Lewis Research Center Research Articles

Creeping to a Critical Point

W hen the space shuttle Columbia touched down at NASA's Kennedy Space Center in Orlando, Fla. on March 9, it returned a remarkable instrument to Earth. Designed to monitor laser light scattered by a dense, compressed gas teetering on the brink of turning into a liquid, this precision apparatus had operated continuously in space for more than 14 days. During this time, researchers had relayed dozens of instructions to the equipment, controlling the temperature of an ultrapure, high-pressure sample of xenon to millionths of a degree. By taking advantage of a setting in which the effects of gravity do not obscure details of a material's activity, they could bring the xenon sample excruciatingly close to its critical temperature-the point at which its liquid and gas phases coexist and blend into one. Robert W Gammon of the Institute for Physical Science and Technology at the University of Maryland in College Park and head of the research team dubbed this project the Zeno experiment in honor of the philosopher of ancient Greece who pondered the paradox of traveling a finite distance in steps that become vanishingly small. The recent shuttle experiment represented the culmination of years of work by a large group of scientists, students, engineers, and technicians at the University of Maryland, NASA's Lewis Research Center in Cleveland, Ball Aerospace in Boulder, Colo., and several other organizations. No other microgravity instrument has logged as many hours as the Zeno experiment, says R. Allen Wilkinson of the space experiments division at Lewis. It's gone through two launches and two landings, and it's gone through hundreds of hours of operation in orbit and more than 10,000 hours of testing on the ground. That's an impressive reliability record, he insists. The data provided by this instrument brought researchers closer to a fundamental understanding of what happens when materials change from one phase to another, whether from gas to liquid, from ordinary conductor of electricity to superconductor, or from nonmagnet to magnet. In particular, Gammon, project scientist Jeffrey N. Shaumeyer of Maryland, and their team observed with unprecedented clarity xenon's behavior as the gas hovered within microkelvins of its critical temperature of 289.72 kelvins, or about 16.7?C.

Selection of papers from the fourteenth space photovoltaic research & technology (SPRAT XIV) conference held in October 1995 at the Lewis Research Center, Cleveland, OH, USA

Selection of papers from the fourteenth space photovoltaic research & technology (SPRAT XIV) conference held in October 1995 at the Lewis Research Center, Cleveland, OH, USA

Characteristics of surface roughness associated with leading-edge ice accretion

Detailed size measurements of surface roughness associated with leading edge ice accretions are presented to provide information on characteristics of roughness and trends of roughness development with various icing parameters. Data was obtained from icing tests conducted in the Icing Research Tunnel (IRT) at NASA Lewis Research Center (LeRC) using a NACA 0012 airfoil. Measurements include diameters, heights, and spacing of roughness elements along with chordwise icing limits. Results confirm the existence of smooth and rough ice zones and that the boundary between the two zones (surface roughness transition region) moves upstream towards stagnation region with time. The height of roughness grows as the air temperature and the liquid water content increase, however, the airspeed has little effect on the roughness height. Results also show that the roughness in the surface roughness transition region grows during a very early stage of accretion but reaches a critical height and then remains fairly constant. Results also indicate that a uniformly distributed roughness model is only valid at a very initial stage of the ice accretion process.

ATM networks and their applications at the NASA Lewis Research Center A case study

This paper provides an overview of ATM networks and a discussion of their role in supporting high-level technological research at the NASA Lewis Research Center in the U.S. NASA Lewis operates a variety of local and wide area networks and is introducing ATM to meet specialized requirements for transporting multimedia traffic, including text, voice, video, real time visualization data, and data collected from scientific experiments. ATM employs fast packet switching and statistical multiplexing to allow many sources to flexibly share network bandwidth and it provides an integrated transport, multiplexing, and switching technology for BISDN. SONET/SDH may be used as a physical layer protocol for optical transmission of ATM cells. ATM, together with SONET, provides standardized high-speed transmission for a wide variety of traffic types and performance requirements. NASA Lewis researchers are experimenting with cluster computing for solving large problems and are testing ATM for use as the transport fabric to support this activity. Video experiments have been conducted over a private ATM network which has links to the Ames and Langley Research Centers. NASA Lewis is exploring the use of ATM to facilitate collaboration between experimental and analytical researchers at geographically dispersed locations and to provide personnel at remote sites with access to NASA Lewis facilities. ATM service classes, the BISDN protocol reference model, and congestion control issues are also addressed in this paper.

Analysis of the effects of surface pitting and wear on the vibration of a gear transmission system

A comprehensive procedure to simulate and analyse the vibrations in a gear transmission system with surface pitting, wear, and partial tooth fracture of the gear teeth is presented. An analytical model was developed where the effects of surface pitting and wear of the gear tooth were simulated by phase and magnitude changes in the gear mesh stiffness. Changes in the gear mesh stiffness were incorporated into each gear-shaft model during the global dynamic simulation of the system. The overall dynamics of the system were evaluated by solving for the transient dynamics of each shaft system simultaneously with the vibration of the gearbox structure. In order to reduce the number of degrees-of-freedom in the system, a modal synthesis procedure was used in the global transient dynamic analysis of the overall transmission ststem. An FFT procedure was used to transform the averaged time signal into the frequency domain for signature analysis. In addition, the Wigner-Ville distribution was also introduced to examine the gear vibration in the joint time-frequency domain for vibration pattern recognition. Experimental results obtained from a gear fatigue test rig at NASA Lewis Research Center were used to evaluate the analytical model.

Microstructural studies of burner-rig thermally fatigued SiC continuous fibre/Si3N4 composite

Ceramic materials a r e a promising candidate for 1400high-temperature structural applications because of their high melting point, high-temperature strength, stiffness, oxidation resistance, and low density. o However, their intrinsic brittleness limits their widespread applications. The incorporation of strong and stiff fibres into a ceramic matrix imparts damage & tolerance capability to the material. The application of continuous fibre ceramic composites (CFCCs) in critical components, such as jet-engine turbine blades and tubular products for heat exchangers, involves thermal or thermomechanical cycling. However, thermal or thermomechanical fatigue studies, especially those simulating the £omponent's cyclic service environment, are scarce. The thermal and thermomechanical fatigue research of CFCCs has been limited to glass-based composites subjected to the photon or induction hcating and thermal cycling (500-1100°C) [1-3]. The cycling lacks the rapid heating and corrosion effects of jet-fuel flame impingement. It is thus inadequate to simulate the complex interaction of the thermomechanical loading, erosive and corrosive environment of a gas turbine engine. To our knowledge, the present study is the first report of burner-rig thermal fatigue behaviour of CFCCs under impinged jet-fuel flame. The [(0/90)5/O]s SCS-9 TM SiC fibre (30 vol%)/ Si3N4 cross-ply laminated composite was fabricated using hot-pressing at 1650 °C and 17.2 MPa for 1 h by Textron Specialty Materials, Lowell, MA, USA. The matrix composition was 92.5wt% Si3N4, 5.0 wt% Y203, 1.5 wt% MgO, and 1.0 wt% A1203. The composite was machined to 12.7 mm x 3.175 mm x 152.4 mm rectangular bars. The burner-rig thermal fatigue tests under constant applied stress were conducted using the Mach 0.3 atmospheric pressure durability test station at NASA 2oo Lewis Research Center, Cleveland, OH, USA. The ~" flame was produced using a "can-type" combustion ~150 chamber. The time to runout was arbitrarily selected 100. as 1000 cycles due to the operating cost of the test facility. The constant tensile stresses of 16.8, 84.0, -~ o B 50 110.0, 125.0, 140.0, and 168.0 MPa were applied to ~. the specimens during thermal cycling. The cyclic 0 temperature profile of 60 s 1350 °C ftame impingement, followed by a 60 s passive, ambient temperature cooling, is shown in Fig. 1. In order to determine the failure mechanism of the

Hydroxyl radical chemiluminescence imaging and the structure of microgravity droplet flames

A procedure is outlined that uses hydroxyl (OH) radical chemiluminescence measurements along with detailed numerical modeling to determine flame position and gain further insight into the structure of microgravity droplet flames. To validate this procedure, microgravity n-heptane and methanol droplet combustion experiments were conducted using the 2.2-second drop tower and the ero Gravity Facility at NASA Lewis Research Center. The spontaneous emission from electronically excited hydroxyl radicals (OHo) within the envelope diffusion flame was measured with a UV-sensitive video camera. The OHo emission data was deconvoluted using an inverse Abel transform to determine the time evolution of the location of peak intensity within the flame. Chemical reactions describing the production, emission, and quenching of OHo were incorporated into a transient, spherically symmetric droplet combustion model. The modeling and experimental results indicate differences in the route of OHo production between n-heptane and methanol flames. For n-heptane, the production of OHo emission-intensity profiles which agree well with experiment (both in terms of shape and location of maximum intensity) and are shifted from the position of maximum ground-state OH concentration. For methanol flames, which produce very little CH, the OHo appears to be the result of thermal excitation within the flame rather than from a specific chemiluminescent reaction. In both cases, the location of maximum OHo emission intensity is very near the location of maximum flame temperature, suggesting that OHo imaging is a good approach for measurement of the flame position.

Soot-field structure in laminar soot-emitting microgravity nonpremixed flames

Due to the intrinsic interest in laminar flame regimes that cannot be attained in normal gravity (1 g), the microgravity environment (0 g) offers an attractive and promising perspective to enhance our understanding of soot formation mechanisms in diffusion flames. An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to define the soot-field structure within 0-g laminar jet nonpremixed flames which operate above their smoke point. Parallel work on earthgravity flames is also presented to facilitate comparisons and define the effect of gravity on the soot fields. The experiment considers jet diffusion flames of nitrogen-diluted acetylene burning in quiescent air at atmospheric pressure. A full-field laser extinction technique is utilized to determine the transient soot spatial distributions in axisymmetric flames with fixed flow conditions corresponding to Rc=O(100). Results are presented for a nonsmoking normal-gravity flame and its nonbuoyant counterpart which releases soot from its blunt tip. Quantitative measurements on the transient postignition character of the soot field in 0 g are presented for the first time and indicate that millimeter-diameter burners result in brief soot-field transients in microgravity. At a fuel flow rate which is nearly twice that at the smoke point in 0 g, the soot field after the initial transient period sustains its annular structure throughout the luminous nonbuoyant-flamezone. The maximum soot volume fraction measured at 0 g is nearly double that at 1 g for the same flow rate, thus attesting to the higher sooting tendency of nonbuoyant flames. The results indicate that the prolonged residence times under microgravity promote soot growth more than oxidation, thus producing more soot in 0 g. Side-by-side comparisons of the soot distributions in 0-g and 1-g flames demonstrate the increased spatial resolution afforded in microgravity flames.

Microgravity combustion of methanol and methanol/water droplets: Drop tower experiments and model predictions

To experimentally validate single and bicomponent droplet combustion models, microgravity methanol and methanol/water droplet combustion experiments were conducted in the 2.2-s drop tower facility at NASA Lewis Research Center. The experiments were then simulated using a transient, bicomponent droplet combustion model developed earlier. Tests were performed in oxidizing environments of 18%–35% O2/N2 with initial liquid water contents of 0–20%. Instantaneous droplet diameter measurements were made using back-lit, high-speed photography. The instantaneous flame position was determined by monitoring the chemiluminescence from electronically excited hydroxyl radicals (OH*). Analysis of the flame and droplet diameter data yielded burning rates and flame standoff ratios for a wide array of methanol and methanol/water droplet combustion conditions. For initially pure methanol droplets, the available burn time (≈1.5 s) was not, in general, sufficient to observe extinction or significant nonlinearity in the regression of diameter squared with respect to time. For each oxygen content, the numerical model predicted the burning rate to within 10% and the flame position to within one normalized diameter without any independent parameter adjustment. In the case of 10% and 20% initial water content, substantial nonlinearity in diameter squared was observed. The numerical model, which accurately accounts for changes in liquid transport properties caused by variable liquid water content, did not predict the nonlinearity to the extent that it was observed. A possible explanation is that with the addition of initial water content to the droplet, the internal mass and thermal transport is enhanced as a result of increased internal mixing. The sources of this internal mixing are likely multifold but are not due to relative gas/liquid convection effects at the droplet surface.

Merits and limitations of optimality criteria method for structural optimization

AbstractThe merits and limitations of the Optimality Criteria (OC) method for the minimum weight design of structures subjected to multiple load conditions under stress, displacement and frequency constraints were investigated by examining several numerical examples. The examples were solved utilizing the OC design code that was developed for this purpose at the NASA Lewis Research Center. This OC code incorporates OC methods available in the literature with generalizations for stress constraints, fully utilized design concepts, and hybrid methods that combine both techniques. It includes multiple choices for Lagrange multiplier and design variable update methods, design strategies for several constraint types, variable linking, displacement and integrated force method analysers, and analytical and numerical sensitivities. On the basis of the examples solved, the optimality criteria for general application were found to be satisfactory for problems with few active constraints or with small numbers of design variables. However, the OC method without stress constraints converged to optimum even for large structural systems. For problems with large numbers of behaviour constraints and design variables, the method appears to follow a subset of active constraints that can result in a heavier design. The computational efficiency of OC methods appears to be similar to some mathematical programming techniques.

Quantitative species measurements in microgravity flames with near-IR diode lasers

Absolute concentrations of water vapor are measured in microgravity (µ-g), nonpremixed methane, and propane jet flames with diode-laser wavelength modulation spectroscopy. These experiments are performed in the 2.2-s µ-g drop facility at the NASA Lewis Research Center. Abel inversion methods are used to determine time-dependent radial profiles from eight line-of-sight projections across the flames. At all measured heights above the nozzle, water vapor spatial distributions in µ-g flames are much wider than their 1-g counterparts. Radial growth of the water signal continues throughout the drop, verifying earlier suggestions that a steady state is not reached during the duration of the test, despite a quasi-steady flame shape. Large amounts of water vapor are observed at larger radii, at odds with visual (video) observations and numerical predictions.

Field tests of a high‐temperature fiber optic lever microphone

The operating principle of a high‐temperature fiber optic lever microphone (FOLM), rated at 538 °C (1000 °F) continuous service, is based on intensity modulation of light reflected from a vibrating membrane. The frequency response of the microphone, calibrated in the laboratory at ambient temperature and 538 °C by the electrostatic actuator method, is typically 60–70 kHz. Applications to high‐temperature field testing include the thermal acoustic fatigue apparatus at Langley Research Center, an advanced short takeoff and vertical landing model at Lewis Research Center, the Mach 22 helium tunnel at Langley (ambient temperature only), interior mixer‐ejector surfaces at GE Aircraft Engines, Cincinnati, and a high temperature acoustic impedance tube at Langley. In most of these applications the FOLM made possible high‐temperature, high‐frequency acoustic data to be taken for the first time. Typical test results will be discussed.

Geometry and flow influences on jet mixing in a cylindrical duct

Geometry and flow influences on jet mixing in a cylindrical duct

3-D Navier-Stokes analysis of crossing glancing shocks/turbulent boundary layer interactions

Three-dimensional viscous flow analysis is performed for a configuration where two crossing and glancing shocks interact with a turbulent boundary layer. A time marching 3-D full Navier—Stokes code, called PARC3D, is used to compute the flow field and the solution is compared to the experimental data obtained at the NASA Lewis Research Center 1 ft × 1 ft supersonic wind tunnel facility. The study is carried out as part of the continuing code assessment program in support of the Generic Hypersonic Research at NASA Lewis. Detailed comparison of static pressure fields and oil flow patterns is made with the corresponding solution on the wall containing the shock/boundary layer interaction in an effort to validate the code for hypersonic inlet applications.

Space power system passes first test

The first successful test of a complete, fully integrated solar dynamic space power system has been carried out in a space simulation vacuum chamber at the National Aeronautics & Space Administration's Lewis Research Center in Cleveland. The 25-foot-diameter concentrator (rear) made up of seven hexagonal reflective panels covered with an aluminum-silicon dioxide coating focuses simulated sunlight onto a receiver (tilted canister at lower left), heating a working gas (a helium-xenon mixture). The gas powers a turboalternator/compressor (foil-wrapped apparatus next to receiver), generating 2 kW of electricity. A salt storage material in the receiver (lithium fluoride-calcium fluoride mixture) provides thermal energy storage, absorbing and releasing energy by melting and freezing to ensure continuous production of electricity through sun/darkness cycles. Being developed under a joint U.S.-Russian program, the solar dynamic system is slated for first use in space on Russian space station Mir in September 1997...

Residual stress measurements in carbon steel : Heyman, J.S.; Namkung, M. Analytical Ultrasonics in Materials Research and Testing. Proceedings of a conference, NASA Lewis Research Center, Ohio (United States), 13–14 Nov. 1984. pp. 61–74. NASA Conference Publication 2383 (1986)

External dc magnetic field-induced changes in natural velocity of Rayleigh surface waves were measured in steel specimens under various stress conditions. The low field slopes of curves representing the fractional changes of natural velocity were proved to provide correct stress information in steels with different metallurgical properties. The slopes of curves under uniaxial compression, exceeding about one third of the yield stress, fell below zero in all the specimens when magnetized along the stress axis. The slopes under tension varied among different steels but remained positive in any circumstances. The stress effect was observed for both applied and residual stress. A physical interpretation of these results is given based on the stress-induced domain structure changes and the delta epsilon effect. Most importantly, it is found that the influence of detailed metallurgical properties cause only secondary effects on the obtained stress information.

Soft Computing in Design and Manufacturing of Advanced Materials

The goal of this paper is to show the potential of fuzzy sets and neural networks, often referred to as soft computing, for aiding in all aspects of manufacturing of advanced materials like ceramics. In design and manufacturing of advanced materials it is desirable to find which of the many processing variables contribute most to the desired properties of the material. There is also interest in real-time quality control of parameters that govern material properties during processing stages. This paper briefly introduces the concepts of fuzzy sets and neural networks and shows how they can be used in the design and manufacturing processes. These two computational methods are alternatives to other methods such as the Taguchi method. The two methods are demonstrated by using data collected at NASA Lewis Research Center. Future research directions are also discussed.

Design optimization of a spacer structure for Space Station Freedom

Structure optimization techniques, which have matured over the last three decades, have been applied successfully to generate a preliminary minimum weight design of a Space Station Freedom component. The design code ‘COMETBOARDS’ which was developed by the Structural Mechanics Branch of NASA Lewis Research Center, was utilized. The component, termed a ‘short spacer truss’ of the Space Station Freedom, was modeled as a space frame, and its minimum-weight design was obtained under prescribed stress, buckling, crippling, displacement, and frequency constraints. The optimum design weight was found to be lighter than that obtained manually. The optimum design also appears to be attractive from manufacturing considerations. A description of the structural component, its design environment, behavior limitations, and the optimum-design process are briefly presented in this paper, along with a brief description of the design code COMETBOARDS that was developed for this purpose.

Applications of fibre optic networks in high technology research

The NASA Lewis Research Center has active programmes in fluid dynamics and solid dynamics. These require a communications network capable of transporting multimedia traffic, including data, voice, interactive and non–interactive video, real–time visualisation, and data gathering from scientific experiments. The use of powerful desktop workstations, which operate as standalone devices, work cooperatively in local clusters, operate in client server mode, access central computers, and address remote sites, also impacts on network requirements. This paper provides an overview of FDDI and SONET networks and investigates their roles in supporting high–level technological research. FDDI's topology, reliability, traffic classes, data encoding, token ring operation, timers, network management issues, and candidate applications are discussed. The multiplexing hierarchy, optical signal format, OAM capability, survivability, and candidate applications of SONET networks are explored. Interoperability issues, with the SDH international standard, ATM packet switching, and BISDN networks, are also addressed.

The Transition to Turbulence of Microgravity Gas Jet Diffusion Flames

Abstract This paper discusses results obtained in a study of the transition to turbulence of gas jet diffusion flames in microgravity. The experiments were conducted at the 2.2-second Drop Tower of the NASA Lewis Research Center. Propane flames burning in quiescent air at atmospheric pressure were investigated. Significant differences have been observed between the transition processes of gas jet diffusion flames in microgravity and normal gravity These include the appearance of large scale structures in microgravity which are manifested at the flame base and are convected downstream. These structures appear intermittently at first, and, then, as Reynolds number is increased, a continuous train of structures is observed along the flame front. The experimental observations are explained in terms of flow transition and instability of the shear layer associated with the injection of the jet flow into quiescent surroundings