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NASA NTRS1996

The Effects of Flame Structure on Extinction of CH4-O2-N2 Diffusion Flames

The effects of flame structure on the extinction limits of CH4-O2-N2 counterflow diffusion flames were investigated experimentally and numerically by varying the stoichiometric mixture fraction Z(sub st), Z(sub st) was varied by varying free-stream concentrations, while the adiabatic flame temperature T(sub ad) was held fixed by maintaining a fixed amount of nitrogen at the flame. Z(sub st) was varied between 0.055 (methane-air flame) and 0.78 (diluted- methane-oxygen flame). The experimental results yielded an extinction strain rate K(sub ext) of 375/s for the methane-air flame, increasing monotonically to 1042/s for the diluted-methane-oxygen flame. Numerical results with a 58-step Cl mechanism yielded 494/s and 1488/s, respectively. The increase in K(sub ext) with Z(sub st) for a fixed T(sub ad) is explained by the shift in the O2 profile toward the region of maximum temperature and the subsequent increase in rates for chain-branching reactions. The flame temperature at extinction reached a minimum at Z(sub st) = 0.65, where it was 200 C lower than that of the methane-air flame. This significant increase in resistance to extinction is seen to correspond to the condition in which the OH and O production zones are centered on the location of maximum temperature.

Du, J.↗

NASA NTRS2016

Arctic Cut-Off High Drives the Poleward Shift of a New Greenland Melting Record

Large-scale atmospheric circulation controls the mass and energy balance of the Greenland ice sheet through its impact on radiative budget, runoff and accumulation. Here, using reanalysis data and the outputs of a regional climate model, we show that the persistence of an exceptional atmospheric ridge, centered over the Arctic Ocean, was responsible for a poleward shift of runoff, albedo and surface temperature records over the Greenland during the summer of 2015. New records of monthly mean zonal winds at 500 hPa and of the maximum latitude of ridge peaks of the 5,700+/-50 m isohypse over the Arctic were associated with the formation and persistency of a cutoff high. The unprecedented (1948-2015) and sustained atmospheric conditions promoted enhanced runoff, increased the surface temperatures and decreased the albedo in northern Greenland, while inhibiting melting in the south, where new melting records were set over the past decade. Subject terms: Earth sciences Atmospheric science Climate science

drainage↗

NASA NTRSDate not supplied

Understanding the Chemistry of PAHs in H2O Ice

Polycyclic aromatic hydrocarbons (PAHs) can be incorporated into H O ice grains and mantles found in dense molecular clouds, comets, interplanetary dust particles (IDPs), and many Solar System objects. However, laboratory infrared (IR) spectroscopic studies of PAHs in ices is still a relatively young field. Early research efforts have shown the rich chemistry of PAHs in H O to be dependent on PAH concentration, with the concentration impacting everything from IR band shifts to the photoproducts produced [1-6]. Similarly, research suggests that a potential top-down synthesis route for complex organic molecules or COMs found within solar systems is possible from PAHs in water ice [7]. In order to fully exploit the observations from the James Webb Space Telescope (JWST) more IR studies of PAHs in H O ice under relevant astrophysical conditions and planetary systems are needed. The NASA ICEE (Institute for Carbon Evolution Experiments), formerly the NASA Ames PAH IR Laboratory, has been at the forefront of this field. Starting in 2019, just before the COVID-19 shutdown, the ICEE laboratory underwent a major renovation, gearing it towards in-depth PAHs in H O ice research under relevant solar systems and interstellar conditions. The ICEE facility enables us to study the processing of PAHs in H O ice using UV photons and charged particle (electron) irradiation either singly or in combination under ultra-high vacuum and low temperature (15K). We use in-situ measurements with IR, Raman, and mass spectra by which we monitor the chemistry of PAHs in H O ice before, during and after the irradiation. This presentation will summarize the past work regarding PAHs in ices and provide an update on the latest ICEE research results in this area, outlining a roadmap for future research efforts.

Andrew Mattioda↗

NASA NTRS2018

Instability Wave–Streak Interactions in a High Mach Number Boundary Layer at Flight Conditions

The interaction of stationary streaks undergoing nonmodal growth with modally unstable instability waves in a hypersonic boundary-layer flow is studied using numerical computations. The geometry and flow conditions are selected to match a relevant trajectory location from the ascent phase of the HIFiRE-1 flight experiment; namely, a 7 degree half-angle, circular cone with 2.5 mm nose radius, freestream Mach number equal to 5.30, unit Reynolds number equal to 13.42 m -1 , and wall-to-adiabatic temperature ratio of approximately 0.35 over most of the vehicle. This paper investigates the nonlinear evolution of initially linear optimal disturbances that evolve into finite-amplitude streaks, followed by an analysis of the modal instability characteristics of the perturbed, streaky boundary-layer flow. The investigation is performed with stationary direct numerical simulations (DNS) and plane-marching parabolized stability equations (PSE), in conjunction with partial-differential-equation-based planar eigenvalue analysis. The overall effect of streaks is to reduce the peak amplification factors of instability waves, indicating a possible downstream shift in the onset of laminar-turbulent transition. The present study confirms previous findings that the mean flow distortion of the nonlinear streak perturbation reduces the amplification rates of the Mack-mode instability. More importantly, however, the present results demonstrate that the spanwise varying component of the streak can produce a larger effect on the Mack-mode amplification. The study with selected azimuthal wavenumbers for the stationary streaks reveals that a wavenumber of approximately 1.4 times larger than the optimal wavenumber is more effective in stabilizing the planar Mack-mode instabilities. In the absence of unstable first-mode waves for the present cold-wall condition, transition onset is expected to be delayed until the peak streak amplitude increases to nearly 35 percent of the freestream velocity, when intrinsic instabilities of the boundary-layer streaks begin to dominate the transition process. For streak amplitudes below that limit a significant net stabilization is achieved, yielding a potential transition delay that can exceed 100 percent of the length of the laminar region in the uncontrolled case.

Boundary layer stability↗

NASA NTRS1986

Observations and mechanisms of GATE waterspouts

The present numerical and observational investigation of interacting cumulus processes implicated in the formation of waterspouts, the GATE database for days 261 and 186 is noted to imply that the existence of cumulus-scale parent vortices is a necessary (albeit not sufficient) condition for the production of waterspouts. A high resolution version of the Schlessinger (1975) three-dimensional cumulus model with a Kessler (1969) type precipitation scheme is used to analyze cumulus-scale vorticity organization, which on the two days in question exhibited contrasting thermal stratification and cloud features. The observations from both days suggest that the waterspouts formed ahead of the wind shift, due to the passage of a gust front.

Simpson, J.↗

NASA NTRS1994

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field B(sub z) and B(sub y)

Continuous optical observations of cusp/cleft auroral activities within approximately equal to 09-15 MLT and 70-76 deg magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF B(sub z) changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field-aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Delta p/p is greater than or equal to 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of approximately equal to 5-10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the approximately equal to 10-14 MLT sector, during negative IMF B(sub z) and B(sub y) conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 is less than Delta p/p is less than 0.5). Simultaneous photometer observations from Ny Alesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (approximately equal to 10-12 MLT), before moving into the postnoon sector in the case we study here, when IMF B(sub y) is less than 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive B(sub y). Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF B(sub y) polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed that dayside magnetosphere in the noon or near-noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

Sandholt, P. E.↗

NASA NTRS1993

Adaptation of Baroreflexes and Orthostatic Hypotension

The footward shift of blood volume and compensatory autonomic reflex responses initiated during the assumption of the upright posture in terrestrial gravity is a common feature in humans. When regular exposure to upright posture is removed, the headward redistribution of blood induces numerous physiological adaptations which compromise this normal response and the development of low blood pressure upon standing (orthostatic hypotension) can ensue. Such microgravity conditions can be produced by prolonged exposure to spaceflight or prolonged bed rest. Since the reduction of blood and plasma volume during exposure to microgravity has been associated with orthostatic instability following spaceflight and bed rest, it has been a reasonable assumption that hypovolemia may be a primary contributing factor to the development of orthostatic hypotension. This view was supported by the observation that the attempt to restore vascular volume in astronauts by drinking saline solutions just prior to re-entry had proven effective in reducing orthostatic instability after spaceflights of short duration. However, as the duration of spaceflight lengthens, orthostatic instability persists despite fluid loading procedures, suggesting that mechanisms other than hypovolemia may contribute to orthostatic hypotension following prolonged exposure to microgravity conditions. Recent evidence has been generated from spaceflight and groundbase experiments that supports the notion that changes in autonomic baroreflexes that control cardiac and vascular responses during orthostatic challenges may be affected by longer periods of microgravity exposure and can contribute to postflight orthostatic instability.

Convertino, Victor A.↗

NASA NTRS1995

Fault Tolerant Characteristics of Artificial Neural Network Electronic Hardware

The fault tolerant characteristics of analog-VLSI artificial neural network (with 32 neurons and 532 synapses) chips are studied by exposing them to high energy electrons, high energy protons, and gamma ionizing radiations under biased and unbiased conditions. The biased chips became nonfunctional after receiving a cumulative dose of less than 20 krads, while the unbiased chips only started to show degradation with a cumulative dose of over 100 krads. As the total radiation dose increased, all the components demonstrated graceful degradation. The analog sigmoidal function of the neuron became steeper (increase in gain), current leakage from the synapses progressively shifted the sigmoidal curve, and the digital memory of the synapses and the memory addressing circuits began to gradually fail. From these radiation experiments, we can learn how to modify certain designs of the neural network electronic hardware without using radiation-hardening techniques to increase its reliability and fault tolerance.

Artificial Neural Network↗

NASA NTRS2020

Design of a Direct-Detection Wind and Aerosol Lidar for Mars Orbit

The present knowledge of the Mars atmosphere is greatly limited by a lack of global measurements of winds and aerosols. Hence, measurements of height-resolved wind and aerosol profiles are a priority for new Mars orbiting missions. We have designed a direct-detection lidar (MARLI) to provide global measurements of dust, winds and water ice profiles from Mars orbit. From a 400-km polar orbit, the instrument is designed to provide wind and backscatter measurements with a vertical resolution of 2 km and with resolution of 2° in latitude along track. The instrument uses a single-frequency, seeded Nd:YAG laser that emits 4 mJ pulses at 1064 nm at a 250 Hz pulse rate. The receiver utilizes a 50-cm diameter telescope and a double edge Fabry-Pérot etalon as a frequency discriminator to measure the Doppler shift of the aerosol-backscatter profiles. The receiver also includes a polarization-sensitive channel to detect the cross-polarized backscatter profiles from water ice. The receiver uses a sensitive 4 × 4 pixel HgCdTe avalanche photodiode array as a detector for all signals. Here we describe the measurement concept, instrument design, and calculate its performance for several cases of Mars atmospheric conditions. The calculations show that under a range of atmospheric conditions MARLI is capable of measuring wind speed profiles with random error of 2–4 m/s within the first three scale heights, enabling vertically resolved mapping of transport processes in this important region of the atmosphere.

Cremons, Daniel R.↗

NASA NTRSDate not supplied

Adjusting a Full Body Model to Mitigate Inverse Kinematics Artifacts in Opensim

BACKGROUND: In support of Vibration Isolation and Stabilization (VIS) system development for Human Health Countermeasures (HHC) exercise systems in space, such as the European Enhanced Exploration Exercise Device (E4D) [1], dynamic quantities required to model the response of a proposed VIS while considering the effect of VIS motion on the forces between the human and VIS platform were obtained using motion capture data [2]. On occasion, large-amplitude oscillatory spikes were found in the subject’s linear and angular momentum derivatives, affecting analyses that depend on forces and moments derived from motion capture. The purpose of this investigation was to identify causes of these artifacts and techniques for their resolution. METHODS AND RESULTS: To obtain the required human dynamic quantities to drive the VIS simulation, motion capture data was collected containing recorded trajectories of passive retroreflective markers on body landmarks of an exercising subject. Since the full body Rajagopal model [3] was originally used to enhance gait analysis, upper body joints did not require large Ranges of Motion (ROM). We thus modified the Rajagopal model [4, 5] to allow it to be used for upper body intensive exercises like those common to the E4D. OpenSim Inverse Kinematics (IK) [6] was performed using these scaled subject models to generate the joint angles throughout the exercise while minimizing marker error. At times, the arms were observed to ‘snap’ from one configuration to another, causing spike artifacts. Following IK, a custom OpenSim plugin [7] was used to determine the required dynamic quantities including the rates of change of the linear and angular momenta of the human. Since motion capture is recorded at a larger time step than required by the VIS simulation, the human center of mass location was fit with splines and a second derivative taken to obtain the momentum derivative, allowing the VIS simulation to maintain conservation of momentum when appropriate. In a few cases during this stage, artifacts much larger than the expected noise of the second derivatives were introduced. Investigation of cases containing artifacts revealed several modifications that could be made to the OpenSim model to improve IK results. Since OpenSim models use Euler angles and rotation sequences, ‘gimbal lock’ would be encountered in the arms when raised 90 degrees to the side (e.g., T-pose, some hang clean exercise, etc.). This was resolved by reorienting the horizontal axes at the shoulder joint by 45 degrees, placing ‘gimbal lock’ outside common arm ROM, with the arm ROMs adjusted following this change. Elbow and wrist ROMs could also be adjusted to allow realistic motion while at the same time limiting the likelihood of unrealistic orientations. On occasion, the arms flipped backwards when raised above the head. This was prevented by using medial elbow markers in scaling and IK. When medial markers were not available, the acromial joint location in the unscaled model was shifted before model scaling to better align the arm with available markers. Lastly, artifacts which became apparent after taking the second derivatives of spline-fit data were found to occur in cases when the pelvis rotation limit prevented the full range of motion of an exercise. These issues were resolved by unclamping the pelvis rotation limit. Through this investigation, an understanding of conditions leading to IK artifacts was acquired allowing the automation of artifact detection. These artifact detection and mitigation techniques can be applied toward modeling of upper body motions in aerospace and other fields for improved IK results.

C A Bell↗

NASA NTRS2013

Association Between Cardiovascular and Intraocular Pressure Changes in a 14-Day 6 deg Head Down Tilt (HDT) Bed Rest Study: Possible Implications in Retinal Anatomy

Visual symptoms and intracranial pressure increase reported in astronauts returning from long duration missions in low Earth-orbit are thought to be related to fluid shifts within the body due to microgravity exposure. Because of this possible relation to fluid shifts, studies conducted in head-down tilt (HDT) bed rest are being monitored for potential changes in ocular health. These measures will also serve to determine whether HDT is a suitable ground-based analog to model subclinical cardiovascular and ocular changes that could shed light on the etiology of the VIIP syndrome observed in spaceflight. Sixteen healthy normotensive (12M, 4F, age range 29-54 years), non-smoker and normal weight subjects, volunteered to participate in a 14 day 6 deg head HDT study conducted at the NASA Flight Analogs Research Unit (FARU). This facility provides standard bed rest conditions (diet, wake/sleep time, time allowed in sunlight) during the time that the subjects stay at the FARU. Cardiovascular parameters were obtained in supine posture at BR-5, BR+0, and BR+3 and ocular monitoring was performed weekly. Intraocular pressure (IOP) increased from pre-bed rest BR-3) to the third day into bed rest (BR+3). Values reached a plateau towards the end of the bed rest phase (BR10) and decreased within the first three days of recovery (BR+2) returning to levels comparable to baseline at BR-3. As expected, most cardiovascular parameters were affected by 14 days of HDT bed rest. Plasma volume decreased as a result of bed rest but recovered to baseline levels by BR+3. Indications of cardiovascular deconditioning included increase in both systolic and diastolic blood pressure and heart rate, and a decrease in stroke volume and cardiac output between BR-5 and BR+3. Due to the experimental design of this study, we were not able to test the hypothesis that fluid shifts might be involved in the IOP increase during the bed rest phase, since cardiovascular measures were not available for those time points. There was no correlation between the largest change in IOP (BR-3 versus BR3) and cardiovascular measure changes between baseline (BR-5) and post bed rest (BR+2). While no clinically relevant visual changes were observed during the study, measurement of various retinal parameters was performed with optical coherence tomography (OCT). A decrease in central subfield retinal thickness was observed between BR+2 and baseline at BR-10, but no association was observed with IOP changes. This work investigates the time course of changes in IOP during 14-day HDT bed rest in an attempt to characterize HDT bed rest as a model of the VIIP syndrome and delve into its etiology.

Cromwell, Ronita↗

NASA NTRS2009

Determining Aliasing in Isolated Signal Conditioning Modules

The basic concept of aliasing is this: Converting analog data into digital data requires sampling the signal at a specific rate, known as the sampling frequency. The result of this conversion process is a new function, which is a sequence of digital samples. This new function has a frequency spectrum, which contains all the frequency components of the original signal. The Fourier transform mathematics of this process show that the frequency spectrum of the sequence of digital samples consists of the original signal s frequency spectrum plus the spectrum shifted by all the harmonics of the sampling frequency. If the original analog signal is sampled in the conversion process at a minimum of twice the highest frequency component contained in the analog signal, and if the reconstruction process is limited to the highest frequency of the original signal, then the reconstructed signal accurately duplicates the original analog signal. It is this process that can give birth to aliasing.

Source record↗

NASA NTRS1985

Interactions among sulfide-oxidizing bacteria

The responses of different phototrophic bacteria in a competitive experimental system are studied, one in which primary factors such as H2S or light limited photometabolism. Two different types of bacteria shared one limited source of sulfide under specific conditions of light. The selection of a purple and a green sulfur bacteria and the cyanobacterium was based on their physiological similarity and also on the fact that they occur together in microbial mats. They all share anoxygenic photosynthesis, and are thus probably part of an evolutionary continuum of phototrophic organisms that runs from, strictly anaerobic physiology to the ability of some cyanobacteria to shift between anoxygenic bacterial style photosynthesis and the oxygenic kind typical of eukaryotes.

Poplawski, R.↗

NASA NTRS1986

Pressure effects on the thermal stability of SiC fibers

Commercially available polymer derived SiC fibers were treated at temperatures from 1000 to 2200 C in vacuum and argon gas pressure of 1 and 1360 atm. Effects of gas pressure on the thermal stability of the fibers were determined through property comparison between the pressure treated fibers and vacuum treated fibers. Investigation of the thermal stability included studies of the fiber microstructure, weight loss, grain growth, and tensile strength. The 1360 atm argon gas treatment was found to shift the onset of fiber weight loss from 1200 to above 1500 C. Grain growth and tensile strength degradation were correlated with weight loss and were thus also inhibited by high pressure treatments. Additional heat treatment in 1 atm argon of the fibers initially treated at 1360 atm argon caused further weight loss and tensile strength degradation, thus indicating that high pressure inert gas conditions would be effective only in delaying fiber strength degradation. However, if the high gas pressure could be maintained throughout composite fabrication, then the composites could be processed at higher temperatures.

Jaskowiak, Martha H.↗

NASA NTRS1989

Pressure effects on the thermal stability of silicon carbide fibers

Jaskowiak, Martha H.↗

NASA NTRS1994

Magnetospheric imaging of high latitude ion outflows

High latitude ion outflows mostly consist of upward streaming O(+) and He(+) emanating from the ionosphere. At heights above 1000 km, these flows consist of cold and hot components which resonantly scatter solar extreme ultraviolet (EUV) light, however, the ion populations respond differently to Doppler shifting resulting from the large relative velocities between the ions and the Sun. The possibility of optical detection of the Doppler effect on the scattering rate will be discussed for the O(+) (83.4 nm) ions. We have contrasted the EUV solar resonance images of these outflows by simulations of the 30.4 nm He(+) and 83.4 nm O(+) emissions for both quiet and disturbed geomagnetic conditions. Input data for the 1000 km level has been obtained from the EICS instrument aboard the Dynamics Explorer (DE) satellite. Our results show emission rates of 50 and 56 milli-Rayleighs at 30.4 nm for quiet and disturbed conditions and 65 and 75 milli-Rayleighs at 83.4 nm for quiet and disturbed conditions, respectively, obtained for a polar orbiting satellite and viewing radially outward. We also find that an imager at an equatorial distance of 9 R(sub E) or more is in a favorable position for detecting ion outflows, particularly when the plasmapause is depressed in latitude. However, an occultation disk is necessary to obscure the bright plasmaspheric emissions.

Garrido, D. E.↗

NASA NTRS1994

Relativistic tidal interaction of a white dwarf with a massive black hole

We compute encounters of a realistic white dwarf model with a massive black hole in the regime where relativistic effects are important, using a three-dimensional, finite-difference, Eulerian, piecewise parabolic method (PPM) hydrodynamical code. Both disruptive and nondisruptive encounters are considered. We identify and discuss relativistic effects important for the problem: relativistic shift of the pericenter distance, time delay, relativistic precession, and the tensorial structure of the tidal forces. In the nondisruptive case, stripping of matter takes place. In the surface layers of the surviving core, complicated hydrodynamical phenomena are revealed. In both disruptive and nondispruptive encounters, material flows out in the form of two thin, S-shaped, supersonic jets. Our results provide realistic initial conditions for the subsequent investigation of the dynamics of the debris in the field of the black hole. We evaluate the critical conditions for complete disruption of the white dwarf, and compare our results with the corresponding results for nonrelativistic encounters.

Frolov, V. P.↗

NASA NTRS2007

Cardiac and Vascular Function in Bedrested Volunteers: Effects of Artificial Gravity Training

Cardiovascular effects of an artificial gravity (AG) countermeasure on deconditioned male volunteers were studied. In two groups of men we measured cardiovascular parameters at rest and in response to 30 minutes of 80 deg. head up tilt (HUT) before, at the end of, and four days following 21 days of 6 deg. head down bed rest (HDBR). One group (N=7) underwent no countermeasure while the other (N=8) received a daily, one hour, dose (2.5 gz at the foot, decreasing to 1.0 gz at the heart) of AG training on the Johnson Space Center short radius centrifuge. Cardiovascular parameters measured included heart rate, blood pressure, stroke volume, cardiac output, peripheral vascular resistance, plasma volume shifts, and vasoactive hormones. Untrained subjects exhibited shorter tilt survival (on average 8 minutes shorter) compared to trained subjects. By the end of bed rest, mean heart rate (MHR) was elevated in both groups (both supine and during tilt). In addition, treated subjects demonstrated lower, tilt-induced, increases in MHR four days following HDBR, indicating a more rapid return to pre bed rest conditions. Results from an index of autonomic balance (percentage of MHR spectral power in the respiratory frequency range) in control of heart rate are consistent with the interpretation that parasympathetic nervous system withdrawal was responsible for both tilt- and bed rest-induced increases in MHR. Our data support our pre-study hypothesis that AG treatment would lessen cardiovascular effects of deconditioning in bed rested men and suggest that AG should be further pursued as a space flight countermeasure.

Meng, M.↗