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NASA NTRS1993

A numerical study of confined turbulent jets

A numerical investigation is reported of turbulent incompressible jets confined in two ducts, one cylindrical and the other conical with a 5 degree divergence. In each case, three Craya-Curtet numbers are considered which correspond, respectively, to flow situations with no moderate and strong recirculation. Turbulence closure is achieved by using the k-epsilon model and a recently proposed realizable Reynolds stress algebraic equation model that relates the Reynolds stresses explicitly to the quadratic terms of the mean velocity gradients and ensures the positiveness of each component of the turbulent kinetic energy. Calculations are carried out with a finite-volume procedure using boundary-fitted curvilinear coordinates. A second-order accurate, bounded convection scheme and sufficiently fine grids are used to prevent the solutions from being contaminated by numerical diffusion. The calculated results are compared extensively with the available experimental data. It is shown that the numerical methods presented are capable of capturing the essential flow features observed in the experiments and that the realizable Reynolds stress algebraic equation model performs much better than the k-epsilon model for this class of flows of great practical importance.

Zhu, J.↗

NASA NTRS1993

A Discrete Probability Function Method for the Equation of Radiative Transfer

A discrete probability function (DPF) method for the equation of radiative transfer is derived. The DPF is defined as the integral of the probability density function (PDF) over a discrete interval. The derivation allows the evaluation of the PDF of intensities leaving desired radiation paths including turbulence-radiation interactions without the use of computer intensive stochastic methods. The DPF method has a distinct advantage over conventional PDF methods since the creation of a partial differential equation from the equation of transfer is avoided. Further, convergence of all moments of intensity is guaranteed at the basic level of simulation unlike the stochastic method where the number of realizations for convergence of higher order moments increases rapidly. The DPF method is described for a representative path with approximately integral-length scale-sized spatial discretization. The results show good agreement with measurements in a propylene/air flame except for the effects of intermittency resulting from highly correlated realizations. The method can be extended to the treatment of spatial correlations as described in the Appendix. However, information regarding spatial correlations in turbulent flames is needed prior to the execution of this extension.

Sivathanu, Y. R.↗

NASA NTRS1992

MIMO system identification using frequency response data

A solution to the problem of obtaining a multi-input, multi-output statespace model of a system from its individual input/output frequency responses is presented. The Residue Identification Algorithm (RID) identifies the system poles from a transfer function model of the determinant of the frequency response data matrix. Next, the residue matrices of the modes are computed guaranteeing that each input/output frequency response is fitted in the least squares sense. Finally, a realization of the system is computed. Results of the application of RID to experimental frequency responses of a large space structure ground test facility are presented and compared to those obtained via the Eigensystem Realization Algorithm.

Medina, Enrique A.↗

NASA NTRS1993

Linear system identification via backward-time observer models

This paper presents an algorithm to identify a state-space model of a linear system using a backward-time approach. The procedure consists of three basic steps. First, the Markov parameters of a backward-time observer are computed from experimental input-output data. Second, the backward-time observer Markov parameters are decomposed to obtain the backward-time system Markov parameters (backward-time pulse response samples) from which a backward-time state-space model is realized using the Eigensystem Realization Algorithm. Third, the obtained backward-time state space model is converted to the usual forward-time representation. Stochastic properties of this approach will be discussed. Experimental results are given to illustrate when and to what extent this concept works.

Juang, Jer-Nan↗

NASA NTRS1921

A New Method of Testing in Wind Tunnels

Now, in existing wind tunnels, using a horsepower of 100 to 300, the models are generally made to a 1/10 scale and the speed is appreciably lower than the speeds currently attained by airplanes. The Reynolds number realized is thus 15 to 25 times smaller than that reached by airplanes in free flight, while the ratio of speed to the velocity of sound is between a third and three quarters of the true ratio. The necessary increases in either the diameter of the wind tunnel or the velocity of the airstream are too costly. However, the author shows that it is possible to have wind tunnels in which the Reynolds number will be greater than that now obtained by airplanes, and in which the ratio of the velocity to the velocity of sound will also be greater than that realized in practice, by employing a gas other than air, at a pressure and temperature different from those of the surrounding atmosphere. The gas is carbonic acid, a gas having a low coefficient of viscosity, high density, and a low ratio of specific heat. The positive results of using carbonic acid in wind tunnel tests are given.

Margoulis, W↗

NASA NTRS1924

The Effect of Slipstream Obstructions on Air Propellers

The screw propeller on airplanes is usually placed near other objects, and hence its performance may be modified by them. Results of tests on propellers free from slip stream obstructions, both fore and aft, are therefore subject to correction, for the effect of such obstructions and the purpose of the investigation was to determine the effect upon the thrust and torque coefficients and efficiency, for previously tested air propellers, of obstructions placed in the slip stream, it being realized that such previous tests had been conducted under somewhat ideal conditions that are impracticable of realization in flight. Simple geometrical forms were used for the initial investigation. Such forms offered the advantage of easy, exact reproduction at another time or in other laboratories, and it was believed that the effects of obstructions usually encountered might be deduced or surmise from those chosen.

Lesley, E P↗

NASA NTRS1952

Automatic control systems satisfying certain general criterions on transient behavior

An analytic method for the design of automatic controls is developed that starts from certain arbitrary criterions on the behavior of the controlled system and gives those physically realizable equations that the control system can follow in order to realize this behavior. The criterions used are developed in the form of certain time integrals. General results are shown for systems of second order and of any number of degrees of freedom. Detailed examples for several cases in the control of a turbojet engine are presented.

Boksenbom, Aaron S↗

NASA NTRS1990

Pass transistor implementations of multivalued logic

A simple straight-forward Karnaugh map logic design procedure for realization of multiple-valued logic circuits is presented in this paper. Pass transistor logic gates are used to realize multiple-valued networks. This work is an extension of pass transistor implementations for binary-valued logic.

Maki, G.↗

NASA NTRS1990

Reliable VLSI sequential controllers

A VLSI architecture for synchronous sequential controllers is presented that has attractive qualities for producing reliable circuits. In these circuits, one hardware implementation can realize any flow table with a maximum of 2(exp n) internal states and m inputs. Also all design equations are identical. A real time fault detection means is presented along with a strategy for verifying the correctness of the checking hardware. This self check feature can be employed with no increase in hardware. The architecture can be modified to achieve fail safe designs. With no increase in hardware, an adaptable circuit can be realized that allows replacement of faulty transitions with fault free transitions.

Whitaker, S.↗

NASA NTRS1990

Auto-programmable impulse neural circuits

Impulse neural networks use pulse trains to communicate neuron activation levels. Impulse neural circuits emulate natural neurons at a more detailed level than that typically employed by contemporary neural network implementation methods. An impulse neural circuit which realizes short term memory dynamics is presented. The operation of that circuit is then characterized in terms of pulse frequency modulated signals. Both fixed and programmable synapse circuits for realizing long term memory are also described. The implementation of a simple and useful unsupervised learning law is then presented. The implementation of a differential Hebbian learning rule for a specific mean-frequency signal interpretation is shown to have a straightforward implementation using digital combinational logic with a variation of a previously developed programmable synapse circuit. This circuit is expected to be exploited for simple and straightforward implementation of future auto-adaptive neural circuits.

Watula, D.↗

NASA NTRS1991

Low cost high efficiency GaAs monolithic RF module for SARSAT distress beacons

Low cost high performance (5 Watts output) 406 MHz beacons are urgently needed to realize the maximum utilization of the Search and Rescue Satellite-Aided Tracking (SARSAT) system spearheaded in the U.S. by NASA. Although current technology can produce beacons meeting the output power requirement, power consumption is high due to the low efficiency of available transmitters. Field performance is currently unsatisfactory due to the lack of safe and reliable high density batteries capable of operation at -40 C. Low cost production is also a crucial but elusive requirement for the ultimate wide scale utilization of this system. Microwave Monolithics Incorporated (MMInc.) has proposed to make both the technical and cost goals for the SARSAT beacon attainable by developing a monolithic GaAs chip set for the RF module. This chip set consists of a high efficiency power amplifier and a bi-phase modulator. In addition to implementing the RF module in Monolithic Microwave Integrated Circuit (MMIC) form to minimize ultimate production costs, the power amplifier has a power-added efficiency nearly twice that attained with current commercial technology. A distress beacon built using this RF module chip set will be significantly smaller in size and lighter in weight due to a smaller battery requirement, since the 406 MHz signal source and the digital controller have far lower power consumption compared to the 5 watt power amplifier. All the program tasks have been successfully completed. The GaAs MMIC RF module chip set has been designed to be compatible with the present 406 MHz signal source and digital controller. A complete high performance low cost SARSAT beacon can be realized with only additional minor iteration and systems integration.

Petersen, W. C.↗

NASA NTRS1993

Computation of confined coflow jets with three turbulence models

A numerical study of confined jets in a cylindrical duct is carried out to examine the performance of two recently proposed turbulence models: an RNG-based K-epsilon model and a realizable Reynolds stress algebraic equation model. The former is of the same form as the standard K-epsilon model but has different model coefficients. The latter uses an explicit quadratic stress-strain relationship to model the turbulent stresses and is capable of ensuring the positivity of each turbulent normal stress. The flow considered involves recirculation with unfixed separation and reattachment points and severe adverse pressure gradients, thereby providing a valuable test of the predictive capability of the models for complex flows. Calculations are performed with a finite-volume procedure. Numerical credibility of the solutions is ensured by using second-order accurate differencing schemes and sufficiently fine grids. Calculations with the standard K-epsilon model are also made for comparison. Detailed comparisons with experiments show that the realizable Reynolds stress algebraic equation model consistently works better than does the standard K-epsilon model in capturing the essential flow features, while the RNG-based K-epsilon model does not seem to give improvements over the standard K-epsilon model under the flow conditions considered.

Zhu, J.↗

NASA NTRS1991

Pulse mode VLSI asynchronous circuits

A new basic VLSI circuit element is presented that can be used to realize pulse mode asynchronous sequential circuits. A synthesis procedure is developed along with an unconventional state assignment procedure. Level input asynchronous sequential circuits can be realized by converting a regular flow table into a differential mode flow table, thereby allowing the new synthesis technique to be general. The new circuits tolerate 1-1 crossovers. This circuit also provides a means for state sequence detection and real time fault detection.

Chen, Q.↗

NASA NTRS1994

Advanced technology for a satellite multichannel demultiplexer/demodulator

Satellite on-board processing is needed to efficiently service multiple users while at the same time minimizing earth station complexity. The processing satellite receives a wideband uplink at 30 GHz and down-converts it to a suitable intermediate frequency. A multichannel demultiplexer then separates the composite signal into discrete channels. Each channel is then demodulated by bulk demodulators, with the baseband signals routed to the downlink processor for retransmission to the receiving earth stations. This type of processing circumvents many of the difficulties associated with traditional bent-pipe repeater satellites. Uplink signal distortion and interference are not retransmitted on the downlink. Downlink power can be allocated in accordance with user needs, independent of uplink transmissions. This allows the uplink users to employ different data rates as well as different modulation and coding schemes. In addition, all downlink users have a common frequency standard and symbol clock on the satellite, which is useful for network synchronization in time division multiple access schemes. The purpose of this program is to demonstrate the concept of an optically implemented multichannel demultiplexer (MCD). A proof-of-concept (POC) model has been developed which has the ability to receive a 40 MHz wide composite signal consisting of up to 1000 40 kHz QPSK modulated channels and perform the demultiplexing process. In addition a set of special test equipment (STE) has been configured to evaluate the performance of the POC model. The optical MCD is realized as an acousto-optic spectrum analyzer utilizing the capability of Bragg cells to perform the required channelization. These Bragg cells receive an optical input from a laser source and an RF input (the signal). The Bragg interaction causes optical output diffractions at angles proportional to the RF input frequency. These discrete diffractions are optically detected and output to individual demodulators for baseband conversion. Optimization of the MCD design was conducted in order to achieve a compromise between two opposing sources of signal degradation: adjacent channel interference and intersymbol interference. The system was also optimized to allow simple, inexpensive ground stations communications with the MCD. These design goals led to the realization of a POC MCD which demonstrates the demultiplexing function with minimal signal degradation. Performance evaluation results using the STE equipment indicate that the dynamic range of the demultiplexer in the presence of adjacent and multiple channel loading is 40 - 50 dB. Measured bit error rate (BER) probabilities varied from the predicted theoretical results by one dB or less. The performance of the proof-of-concept model indicate that the development of a space qualified optically implemented MCD are feasible. The advantages to such an implementation include reduced size, weight and power and increased reliability when compared with electronic approaches. All of these factors are critical to on-board satellite processors. Further optimization can be conducted which trade ground station complexity and MCD performance to achieve desired system results.

Abramovitz, Irwin J.↗

NASA NTRS1993

Appendix A: Proposed statement of work, 1994

This NRA effort is devoted to developing new techniques and methodologies which utilize and/or provide support to Integrated Vehicle Health Management (IVHM) concepts and techniques, modern design processes, and open architectures to realize an avionics system architecture that relieves the flight control system (FCS) of the requirement of maintaining intimate knowledge and control of the vehicle subsystems (for instance, the reaction control system (RCS)). The benefit of this architecture is that future upgrades and enhancements to the system(s) or to individual components within the system(s) are greatly simplified. This approach also allows a much more straightforward treatment of failure analysis, system diagnosis, and the design of fault containment domains. This NRA effort is also devoted to realizing capabilities to provide an available avionics system (and subsystem(s)) at minimum operational cost. This thrust provides a direct benefit to NASA in that it seeks to accelerate the design cycle to allow state of the art components and designs to actually appear in the fielded system rather than merely in the initial design. To achieve this, this effort is intended to benefit from efforts already underway at Lockheed and other major contractors. For instance, Lockheed Sanders is currently engaged in a major DoD funded development program which has the goal of cutting design cycle time of high performance electronics by a factor of four while simultaneously improving quality also by a factor of four. The early work on this program was used to enable the rapid prototyping of the Reaction Jet Drive Controller which was accomplished in 1993. Similarly, maximum leverage will be derived from recent NASA and DoD efforts to increase the content of high quality commercial grade electronic components in systems for aerospace applications. Both of these goals result in a system with enhanced cost effectiveness, increased reliability, and greatly increased performance compared to a system developed using a more conventional approach.

Source record↗

NASA NTRS1994

A rapid-pressure correlation representation consistent with the Taylor-Proudman theorem materially-frame-indifferent in the 2D limit

A nonlinear representation for the rapid-pressure correlation appearing in the Reynolds stress equations, consistent with the Taylor-Proudman theorem, is presented. The representation insures that the modeled second-order equations are frame-invariant with respect to rotation when the flow is two-dimensional in planes perpendicular to the axis of rotation. The representation satisfies realizability in a new way: a special ansatz is used to obtain analytically, the values of coefficients valid away from the realizability limit: the model coefficients are functions of the state of the turbulence that are valid for all states of the mechanical turbulence attaining their constant limiting values only when the limit state is achieved. Utilization of all the mathematical constraints are not enough to specify all the coefficients in the model. The unspecified coefficients appear as free parameters which are used to insure that the representation is asymptotically consistent with the known equilibrium states of a homogeneous sheared turbulence. This is done by insuring that the modeled evolution equations have the same fixed points as those obtained from computer and laboratory experiments for the homogeneous shear. Results of computations of the homogeneous shear, with and without rotation, and with stabilizing and destabilizing curvature, are shown. Results are consistently better, in a wide class of flows which the model not been calibrated, than those obtained with other nonlinear models.

Ristorcelli, J. R.↗

NASA NTRS1994

The use of biomechanics in the study of movement in microgravity

As biomechanists interested in the adaptability of the human body to microgravity conditions, it appears that our job is not only to make sure that the astronauts can function adequately in space but also that they can function upon their return to Earth. This is especially significant since many of the projects now being designed at NASA concern themselves with humans performing for up to 3 years in microgravity. While the Extended Duration Orbiter flights may last 30 to 60 days, future flights to Mars using current propulsion technology may last from 2 to 3 years. It is for this range of time that the adaptation process must be studied. Specifically, biomechanists interested in space travel realize that human performance capabilities will change as a result of exposure to microgravity. The role of the biomechanist then is to first understand the nature of the changes realized by the body. These changes include adaptation by the musculoskeletal system, the nervous system, cardiorespiratory system, and the cardiovascular system. As biomechanists, it is also our role to take part in the development of countermeasure programs that involve some form of regular exercise. Exercise countermeasure programs should include a variety of modalities with full knowledge of the loads imposed on the body by these modalities. Any exercise programs that are to be conducted by the astronauts during space travel must consider the fact that the musculoskeletal and neuromuscular systems degrade as a function of flight duration. Additionally, it must be understood that the central nervous system modifies its output in the control of the human body during space flight and most importantly, we must prepare the astronauts for their return to one g.

Gregor, R. J.↗

NASA NTRS1994

SeaWiFS technical report series. Volume 16: The second SeaWiFS Intercalibration Round-Robin Experiment, SIRREX-2, June 1993

The results of the second Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Intercalibration Round-Robin Experiment (SIRREX-2), which was held at the Center for Hydro-Optics and Remote Sensing (CHORS) at San Diego State University on 14-25 Jun. 1993 are presented. SeaWiFS is an ocean color radiometer that is scheduled for launch in 1994. The SIRREXs are part of the SeaWiFS Calibration and Validation Program that includes the GSFC, CHORS, NIST, and several other laboratories. GSFC maintains the radiometric scales (spectral radiance and irradiance) for the SeaWiFS program using spectral irradiance standards lamps, which are calibrated by NIST. The purpose of each SIRREX is to assure that the radiometric scales which are realized by the laboratories who participate in the SeaWiFS Calibration and Validation Program are correct; that is, the uncertainties of the radiometric scales are such that measurements of normalized water-leaving radiance using oceanographic radiometers have uncertainties of 5%. SIRREX-1 demonstrated, from the internal consistency of the results, that the program goals would not be met without improvements to the instrumentation. The results of SIRREX-2 demonstrate that spectral irradiance scales realized using the GSFC standard irradiance lamp (F269) are consistent with the program goals, as the uncertainty of these measurements is assessed to be about 1%. However, this is not true for the spectral radiance scales, where again the internal consistency of the results is used to assess the uncertainty. This is attributed to inadequate performance and characterization of the instrumentation. For example, spatial nonuniformities, spectral features, and sensitivity to illumination configuration were observed in some of the integrating sphere sources. The results of SIRREX-2 clearly indicate the direction for future work, with the main emphasis on instrument characterization and the assessment of the measurement uncertainties so that the results may be stated in a more definitive manner.

Hooker, Stanford B.↗