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NASA NTRS1988

Monolithic GaAs digitizer for space-based laser pulse spreading effect

A 6-bit 1-GHz digitizer was designed to analyze the 1-ns pulse spreading effects in a space based altimeter. The digitizer consisted of four 4-bit flash A/D converters and a 6-bit encoder. Also, the converter utilized four 4-bit converters and a 4-to-6 bit encoder to achieve 6 bit resolution at the 1 GHz sample rate. The design was unique because it utilized only the inverters and NOR gates for the converters and encoder, hence it could be fabricated using the existing state-of-the-art GaAs processing techniques. This GHz digitizer has many commercial applications. It could be applicable to: (1) digital microwave transmission system for the telecommunication industries, (2) pulse monitoring in high kinetic chemical reactions, (3) transient signals in the medical field, and (4) microwave signals in astronomy.

Lao, Bennig↗

NASA NTRS1992

Forced unsteady deceleration of a turbulent boundary layer from a temporal perspective

The behavior of a turbulent boundary layer which has been subjected to a local ramp-like deceleration in the external velocity field, which leads to forced separation, has been studied experimentally. The data of this study are re-interpreted in light of more recent findings concerning the temporal nature of boundary layer turbulence in the presence of forced unsteady shear. In particular, the robustness of the near-wall turbulent motions to organized deformation is recognized. Their resilence during unsteady shearing action promotes continued efficient turbulent mixing and rapid redistribution of turbulent kinetic energy during forced transients. In aerodynamic problems, the rapid nature of the adjustment of the turbulence field to a new temporal boundary condition necessitates equally rapid remedial measures to be taken if means of control/prevention of forced unsteady separation are to be deployed to maximum effect. This requirement suggests exploration of the use of simple, real-time statistical forecasting techniques, based upon time-series analysis of easily-measurable features of the flow, to help assure timely deployment of mechanisms of boundary-layer control. This paper focuses upon the nature of turbulence in boundary layers undergoing forced deceleration which would lead to separation. A preliminary form of a forecasting model is presented and evaluated. Using observations of the previous two large eddies passing a detector, it forecasts the behavior of the future large eddy rather well.

Brereton, G. J.↗

NASA NTRS1988

The seasonal cycle of energetics from the GLAS/UMD climate GCM

The annual cycle of atmospheric energetics from a 2-year integration of the GLAS/UMD Climate GCM is computed and compared to results from the European Centre analyses of the GWE year, and to previously published results on a global basis. All calculations are done in the mixed space-time domain. The main conclusions are: (1) the seasonal cycle of today's eddy kinetic energy (in both hemispheres), and of the transient eddy available potential energy and the potential-to-kinetic energy conversions (mean and eddy) in the Northern Hemisphere are well simulated by the GCM; (2) the GCM's tendency to have anomalously large mean u-winds at upper levels in high latitudes leads to excessive wintertime values of mean kinetic and available potential energies, and causes distortions in the GCM latitude-height distribution of kinetic energy and of many of the conversions; (3) the eddy conversion of available potential-to-kinetic energy obtained from the ageostrophic wind in these analyses; and (4) the conversions in the Southern Hemisphere are not well simulated by the GCM, although the observations are somewhat questionable.

Straus, David M.↗

NASA NTRS1985

Mephisto: Interfacial Destabilization in Metal Alloys

The destabilizing mechanisms at a solidification interface were studied to obtain information on the kinetics and morphologies in the transient and steady state, and to separate the influences of liquid phase instabilities from interfacial instabilities. A differential seebeck voltage measurements technique was developed to provide a continuous record of the solid-liquid interface temperature as the solidification rate is varied to determine the kinetic coefficients. Signal processing and noise suppression techniques allow nonovolt precision which corresponds to mK accuracy for the interfacial temperature.

Favier, J. J.↗

NASA NTRS1993

Transient development of perturbations in stratified shear flow

Transient development of perturbations in inviscid stratified shear flow is investigated. Use is made of closed form analytic solutions that allow concise identification of optimally growing plane-wave solutions for the case of an unbounded flow with constant shear and stratification. For the case of channel flow, variational techniques are employed to determine the optimally growing disturbances. The maximum energy growth attained over a specific time interval decreases continuously with increasing stratification, and no special significance attaches to Ri = 0.25. Indeed, transient growth can be substantial even for Ri = O(1). A general lower bound on the energy growth attained by an optimal perturbation in a stratified flow over a given time interval is the square root of the growth attained by the corresponding perturbation in unstratified flow. Enhanced perturbation persistence is found for mean-flow stratification lying in the range Ri between 0.1 and 0.3. Small but finite perturbations in mean flow with Ri less than 0.4 produce regions with locally negative total density gradient, which are expected to overturn. Although the perturbations are of wave form, buoyancy fluxes mediate transfer between perturbation kinetic and potential energy during transient development, thus implying that buoyancy flux is not a determinative diagnostic for distinguishing between waves and turbulence in stratified flows.

Farrell, Brian F.↗

NASA NTRS2022

Hybrid simulations of a tangential discontinuity-driven foreshock bubble formation in comparison with a hot flow anomaly formation

Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are significant foreshock transients that can accelerate particles and disturb the magnetosphere-ionosphere system. Yet, their early formation mechanisms are still not fully understood. To investigate the formation of tangential discontinuity (TD)-driven FBs and HFAs, we use 2-D local hybrid simulations where a reflected or an injected warm foreshock ion beam can interact with a TD whose half-thickness is comparable to the ion inertial scale. We show that the foreshock ions perform a partial gyration within, or across, the TD. Bulk motion differences between partially-gyrating foreshock ions and fluid-electrons lead to the generation of currents. As the trigger, these foreshock-driven currents change the magnetic field topology around the TD and force the frozen-in solar wind plasma to redistribute along with the field lines, shaping the foreshock transient. This confirms a recently proposed kinetic formation model. The extent of the magnetic field direction change across the TD within the foreshock ion gyromotion determines the current profile and thus the type of foreshock transient that forms. For a thin TD, the foreshock ions generate a current that is much stronger on the upstream side than the downstream side, forming an FB with one upstream compressional boundary. For the same foreshock ion gyroradius and magnetic shear, a thick TD yields comparable foreshock-driven currents on the upstream and downstream sides, forming an HFA with two compressional boundaries. Our study suggests that the TD thickness is one of the factors that determine the formation of FBs and HFAs

Andrew Vu↗

NASA NTRS1983

A study of the kinetic energy generation with general circulation models

The history data of winter simulation by the GLAS climate model and the NCAR community climate model are used to examine the generation of atmospheric kinetic energy. The contrast between the geographic distributions of the generation of kinetic energy and divergence of kinetic energy flux shows that kinetic energy is generated in the upstream side of jets, transported to the downstream side and destroyed there. The contributions from the time-mean and transient modes to the counterbalance between generation of kinetic energy and divergence of kinetic energy flux are also investigated. It is observed that the kinetic energy generated by the time-mean mode is essentially redistributed by the time-mean flow, while that generated by the transient flow is mainly responsible for the maintenance of the kinetic energy of the entire atmospheric flow.

Chen, T.-C.↗

NASA NTRS1981

Radio and visible light observations of matter ejected from the sun

An initial set of visible light and radio observations of a coronal transient made with the Coronagraph/Polarimeter experiment on SMM and the radioheliograph at Culgoora, Australia is presented. It is noted that an enormous loop-shaped transient observed on April 7, 1980, exhibited bright material having whiplike, nonradial motions, as well as moving and stationary radio sources. The data make it possible to establish that a moving type IV radio source was located on or very close to the fast-moving loop. The thermal, kinetic, and magnetic energies in the transient are estimated and, for the first time, compared with the radiative energy of the associated flare.

Wagner, W. J.↗

NASA NTRS2006

Dynamic Response Testing in an Electrically Heated Reactor Test Facility

Non-nuclear testing can be a valuable tool in development of a space nuclear power or propulsion system. In a non-nuclear test bed, electric heaters are used to simulate the heat from nuclear fuel. Standard testing allows one to fully assess thermal, heat transfer, and stress related attributes of a given system, but fails to demonstrate the dynamic response that would be present in an integrated, fueled reactor system. The integration of thermal hydraulic hardware tests with simulated neutronic response provides a bridge between electrically heated testing and full nuclear testing. By implementing a neutronic response model to simulate the dynamic response that would be expected in a fueled reactor system, one can better understand system integration issues, characterize integrated system response times and response characteristics, and assess potential design improvements at a relatively small fiscal investment. Initial system dynamic response testing was demonstrated on the integrated SAFE-100a heat pipe cooled, electrically heated reactor and heat exchanger hardware, utilizing a one-group solution to the point kinetics equations to simulate the expected neutronic response of the system (Bragg-Sitton, 2005). The current paper applies the same testing methodology to a direct drive gas cooled reactor system, demonstrating the applicability of the testing methodology to any reactor type and demonstrating the variation in system response characteristics in different reactor concepts. In each testing application, core power transients were controlled by a point kinetics model with reactivity feedback based on core average temperature; the neutron generation time and the temperature feedback coefficient are provided as model inputs. Although both system designs utilize a fast spectrum reactor, the method of cooling the reactor differs significantly, leading to a variable system response that can be demonstrated and assessed in a non-nuclear test facility.

Bragg-Sitton, Shannon M.↗

NASA NTRS1983

Crystal nucleation in amorphous (Au/100-y/Cu/y/)77Si9Ge14 alloys

Because, unlike most metallic glasses, melt-spun alloys of the series (Au/100-y/Cu/y/)77Si9Ge14 exhibit well separated glass transition and kinetic crystallization temperatures, crystallization can be studied in the fully relaxed amorphous phase. An isothermal calorimetric analysis of the devitrification kinetics of the amorphous alloy indicates sporadic nucleation and a constant growth rate. It is found for the cases of alloys with y values lower than 25 that the classical theory of homogeneous nucleation is consistent with observations, including transient effects. An analysis of the crystallization kinetics shows that slow crystal growth rates play an important role in glass formation in these alloys. Although the reduced glass transition temperature increases with Cu content, glass formation is more difficult at high Cu contents, perhaps because of a difference in nucleus composition.

Thompson, C. V.↗

NASA NTRS1977

Anodic growth of niobium oxide

Report documents experimental examination of growth kinetics of niobium oxide, using current transient technique.

Davidson, M. C.↗

NASA NTRS1994

Galactic oscillations

A stable galaxy, if excited above its ground state, oscillates about that ground state. If it is resonably robust, it can support oscillations of large amplitude. Normal mode oscillations, with surprisingly large amplitudes, have been seen in numerical experiments. Observational evidence shows that real galaxies also oscillate. Galaxies ring like a bell in the experiments, and ringing continues undamped long after initial transients have died out. Their total kinetic energy oscillates with an amplitude as large as 10% of the mean. A fundamental mode dominates. It is homologous expansion/contraction of the entire galaxy (no nodes). Inward or outward velocities due to this mode are sufficiently large in the outer reaches of a galaxy to account for kinematic warps in observed velocity fields. A second spherically symmetrical mode has one node and is important near the center of the galaxy. It may be the driving force behind bulges in spiral galaxies. Two other normal modes have been identified as well. This appears to be the first experimental demonstration of normal mode oscillations within stable galaxy models.

Miller, R. H.↗

NASA NTRS2003

Combustion Of Metals In Reduced Gravity And Extraterrestrial Environments

The recent focus of this research project has been to model the combustion of isolated metal droplets and, in particular, to couple the existing theories and formulations of phenomena such as condensation, reaction kinetics, radiation, and surface reactions to formulate a more complete combustion model. A fully transient, one-dimensional (spherical symmetry) numerical model that uses detailed chemical kinetics, multi-component molecular transport mechanisms, condensation kinetics, and gas phase radiation heat transfer was developed. A coagulation model was used to simulate the particulate formation of MgO. The model was used to simulate the combustion of an Mg droplet in pure O2 and CO2. Methanol droplet combustion is considered as a test case for the solution method for both quasi-steady and fully transient simulations. Although some important processes unique to methanol combustion, such as water absorption at the surface, are not included in the model, the results are in sufficient agreement with the published data. Since the major part of the heat released in combustion of Mg, and in combustion of metals in general, is due to the condensation of the metal oxide, it is very important to capture the condensation processes correctly. Using the modified nucleation theory, an Arrhenius type rate expression is derived to calculate the condensation rate of MgO. This expression can be easily included in the CHEMKIN reaction mechanism format. Although very little property data is available for MgO, the condensation rate expression derived using the existing data is able to capture the condensation of MgO. An appropriate choice of the reference temperature to calculate the rate coefficients allows the model to correctly predict the subsequent heat release and hence the flame temperature.

Abbud-Madrid, A.↗

NASA NTRS1973

An analysis of a charring ablator with thermal nonequilibrium, chemical kinetics, and mass transfer

The differential equations governing the transient response of a one-dimensional ablative thermal protection system are presented for thermal nonequilibrium between the pyrolysis gases and the char layer and with finite rate chemical reactions occurring. The system consists of three layers (the char layer, the uncharred layer, and an optical insulation layer) with concentrated heat sinks at the back surface and between the second and third layers. The equations are solved numerically by using a modified implicit finite difference scheme to obtain solutions for the thickness of the charred and uncharred layers, surface recession and pyrolysis rates, solid temperatures, porosity profiles, and profiles of pyrolysis-gas temperature, pressure, composition, and flow rate. Good agreement is obtained between numerical results and exact solutions for a number of simplified cases. The complete numerical analysis is used to obtain solutions for an ablative system subjected to a constant heating environment. Effects of thermal, chemical, and mass transfer processes are shown.

Clark, R. K.↗

NASA NTRS1992

A model for nonlinear collisionless reconnection

The time dependence and scaling of the electric and magnetic field components during the nonlinear phase of reconnection of 'thin' unstable neutral layers (thickness approximately equals electron gyroradius) are investigated. Solutions to model equations show an initial interval of rapid growth and subsequent large-amplitude fluctuations of the trapped magnetic flux and the ion kinetic energy. The fluctuations arise from the transient formation, merging, and dissolution of magnetic islands.

Lovelace, R. V. E.↗

NASA NTRS1993

Mars aqueous chemistry experiment

The Mars Aqueous Chemistry Experiment (MACE) is designed to conduct a variety of measurements on regolith samples, encompassing mineral phase analyses, chemical interactions with H2O, and physical properties determinations. From these data, much can be learned or inferred regarding the past weathering environment, the contemporaneous soil micro-environments, and the general chemical and physical state of the Martian regolith. By analyzing both soil and duricrust samples, the nature of the latter may become more apparent. Sites may be characterized for comparative purposes and criteria could be set for selection of high priority materials on future sample return missions. Progress for the first year MACE PIDDP is reported in two major areas of effort: (1) fluids handling concepts, definition, and breadboard fabrication and (2) aqueous chemistry ion sensing technology and test facility integration. A fluids handling breadboard was designed, fabricated, and tested at Mars ambient pressure. The breadboard allows fluid manipulation scenarios to be tested under the reduced pressure conditions expected in the Martian atmosphere in order to validate valve operations, orchestrate analysis sequences, investigate sealing integrity, and to demonstrate efficacy of the fluid handling concept. Additional fluid manipulation concepts have also been developed based on updated MESUR spacecraft definition. The Mars Aqueous Chemistry Experiment Ion Selective Electrode (ISE) facility was designed as a test bed to develop a multifunction interface for measurements of chemical ion concentrations in aqueous solution. The interface allows acquisition of real time data concerning the kinetics and heats of salt dissolution, and transient response to calibration and solubility events. An array of ion selective electrodes has been interfaced and preliminary calibration studies performed.

Clark, Benton C.↗

NASA NTRS2005

Heat Pipe Reactor Dynamic Response Tests: SAFE-100 Reactor Core Prototype

The SAFE-I00a test article at the NASA Marshall Space Flight Center was used to simulate a variety of potential reactor transients; the SAFEl00a is a resistively heated, stainless-steel heat-pipe (HP)-reactor core segment, coupled to a gas-flow heat exchanger (HX). For these transients the core power was controlled by a point kinetics model with reactivity feedback based on core average temperature; the neutron generation time and the temperature feedback coefficient are provided as model inputs. This type of non-nuclear test is expected to provide reasonable approximation of reactor transient behavior because reactivity feedback is very simple in a compact fast reactor (simple, negative, and relatively monotonic temperature feedback, caused mostly by thermal expansion) and calculations show there are no significant reactivity effects associated with fluid in the HP (the worth of the entire inventory of Na in the core is .<el, so fluid movement and temperature changes will cause very minor effects). In previous SAFE-100 tests, the point kinetics model was based on core thermal expansion via deflection measurements. It was found that core deflection was a strung function of how the SAFE-100 modules were fabricated and assembled (in terms of straightness, gaps, and other tolerances). To remove the added variable of how this particular core expands as compared to a different concept, it was decided to use a temperature based feedback model (based on several thermocouples placed throughout the core).

Bragg-Sitton, Shannon M.↗

NASA NTRS1988

Optical emission diagnostics of electron cyclotron resonance and glow discharge plasmas for a-Si:H and a-SiC:H film depositions

It is demonstrated that the steady-state and kinetic characteristics of ECR (electron cyclotron resonance) and RF glow discharge plasmas can be readily monitored by OES (optical emission spectroscopy) in real time during a-Si:H and a-SiC:H film depositions using an OMA detection system. The ECR and RF glow discharge plasmas used for a-Si:H and a-SiC:H film depositions were studied by monitoring the emission of SiH(asterisk), H(asterisk), H(asterisk)2, and CH(asterisk) excited states. The OES of the ECR plasma shows a strong emission at 434 nm from H(asterisk), which is not detectable in the glow discharge plasma. Steady-state OES studies have established preliminary correlations between SiH(asterisk) and CH(asterisk) emission intensities and the film deposition rate. Transient OES spectra of SiH4 and CH4 plasmas have shown different kinetics in SiH(asterisk) and CH(asterisk) emission intensities. Transient studies of the SiH(asterisk) emission intensity have indicated that additional mechanisms for producing the SiH(asterisk) species become evident in hydrogen-diluted silane plasmas.

Yang, C. L.↗