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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

A Complementary Note to 'A Lag-1 Smoother Approach to System-Error Estimation': The Intrinsic Limitations of Residual Diagnostics

Recently, this author studied an approach to the estimation of system error based on combining observation residuals derived from a sequential filter and fixed lag-1 smoother. While extending the methodology to a variational formulation, experimenting with simple models and making sure consistency was found between the sequential and variational formulations, the limitations of the residual-based approach came clearly to the surface. This note uses the sequential assimilation application to simple nonlinear dynamics to highlight the issue. Only when some of the underlying error statistics are assumed known is it possible to estimate the unknown component. In general, when considerable uncertainties exist in the underlying statistics as a whole, attempts to obtain separate estimates of the various error covariances are bound to lead to misrepresentation of errors. The conclusions are particularly relevant to present-day attempts to estimate observation-error correlations from observation residual statistics. A brief illustration of the issue is also provided by comparing estimates of error correlations derived from a quasi-operational assimilation system and a corresponding Observing System Simulation Experiments framework.

Fixed-lag smoothing↗
NASA NTRS1994

(abstract) Optimal Low Thrust Trajectories Using Differential Inclusion Concepts

Low thrust propulsion systems typically have their greatest benefit for high energy missions or missions with large post-launch maneuver requirements. Missions which have been examined include main belt asteroid rendezvous, comet rendezvous, outer planet and Mercury orbiters, Pluto flyby, and solar probe missions. Low thrust mission design software used to determine these trajectories is based on two distinct formulations of the optimal control problem: the indirect and direct methods. The traditional approach (indirect) is to use the calculus of variations to obtain first order necessary conditions on the states and costates. In contrast, direct methods are conceptually different in that no explicit integration takes place. A direct method based on differential inclusion concepts has been developed and successfully used to compute low thrust trajectories. This new approach removes explicit control dependence from the problem thereby reducing the dimension of the parameter space for the nonlinear programming problem. Also when compared to other direct methods, fewer nonlinear constraints are required to represent the dynamics of the problem.

low↗
NASA NTRSDate not supplied

Variational Coupled Loads Analysis using the Hybrid Parametric Variation Method

Time-domain coupled loads analysis (CLA)is used to determine the response of a launch vehicle and payload system to transient forces, such as liftoff, engine ignitions and shutdowns, jettison events, and atmospheric flight loads, such as buffet. CLA, using Hurty/Craig-Bampton (HCB)component models, is the accepted method for the establishment of design-level loads for launch systems. However, uncertainty in the component models flows into uncertainty in predicted system results. Uncertainty in the structural responses during launch is a significant concern because small variations in launch vehicle and payload mode shapes and their interactions can result in significant variations in system loads. Uncertainty quantification (UQ)is used to determine statistical bounds on prediction accuracy based on model uncertainty. In this paper uncertainty is treated at the HCB component-model level. In an effort to account for model uncertainties and statistically bound their effect on CLA predictions, this work combines CLA with UQ in a process termed variational coupled loads analysis (VCLA). The modeling of uncertainty using a parametric approach, in which input parameters are represented by random variables, is common, but its major drawback is the resulting uncertainty is limited to the form of the nominal model. Uncertainty in model form is one of the biggest contributors to uncertainty in complex built-up structures. Model-form uncertainty can be represented using a nonparametric approach based on random matrix theory (RMT). In this work, UQ is performed using the hybrid parametric variation (HPV)method, which combines parametric with nonparametric uncertainty at the HCB component model level. The HPV method requires the selection of dispersion values for the HCB fixed-interface (FI)eigenvalues, and the HCB mass and stiffness matrices. The dispersions are based upon component test-analysis modal correlation results. During VCLA, random component models are assembled into an ensemble of random systems using a Monte Carlo (MC)approach. CLA is applied to each of the ensemble members to produce an ensemble of system-level responses for statistical analysis. The proposed methodology is demonstrated through its application to a buffet loads analysis of NASA’s Space Launch System (SLS)during the transonic regime fifty seconds after liftoff. Core stage (CS)section shears and moments are recovered, and statistics are computed.

Uncertainty Quantification↗
NASA NTRS2002

Computational Relativistic Astrophysics Using the Flow Field-Dependent Variation Theory

We present our method for solving general relativistic nonideal hydrodynamics. Relativistic effects become pronounced in such cases as jet formation from black hole magnetized accretion disks which may lead to the study of gamma-ray bursts. Nonideal flows are present where radiation, magnetic forces, viscosities, and turbulence play an important role. Our concern in this paper is to reexamine existing numerical simulation tools as to the accuracy and efficiency of computations and introduce a new approach known as the flow field-dependent variation (FDV) method. The main feature of the FDV method consists of accommodating discontinuities of shock waves and high gradients of flow variables such as occur in turbulence and unstable motions. In this paper, the physics involved in the solution of relativistic hydrodynamics and solution strategies of the FDV theory are elaborated. The general relativistic astrophysical flow and shock solver (GRAFSS) is introduced, and some simple example problems for computational relativistic astrophysics (CRA) are demonstrated.

Richardson, G. A.↗
NASA NTRS1974

Data and results from a study of internal convective cooling systems for hypersonic aircraft

An extensive survey of current and future airframe construction materials and coolants was conducted, so that the most promising candidates could be examined for cooled-panel, cooling-system and airframe concepts. Consideration was given to over 100 structural materials, 50 coolants, 6 classes of structural panel concepts, 4 classes of thermal panel concepts with numerous variations, and 3 overall cooled airframe design approaches, including unshielded, shielded, and dual temperature types. The concept identification and parametric comparison phase examined all major elements of the convectively cooled airframe, including the differing requirements at various locations on the aircraft. The parametric results were used for the investigation to two separate vehicles, a hypersonic transport with a length of 96 meters (314 feet) and a weight of 24,000 kg (528,600 lb) and a hypersonic research airplane, with a length of 25m (80 ft) and a weight of 20,300 kg (447,000 lb).

Anthony, F. M.↗
NASA NTRS1976

Ruby laser induced emission from NO2

Two different types of emission from excited NO2 were observed using pulsed ruby laser light at 6943 A. The first type of fluorescence was seen in the near IR and results from the single photon excitation of NO2 from the ground 2-A1 state. By observing the emission as a function of time an unexpected behavior was observed in the near IR and could be explained by a consecutive deactivation mechanism, wherein a secondary species is preferentially detected. A second type of emission recently observed in the blue spectral region is weaker and is due to a multiphoton process. The intensity of the blue emission is a function of the cube of the laser intensity at low pressures and approaches the square at high pressures. This variation is attributed to simultaneous deactivation of the excited NO2 intermediate by collision (square) and by anti-Stokes Raman scattering off of the excited NO2 (cube).

Hakala, D. F.↗
NASA NTRS1985

Aerosol pattern correlation techniques of wind measurement

This paper reviews the current status of lidar image correlation techniques of remote wind measurement. It also examines the potential use of satellite borne lidar global wind measurements using this approach. Lidar systems can easily detect spatial variations in the volume scattering cross section of naturally occurring aerosols. Lidar derived RHI, PPI and range-time displays of aerosol backscatter have been extensively employed in the study of atmospheric structure. Descriptions of this type of data can be obtained in many references including Kunkel et al. (1977), Kunkel et al. (1980), Boers et al. (1984), Uthe et al. (1980), Melfi et al. (1985) and Browell et al. (1983). It is likely that the first space-borne lidars for atmospheric studies will observe aerosol backscatter to measure parameters such as boundary layer depth and cloud height. This paper examines the potential application of these relatively simple aerosol backscatter lidars to global wind measurements.

Eloranta, Edwin W.↗
NASA NTRS1957

Comparison of effect of a turbojet engine and three cold-flow configurations on the stability of a full-scale supersonicle inlet

Increasing the volume and length of the duct behind the inlet affected the inlet stability at Mach 2.0 and zero angle of attack. Close approximation of the inlet stability limit of the J34 engine-inlet configuration was obtained by a cold-pipe configuration having a length and volume approaching that measured to the engine turbine. Variation of these parameters had a small effect on the minimum subcritical stable mass flow below a cowl-lip-position parameter of 44 degrees and appeared to have a negligible effect on the inlet pressure-recovery - mass-flow curve. Initial buzz frequency and minimum cowl-lip-position parameter for complete buzz-free operation varied with configuration.

Musial, Norman T↗
NASA NTRS1993

A new Lagrangian method for three-dimensional steady supersonic flows

In this report, the new Lagrangian method introduced by Loh and Hui is extended for three-dimensional, steady supersonic flow computation. The derivation of the conservation form and the solution of the local Riemann solver using the Godunov and the high-resolution TVD (total variation diminished) scheme is presented. This new approach is accurate and robust, capable of handling complicated geometry and interactions between discontinuous waves. Test problems show that the extended Lagrangian method retains all the advantages of the two-dimensional method (e.g., crisp resolution of a slip-surface (contact discontinuity) and automatic grid generation). In this report, we also suggest a novel three dimensional Riemann problem in which interesting and intricate flow features are present.

Loh, Ching-Yuen↗
NASA NTRS1994

Flexural rigidity of the Basin and Range-Colorado Plateau-Rocky Mountain transition from coherence analysis of gravity and topography

Stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography. However, the spatial resolution of coherence analysis has been limited by use of two-dimensional periodogram spectra from very large (greater than 10(exp 5)sq km) windows that generally include multiple tectonic features. Using a two-dimensional spectral estimator based on the maximum entropy method, the spatial resolution of flexural proerties can be enhanced by a factor of 4 or more, enabling more detailed analysis at the scale of individual tectonic features. This new approach is used to map the spatial variation of flexural rigidity along the Basin and Range transition to the Colorado Plateau and Middle Rocky Mountains physiographic provinces. Large variations in flexural isostatic responses are found, with rigidities ranging from as low as 8.7 x 10(exp 20) N m (elastic thickness (T(sub e) = 4.6 km) in the Basin and Range to as high as 4.1 x 10(exp 24) N m T(sub e) = 77 km) in the Middle Rocky Mountains. These results compare favorably woith independent determinations of flexural rigidity in the region. Areas of low flexural rigidity correlate strongly with areas of high surface heat flow, as is expected from the contingence of flexural rigidity on a temperature-dependent flow law. Also, late Cenozoic normal faults with large displacements are found primarily in area of low flexural rigidity region. The highest flexural rigidity is found within the Archean Wyoming craton, where evidence suggests that deeply rooted cratonic lithosphere may play a role in determining the distribution of tectonism at the surface.

Lowery, Anthony R.↗
NASA NTRS1995

Active Control of Fan Noise-Feasibility Study. Volume 2: Canceling Noise Source-Design of an Acoustic Plate Radiator Using Piezoceramic Actuators

The feasibility of using acoustic plate radiators powered by piezoceramic thin sheets as canceling sources for active control of aircraft engine fan noise is demonstrated. Analytical and numerical models of actuated beams and plates are developed and validated. An optimization study is performed to identify the optimum combination of design parameters that maximizes the plate volume velocity for a given resonance frequency. Fifteen plates with various plate and actuator sizes, thicknesses, and bonding layers were fabricated and tested using results from the optimization study. A maximum equivalent piston displacement of 0.39 mm was achieved with the optimized plate samples tested with only one actuator powered, corresponding to a plate deflection at the center of over 1 millimeter. This is very close to the deflection required for a full size engine application and represents a 160-fold improvement over previous work. Experimental results further show that performance is limited by the critical stress of the piezoceramic actuator and bonding layer rather than by the maximum moment available from the actuator. Design enhancements are described in detail that will lead to a flight-worthy acoustic plate radiator by minimizing actuator tensile stresses and reducing nonlinear effects. Finally, several adaptive tuning methods designed to increase the bandwidth of acoustic plate radiators are analyzed including passive, active, and semi-active approaches. The back chamber pressurization and volume variation methods are investigated experimentally and shown to be simple and effective ways to obtain substantial control over the resonance frequency of a plate radiator. This study shows that piezoceramic-based plate radiators can be a viable acoustic source for active control of aircraft engine fan noise.

Pla, F. G.↗
NASA NTRS1995

Viscous aerodynamic design using the adjoint variable approach

The use of classical optimal control methods, in particular variational methods, to solve the airfoil optimization problem, by deriving a set of adjoint (costate) equations and boundary conditions has already been done for inviscid (potential and Euler flows) and two dimensional, steady state, incompressible flow governed by the Navier-Stokes equations. The interior and boundary terms of the volume integral have been derived (in this work) for the steady Navier-Stokes equations in three dimensions for a viscous, compressible heat conducting fluid. This can be used to derive the adjoint equations and numerical boundary conditions for general classes of problems and hence paves the way for a solution to the aerodynamic optimization problem for compressible viscous flows. The next steps to the realization of that goal are projected as below. The usual square integral pressure functional as an objective function is being replaced by a more realistic drag functional subject to a lift constraint. The feasibility of attempting the more difficult time dependent problem is being investigated. It remains to get the full system of adjoint equations and boundary conditions with the new functional. The state and adjoint equations must be discretized and coded. An appropriate optimization program must be used (steepest descents seems inadequate) and various known airfoil shapes should be recovered in test cases of the computer program.

DeRise, George↗
NASA NTRS1996

Computational Modeling of Multi-Phase/Multi-Species Flows with Applications to Liquid Rocket Engines

Accurate prediction of all physical phenomena in a combustion chamber is essential for better understanding of the system performance. Atomization, evaporation, combustion, chemical kinetics, and turbulence are those processes of great importance that need to be well understood. Processes involving the liquid phase in a combustion chamber will be further complicated under supercritical conditions. More advanced and accurate numerical techniques are required to extend our understanding of the above phenomena. A computer program for multi-species/multi-phase flow was developed for NASA/MSFC in 1992. This code, called Liquid Thrust Chamber Performance (LTCP) program takes an Eulerian- Eulerian approach and is based on the Total Variation Diminishing (TVD) technique with Lax-Friedrichs upwind method. Under the NASA/ASEE SFFP the LTCP code was used to predict the performance characteristics of several engines that were of particular interest to NASA. This code was also successful in a combustion detonation study. Converting the program to the PC platform was accomplished which extends usability and makes it available to a wider range of users. The Eulerian formulation of the liquid phase provides a suitable model that can be extended to include combustion modeling under supercritical conditions. The results have been compared against the ones of other codes and available measured data. The algorithm proved to be robust and efficient for problems with stiff source terms.

Navaz, Homayun K.↗
NASA NTRS2003

Simulation of Local Blood Flow in Human Brain under Altered Gravity

In addition to the altered gravitational forces, specific shapes and connections of arteries in the brain vary in the human population (Cebral et al., 2000; Ferrandez et al., 2002). Considering the geometric variations, pulsatile unsteadiness, and moving walls, computational approach in analyzing altered blood circulation will offer an economical alternative to experiments. This paper presents a computational approach for modeling the local blood flow through the human brain under altered gravity. This computational approach has been verified through steady and unsteady experimental measurements and then applied to the unsteady blood flows through a carotid bifurcation model and an idealized Circle of Willis (COW) configuration under altered gravity conditions.

Kim, Chang Sung↗
NASA NTRS2004

Analysis of Orientation-dependence of Martian Gullies

The recent discovery of small Martian gullies has stimulated debate about the role that water plays on the Martian surface under current or recent conditions. Of critical importance in evaluating various gully hypotheses is reliable morphometric and orientation data. The former centers on such questions as whether the water (or another fluid) emanated from a surface or sub-surface source and the duration of flow. The latter ties into whether solar insolation has an important effect on formation of the initial water source and subsequent mobilization. Initial studies of gullies indicated a poleward orientation dependence, an observation which has recently been challenged. Herein we investigate the orientation of Martian gullies and the dependence of various parameters on the orientation. Whereas previous studies have been global or through most of the southern hemisphere, we focus on several specific regions. This approach offers some advantages in that regional variations are factored out, such that of lithology, ground water table depth (if any), surface thermal properties, and other parameters are more or less the same in a given region. Differences in gully attributes as a function of orientation within a region can more easily be attributable to solar insolation effects than is the case for global statistics. We use the orientation to constrain several classes of gully formation hypotheses. 1) A favored orientation toward the pole across all regions could indicate a process dominated by melting of cold trapped ice, snow, or condensed volatiles from incident sunlight during summer under current conditions. 2) Variations among all regions would be more consistent with mechanisms less strongly tied to current solar insolation, such as geothermal heating of ice. 3) Favored orientations within specific regions, but differing among regions, could indicate a preference for poleward ices and melting, with orientation being a function of age and dependent on variations in obliquity and precision. We find that the gullies fall into either categories 2 or 3, but not 1, indicating the recent melting of cold trapped condensates is unlikely the sole formation mechanism.

Mohan, S.↗
NASA NTRS2004

Induced Power of the Helicopter Rotor

A simplified rotor model was used to explore fundamental behavior of lifting rotor induced power at moderate and high advance ratios. Several rotor inflow theories, including dynamic inflow theory and prescribed-wake vortex theory, together with idealized notional airfoil stall models were employed. A number of unusual results were encountered at high advance ratios including trim control reversal and multiple trim solutions. Significant increases in rotor induced power (torque) above the ideal minimum were observed for moderately high advance ratio. Very high induced power was observed near and above unity advance ratio. The results were sensitive to the stall characteristics of the airfoil models used. An equivalent wing analysis was developed to determine induced power from Prandtl lifting line theory and help interpret the rotor induced power behavior in terms of the spanwise airload distribution. The equivalent wing approach was successful in capturing the principal variations of induced power for different configurations and operating conditions. The effects blade root cutout were found to have a significant effect on rotor trim and induced power at high advance ratios.

Ormiston, Robert A.↗