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NASA NTRS1994

Structure and Dynamics of the Solar Corona

Advanced computational techniques were used to study solar coronal heating and coronal mass ejections. A three dimensional, time dependent resistive magnetohydrodynamic code was used to study the dynamic response of a model corona to continuous, slow, random magnetic footpoint displacements in the photosphere. Three dimensional numerical simulations of the response of the corona to simple smooth braiding flows in the photosphere were calculated to illustrate and understand the spontaneous formation of current filaments. Two dimensional steady state helmet streamer configurations were obtained by determining the time asymptotic state of the interaction of an initially one dimensinal transponic solar wind with a spherical potential dipole field. The disruption of the steady state helmet streamer configuration was studied as a response to shearing of the magnetic footpoints of the closed field lines under the helmet.

Schnack, D. D.↗

NASA NTRS1994

Peripheral innervation patterns of vestibular nerve afferents in the bullfrog utriculus

Vestibular nerve afferents innervating the bullfrog utriculus differ in their response dynamics and sensitivity to natural stimulation. They also supply hair cells that differ markedly in hair bundle morphology. To examine the peripheral innervation patterns of individual utricular afferents more closely, afferent fibers were labeled by the extracellular injection of horseradish peroxidase (HRP) into the vestibular nerve after sectioning the vestibular nerve medial to Scarpa's ganglion to allow the degeneration of sympathetic and efferent fibers. The peripheral arborizations of individual afferents were then correlated with the diameters of their parent axons, the regions of the macula they innervate, and the number and type of hair cells they supply. The utriculus is divided by the striola, a narrow zone of distinctive morphology, into media and lateral parts. Utiricular afferents were classified as striolar or extrastriolar according to the epithelial entrance of their parent axons and the location of their terminal fields. In general, striolar afferents had thicker parent axons, fewer subepithelial bifurcations, larger terminal fields, and more synaptic endings than afferents in extrstriolar regions. Afferents in a juxtastriolar zone, immediately adjacent to the medial striola, had innervation patterns transitional between those in the striola and more peripheral parts of the medial extrastriola. moast afferents innervated only a single macular zone. The terminal fields of striolar afferents, with the notable exception of a few afferents with thin parent axons, were generally confined to one side of the striola. Hair cells in the bullfrog utriculus have perviously been classified into four types based on hair bundle morphology. Afferents in the extrastriolar and juxtastriolar zones largely or exclusively innervated Type B hair cells, the predominant hair cell type in the utricular macula. Striolar afferents supplied a mixture of four hair cell types, but largely contacted Type B and Type C hair cells, particularly on the outer rows of the medial striola. Afferents supplying more central striolar regions innervated fewer Type B and larger numbers of Type E and Type F hair cells. Striolar afferents with thin parent axons largely supplied Type E hair cells with bulbed kniocilia in the innermost striolar rows.

Baird, Richard A.↗

NASA NTRS1992

Vibrational behavior of adaptive aircraft wing structures modelled as composite thin-walled beams

The vibrational behavior of cantilevered aircraft wings modeled as thin-walled beams and incorporating piezoelectric effects is studied. Based on the converse piezoelectric effect, the system of piezoelectric actuators conveniently located on the wing yield the control of its associated vertical and lateral bending eigenfrequencies. The possibility revealed by this study enabling one to increase adaptively the eigenfrequencies of thin-walled cantilevered beams could play a significant role in the control of the dynamic response and flutter of wing and rotor blade structures.

Song, O.↗

NASA NTRS1995

Development of a composite tailoring procedure for airplane wing

The development of a composite wing box section using a higher order-theory is proposed for accurate and efficient estimation of both static and dynamic responses. The theory includes the effect of through-the-thickness transverse shear deformations which is important in laminated composites and is ignored in the classical approach. The box beam analysis is integrated with an aeroelastic analysis to investigate the effect of composite tailoring using a formal design optimization technique. A hybrid optimization procedure is proposed for addressing both continuous and discrete design variables.

Chattopadhyay, Aditi↗

NASA NTRS1994

Equatorial wave activity derived from fluctuations in observed convection

The spectrum of equatorial wave activity propagating vertically into the stratosphere is calculated from high-resolution imagery of the global convective pattern. Synoptic Global Cloud Imagery (GCI), constructed from six satellites simultaneously observing the earth, is used to diabatically force the linearized primitive equations. Having resolution of 0.5 deg and 3 h, that imagery captures the dominant scales of organized convection, including several harmonics of the diurnal cycle. Its global coverage with high space-time resolution allows the GCI to represent heating variability and dynamical behavior excited by it over a wide range of scales. The dynamical response above the heating is evaluated globally in terms of a space-time spectrum of Hough modes, one which includes planetary-scale Kelvin waves, Rossby waves, and gravity waves down to the resolution of the GCI. The geopotential response, which is indicative of temperature fluctuations observed by satellite, is very red in frequency. Therefore, planetary-scale waves with periods longer than two days dominate the spectrum of geopotential, while high-frequency gravity waves make a comparatively small contribution. Some 80% of the geopotential variance is accounted for by the Kelvin and gravest-symmetric Rossby modes, while the Rossby-gravity mode is comparatively weak. In horizontal eddy motion, the excited wave spectrum is still dominated by planetary-scale components. However, meridional wind fluctuations associated with the Rossby-gravity mode have variance comparable to that of zonal wind fluctuations associated with the Kelvin mode, even though the Rossby-gravity mode is nearly invisible in the geopotential response. Estimates of tropospheric heating lead to amplitudes and propagation characteristics that are broadly consistent with satellite and radiosonde observations of wave activity in the lower stratosphere. The space-time spectrum of EP flux is significantly whiter than the response in either geopotential or motion. Gravity waves of small scale and high frequency carry a large fraction of the upward flux. Although it dominates eastward variance of geopotential and motion, the Kelvin mode carries only about 50% of the eastward EP flux at phase speeds of 20-40 m/s and only 35% of the total eastward flux transmitted to the stratosphere. The remainder is carried by the gravity wave spectrum, which carries nearly all of the westward flux at phase speeds greater than 20 m/s. The gravity wave spectrum also contributes significantly at phase speeds of 10-20 m/s, where only 25% of the flux is accounted for by zonal wavenumbers less than 20. The broad nature of the gravity wave spectrum suggests its absorption at critical levels will be distributed over a deep layer of the middle atmosphere.

Bergman, John W.↗

NASA NTRS1994

Hybrid simulations of whistler waves generation and current closure by a pulsed tether in the ionosphere

The dynamic response of a magnetized collisionless plasma to an externally driven, finite size, sudden switch-on current source across the magnetic field has been studied using a two dimensional hybrid code. It was found that the predominant plasma response was the excitation of whistler waves and the formation of current closure by induced currents in the plasma. The results show that the current closure path consists of: (a) two antiparallel field-aligned current channels at the end of the imposed current sheet; and (b) a cross-field current region connecting these channels. The formation of the current closure path occured in the whistler timescale much shorter than that of MHD and the closure region expanded continuously in time. The current closure process was accompanied by significant energy loss due to whistler radiation.

Chang, C. L.↗

NASA NTRS1995

Characterization of a hot-film probe for hypersonic flow

The critical issues concerning the application of constant-temperature hot-film anemometry to hypersonic flow are reviewed and extended. Mass-flux static calibrations were conducted in a Mach 10 helium flow, while mass-flux and total-temperature static calibrations were made in a Mach 6 air flow. In addition, comparative hot-film/hot-wire turbulence measurements were made in a Mach 11 helium boundary layer to provide insight into the dynamic response of the hot film. The measurements indicate that substrate conduction 'losses' dominate the static response of the hot-film probe, thus resulting in poor sensitivity to mass-flux and total temperature. Furthermore, it has been found that it is not possible to isolate mass-flux fluctuations at high overheat ratios for the current hot-film design. Thus, the sapphire-substrate hot-film anemometer is a robust, high-bandwidth instrument limited to qualitative transition and turbulence measurements. Finally, the extension of this technique to providing quantitative information is dependent upon the development of lower thermal-conductivity substrate materials.

Sheplak, M.↗

NASA NTRS1995

Sea surface topography fields of the tropical Pacific from data assimilation

Time series of maps of monthly tropical Pacific dynamic topography anomalies from 1979 through 1985 were constructed by means of assimilation of the tide gauge and expendable bathythermograph (XBT) data into a linear model driven by observed winds. Estimates of error statistics were calculated and compared to actual differences between hindcasts and observations. Four experiements were performed as follows: one with no assimilation, one with assimiation of sea level anomaly data from eight selected island tide gauge stations, one with assimilation of dynamic height anomalies derived from XBT data, and one with both XBT and tide gauge data assimilated. Data from seven additional tide gauge stations were withheld from the assimilation process and used for verification in all four experiements. Statistical objective maps based on data alone were also constructed for comparison purposes. The dynamic response of the model without assimilation was, in general, weaker than the observed response. Assimilation resulted in enhanced signal amplitude in all three assimilation experiments. With few exceptions the error estimated generated by the Kalman filter appeared quite reasonable. Since the error processes cannot be assumed to be white or stationary, we could find no straightforward way to test the formal statistical hypothesis that the time series of differences between the filter ouput and the actual observations were drawn from a population with statistics given by the Kalman filter estimaes. We found that the best filter we could devise was still short of the goal of producing a white innovation sequence. Extensive changes in the assumed error statistics make only marginal differences. The same is true for long time and space scale behavior of different models with richer physics and finer resolution. Better data assimilation results will probably require relaxation of the assumptions of stationarity and serial independence of the errors. Formation of such detailed noise models will require longer time series, with the attendant problems of matching very different data sets.

Miller, Robert N.↗

NASA NTRS1995

Testing And Analysis Of Rubbing Of Turbine-Blade Tips

Unique combination of established techniques of experimentation and theoretical analysis devised to study stresses induced in turbine blades by intermittent or periodic rubbing of tips of blades in turbine housings. Rubbing is inevitable consequence of modern high-speed, high-efficiency turbine designs, which call for blade-tip clearances as small as possible. Necessary to quantify stresses caused by rubbing in order to predict fatigue lives of blades. Testing apparatus simulates blade-tip rubbing and measures dynamic responses of blades.

Davis, Gary A.↗

NASA NTRS1995

Moving base simulation of an ASTOVL lift-fan aircraft

Using a generalized simulation model, a moving-base simulation of a lift-fan short takeoff/vertical landing fighter aircraft was conducted on the Vertical Motion Simulator at Ames Research Center. Objectives of the experiment were to (1) assess the effects of lift-fan propulsion system design features on aircraft control during transition and vertical flight including integration of lift fan/lift/cruise engine/aerodynamic controls and lift fan/lift/cruise engine dynamic response, (2) evaluate pilot-vehicle interface with the control system and head-up display including control modes for low-speed operational tasks and control mode/display integration, and (3) conduct operational evaluations of this configuration during takeoff, transition, and landing similar to those carried out previously by the Ames team for the mixed-flow, vectored thrust, and augmentor-ejector concepts. Based on results of the simulation, preliminary assessments of acceptable and borderline lift-fan and lift/cruise engine thrust response characteristics were obtained. Maximum pitch, roll, and yaw control power used during transition, hover, and vertical landing were documented. Control and display mode options were assessed for their compatibility with a range of land-based and shipboard operations from takeoff to cruise through transition back to hover and vertical landing. Flying qualities were established for candidate control modes and displays for instrument approaches and vertical landings aboard an LPH assault ship and DD-963 destroyer. Test pilot and engineer teams from the Naval Air Warfare Center, Boeing, Lockheed, McDonnell Douglas, and the British Defence Research Agency participated in the program.

Chung, William W. Y.↗

NASA NTRS1995

A kinematically driven anisotropic viscoelastic constitutive model applied to tires

Aircraft tires are composite structures manufactured with viscoelastic materials such as carbon black filled rubber and nylon cords. When loaded they experience large deflections and moderately large strains. Detailed structural models of tires require the use of either nonlinear shell or nonlinear three dimensional solid finite elements. Computational predictions of the dynamic response of tires must consider the composite viscoelastic material behavior in a realistic fashion. We describe a modification to a nonlinear anisotropic shell finite element so it can be used to model viscoelastic stresses during general deformations. The model is developed by introducing internal variables of the type used to model elastic strain energy. The internal variables are strains, curvatures, and transverse shear angles which are in a one-to-one correspondence with the generalized coordinates used to model the elastic strain energy for nonlinear response. A difference-relaxation equation is used to relate changes in the observable strain field to changes in the internal strain field. The internal stress state is introduced into the equilibrium equations by converting it to nodal loads associated with the element's displacement degrees of freedom. In this form the tangent matrix in the Newton-Raphson solution algorithm is not modified from its form for the nonlinear statics problem. Only the gradient vector is modified and the modification is not computationally costly. The existing finite element model for the Space Shuttle nose gear tire is used to provide examples of the algorithm. In the first example, the tire's rim is displaced at a constant rate up to a fixed value. In the second example, the tire's rim is enforced to follow a saw tooth load and unload curve to generate hysteresis loops.

Johnson, Arthur R.↗

NASA NTRS1996

Inertial Head-Tracker Sensor Fusion by a Complementary Separate-Bias Kalman Filter

Current virtual environment and teleoperator applications are hampered by the need for an accurate, quick-responding head-tracking system with a large working volume. Gyroscopic orientation sensors can overcome problems with jitter, latency, interference, line-of-sight obscurations, and limited range, but suffer from slow drift. Gravimetric inclinometers can detect attitude without drifting, but are slow and sensitive to transverse accelerations. This paper describes the design of a Kalman filter to integrate the data from these two types of sensors in order to achieve the excellent dynamic response of an inertial system without drift, and without the acceleration sensitivity of inclinometers.

Foxlin, Eric↗

NASA NTRS1995

Gain properties of doped GaAs/AlGaAs multiple quantum well avalanche photodiode structures

A comprehensive characterization has been made of the static and dynamical response of conventional and multiple quantum well (MQW) avalanche photodiodes (APDs). Comparison of the gain characteristics at low voltages between the MQW and conventional APDs show a direct experimental confirmation of a structure-induced carrier multiplication due to interband impact ionization. Similar studies of the bias dependence of the excess noise characteristics show that the low-voltage gain is primarily due to electron ionization in the MQW-APDS, and to both electron and hole ionization in the conventional APDS. For the doped MQW APDS, the average gain per stage was calculated by comparing gain data with carrier profile measurements, and was found to vary from 1.03 at low bias to 1.09 near avalanche breakdown.

Menkara, H. M.↗

NASA NTRS1996

Inertial head-tracker sensor fusion by a complementary separate-bias Kalman filter

Current virtual environment and teleoperator applications are hampered by the need for an accurate, quick responding head-tracking system with a large working volume. Gyroscopic orientation sensors can overcome problems with jitter, latency, interference, line-of-sight obscurations, and limited range, but suffer from slow drift. Gravimetric inclinometers can detect attitude without drifting, but are slow and sensitive to transverse accelerations. This paper describes the design of a Kalman filter to integrate the data from these two types of sensors in order to achieve the excellent dynamic response of an inertial system without drift, and without the acceleration sensitivity of inclinometers.

Foxlin, Eric↗

NASA NTRS1996

Control of Thermal Deflection, Panel Flutter and Acoustic Fatigue at Elevated Temperatures Using Shape Memory Alloys

The High Speed Civil Transport (HSCT) will have to be designed to withstand high aerodynamic load at supersonic speeds (panel flutter) and high acoustic load (acoustic or sonic fatigue) due to fluctuating boundary layer or jet engine acoustic pressure. The thermal deflection of the skin panels will also alter the vehicle's configuration, thus it may affect the aerodynamic characteristics of the vehicle and lead to poor performance. Shape memory alloys (SMA) have an unique ability to recover large strains completely when the alloy is heated above the characteristic transformation (austenite finish T(sub f)) temperature. The recovery stress and elastic modulus are both temperature dependent, and the recovery stress also depends on the initial strain. An innovative concept is to utilize the recovery stress by embedding the initially strained SMA wire in a graphite/epoxy composite laminated panel. The SMA wires are thus restrained and large inplane forces are induced in the panel at elevated temeperatures. By embedding SMA in composite panel, the panel becomes much stiffer at elevated temperatures. That is because the large tensile inplane forces induced in the panel from the SMA recovery stress. A stiffer panel would certainly yield smaller dynamic responses.

Mei, Chuh↗

NASA NTRS1996

Fiber Optic Sensors for Smart Materials and Structures

In this paper we describe recently developed fiber sensors which are capable of monitoring the health of smart-structures. The unobstrusive geometry of these sensors make them an excellent choice for embedding the sensor in composite materials to measure internal states of strain in structures and materials. Some of these sensors have gage lengths that can be tailored from tens of microns to many meters. We will describe various demodulation schemes (Pseudo-Heterodyne, Synthetic-Heterodyne, Homodyne, Differential-Cross Multiplier, and Single Channel Phase-Tracker) to obtain high bandwidth measurements, enabling measurement of static to high frequency impact generated strains with a dynamic response exceeding tens of thousands of microstrains. In addition, we will show that we can tailor the fiber sensor to either measure only strain and reject temperature response or measure only the temperature, or measure both temperature and strain simultaneously. We will also demonstrate the ability to measure multiple strain components inside a host simultaneously using a single fiber sensor embedded in the host using a certain sensor type and transverse strain immunity using another sensor type. Additionally we will show the ability to measure temperature up to 100 C using fiber optic sensors.

Singh, H.↗

NASA NTRS1996

Kinematic/Dynamic Characteristics for Visual and Kinesthetic Virtual Environments

Work was carried out on two topics of principal importance to current progress in virtual environment research at NASA Ames and elsewhere. The first topic was directed at maximizing the temporal dynamic response of visually presented Virtual Environments (VEs) through reorganization and optimization of system hardware and software. The final results of this portion of the work was a VE system in the Advanced Display and Spatial Perception Laboratory at NASA Ames capable of updating at 60 Hz (the maximum hardware refresh rate) with latencies approaching 30 msec. In the course of achieving this system performance, specialized hardware and software tools for measurement of VE latency and analytic models correlating update rate and latency for different system configurations were developed. The second area of activity was the preliminary development and analysis of a novel kinematic architecture for three Degree Of Freedom (DOF) haptic interfaces--devices that provide force feedback for manipulative interaction with virtual and remote environments. An invention disclosure was filed on this work and a patent application is being pursued by NASA Ames. Activities in these two areas are expanded upon below.

Bortolussi, Michael R.↗

NASA NTRS1993

Interannual Sea Level Variations in the Tropical Indian Ocean from Geosat and Shallow Water Simulations

Sea level variations of the Indian Ocean north of 20 deg S are analyzed from Geosat satellite altimeter data over April 1985-September 1989. These variations are compared and interpreted with numerical simulations derived from a reduced gravity model forced by FSU observed winds over the same period. After decomposition into complex empirical orthogonal functions, the low-frequency anomalies are described by the first two modes for observations as well as for simulations. The sums of the two modes contain 34% and 40% of the observed and simulated variances, respectively. Averaged over the basin, the observed and simulated sea level changes are correlated by 0.92 over 1985-1988. The strongest change happens during the El Ninio 1986-1987: between winter 1986 and summer 1987 the basin-averaged sea level rises by approx. 1 cm. These low-frequency variations can partly be explained by changes in the Sverdrup circulation. The southern tropical Indian Ocean between 1O deg and 20 deg S is the domain where those changes are strongest: the averaged sea level rises by approx. 4.5 cm between winter 1986 and winter 1987. There, the signal propagates southwestward across the basin at a speed similar to free Rossby waves. Sensitivity of observed anomalies is examined over 1987-1988, with different orbit ephemeris, tropospheric corrections, and error reduction processes. The uncertainty of the basin-averaged sea level estimates is mostly due to the way the orbit error is reduced and reaches approx. 1 cm. Nonetheless, spatial correlation is good between the various observations and better than between observations and simulations. Sensitivity of simulated anomalies to the wind uncertainty, examined with Former Soviet Union (FSU) and European Center for Medium-Range Weather Forecasting (ECMWF) forcings over 1985-1988, shows that the variance of the simulations driven by ECMWF is 52% smaller, as FSU winds are stronger than ECMWF. Results show that the wind strength also affects the dynamic response of the ocean: anomalies propagate westward across the basin more than twice as fast with FSU than with ECMWF. It is found that the discrepancy is larger between ECMWF and FSU simulations than between observations and FSU simulations.

Perigaud, Claire↗