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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 829 records · Page 46

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NASA NTRS1980

System design for a million channel digital spectrum analyzer /MCSA/

The system design of a wideband (8 MHz) million-channel digital spectrum analyzer for use with a SETI receiver is presented. The analyzer makes use of a digital bandpass filter bank for transforming the wideband input signal into a specified number (120) of uniform narrowband output channels by the use of a Fourier transform digital processor combined with a prototype digital weighting network (finite impulse response filter). The output is then processed separately by 120 microprocessor-based discrete Fourier transform computers, each producing 8190 output channels of approximately 8 Hz bandwidth by the use of an 8190-point prime factor algorithm.

Peterson, A.↗

NASA NTRS1981

Steady-state magma discharge at Etna 1971-81

Throughout the past decade Mount Etna has been in almost continuous activity and even during periods of repose incandescent lava has often been visible in at least one of the summit vents. Using observations by Italian, British and French volcanological teams, the volumes of lava produced by each eruption from 1971 to July 1981 have been estimated. The computed output of magma for this period approximates to a rate of 0.7 cu m/s. This is compared with the output rate estimates for Etna's historic past. The steady-state nature of the output during the past decade has implications for the interpretation of the volcano's internal plumbing and the petrology of its lavas, and the assumption that this state will be maintained allows a discussion of the timing and magnitude of future eruptions.

Wadge, G.↗

NASA NTRS1980

Analytical simulation of nonlinear satellite links

It is noted that performance predictions for nonlinear digital satellite channels are usually obtained by either purely analytical methods or Monte Carlo simulations. When used with other than very simple channel models, however, they become extremely costly. It is shown that, through analytical modeling of the effect of the nonlinearity on the uplink waveform, it is possible to obtain approximate performance predictions with a minimum of computer time. It is also demonstrated that the results agree quite well with Monte Carlo simulations and that significant trade-offs can be performed with this tool. An analytical simulation based on Blachman's (1968) definition of signal x signal, signal x noise, and noise x noise terms is shown to provide the required flexibility for modeling the effect of signal distortions on the link performance and to produce good results over a wide range of nonlinearities and channel conditions.

Braun, W. R.↗

NASA NTRS1982

A vectorized, finite-volume, adaptive-grid algorithm for Navier-Stokes calculations

An adaptive grid, finite-volume method has been used to solve the Navier-Stokes equations for complete (forebody and afterbody) flowfields around blunt bodies. The code, which is applicable for axisymmetric or two-dimensional flows, allows the mesh to adjust during the computation to provide a closer spacing of mesh points in regions of high gradients, thus minimizing the number of required computational points. The solution technique is explicit, utilizing a maximum time-step advancement at each grid point to accelerate convergence to the steady state. The code has been fully vectorized for efficient solution on the CYBER 203 computer. A very flexible rezoning routine is used to concentrate mesh points anywhere in the field, either by a user-defined weighting function or by allowing high gradient regions to adjust the grid. The grid adjustment routine is implicit in nature and represents a very small portion of the total computational cost. Currently, the code runs in approximately 0.000016 seconds per grid point per iteration.

Gnoffo, P. A.↗

NASA NTRS1982

The delta-four-stream approximation for radiative flux transfer

An approximate technique for solving the radiative transfer problem for fluxes reflected, transmitted, and absorbed by a homogeneous scattering layer is presented. The technique is a straightforward delta-function modification of a solution to the transfer equation in closed form via the discrete-ordinates method with four streams. By the use of four streams, significant improvement in accuracy is obtained for anisotropic phase functions over that obtained by similarly employed two-stream approximations with delta-functions. However, the computational effort is increased only slightly. Equations are developed for the general case of a layer underlaid by a reflecting Lambert surface. Graphical comparisons are given of fractional error resulting from the use of this method with that resulting from the use of typical four-stream and delta-two-stream techniques.

Cuzzi, J. N.↗

NASA NTRS1982

Optical alignment measurements at Goddard Space Flight Center

The significant changes that have been made in the Optical Alignment Facility at the Goddard Space Flight Center since it was last described (Appler and Howell, 1968) are discussed. The changes are reflected in both the hardware and the software involved in making alignment measurements. The data analysis procedures can now be done on a desktop computer with turn-around times of approximately 4 h. Arbitrary coordinate transformations and corrections for facility errors are also feasible while retaining absolute accuracies of less than 6 sec of arc.

Eichhorn, W. L.↗

NASA NTRS1983

Introductory aerothermodynamics of advanced space transportation systems

An assessment is made of the severity of the physical phenomena affecting the aerothermodynamics of advanced space transportation system hypervelocity flights at low ambient density, and flight domains corresponding to these phenomena are mapped. The development of advanced computational codes that will be needed when approximate conceptual studies have defined advantageous configurations is discussed. The advanced space transportation systems considered are an aeroassisted orbital transfer vehicle and a small, rapid response maneuverable craft which is launched either from earth or a conventional aircraft, assumes near-earth orbit, and finally reenters with lift and cross range capability to land on an airstrip.

Howe, J. T.↗

NASA NTRS1982

Modelling of TGS growth in space

Attention is called to the necessity of programming the temperature downward in order to maintain a constant growth rate in the absence of convection. Trial and error numerical computations are performed to find the first approximation for the required isothermal dissolution period, linear ramp, and polynomial temperature variation period. The linear ramp rate is limited by the specified maximum temperature gradient. The isothermal dissolution turns out to be unnecessary. The linear ramp must be stopped before the polynomial period is begun in order to avoid overshooting the specified maximum growth rate. After a few hours, the temperature profile approaches steady-state behavior.

Liu, L. C.↗

NASA NTRS1984

Generalized wake geometry for a helicopter in forward flight and effect of wake deformation on airloads

An analytical investigation was conducted to develop a first level generalization of the predicted distorted wake geometry of a helicopter rotor in level steady forward flight and to demonstrate the influence of wake deformation on the prediction of rotor airloads. Distortions of the tip vortex relative to the classical undistorted geometry are generalized in terms of vortex age, blade azimuth, advance ratio, thrust coefficient, and number of blades for a representative rotor. A computer module and charts were developed for approximating wake geometry and identifying wake boundaries and locations of blade-vortex passage. Predicted H-34 airloads for several inflow/wake models are compared with data for selected flight conditions. The occurrence of close blade-vortex passages and the high sensitivity of predicted airloads to small deviations of blade to tip vortex distance demonstrate the requirement for improved blade-vortex interaction models. The airload correlations indicate that refined distorted wake methodology has the potential to provide more accurate airload prediction.

Egolf, T. A.↗

NASA NTRS1985

Thermally induced stresses and deformations in layered composite tubes

The thermally induced stresses and deformations in layered, orthotropic tubes are studied. The motivation for studying tubes is their likely application for use in space structures. Tubes are a strong candidate for this application because of their high structural efficiency, as measured by stiffness per unit weight, and their relative ease of fabrication. Also, tubes have no free edges to deteriorate or delaminate. An anticipated thermal condition for tubes in space is a circumferential temperature gradient. This type of gradient will introduce dimensional changes into the structure and may cause stresses large enough to cause damage to the material. There are potentially large differences in temperatures at different circumferential locations on the tube. Because of this, the effects of temperature dependent material properties on the stresses and deformations may be important. The study is composed of three parts: experiments to determine the functional form of the circumferential gradient and to measure tube deflections; an elasticity solution to compute the stresses and deformations; and an approximate approach to determine the effects of temperature dependent material properties.

Cooper, D. E.↗

NASA NTRS1984

STAEBL: Structural tailoring of engine blades, phase 2

The Structural Tailoring of Engine Blades (STAEBL) program was initiated at NASA Lewis Research Center in 1980 to introduce optimal structural tailoring into the design process for aircraft gas turbine engine blades. The standard procedure for blade design is highly iterative with the engineer directly providing most of the decisions that control the design process. The goal of the STAEBL program has been to develop an automated approach to generate structurally optimal blade designs. The program has evolved as a three-phase effort with the developmental work being performed contractually by Pratt & Whitney Aircraft. Phase 1 was intended as a proof of concept in which two fan blades were structurally tailored to meet a full set of structural design constraints while minimizing DOC+I (direct operating cost plus interest) for a representative aircraft. This phase was successfully completed and was reported in reference 1 and 2. Phase 2 has recently been completed and is the basis for this discussion. During this phase, three tasks were accomplished: (1) a nonproprietary structural tailoring computer code was developed; (2) a dedicated approximate finite-element analysis was developed; and (3) an approximate large-deflection analysis was developed to assess local foreign object damage. Phase 3 is just beginning and is designed to incorporated aerodynamic analyses directly into the structural tailoring system in order to relax current geometric constraints.

Hirschbein, M. S.↗

NASA NTRS1987

Adaptive momentum management for large space structures

Momentum management is discussed for a Large Space Structure (LSS) with the structure selected configuration being the Initial Orbital Configuration (IOC) of the dual keel space station. The external forces considered were gravity gradient and aerodynamic torques. The goal of the momentum management scheme developed is to remove the bias components of the external torques and center the cyclic components of the stored angular momentum. The scheme investigated is adaptive to uncertainties of the inertia tensor and requires only approximate knowledge of principle moments of inertia. Computational requirements are minimal and should present no implementation problem in a flight type computer and the method proposed is shown to be effective in the presence of attitude control bandwidths as low as .01 radian/sec.

Hahn, E.↗

NASA NTRS1985

Evaluation of fault-tolerant system performance by approximate techniques

An approximate method for calculating the statistics of the performance of a fault-tolerant system is developed. An approximate method is necessary because the statistical model of the system behavior is large-scale and the time horizon of interest encompasses many cycles of the Redundancy Management logic. In the development, a compact representation of the necessary information called the v-transform is introduced and discussed. Based upon this representation, an approximation that leads to a very efficient computational procedure is suggested and numerically analyzed. A very brief discussion of other related work is also presented.

Walker, B. K.↗

NASA NTRS1987

Adaptive momentum management for the dual keel Space Station

The report discusses momentum management for a large space structure with the structure selected configuration being the Initial Orbital Configuration of the dual-keel Space Station. The external torques considered were gravity gradient and aerodynamic torques. The goal of the momentum management scheme developed is to remove the bias components of the external torques and center the cyclic components of the stored angular momentum. The scheme investigated is adaptive to uncertainties of the inertia tensor and requires only approximate knowledge of principal moments of inertia. Computational requirements are minimal and should present no implementation problem in a flight-type computer. The method proposed is shown to be effective in the presence of attitude control bandwidths as low as 0.01 radian/sec.

Hopkins, M.↗

NASA NTRS1988

A diagonally inverted LU implicit multigrid scheme

A new Diagonally Inverted LU Implicit scheme is developed within the framework of the multigrid method for the 3-D unsteady Euler equations. The matrix systems that are to be inverted in the LU scheme are treated by local diagonalizing transformations that decouple them into systems of scalar equations. Unlike the Diagonalized ADI method, the time accuracy of the LU scheme is not reduced since the diagonalization procedure does not destroy time conservation. Even more importantly, this diagonalization significantly reduces the computational effort required to solve the LU approximation and therefore transforms it into a more efficient method of numerically solving the 3-D Euler equations.

Yokota, Jeffrey W.↗

NASA NTRS1988

A diagonally inverted LU implicit multigrid scheme

A new Diagonally Inverted LU Implicit scheme is developed within the framework of the multigrid method for the three-dimensional unsteady Euler equations. The matrix systems that are to be inverted in the LU scheme are treated by local diagonalizing transformations that decouple them into systems of scalar equations. Unlike the Diagonalized ADI method, the time accuracy of the LU scheme is not reduced since the diagonalizing procedure does not destroy time conservation. Even more importantly, this diagonalization significantly reduces the computational effort required to solve the LU approximation and therefore transforms it into a more efficient method of numerically solving the three-dimensional Euler equations.

Yokota, J. W.↗

NASA NTRS1989

Thickening oscillation of a delta wing using Navier-Stokes and Navier-displacement equations

The problem of unsteady, supersonic, locally conical, vortical flow around a delta wing undergoing thickening oscillation is solved using the unsteady, thin-layer Navier-Stokes equations and the unsteady linearized Navier-displacement equations. The unsteady, thin-layer Navier-Stokes equations are solved using the implicit approximate-factorization finite-volume scheme to compute the conservative components of the flow vector field. With the conservative components known at any time step, the linearized, Navier-displacement equations are solved using the alternating, direction-implicit scheme to obtain the grid points displacements due to known displacement boundary conditions. A grid-displacements limiter, in the form of a low mesh Reynolds number, is used to limit grid-folding in regions of highly reversed flow.

Chuang, Hsin-Kung A.↗

NASA NTRS1989

Two-target game model of an air combat with fire-and-forget all-aspect missiles

An air combat duel between similar aggressive fighter aircraft, both equipped with the same type of guided missiles, is formulated as a two-target differential game using the dynamic model of the game of two identical cars. Each of the identical target sets represents the effective firing envelope of an all-aspect fire-and-forget air-to-air missile. The firing range limits depend on the target aspect angle and are approximated by analytical functions. The maximum range, computed by taking into account the optimal missile avoidance maneuver of the target, determines the no-escape firing envelope. The solution consists of the decomposition of the game space into four regions: the respective winning zones of the two opponents, the draw zone, and the region where the game terminates by a mutual kill. The solution provides a new insight for future air combat analysis.

Davidovitz, A.↗