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At least 217 records · Page 12

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NASA NTRS1985

Development of a sensitivity analysis technique for multiloop flight control systems

This report presents the development and application of a sensitivity analysis technique for multiloop flight control systems. This analysis yields very useful information on the sensitivity of the relative-stability criteria of the control system, with variations or uncertainties in the system and controller elements. The sensitivity analysis technique developed is based on the computation of the singular values and singular-value gradients of a feedback-control system. The method is applicable to single-input/single-output as well as multiloop continuous-control systems. Application to sampled-data systems is also explored. The sensitivity analysis technique was applied to a continuous yaw/roll damper stability augmentation system of a typical business jet, and the results show that the analysis is very useful in determining the system elements which have the largest effect on the relative stability of the closed-loop system. As a secondary product of the research reported here, the relative stability criteria based on the concept of singular values were explored.

Vaillard, A. H.↗

NASA NTRS1986

Effects of time delay and pitch control sensitivity in the flared landing

Between December 1985 and January 1986, a flared landing program was conducted, using the USAF Total In-Flight simulator airplane, to examine time delay effects in a formal manner. Results show that as pitch sensitivity is increased, tolerance to time delay decreases. With the proper selection of pitch sensitivity, Level I performance was maintained with time delays ranging from 150 milliseconds to greater than 300 milliseconds. With higher sensitivity, configurations with Level I performance at 150 milliseconds degraded to level 2 at 200 milliseconds. When metrics of time delay and pitch sensitivity effects are applied to enhance previously developed predictive criteria, the result is an improved prediction technique which accounts for significant closed loop items.

Berthe, C. J.↗

NASA NTRS1984

Structural sensitivity analysis: Methods, applications, and needs

Some innovative techniques applicable to sensitivity analysis of discretized structural systems are reviewed. These techniques include a finite-difference step-size selection algorithm, a method for derivatives of iterative solutions, a Green's function technique for derivatives of transient response, a simultaneous calculation of temperatures and their derivatives, derivatives with respect to shape, and derivatives of optimum designs with respect to problem parameters. Computerized implementations of sensitivity analysis and applications of sensitivity derivatives are also discussed. Finally, some of the critical needs in the structural sensitivity area are indicated along with Langley plans for dealing with some of these needs.

Adelman, H. M.↗

NASA NTRS1987

Sensitivity method for integrated structure/active control law design

The development is described of an integrated structure/active control law design methodology for aeroelastic aircraft applications. A short motivating introduction to aeroservoelasticity is given along with the need for integrated structures/controls design algorithms. Three alternative approaches to development of an integrated design method are briefly discussed with regards to complexity, coordination and tradeoff strategies, and the nature of the resulting solutions. This leads to the formulation of the proposed approach which is based on the concepts of sensitivity of optimum solutions and multi-level decompositions. The concept of sensitivity of optimum is explained in more detail and compared with traditional sensitivity concepts of classical control theory. The analytical sensitivity expressions for the solution of the linear, quadratic cost, Gaussian (LQG) control problem are summarized in terms of the linear regulator solution and the Kalman Filter solution. Numerical results for a state space aeroelastic model of the DAST ARW-II vehicle are given, showing the changes in aircraft responses to variations of a structural parameter, in this case first wing bending natural frequency.

Gilbert, Michael G.↗

NASA NTRS1987

Sensitivity and optimization of composite structures using MSC/NASTRAN

Design sensitivity analysis for composites will soon be available in MSC/NASTRAN. The design variables for composites can be lamina thicknesses, orientation angles, material properties or a combination of all three. With the increasing use of composites in aerospace and automotive industries, this general capability can be used in its own right for carrying out sensitivity analysis of complicated real-life structures. As part of a research effort, the sensitivity analysis was coupled with a general purpose optimizer. This preliminary version of the optimizer is capable of dealing with minimum weight structural design with a rather general design variable linking capability at the element level or system level. Only sizing type of design variables (i.e., lamina thicknesses) can be handled by the optimizer. Test cases were run and validated by comparison with independent finite element packages. The linking of design sensitivity capability for composites in MSC/NASTRAN with an optimizer would give designers a powerful automated tool to carry out practical opitmization design of real-life complicated composite structures.

Nagendra, Gopal K.↗

NASA NTRS1987

Design sensitivity analysis of rotorcraft airframe structures for vibration reduction

Optimization of rotorcraft structures for vibration reduction was studied. The objective of this study is to develop practical computational procedures for structural optimization of airframes subject to steady-state vibration response constraints. One of the key elements of any such computational procedure is design sensitivity analysis. A method for design sensitivity analysis of airframes under vibration response constraints is presented. The mathematical formulation of the method and its implementation as a new solution sequence in MSC/NASTRAN are described. The results of the application of the method to a simple finite element stick model of the AH-1G helicopter airframe are presented and discussed. Selection of design variables that are most likely to bring about changes in the response at specified locations in the airframe is based on consideration of forced response strain energy. Sensitivity coefficients are determined for the selected design variable set. Constraints on the natural frequencies are also included in addition to the constraints on the steady-state response. Sensitivity coefficients for these constraints are determined. Results of the analysis and insights gained in applying the method to the airframe model are discussed. The general nature of future work to be conducted is described.

Murthy, T. Sreekanta↗

NASA NTRS1987

Decoupled direct method for sensitivity analysis in combustion kinetics

An efficient, decoupled direct method for calculating the first order sensitivity coefficients of homogeneous, batch combustion kinetic rate equations is presented. In this method the ordinary differential equations for the sensitivity coefficients are solved separately from , but sequentially with, those describing the combustion chemistry. The ordinary differential equations for the thermochemical variables are solved using an efficient, implicit method (LSODE) that automatically selects the steplength and order for each solution step. The solution procedure for the sensitivity coefficients maintains accuracy and stability by using exactly the same steplengths and numerical approximations. The method computes sensitivity coefficients with respect to any combination of the initial values of the thermochemical variables and the three rate constant parameters for the chemical reactions. The method is illustrated by application to several simple problems and, where possible, comparisons are made with exact solutions and those obtained by other techniques.

Radhakrishnan, Krishnan↗

NASA NTRS1984

Design sensitivity analysis with Applicon IFAD using the adjoint variable method

A numerical method is presented to implement structural design sensitivity analysis using the versatility and convenience of existing finite element structural analysis program and the theoretical foundation in structural design sensitivity analysis. Conventional design variables, such as thickness and cross-sectional areas, are considered. Structural performance functionals considered include compliance, displacement, and stress. It is shown that calculations can be carried out outside existing finite element codes, using postprocessing data only. That is, design sensitivity analysis software does not have to be imbedded in an existing finite element code. The finite element structural analysis program used in the implementation presented is IFAD. Feasibility of the method is shown through analysis of several problems, including built-up structures. Accurate design sensitivity results are obtained without the uncertainty of numerical accuracy associated with selection of a finite difference perturbation.

Frederick, Marjorie C.↗

NASA NTRS1985

Shape design sensitivity analysis using domain information

A numerical method for obtaining accurate shape design sensitivity information for built-up structures is developed and demonstrated through analysis of examples. The basic character of the finite element method, which gives more accurate domain information than boundary information, is utilized for shape design sensitivity improvement. A domain approach for shape design sensitivity analysis of built-up structures is derived using the material derivative idea of structural mechanics and the adjoint variable method of design sensitivity analysis. Velocity elements and B-spline curves are introduced to alleviate difficulties in generating domain velocity fields. The regularity requirements of the design velocity field are studied.

Seong, Hwal-Gyeong↗

NASA NTRS1986

The thermoluminescence sensitivity-metamorphism relationship in ordinary chondrites - Experimental data on the mechanism and implications for terrestrial systems

Hydrothermal annealing experiments have been performed on samples of the Sharps meteorite in order to investigate the mechanism responsible for the metamorphism-related, 10-to-the-5th-fold range in the thermoluminescence (TL) sensitivity in ordinary chondrites. Duplicate 50 mg samples of meteorite were annealed under the following conditions: (1) 168 h at 785 C and 1 kbar; (2) the same time, temperature and pressure, but with 2 wt pct water; (3) 174 h at 855 C and 0.77 kbar with 2 wt pct water and 2 molal sodium disilicate (NadiSi); (4) the same time, temperature and pressure as the preceding samples, but with 10 wt pct H2O and 2 molal NadiSi. Samples annealed under the first three sets of conditions showed little or no change in their TL sensitivities, however the samples annealed with 10 wt pct water and 2 molal NadiSi showed a three-fold to 10-fold increase in TL sensitivity, and the temperature of the TL peak was suggestive of feldspar in the high-temperature form. It is suggested that these data are consistent with the TL sensitivity-metamorphism relationship in ordinary chondrites being due to the formation of the TL phosphor, feldspar, by the crystallization of chondrule glass.

Guimon, R. K.↗

NASA NTRS1985

Implementation of design sensitivity analysis with existing finite element codes

A numerical method is presented to implement structural design sensitivity analysis theory, using the versatility and convenience of existing finite element structural analysis programs. Design variables such as thickness and cross-sectional areas of components of individual members and built-up structures are considered. Structural performance functionals considered include displacement and stress. The method is also applicable for eigenvalue problem design sensitivity analysis. It is shown that calculations can be carried out outside existing finite element codes, using postprocessing data only. Thus, design sensitivity analysis software does not have to be embedded in an existing finite element code. Feasability of the method is shown through analysis of several problems, including a built-up structure. Accurate design sensitivity results are obtained without the uncertainty of numerical accuracy associated with selection of finite difference perturbations.

Choi, Kyung K.↗

NASA NTRS1988

Sensitivity of control-augmented structure obtained by a system decomposition method

The verification of a method for computing sensitivity derivatives of a coupled system is presented. The method deals with a system whose analysis can be partitioned into subsets that correspond to disciplines and/or physical subsystems that exchange input-output data with each other. The method uses the partial sensitivity derivatives of the output with respect to input obtained for each subset separately to assemble a set of linear, simultaneous, algebraic equations that are solved for the derivatives of the coupled system response. This sensitivity analysis is verified using an example of a cantilever beam augmented with an active control system to limit the beam's dynamic displacements under an excitation force. The verification shows good agreement of the method with reference data obtained by a finite difference technique involving entire system analysis. The usefulness of a system sensitivity method in optimization applications by employing a piecewise-linear approach to the same numerical example is demonstrated. The method's principal merits are its intrinsically superior accuracy in comparison with the finite difference technique, and its compatibility with the traditional division of work in complex engineering tasks among specialty groups.

Sobieszczanskisobieski, Jaroslaw↗

NASA NTRS1988

Shape sensitivity analysis of wing static aeroelastic characteristics

A method is presented to calculate analytically the sensitivity derivatives of wing static aeroelastic characteristics with respect to wing shape parameters. The wing aerodynamic response under fixed total load is predicted with Weissinger's L-method; its structural response is obtained with Giles' equivalent plate method. The characteristics of interest include the spanwise distribution of lift, trim angle of attack, rolling and pitching moments, wind induced drag, as well as the divergence dynamic pressure. The shape parameters considered are the wing area, aspect ratio, taper ratio, sweep angle, and tip twist angle. Results of sensitivity studies indicate that: (1) approximations based on analytical sensitivity derivatives can be used over wide ranges of variations of the shape parameters considered, and (2) the analytical calculation of sensitivity derivatives is significantly less expensive than the conventional finite-difference alternative.

Barthelemy, Jean-Francois M.↗

NASA NTRS1988

Loading rate sensitivity of open hole composites in compression

The results are reported of an experimental study on the compressive, time-dependent behavior of graphite fiber reinforced polymer composite laminates with open holes. The effect of loading rate on compressive strength was determined for six material systems ranging from brittle epoxies to thermoplastics at both 75 F and 220 F. Specimens were loaded to failure using different loading rates. The slope of the strength versus elapsed time-to-failure curve was used to rank the materials' loading rate sensitivity. All of the materials had greater strength at 75 F than at 220 F. All the materials showed loading rate effects in the form of reduced failure strength for longer elapsed-time-to-failure. Loading rate sensitivity was less at 220 F than the same material at 70 F. However, C12000/ULTEM and IM7/8551-7 were more sensitive to loading rate than the other materials at 220 F. AS4/APC2 laminates with 24, 32, and 48 plies and 1/16 and 1/4 inch diameter holes were tested. The sensitivity to loading rate was less for either increasing number of plies or larger hole size. The failure of the specimens made from brittle resins was accompanied by extensive delaminations while the failure of the roughened systems was predominantly by shear crippling. Fewer delamination failures were observed at the higher temperature.

Lubowinski, Steve J.↗

NASA NTRS1988

Shape sensitivity analysis of wing static aeroelastic characteristics

A method is presented to calculate analytically the sensitivity derivatives of wing static aeroelastic characteristics with respect to wing shape parameters. The wing aerodynamic response under fixed total load is predicted using Weissinger's L-method; its structural response is obtained with Giles' equivalent plate method. The characteristics of interest in this study include the spanwise distribution of lift, trim angle of attack, rolling and pitching moments, wing induced drag as well as the divergence dynamic pressure. The shape parameters considered are the wing area, aspect ratio, taper ratio, sweep angle and tip twist angle. Results of sensitivity studies indicate that (1) approximations based on analytical sensitivity derivatives can be used for wide ranges of variations of the shape parameters considered and (2) the analytical calculation of sensitivity derivatives is significantly less expensive than the conventional finite difference alternative.

Barthelemy, Jean-Francois M.↗

NASA NTRS1988

The doubled CO2 climate and the sensitivity of the modeled hydrologic cycle

Four doubled CO2 experiments with the GISS general circulation model are compared to investigate the consistency of changes in water availability over the United States. The experiments compare the influence of model sensitivity, model resolution, and the sea-surface temperature gradient. The results show that the general mid-latitude drying over land is dependent upon the degree of mid-latitude eddy energy decrease, and thus the degree of high-latitude temperature change amplification. There is a general tendency in the experiments for the northern and western United States to become wetter, while the southern and eastern portions dry. However, there is much variability from run to run, with different regions showing different degrees of sensitivity to the parameters tested. The results for the western United States depend most on model resolution; those for the central United States, on the sea-surface temperature gradient and the degree of mid-latitude ocean warming; and those for the eastern United States, on model sensitivity. The changes in particular seasons depend on changes in other seasons, and will therefore be sensitive to the realism of the ground hydrology parameterization.

Rind, D.↗

NASA NTRS1989

Iterative methods for design sensitivity analysis

A numerical method is presented for design sensitivity analysis, using an iterative-method reanalysis of the structure generated by a small perturbation in the design variable; a forward-difference scheme is then employed to obtain the approximate sensitivity. Algorithms are developed for displacement and stress sensitivity, as well as for eignevalues and eigenvector sensitivity, and the iterative schemes are modified so that the coefficient matrices are constant and therefore decomposed only once.

Belegundu, A. D.↗

NASA NTRS1989

Design sensitivity analysis of boundary element substructures

The ability to reduce or condense a three-dimensional model exactly, and then iterate on this reduced size model representing the parts of the design that are allowed to change in an optimization loop is discussed. The discussion presents the results obtained from an ongoing research effort to exploit the concept of substructuring within the structural shape optimization context using a Boundary Element Analysis (BEA) formulation. The first part contains a formulation for the exact condensation of portions of the overall boundary element model designated as substructures. The use of reduced boundary element models in shape optimization requires that structural sensitivity analysis can be performed. A reduced sensitivity analysis formulation is then presented that allows for the calculation of structural response sensitivities of both the substructured (reduced) and unsubstructured parts of the model. It is shown that this approach produces significant computational economy in the design sensitivity analysis and reanalysis process by facilitating the block triangular factorization and forward reduction and backward substitution of smaller matrices. The implementatior of this formulation is discussed and timings and accuracies of representative test cases presented.

Kane, James H.↗