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At least 307 records · Page 17

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NASA NTRS1985

Modal response and noise transmission of composite panels

Noise transmission through flat, angular, fiber-reinforced composite panels is investigated experimentally and analytically. A modal decomposition technique was used to obtain solutions to the governing differential equation of motion. Experimental modal analysis was performed in order to confirm the theoretical results. The test specimens were cross-ply and angle-ply composite panels made of various concentrations of fiberglass, graphite, or aramid fibers embedded in epoxy resion. The experimental results showed good agreement with the theoretical calculations. Graphs of the transmission loss characteristics of the different composite panels are provided.

Grosveld, F. W.↗

NASA NTRS1984

Modal insensitivity with optimality

This paper deals with the design of a constant gain, feedback controller which results in selected modal insensitivity, and at the same time optimizes a quadratic performance index representative of desired system performance for nominal plant parameter values. Both full state and output feedback control are considered. A constraint is established for the feedback gain matrix that results in modal insensitivity, and necessary conditions for optimality subject to this constraint are given. This forms the basis for a numerical algorithm to compute the optimal feedback gain. To illustrate the procedure, a design is carried out using the lateral dynamics of an L-1011 aircraft.

Calise, A. J.↗

NASA NTRS1986

Modal test/analysis correlation for Centaur G Prime launch vehicle

A modal test was performed on the Centaur G Prime launch vehicle for the purpose of verifying the loads analysis model. This paper describes the procedure by which modal parameters obtained in this test were correlated with the corresponding analytical predictions. Based on this correlation the stiffness model of the shuttle trunnion system has been modified. The evolution of the model updating and the final results are described.

Chen, J.↗

NASA NTRS1986

Effects of noise on modal parameters identified by the Eigensystem Realization Algorithm

The basic concept of the Eigensystem Realization Algorithm for modal parameter identification and model reduction is extended to minimize the distortion of the identified parameters caused by noise. The mathematical foundation for the properties of accuracy indicators, such as the singular values of the data matrix and modal amplitude coherence, is provided, based on knowledge of the noise characteristics. These indicators quantitatively discriminate noise from system information and are used to reduce the realized system model to a better approximation of the true model. Monte Carlo Simulations are included to support the analytical studies.

Juang, J.-N.↗

NASA NTRS1986

Effects of noise on ERA-identified modal parameters

The basic concept of the Eigensystem Realization Algorithm (ERA) for modal parameter identification and model reduction is extended to minimize the distortion of the identified parameters caused by noise. The mathematical foundation for the properties of accuracy indicators such as the singular values of the data matrix and modal amplitude coherence is provided based on knowledge of the noise characteristics. These indicators quantitatively discriminate noise from system information and are used to reduce the realized system model to a better approximation of the true model. Monte Carlo simulations are included to support the analytical studies.

Juang, J.-N.↗

NASA NTRS1986

An Eigensystem Realization Algorithm in Frequency Domain for modal parameter identification

This paper demonstrates the close conceptual relationships between time domain and frequency domain approaches to identification of modal parameters for linear systems. A frequency domain eigensystem realization algorithm, via transfer functions, is developed using a known procedure formulated for a time domain eigensystem realization algorithm, via free decay measurement data. An important feature is the capability of windowing to concentrate analysis on the frequency range of interest. The procedure of overlap averaging is used to produce smoother spectra to reduce the effect of noise on identified modal parameters. Examples from simulation and experiments are given to illustrate the validity of formulations derived in the paper.

Juang, J.-N.↗

NASA NTRS1986

Comparison of modal identification techniques using a hybrid-data approach

Modal identification of seemingly simple structures, such as the generic truss is often surprisingly difficult in practice due to high modal density, nonlinearities, and other nonideal factors. Under these circumstances, different data analysis techniques can generate substantially different results. The initial application of a new hybrid-data method for studying the performance characteristics of various identification techniques with such data is summarized. This approach offers new pieces of information for the system identification researcher. First, it allows actual experimental data to be used in the studies, while maintaining the traditional advantage of using simulated data. That is, the identification technique under study is forced to cope with the complexities of real data, yet the performance can be measured unquestionably for the artificial modes because their true parameters are known. Secondly, the accuracy achieved for the true structural modes in the data can be estimated from the accuracy achieved for the artificial modes if the results show similar characteristics. This similarity occurred in the study, for example, for a weak structural mode near 56 Hz. It may even be possible--eventually--to use the error information from the artificial modes to improve the identification accuracy for the structural modes.

Pappa, Richard S.↗

NASA NTRS1987

Modal sensitivity for structural systems with repeated frequencies

Repeated or closely packed modal frequencies are common physical occurrences for vibrating structures which are complex or possess multi-planes of symmetry. The computation of the sensitivity to structural modifications for these frequencies and mode shapes is made difficult by the fact that the mode shapes are not unique, since any linear combination of eigenvectors corresponding to a repeated eigenvalue is also an eigenvector. The work of Chen and Pan is extended, who used modal expansion techniques for accommodating the sensitivity analysis of structures with repeated eigenvalues. Starting with a discussion of the physical significance of sensitivity analysis for repeated frequency modes, a derivation is presented of the governing equations for the derivatives of a repeated eigenvalue. This is followed with a small example to illustrate the results. An efficient computation procedure, based upon an expansion of Nelson's ideas for large banded systems, is then proposed for systems with repeated or closely spaced eigenvalues.

Ojalvo, I. U.↗

NASA NTRS1987

Modified independent modal space control method for active control of flexible systems

A modified independent modal space control (MIMSC) method is developed for designing active vibration control systems for large flexible structures. The method accounts for the interaction between the controlled and residual modes. It incorporates also optimal placement procedures for selecting the optimal locations of the actuators in the structure in order to minimize the structural vibrations as well as the actuation energy. The MIMSC method relies on an important feature which is based on time sharing of a small number of actuators, in the modal space, to control effectively a large number of modes. Numerical examples are presented to illustrate the application of the method to generic flexible systems. The results obtained suggest the potential of the devised method in designing efficient active control systems for large flexible structures.

Baz, A.↗

NASA NTRS1987

Computer animation of modal and transient vibrations

An interactive computer graphics processor is described that is capable of generating input to animate modal and transient vibrations of finite element models on an interactive graphics system. The results from NASTRAN can be postprocessed such that a three dimensional wire-frame picture, in perspective, of the finite element mesh is drawn on the graphics display. Modal vibrations of any mode shape or transient motions over any range of steps can be animated. The finite element mesh can be color-coded by any component of displacement. Viewing parameters and the rate of vibration of the finite element model can be interactively updated while the structure is vibrating.

Lipman, Robert R.↗

NASA NTRS1987

An eigensystem realization algorithm using data correlations (ERA/DC) for modal parameter identification

A modification to the Eigensystem Realization Algorithm (ERA) for modal parameter identification is presented in this paper. The ERA minimum order realization approach using singular value decomposition is combined with the philosophy of the Correlation Fit method in state space form such that response data correlations rather than actual response values are used for modal parameter identification. This new method, the ERA using data correlations (ERA/DC), reduces bias errors due to noise corruption significantly without the need for model overspecification. This method is tested using simulated five-degree-of-freedom system responses corrupted by measurement noise. It is found for this case that, when model overspecification is permitted and a minimum order solution obtained via singular value truncation, the results from the two methods are of similar quality.

Juang, Jer-Nan↗

NASA NTRS1987

Acoustic power measurement for single and annular stream duct-nozzle systems utilizing a modal decomposition scheme

A refined acoustic impulse technique was used to evaluate the adequacy of single point in-duct pressure measurements for determining the acoustic power for incident, reflected, and transmitted fields for single and annular stream duct-nozzle systems at various flow conditions. The spatial distributions of incident and reflected pressure fields were measured at several radial and azimuthal locations inside the duct. A modal decomposition scheme was used to derive the acoustic power of each duct mode from the complex pressure measurements. The total sums of these individual modal acoustic powers were compared with the area-weighted acoustic power distributions that were evaluated from measured pressure data. It was found that a single-point measurement near the duct wall is adequate for estimating the transmitted power for both single and annular stream duct-nozzle systems.

Salikuddin, M.↗

NASA NTRS1987

Modal decoupling insensitivity

This note deals with the design of an output feedback control system such that selected closed-loop eigenvalues are insensitive, while the corresponding eigenvectors preserve a prescribed decoupling under parameter variation. This results in restricting specified elements of an eigenvector to be insensitive, rather than the entire eigenvector. This requirement is henceforth referred to as modal decoupling insensitivity. Sufficient conditions for modal decoupling insensitivity are derived. These conditions offer the advantage of increased flexibility in eigenvector assignment, but the constraint on the feedback matrix is more restrictive. A numerical example is presented to illustrate the design procedure.

Raman, K. V.↗

NASA NTRS1987

Centaur G Prime modal test

The Centaur G Prime modal test resulted in sets of modes (frequencies, mode shapes and damping) with an accuracy similiar to or better than that normally obtained from the modal testing of linear structures with no backlash and small damping. In other words, performing the test at high level greatly minimized the backlash effect and provided a valid, simple linearization of the trunnion friction problem for the Centaur in the Shuttle Cargo Bay. All the most important modes (target modes) were measured and provided the data base for updating the finite element model for the pre-flight verification loads analysis.

Trubert, Marc↗

NASA NTRS1988

Modal forced response of propfans in yawed flow

A modal forced response method for propfans in yawed flow is presented. This capability exists in the Aeroelastic Stability and Response of Propfan (ASTROP3) code developed at the Lewis Research Center. The code uses three-dimensional steady and unsteady cascade aerodynamics by Williams and Hwang (1986) and a NASTRAN finite element model to represent the blade structure. In addition, many utility programs exist in ASTROP3 that help in both the preprocessing of the NASTRAN model and the postprocessing of modal response results. The postprocessing work that computes the blade vibratory displacements and stresses in yawed flow are highlighted here.

Narayanan, G. V.↗

NASA NTRS1988

Single-Mode Projection Filters for Modal Parameter Identification for Flexible Structures

Single-mode projection filters are developed for eigensystem parameter identification from both analytical results and test data. Explicit formulations of these projection filters are derived using the orthogonal matrices of the controllability and observability matrices in the general sense. A global minimum optimization algorithm is applied to update the filter parameters by using the interval analysis method. The updated modal parameters represent the characteristics of the test data. For illustration of this new approach, a numerical simulation for the MAST beam structure is shown by using a one-dimensional global optimization algorithm to identify modal frequencies and damping. Another numerical simulation of a ten-mode structure is also presented by using a two-dimensional global optimization algorithm to illustrate the feasibility of the new method. The projection filters are practical for parallel processing implementation.

Huang, Jen-Kuang↗

NASA NTRS1988

Expertise and reasoning with possibility: An explanation of modal logic and expert systems

Recently systems of modal reasoning have been brought to the foreground of artificial intelligence studies. The intuitive idea of research efforts in this area is that in addition to the actual world in which sentences have certain truth values there are other worlds in which those sentences have different truth values. Such alternative worlds can be considered as possible worlds, and an agent may or may not have access to some or all of them. This approach to reasoning can be valuable in extending the expert system paradigm. Using the scheme of reasoning proposed by Toulmin, Reike and Janick and the modal system T, a scheme is proposed for expert reasoning that mitigates some of the criticisms raised by Schank and Nickerson.

Rochowiak, Daniel↗

NASA NTRS1988

Asymptotic modal analysis and statistical energy analysis

Statistical Energy Analysis (SEA) is defined by considering the asymptotic limit of Classical Modal Analysis, an approach called Asymptotic Modal Analysis (AMA). The general approach is described for both structural and acoustical systems. The theoretical foundation is presented for structural systems, and experimental verification is presented for a structural plate responding to a random force. Work accomplished subsequent to the grant initiation focusses on the acoustic response of an interior cavity (i.e., an aircraft or spacecraft fuselage) with a portion of the wall vibrating in a large number of structural modes. First results were presented at the ASME Winter Annual Meeting in December, 1987, and accepted for publication in the Journal of Vibration, Acoustics, Stress and Reliability in Design. It is shown that asymptotically as the number of acoustic modes excited becomes large, the pressure level in the cavity becomes uniform except at the cavity boundaries. However, the mean square pressure at the cavity corner, edge and wall is, respectively, 8, 4, and 2 times the value in the cavity interior. Also it is shown that when the portion of the wall which is vibrating is near a cavity corner or edge, the response is significantly higher.

Dowell, Earl H.↗