Showing papers on "Modal testing published in 1982"

Finite Element Prediction of Damping in Structures with Constrained Viscoelastic Layers

Abstract: An efficient method is described for finite element modelling of three-layer laminates containing a viscoelastic layer. Modal damping ratios are estimated from undamped normal mode results by means of the modal strain energy method. Comparisons are given between results obtained by the MSE method implemented in NASTRAN, by various exact solutions for approximate governing differential equations, and by experiment. Results are in terms of frequencies, modal damping ratios, and mechanical admittances for simple beams, plates, and rings. Application of the finite element -- MSE method in design of integrally damped structures is discussed.

Finite-element analysis and modal testing of a rotating wind turbine

Abstract: A finite element procedure, which includes geometric stiffening, and centrifugal and Coriolis terms resulting from the use of a rotating coordinate system, has been developed to compute the mode shapes and frequencies of rotating structures. Special application of this capability has been made to Darrieus, vertical axis wind turbines. In a parallel development effort, a technique for the modal testing of a rotating vertical axis wind turbine has been established to measure modal parameters directly. Results from the predictive and experimental techniques for the modal frequencies and mode shapes are compared over a wide range of rotational speeds.

Modal damping for torsionally coupled buildings on elastic foundation

Abstract: A method to determine the approximate normal modes and the modal damping for torsionally coupled buildings on an elastic foundation is presented. The modal damping is determined by an iterative procedure which matches the approximate normal mode solution with the rigorous solution. The response quantity to be matched is selected in a consistent and logical manner. The normal modes and the damping ratios thus found are then used to determine the seismic response of the interaction system by the response spectrum technique.

Modal testing of a rotating wind turbine

Abstract: A testing technique has been developed to measure the modes of vibration of a rotating vertical-axis wind turbine. This technique has been applied to the Sandia Two-Meter Turbine, where the changes in individual modal frequencies as a function of the rotational speed have been tracked from 0 rpm (parked) to 600 rpm. During rotational testing, the structural response was measured using a combination of strain gages and accelerometers, passing the signals through slip rings. Excitation of the turbine structure was provided by a scheme which suddenly released a pretensioned cable, thus plucking the turbine as it was rotating at a set speed. In addition to calculating the real modes of the parked turbine, the modes of the rotating turbine were also determined at several rotational speeds. The modes of the rotating system proved to be complex due to centrifugal and Coriolis effects. The modal data for the parked turbine were used to update a finite-element model. Also, the measured modal parameters for the rotating turbine were compared to the analytical results, thus verifying the analytical procedures used to incorporate the effects of the rotating coordinate system.

Direct system parameter identification of mechanical structures with application to modal analysis

Abstract: In this paper a method is described to estimate mechanical structure characteristics in terms of mass, stiffness and damping matrices using measured force input and response data. The estimated matrices can be used to calculate a consistent set of damped natural frequencies and damping values, mode shapes and modal scale factors for the structure. The proposed technique is attractive as an experimental modal analysis method since the estimation of the matrices does not require previous estimation of frequency responses and since the method can be used, without any additional complications, for multiple force input structure testing.

Comparison of modal analysis and equivalent circuit representation of E-plane arm of the jerusalem cross

Abstract: Transmission coefficients predicted by modal analysis agree well with measurements, but an LC equivalent circuit gives satisfactory results for normal incidence only. The two methods predict similar phases for the induced currents. The modal method's account of the current distribution is illustrated for two resonant frequencies in the E-plane.

A Study of the Modal Truncation Error in the Component Mode Analysis of a Dual-Rotor System

Abstract: In the component mode synthesis method, the equation of motion in the generalized coordinates is built upon the undamped eigenvalue data of the component structures. Error is inevitable when truncated modes are used. In this paper, two modal truncation schemes were evaluated with regard to the critical speed, stability, and unbalance response of a two-spool gas turbine engine. The numbers of modes required to yield acceptable accuracy in these cases were determined. Guidelines for modal truncation were derived from these results.

Acoustic modal analysis experiment

Abstract: This paper proposes an experimental modal technique for acoustic ducts, mufflers, and resonators over the plane‐wave frequency regime. Global modal properties, such as natural frequencies and modes of gas oscillation, are extracted from the coincident‐quadrature response curves of measured cross‐point acoustic impedances at a number of observation locations. The acoustic system is excited by a vibrating piston which is driven by an electromagnetic shaker with bandlimited binary random noise signal. The acoustic impedance is determined using the following two transducers: (i) an accelerometer attached to the piston—its signal is processed to yield volume velocity information, and (ii) a microphone traverse. Digital data acquisition and processing techniques are used to generate the necessary impedance data at a number of locations for modal analysis. In order to demonstrate the validity of our experimental technique, we have applied it to the following example cases, and obtained excellent correlation between theory and experiment: closed–closed tube, closed–open tube, and symmetrical and unsymmetrical lumped parameter systems. In this paper, we discuss the conceptual, analytical, physical, and measurement considerations of the acoustic modal analysis. We also point out some areas of further research.

Wave and Earthquake Response of Offshore Structures: Evaluation of Modal Solutions

Abstract: Direct numerical integration of the equations of motion is used to evaluate the adequacy of modal solutions, as applied to the analysis and design of large offshore structures under wave and earthquake loadings. Two such solutions are considered: mode acceleration and simple modal superposition. Both these methods are widely used by the offshore industry but recently their adequacy in predicting member forces under wave loadings has been questioned. After reviewing the underlying assumptions and the differences between the two solutions in relation to the two types of loading, results from analyses of a template platform, modeled with 1,428 degrees of freedom, are compared to those obtained by a direct solution in geometric coordinates. It is found that for wave loadings the mode acceleration method gives excellent results, while simple modal superposition can lead to erroneous member forces and stresses even if a large number of modes is used. For earthquake loadings, on the other hand, mode acceleration is not any better than simple modal superposition, which gives answers that may be considered acceptable for design applications.

Tire models for the determination of vehicles' structural loads

Abstract: Finite element techniques and modal testing lead to models capable of simulating the vibrational response of passenger cars due to forces input through the tires. These techniques are well advanced, offering very useful solutions if the applied forces are known. The objective of this work is the development of a tire model capable of supplying good estimates of the magnitude and frequency of forces applied to the wheel spindle during tire/rough-road interactions. This paper presents testing and calculations which show that a modified version of the so-called radial spring model is very useful for this purpose. But the paper also indicates that further work is required to aid in the understanding of high speed impacts because the changing radius of the tire causes longitudinal slip with corresponding potentially large longitudinal forces. For the covering abstract of the conference see TRIS 378303. (Author/TRRL)

Modal Synthesis Modal Correction — Modal Coupling

Abstract: Modal synthesis is a technical term closely related to the development of the finite element technique. Especially in dealing with large structural systems with some thousand physical degrees of freedom, modal synthesis has proved to be an efficient means to reduce computational errors and to minimize computer costs by decomposing the system into several subsystems, each of which is analyzed separately. Based on the results of these component analyses the subsystems can be brought together again to the complete system by means of modal synthesis methods.

Modal vector estimation for closely-spaced-frequency modes

Abstract: Techniques for obtaining improved modal vector estimates for systems with closely spaced frequency modes are discussed. In describing the dynamical behavior of a complex structure modal parameters are often analyzed: undamped natural frequency, mode shape, modal mass, modal stiffness and modal damping. From both an analytical standpoint and an experimental standpoint, identification of modal parameters is more difficult if the system has repeated frequencies or even closely spaced frequencies. The more complex the structure, the more likely it is to have closely spaced frequencies. This makes it difficult to determine valid mode shapes using single shaker test methods. By employing band selectable analysis (zoom) techniques and by employing Kennedy-Pancu circle fitting or some multiple degree of freedom (MDOF) curve fit procedure, the usefulness of the single shaker approach can be extended.

Modal Balancing of a Multi-Mass Flexible Rotor Without Trial Weights

Abstract: In this paper, a procedure is presented to determine the unbalance distribution in a multi-mass flexible rotor system without requiring that trial weights be placed upon the shaft to first determine the influence coefficient matrix of the various balance planes. A modified Nyquist plotting procedure is presented to generate a polar plot of proximity probe measurements for determination of the 90-deg phase shift position of the modal eccentricity. By knowing the rotor modal mass and mode shape, a modal balancing distribution can be calculated. This relationship provides a quick procedure in estimating a first mode balance correction weight in both magnitude and angular location. An application is presented for a steam turbine during the startup of a hydrogen gas compression train. Higher order modal unbalance corrections are shown to be calculable in a similar manner.Copyright © 1982 by ASME

Optimum damping locations for structural vibration control

Abstract: A technique for determining the optimum damper locations and damping rates for a flexible structure has been developed. Using a nonlinear-mathematical-programming algorithm, a diagonal damping matrix is determined such that specified modes have a prescribed modal damping ratio. The design objective is to minimize the total damping effort while constraining the modal damping ratio to be equal or greater than the prescribed amount. Additional constraints require the diagonal elements of the damping matrix to be positive which guarantees that all modes of the damped system will be stable. Results are shown for a uniform free-free beam.

Analysis of Vibration by Component Mode Synthesis Method : Part.2 Forced Vibration, (I)

Abstract: A method is presented to analyze the forced vibration of a complex mechanical structure by using the natural modes of its components. The structure is divided into some components. All components are classified into master components and branch ones. The natural modes of each component are determined separately by the finite element method. The natural modes of all components are synthesized to form generalized system coordinates. The equation of motion under these system coordinates is solved to know the natural frequencies and the natural modes under these system coordinates. Using these natural modes, the response of the forced vibration is solved by the technique of the modal analysis. The forced vibration of three specimens is analyzed by the present method. On the other hand, the forced vibrations of these specimens are measured experimentally under the harmonic exciting force to get the compliances. The calculative results are compared with the experimental ones to check up on the accuracy of the calculation and availability of the proposed method.

Determination of normal modes from measured complex modes

Abstract: A technique is presented for computing a set of normal modes from a set of measured complex modes The number of elements in the modal vectors, which is equal to the number of measurements, can be larger than the number of modes under consideration It is also shown that the practice of normal mode approximation to complex modes can lead to very large errors when the modes are too complex A numerical example and a simulated experiment illustrate the concepts discussed and support the theory presented

Critical parameter selection in the vibration suppression problem of large flexible space structures

Abstract: The paper addresses the application of parameter sensitivity analysis to large flexible space structure models with uncertain parameters such as modal dampings, modal frequencies, mode shape slopes at actuator (sensor) locations. A criterion that allows the specific control objective to influence the parameter selection process is given to delineate the critical parameters in linear regulator problems. The quantitative measure is labeled 'Parameter Error Index (PEI) '. Explicit and simple formulas are obtained in terms of modal data for PEI for the vibration suppression problem of an LSS. The proposed procedure is applied to the 'Purdue Model', a generic two dimensional LSS model. Results are presented which indicate that, for this problem modal frequencies are more critical parameters than mode shapes. This type of information is useful in parameter estimation, robust control design, structure redesign, etc.

DIRECI SYSrrPl PARAMETER IDEBTIPICATIOH OF ~~ICAL STRUCTURES with APPLICATIOE TO MODAL AHALYSIS

Abstract: In this paper a method is described to estimate mechanical structure c haracteristics in terms of mass, stiffness and damping matrices using measured force i nput and response data. The estimated matrices can be used to calculate a consistent set of damped natural frequencies and damping values. mode shapes and modal scale factors for the s tructure. The proposed technique is attractive as an experimental modal analysis method since the estimation of the matrices does not require previous estimation of frequency responses and since the method can be used, without any additional complications, for multiple force i nput structure testing.