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At least 1,513 records · Page 84

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NASA NTRS1990

Microdynamic modelling of joint-dominated truss-based structures

A methodology for the mathematical modeling of preloaded joint-dominated truss-based space structures applicable in the microdynamic regime is presented. First, various factors which could affect the microdynamic behavior of joint-dominated truss structures are examined. Then, mathematical models for various types of joints involving contact deformations are derived from Hertzian contact theory. Their dynamic behavior is studied analytically and numerically by means of computer simulation. It was found that both synchronous and asynchronous oscillations having the same orders of magnitude are excited by sinusoidal load perturbations in the microdynamic regime. These oscillations persist even in the presence of light linear damping. The integration of the derived joint models with those for the elastic links of preloaded planar truss structures is discussed.

Wang, P. K. C.↗

NASA NTRS1990

Coupled effects of conduction in the crystal and thermo-solutal convection in a rectangular inclined enclosure

To date modeling of crystal growth of optoelectronic materials using Physical Vapor Transport has been limited to the study of the fluid phase. To achieve it, the equations of coupled heat, mass and momentum transfer in the gas have to be solved. The first objective of this study is to examine the effect of heat conduction in the crystal on the fluid flow in the neighborhood of the interface. Heat transfer boundary conditions on both interfaces were modified to take into account the additional heat flux between gas and solid. It is proved that heat conduction does not affect the fluid flow. In the presence of gravity, density gradients in the fluid phase generate convection responsible for the problem of a nonplanar growth of the interface. The second objective is to study systematically under one-g the different possible flows in order to solve this problem. Depending on the parameters, a diffusive mode and three convective modes (thermal, solutal and thermo-solutal) are observed. The competition between thermal and solutal convections leads to a mathematical condition which can be used to achieve a planar growth. It is proven that, under the physical conditions chosen, this mathematical condition cannot be thermodynamically satisfied.

Mennetrier, Christophe↗

NASA NTRS1989

Flutter suppression control law synthesis for the active flexible wing model

The Active Flexible Wing Project is a collaborative effort between the NASA Langley Research Center and Rockwell International. The objectives are the validation of methodologies associated with mathematical modeling, flutter suppression control law development and digital implementation of the control system for application to flexible aircraft. A flutter suppression control law synthesis for this project is described. The state-space mathematical model used for the synthesis included ten flexible modes, four control surface modes and rational function approximation of the doublet-lattice unsteady aerodynamics. The design steps involved developing the full-order optimal control laws, reducing the order of the control law, and optimizing the reduced-order control law in both the continuous and the discrete domains to minimize stochastic response. System robustness was improved using singular value constraints. An 8th order robust control law was designed to increase the symmetric flutter dynamic pressure by 100 percent. Preliminary results are provided and experiences gained are discussed.

Mukhopadhyay, Vivek↗

NASA NTRS1991

Microwave Brightness Of Land Surfaces From Outer Space

Mathematical model approximates microwave radiation emitted by land surfaces traveling to microwave radiometer in outer space. Applied to measurements made by Scanning Multichannel Microwave Radiometer (SMMR). Developed for interpretation of microwave imagery of Earth to obtain distributions of various chemical, physical, and biological characteristics across its surface. Intended primarily for use in mapping moisture content of soil and fraction of Earth covered by vegetation. Advanced Very-High-Resolution Radiometer (AVHRR), provides additional information on vegetative cover, thereby making possible retrieval of soil-moisture values from SMMR measurements. Possible to monitor changes of land surface during intervals of 5 to 10 years, providing significant data for mathematical models of evolution of climate.

Kerr, Yann H.↗

NASA NTRS1991

Computing Flow In A Labyrinth Seal

Mathematical model and computer program developed to simulate flow in labyrinth seal. Primary purpose to determine drop in pressure across primary labyrinth seal of high-pressure-oxygen turbopump of Space Shuttle main engine, including portion of drop caused by resistance to flow in gaseous-oxygen drain. Variety of geometries, including multiple chokes, accommodated. Provides for improved study of various types of seal failures and for reviews of mechanics of seal regions. Flexibility of program makes it applicable to other machinery. Further development of program includes providing for lands of various area (variable radius) and improving mathematical model of drain system.

Smith, Tyn S.↗

NASA NTRS1991

Computer Language For Optimization Of Design

SOL is computer language geared to solution of design problems. Includes mathematical modeling and logical capabilities of computer language like FORTRAN; also includes additional power of nonlinear mathematical programming methods at language level. SOL compiler takes SOL-language statements and generates equivalent FORTRAN code and system calls. Provides syntactic and semantic checking for recovery from errors and provides detailed reports containing cross-references to show where each variable used. Implemented on VAX/VMS computer systems. Requires VAX FORTRAN compiler to produce executable program.

Scotti, Stephen J.↗

NASA NTRS1991

Spatial-Operator Algebra For Robotic Manipulators

Report discusses spatial-operator algebra developed in recent studies of mathematical modeling, control, and design of trajectories of robotic manipulators. Provides succinct representation of mathematically complicated interactions among multiple joints and links of manipulator, thereby relieving analyst of most of tedium of detailed algebraic manipulations. Presents analytical formulation of spatial-operator algebra, describes some specific applications, summarizes current research, and discusses implementation of spatial-operator algebra in the Ada programming language.

Rodriguez, Guillermo↗

NASA NTRS1991

Adaptive Control Of Large Vibrating, Rotating Structures

Globally convergent theoretical method provides for adaptive set-point control of orientation of, along with suppression of the vibrations of, large structure. Method utilizes inherent passivity properties of structure to attain mathematical condition essential to adaptive convergence on commanded set point. Maintains stability and convergence in presence of errors in mathematical model of dynamics of structure and actuators. Developed for controlling attitudes of large, somewhat flexible spacecraft, also useful in such terrestrial applications as controlling movable bridges or suppressing earthquake vibrations in bridges, buildings, and other large structures.

Bayard, David S.↗

NASA NTRS1991

Dynamic Analyses Including Joints Of Truss Structures

Method for mathematically modeling joints to assess influences of joints on dynamic response of truss structures developed in study. Only structures with low-frequency oscillations considered; only Coulomb friction and viscous damping included in analysis. Focus of effort to obtain finite-element mathematical models of joints exhibiting load-vs.-deflection behavior similar to measured load-vs.-deflection behavior of real joints. Experiments performed to determine stiffness and damping nonlinearities typical of joint hardware. Algorithm for computing coefficients of analytical joint models based on test data developed to enable study of linear and nonlinear effects of joints on global structural response. Besides intended application to large space structures, applications in nonaerospace community include ground-based antennas and earthquake-resistant steel-framed buildings.

Belvin, W. Keith↗

NASA NTRS1990

Improving system reliability through formal analysis and use of checks in software

Software is playing increasingly important roles in avionics systems. It is widely used in navigation and, in some cases, in control loops that maintain aircraft stability. To guarantee the safety of flight systems, the FAA requires that critical components have a probability of failure no greater than 10(exp -9) per hour of flight. Software is being used to diagnose system components for failure. SIFT (Software Implemented Fault Tolerance) was a computer system developed to study the use of software to check for failure and manage processor reconfiguration. To guarantee that software satisfies its specifications, formal verification can be used. With this a program and its specification are viewed as mathematical objects, and a mathematical proof is used to show that the program and its specification are equivalent. In previous research, a theory of checking was developed to offer assistance in analyzing specifications and designing run-time checks. In the theory, checking is considered abstractly in terms of n-ary relations much like those of relational database theory. Within the theory check are categorized, checks on input and checks on results are considered, and formal attention is given to the minimization and logical combination of checks. The focus is upon input checks and the obstacles in checking input to critical systems. A central concern is with a property referred to as independence. The concern is with circumstances under which it is possible to apply isolated, independent checks to separate sensor inputs and be assure that all illegal input will be properly detected. Presently, independence is being investigated and checked in the context of the GCS (Guidance and Control System). The GCS simulator is intended for testing software that implements control laws for landing spacecraft. The large number of inputs and their complex interrelationships provide an exciting context in which to investigate independence and the difficulties of supplying input checks.

Staknis, Mark E.↗

NASA NTRS1988

Heat pipe dynamic behavior

The vapor flow in a heat pipe was mathematically modeled and the equations governing the transient behavior of the core were solved numerically. The modeled vapor flow is transient, axisymmetric (or two-dimensional) compressible viscous flow in a closed chamber. The two methods of solution are described. The more promising method failed (a mixed Galerkin finite difference method) whereas a more common finite difference method was successful. Preliminary results are presented showing that multi-dimensional flows need to be treated. A model of the liquid phase of a high temperature heat pipe was developed. The model is intended to be coupled to a vapor phase model for the complete solution of the heat pipe problem. The mathematical equations are formulated consistent with physical processes while allowing a computationally efficient solution. The model simulates time dependent characteristics of concern to the liquid phase including input phase change, output heat fluxes, liquid temperatures, container temperatures, liquid velocities, and liquid pressure. Preliminary results were obtained for two heat pipe startup cases. The heat pipe studied used lithium as the working fluid and an annular wick configuration. Recommendations for implementation based on the results obtained are presented. Experimental studies were initiated using a rectangular heat pipe. Both twin beam laser holography and laser Doppler anemometry were investigated. Preliminary experiments were completed and results are reported.

Issacci, F.↗

NASA NTRS1990

Estimability of geodetic parameters from space VLBI observables

The feasibility of space very long base interferometry (VLBI) observables for geodesy and geodynamics is investigated. A brief review of space VLBI systems from the point of view of potential geodetic application is given. A selected notational convention is used to jointly treat the VLBI observables of different types of baselines within a combined ground/space VLBI network. The basic equations of the space VLBI observables appropriate for convariance analysis are derived and included. The corresponding equations for the ground-to-ground baseline VLBI observables are also given for a comparison. The simplified expression of the mathematical models for both space VLBI observables (time delay and delay rate) include the ground station coordinates, the satellite orbital elements, the earth rotation parameters, the radio source coordinates, and clock parameters. The observation equations with these parameters were examined in order to determine which of them are separable or nonseparable. Singularity problems arising from coordinate system definition and critical configuration are studied. Linear dependencies between partials are analytically derived. The mathematical models for ground-space baseline VLBI observables were tested with simulation data in the frame of some numerical experiments. Singularity due to datum defect is confirmed.

Adam, Jozsef↗

NASA NTRS1990

Extension of transonic flow computational concepts in the analysis of cavitated bearings

An analogy between the mathematical modeling of transonic potential flow and the flow in a cavitating bearing is described. Based on the similarities, characteristics of the cavitated region and jump conditions across the film reformation and rupture fronts are developed using the method of weak solutions. The mathematical analogy is extended by utilizing a few computational concepts of transonic flow to numerically model the cavitating bearing. Methods of shock fitting and shock capturing are discussed. Various procedures used in transonic flow computations are adapted to bearing cavitation applications, for example, type differencing, grid transformation, an approximate factorization technique, and Newton's iteration method. These concepts have proved to be successful and have vastly improved the efficiency of numerical modeling of cavitated bearings.

Vijayaraghavan, D.↗

NASA NTRS1990

A Finite Element Procedure for Calculating Fluid-Structure Interaction Using MSC/NASTRAN

This report is intended to serve two purposes. The first is to present a survey of the theoretical background of the dynamic interaction between a non-viscid, compressible fluid and an elastic structure is presented. Section one presents a short survey of the application of the finite element method (FEM) to the area of fluid-structure-interaction (FSI). Section two describes the mathematical foundation of the structure and fluid with special emphasis on the fluid. The main steps in establishing the finite element (FE) equations for the fluid structure coupling are discussed in section three. The second purpose is to demonstrate the application of MSC/NASTRAN to the solution of FSI problems. Some specific topics, such as fluid structure analogy, acoustic absorption, and acoustic contribution analysis are described in section four. Section five deals with the organization of the acoustic procedure flowchart. Section six includes the most important information that a user needs for applying the acoustic procedure to practical FSI problems. Beginning with some rules concerning the FE modeling of the coupled system, the NASTRAN USER DECKs for the different steps are described. The goal of section seven is to demonstrate the use of the acoustic procedure with some examples. This demonstration includes an analytic verification of selected FE results. The analytical description considers only some aspects of FSI and is not intended to be mathematically complete. Finally, section 8 presents an application of the acoustic procedure to vehicle interior acoustic analysis with selected results.

Chargin, Mladen↗

NASA NTRS1991

Model correlation and damage location for large space truss structures: Secant method development and evaluation

On-orbit testing of a large space structure will be required to complete the certification of any mathematical model for the structure dynamic response. The process of establishing a mathematical model that matches measured structure response is referred to as model correlation. Most model correlation approaches have an identification technique to determine structural characteristics from the measurements of the structure response. This problem is approached with one particular class of identification techniques - matrix adjustment methods - which use measured data to produce an optimal update of the structure property matrix, often the stiffness matrix. New methods were developed for identification to handle problems of the size and complexity expected for large space structures. Further development and refinement of these secant-method identification algorithms were undertaken. Also, evaluation of these techniques is an approach for model correlation and damage location was initiated.

Smith, Suzanne Weaver↗

NASA NTRS1991

Identification of linear multivariable systems from a single set of data by identification of observers with assigned real eigenvalues

A formulation is presented for identification of linear multivariable from a single set of input-output data. The identification method is formulated with the mathematical framework of learning identifications, by extension of the repetition domain concept to include shifting time intervals. This method contrasts with existing learning approaches that require data from multiple experiments. In this method, the system input-output relationship is expressed in terms of an observer, which is made asymptotically stable by an embedded real eigenvalue assignment procedure. Through this relationship, the Markov parameters of the observer are identified. The Markov parameters of the actual system are recovered from those of the observer, and then used to obtain a state space model of the system by standard realization techniques. The basic mathematical formulation is derived, and numerical examples presented to illustrate.

Phan, Minh↗

NASA NTRS1989

The transient performance of a two-phase fluid reservoir

Thermal control of future large, high power spacecraft will require a two-phase fluid central bus. The two-phase fluid reservoir is a critical component in the two-phase fluid bus. It both controls the saturation temperature and provides a space for volumetric changes. A dynamic reservoir simulation model does not currently exist, but it is needed to expedite efforts and reduce risk. During 1989 an effort was made to develop a simulation model of the transient performance of a two-phase fluid reservoir. As a beginning, a preliminary model was developed. It is based upon component mathematical models in lumped parametric form and build upon five component mathematical models for calculating dynamic responses of two-phase fluid reservoirs, primary feedback elements, controller commands, heater actuators, and reservoir heaters. As much as possible, the model took advantage of the available SINDA'85/FLUINT thermal/fluid integrator. Additional calculation logic and computer subroutines were developed to complete implementation of the model. The model is capable of simulating dynamic response of an equilibrium two-phase fluid reservoir. Modification of the model to include the liquid/vapor nonequilibrium is required for applications of the model to simulate performance of reservoir in which the liquid and vapor phases of the reservoir fluid are not in equilibrium. In addition, the model in its present form, needs to be refined in several respects. More empirical data are needed to guide the model development. The model may then be used to conduct a full parametric study of two-phase fluid reservoirs. More complexities in two-phaes flow regions in laboratory and flight conditions may have to be considered eventually if empirical data cannot be simulated satisfactorily. System with other components arrangement also need to be simulated if optimization is ever to be attained. The present model does, however, preliminarily demonstrates that such analyses are quite possible and offers a far less expensive method to understand the transient of a two-phase fluid reservoir system than a totally headware approach.

Chi, Joseph↗

NASA NTRS1989

A successful intervention program for high ability minority students

Among professional occupations in the United States, non-Asian minorities are least represented in science and engineering fields. The Bureau of Labor Statistics predicts that over the next decade, civilian employment of scientists and engineers has the potential to grow by 40 percent. Furthermore, projections for the year 2000 indicate that 100,000 fewer B.S. and B.A. degrees will be awarded than were awarded in 1984. The latter projection takes into consideration the overall declining proportion of all 18 year old college students. Within this shrinking pool of 18 year old potential college students will be an increasing proportion of Blacks and Hispanics. In order to change the educational patterns for minority youth, an intense look at the factors that affect the science and mathematics performance of minorities. Furthermore, the work of programs that are successful at producing minority scientists and engineers must be examined and documented with the intent of replicating these programs. The fundamental concern at this time appears to be the quality of precollege experience because research has shown that lack of precollege preparation is the single most important cause of underrepresentation of minorities in science and engineering careers. For many years, intervention programs have attempted to improve the quality of the minority precollege experience by latter year intervention in grades eleven and twelve. Later efforts, such as this one, have concentrated on earlier years. The effectiveness of intervention programs is widely accepted but not rigorously documented. The mechanisms these programs have developed need to be identified and their potential for broader use evaluated. The ultimate goal of such studies would be to provide the different educational communities with a set of proven cost-effective state of the art mechanisms designed to increase participation and success of minority students in science and mathematics-related courses. One such intervention program is the Saturday Academy program for high ability minority students in the Washington, D.C. area. A description of the Saturday Academy is provided with the intent of making it available to personnel who are considering the development of similar projects. The effect of participation in the program on high school graduate rates, college enrollment, and choice of quantitative major is examined.

Coleman, Winson R.↗