies (<200 Hz). The device was applied to control noise transmission through a panel mounted in a transmission loss test facility. Experimental results are presented for the bare panel, and combinations of passive treatment, reactive treatment, and active control. Results indicate that three RAP devices were able to increase the overall broadband (15-1000 Hz) transmission loss by 9.4 dB. These three devices added a total of 285 grams to the panel mass of 6.0 kg, or approximately 5%, not including control electronics.

Carneal, James P.↗
DOE OSTI2025

Nonideal stability analysis of differentially rotating plasmas with global curvature effects

The linear stability of global nonaxisymmetric modes in differentially rotating, magnetized, nonideal plasma is critical to classifying turbulence and transport phenomena. We investigate the competition between the local magneto-rotational instability (MRI) and the magneto-curvature instability (MCI)—a distinct nonaxisymmetric low-frequency curvature-driven global branch that appears alongside MRI. Here, to accomplish this, we developed a nonideal global spectral method, which is validated against NIMROD code simulations. This spectral approach allows for the direct derivation of an extended effective potential formalism and a resistive Alfvénic resonance condition, providing a framework for direct analysis of energy contributions and confinement mechanisms. Our study reveals that the global, low-frequency MCI persists at low magnetic Reynolds numbers (Rm), whereas the localized, high-frequency MRI is stabilized by diffusive broadening of its structure around its Alfvénic resonances. Consequently, we identify the global MCI branch as the primary onset mechanism for nonaxisymmetric magnetohydrodynamic instability in systems with finite curvature, e.g., astrophysical rotators. We establish distinct parameter regimes for mode dominance: MCI prevails in geometrically moderate-thickness disks with intermediate curvature and radial gaps, while MRI dominates in thin, low-curvature disks with large radial gaps. Mode competition is also highly sensitive to the flow profile, particularly vorticity and its gradient, with nonuniform shear profiles exhibiting more robust instability due to flow curvature (i.e., the second derivative of the flow profile) and shear contributions. A key outcome is the development of spectral diagrams derived from the global spectral method. These diagrams comprehensively map dominant instabilities and their characteristics, offering a predictive tool for critical onset parameters (i.e., flow curvature, magnetic field, and Rm) and facilitating the interpretation of experimental and simulation results. Notably, these diagrams demonstrate that the global MCI is generally the sole unstable mode at the initial onset of nonaxisymmetric instability.

Haywood, Alexander [Princeton Univ., NJ (United St↗
DOE OSTI2023

A Hybrid Transmitter With Voltage-Mode SST Preemphasis and Current-Mode Transmitter Equalization Capable of Operating at 77 K in DUNE

The Deep Underground Neutrino Experiment (DUNE) requires that the front-end transmitters operate at cryogenic temperature and drive 25–35 m long twin-axial (twinax) cables. Here, to compensate the frequency-dependent channel loss over the long cables and alleviate the de-emphasizing of the low-frequency signal magnitude, a hybrid of a current-mode (CM) transmitter equalization (TXEQ) and a voltage-mode (VM) preemphasis is proposed. The TXEQ employs a finite-impulse response (FIR) filter to boost the high-frequency components while de-emphasizing the low-frequency signal magnitude, thereby flattening the overall channel frequency response and reducing the intersymbol interference (ISI). The VM preemphasis is proposed to further mitigate ISI by boosting the high-frequency portion without degrading the signal magnitude, allowing for high signal swing. The main driver utilizes VM source-series-terminated (SST) output stages, which offers higher signal swing and better power efficiency than the conventional CM logic (CML) drivers. To ensure the lifetime and reliability at cryogenic temperature, the transmitter is implemented in a 65-nm CMOS process operating at 1.1 V of supply voltage and employing transistors with larger than minimum lengths. Silicon measurement results at 77 K have validated the proposed approaches.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1997

Time Correlations and the Frequency Spectrum of Sound Radiated by Turbulent Flows

Theories of turbulent time correlations are applied to compute frequency spectra of sound radiated by isotropic turbulence and by turbulent shear flows. The hypothesis that Eulerian time correlations are dominated by the sweeping action of the most energetic scales implies that the frequency spectrum of the sound radiated by isotropic turbulence scales as omega(exp 4) for low frequencies and as omega(exp -3/4) for high frequencies. The sweeping hypothesis is applied to an approximate theory of jet noise. The high frequency noise again scales as omega(exp -3/4), but the low frequency spectrum scales as omega(exp 2). In comparison, a classical theory of jet noise based on dimensional analysis gives omega(exp -2) and omega(exp 2) scaling for these frequency ranges. It is shown that the omega(exp -2) scaling is obtained by simplifying the description of turbulent time correlations. An approximate theory of the effect of shear on turbulent time correlations is developed and applied to the frequency spectrum of sound radiated by shear turbulence. The predicted steepening of the shear dominated spectrum appears to be consistent with jet noise measurements.

Rubinstein, Robert↗
NASA NTRS1998

Nonlinear Interaction of Detuned Instability Waves in Boundary-Layer Transition: Resonant-Triad Interaction

The non-equilibrium critical-layer analysis of a system of frequency-detuned resonant-triads is presented using the generalized scaling of Lee. It is shown that resonant-triads can interact nonlinearly within the common critical layer when their (fundamental) Strouhal numbers are different by a factor whose magnitude is of the order of the growth rate multiplied by the wavenumber of the instability wave. Since the growth rates of the instability modes become larger and the critical layers become thicker as the instability waves propagate downstream, the frequency-detuned resonant-triads that grow independently of each other in the upstream region can interact nonlinearly in the later downstream stage. In the final stage of the non-equilibrium critical-layer evolution, a wide range of instability waves with the scaled frequencies differing by almost an Order of (l) can nonlinearly interact. Low-frequency modes are also generated by the nonlinear interaction between oblique waves in the critical layer. The system of partial differential critical-layer equations along with the jump equations are presented here. The amplitude equations with their numerical solutions are given in Part 2. The nonlinearly generated low-frequency components are also investigated in Part 2.

Lee, Sang Soo↗
DOE OSTI2024

Low flow characteristics for regulated and unregulated streams in North Carolina and prediction using climate signals

In this work, low flow statistics of regulated and unregulated streams in the state of North Carolina were updated with streamflow data through 2019. About 22% of the streams considered show a significant downward trend, but considerable low-frequency variability confounding trends. The relationship between regional index time series of groundwater-depth low flow and the Atlantic Multidecadal Oscillation (AMO) was examined to determine whether low-frequency climate modes can account for the long-term pattern in low flows. Consequently, a significant correlation was found between AMO and groundwater-depth low flow, such that positive AMO is associated with lower groundwater-depth low flow and vice versa, particularly for the Piedmont region. Predictive equations for annual low flows at the ecoregion level shows that springtime average streamflow and AMO were selected as the primary predictors of low flow for coastal and Piedmont regions, whereas springtime average streamflow and the November–December–January average Oceanic Niño Index were used as the primary predictors for the mountain region. The relative root mean square error (RMSE) of the disaggregated predictions to US Geologic Survey gauge locations was <23% at 79% of the stations, between 24% and 43% at 10% of the stations, and greater than 44% at 1% of the stations. The remaining 10% of stations showed large RMSEs. This latter percentage is characterized by smaller drainage basins and intermittent flows, suggesting the prediction models are not applicable to drainage basins smaller than roughly 20 square km and intermittent streams.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1981

Space-time spectral structure of a GLAS general circulation model and a comparison with observations

The wavenumber-frequency spectra of geopotential height computed from a winter simulation of a general circulation model are compared with the observed winter spectra averaged over 15 winters. The space and time scales studied include: (1) stationary planetary waves; (2) stationary synoptic-scale waves; (3) low-frequency planetary waves; (4) low frequency synoptic-scale waves; (5) medium-frequency planetary waves; and (6) medium frequency synoptic-scale waves. Variances in these categories are presented and their distributions with latitude and height are discussed.

Straus, D. M.↗
DOE OSTI2026

An adaptive model-free robotic force control strategy for hydrodynamic real-time hybrid simulation of floating offshore wind turbines

Real-time hybrid simulation (RTHS) - a cyber-physical testing approach - promises to enhance the simulation fidelity of the model-scale experiments used to prototype floating offshore wind turbines (FOWTs). In hydrodynamic RTHS (hydro-RTHS), actuators emulate aerodynamic forces on model-scale FOWT specimens subjected to physical waves in a hydrodynamic laboratory. Robotic arms are promising candidates for actuation in hydro-RTHS due to their compact multi-degree-of-freedom (DOF) capabilities. Unlike classical RTHS for seismic applications, which typically relies on displacement control, hydro-RTHS requires 6-DOF force control on newly designed floating prototypes in a model-scale setting, which presents significant challenges, including modeling uncertainties, directional asymmetry, configuration drift, bandwidth limitations, and time-varying delays. To mitigate these constraints without extensive pre-test calibration, this study proposes an adaptive model-free robotic force control strategy that combines task-space explicit force control with a secondary joint-space pose-keeping task. The Adaptive Feedforward Compensator (AFC) is integrated into the force control loop to compensate for time-varying delay. Experimental testing was conducted using a Franka Emika Panda robotic arm with a 1:50 scale FOWT specimen under operational wind and wave conditions. Results demonstrate stable and consistent 6-DOF force tracking. Effective delay compensation was observed, with low-frequency delay reductions ranging from 71.4% to 91.8% and improvements in low-frequency surge force tracking of 25.0% to 52.1%. This study enhances robotic actuation performance in hydro-RTHS and introduces a force control strategy that supports reliable robotic operation in uncertain floating environments. Future work will explore disturbance-observer mechanisms to further enhance wave rejection capabilities under extreme wind and wave conditions.

17 WIND ENERGY↗