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At least 451 records · Page 25

The influence of nonlinear damping on the random response of panels by time domain simulation

The effects of nonlinear damping on the large-deflection response of symmetrically laminated composite panels is studied by time-domain simulation in an attempt to quantify experimentally observed phenomena. A single-mode deflection approach is used in the equations of motion and in the linear and nonlinear damping models. The probability, peak-probability, and spectral-density data are compiled for both strains and deflection. The rms responses as a function of input excitation level are also obtained and compared to linear, equivalent-linearization, and Fokker-Planck equation solutions.

Robinson, J.↗

Component mode synthesis for model order reduction of nonclassically damped systems

Based on a component mode synthesis approach, a model-order-reduction method for linear structures with arbitrary linear damping has been developed. Projection matrices are introduced to make the method applicable to systems having rigid-body freedom. To test the method, eigenvalues of a reduced-order model of a free-free beam with nonproportional damping were compared to exact eigenvalues and to eigenvalues obtained using two other model-reduction strategies. The present model-reduction strategy proved to be decidedly superior.

Craig, Roy R., Jr.↗

Sensitivity of actively damped structures to imperfections and modeling errors

The sensitivity of actively damped response of structures with respect to errors in the structural modeling is studied. Two ways of representing errors are considered. The first approach assumes errors in the form of spatial variations (or imperfections) in the assumed mass and stiffness properties of the structures. The second approach assumes errors due to such factors as unknown joint stiffnesses, discretization errors, and nonlinearities. These errors are represented here as discrepancies between experimental and analytical mode shapes and frequencies. The actively damped system considered here is a direct-rate feedback regulator based on a number of colocated velocity sensors and force actuators. The response of the controlled structure is characterized by the eigenvalues of the closed-loop system. The effects of the modeling errors are thus presented as the sensitivity of the eigenvalues of the closed-loop system. Results are presented for two examples: (1) a three-span simply supported beam controlled by three sensors and actuators, and (2) a laboratory structure consisting of a cruciform beam supported by cables.

Haftka, Raphael T.↗

Nonobstructive Damping For Parts Vibrating In Flows

Vibration-prone parts in fast-flowing liquids or gases damped by simple provision: drill or cast small holes in them and fill holes with particles. Particles absorb vibration energy without obstructing flow. Damping holes add little to cost of manufacturing parts. Reduces masses of parts because masses of inserted particles less than materials removed to make holes.

Panossian, Hagop V.↗

Inertia-Wheel Vibration-Damping System

Proposed electromechanical system would damp vibrations in large, flexible structure. In active vibration-damping system motors and reaction wheels at tips of appendages apply reaction torques in response to signals from accelerometers. Velocity signal for vibrations about one axis processes into control signal to oppose each of n vibrational modes. Various modes suppressed one at a time. Intended primarily for use in spacecraft that has large, flexible solar panels and science-instrument truss assembly, embodies principle of control interesting in its own right and adaptable to terrestrial structures, vehicles, and instrument platforms.

Fedor, Joseph V.↗

Damping Seals And Bearings For A Turbomachine

Improved design for support of rotor in turbopump integrates ball bearings with damping seals and damping bearings. Reduces radial (side) loads on ball bearings, making it possible to increase contact angles to withstand increased transient axial loads. Service lives of bearings prolonged.

Von Pragenau, George L.↗

Enthalpy damping for high Mach number Euler solutions

An improvement on the enthalpy damping procedure currently in use in solving supersonic flow fields is described. A correction based on entropy values is shown to produce a very efficient scheme for simulation of high Mach number three-dimensional flows. Substantial improvements in convergence rates have been achieved by incorporating this enthalpy damping scheme in a finite-volume Runge-Kutta method for solving the Euler equations. Results obtained for blended wing-body geometries at very high Mach numbers are presented.

Moitra, Anutosh↗

Bending rate damping in elastic systems

Preliminary results of an investigation of the bending rate damping model for elastic structures are presented. A model for which the internal damping term is physically plausible and which can accomodate cantilevered boundary conditions is discussed. The model formulation and mathematical foundations are given, and numerical results are discussed.

Banks, H. T.↗

Active damping application to the shuttle RMS

Control Structure Interaction (CSI) is a relatively new technology developed over the last 10 to 15 years for application to large flexible space vehicles. The central issue is recognition that high performance control systems necessary for good spacecraft performance may adversely interact with the dynamics of the spacecraft structures, a problem increasingly aggravated by the large size and reduced stiffness of modern spacecraft structural designs. The CSI analysis and design methods were developed to avoid interactions while maintaining spacecraft performance without exceeding structural capabilities, but they remain largely unvalidated by hardware experiments or demonstrations, particularly in-space flight demonstrations. One recent proposal for a low cost flight validation of CSI technology is to demonstrate active damping augmentation of the Space Shuttle Remote Manipulator System (RMS). An analytical effort to define the potential for such an active damping augmentation demonstration to improve the structural dynamic response of the RMS following payload maneuvers is described. It is hoped that this study will lead to an actual inflight CSI test with the RMS using existing shuttle hardware to the maximum extent possible. By using the existing hardware, the flight demonstration results may eventually be of direct benefit to actual Space Shuttle RMS operations, especially during the construction of the Space Station Freedom.

Gilbert, Michael G.↗

Assessing damping uncertainty in space structures with fuzzy sets

NASA has been interested in the development of methods for evaluating the predictive accuracy of structural dynamic models. This interest stems from the use of mathematical models in evaluating the structural integrity of all spacecraft prior to flight. Space structures are often too large and too weak to be tested fully assembled in a ground test lab. The predictive accuracy of a model depends on the nature and extent of its experimental verification. The further the test conditions depart from in-service conditions, the less accurate the model will be. Structural damping is known to be one source of uncertainty in models. The uncertainty in damping is explored in order to evaluate the accuracy of dynamic models. A simple mass-spring-dashpot system is used to illustrate a comparison among three methods for propagating uncertainty in structural dynamics models: the First Order Method, the Numerical Simulation Method, and the Fuzzy Set Method. The Fuzzy Set Method is shown to bound the range of possible responses and thus to provide a valuable limiting check on the First Order Method near resonant conditions. Fuzzy Methods are a relative inexpensive alternative to numerical simulation.

Ross, Timothy J.↗

Surface tension and viscosity from damped free oscillations of viscous droplets

Damped oscillations of a viscous droplet in vacuum or in an inert gas of negligible density are considered. The dependence of the complex decay factor on the properties of the liquid is investigated for the first time, and numerical results are compared with earlier studies for special cases. A new method is developed to determine both surface tension and viscosity from a single experiment in which the damping rate and frequency of oscillations are measured. The procedure to determine surface tension and viscosity from oscillating levitated liquids is outlined, and results presented for various modes of shape oscillations.

Suryanarayana, P. V. R.↗

Sound transmission loss of integrally damped, curved panels

Results are reported from acoustic transmission-loss measurements on 13 curved Al and composite aircraft-type panels (height 0.81 m, arc length 1.32 m, and curvature radius 2.29 m) without and with integral damping of various types. The fabrication of the panels and the test procedures are described and illustrated with photographs, and the results are presented in graphs. It is found that the loss of a curved panel exceeds the mass-law-predicted loss for a flat panel of the same material and thickness at frequencies below the ring frequency. At higher frequencies, the curved-panel loss is lower than the mass-law loss, being proportional to 20 time the log of thickness. Integral damping is found to be effective both below and above the ring frequency, but different mechanisms are responsible in each case.

Grosveld, Ferdinand W.↗

Discovery of a Ly-alpha galaxy near a damped Ly-alpha absorber at z = 2.3

The detection of a galaxy associated with the damped Ly-alpha absorbing cloud seen at z = 2.309 toward the QSO PHL 957 is reported. In addition to a strong but narrow Ly-alpha emission line and weaker C IV and He II lines, the object shows continuum at V about 24, with a slope rising slightly toward the red. This is similar to what is seen in high-redshift radio galaxies, but this galaxy does not correspond to any known radio source. The detected emission lines and continuum are most easily interpreted as light from hot, recently formed stars, implying a sizable star formation rate and a scarcity of dust. The spatial correlation of the absorbing cloud and the companion galaxy supports the interpretation of damped Ly-alpha clouds as objects fundamentally different from the lower column density Ly-alpha forest clouds, which show weak or no clustering.

Lowenthal, James D.↗

Effect of heat treatment on stiffness and damping of SiC/Ti-15-3

The effect of heat treatment on material properties of SiC/Ti-15-3 was measured by vibration tests. Heat treatment changes the microstructure, which was found to stiffen the matrix and reduce its damping capacity. Test results indicate how these changes in the matrix affect the corresponding properties of the composite. Measurements show that heat treatment affects damping properties of the composite to a greater extent than stiffness properties. The extent of change in mechanical properties is shown to depend on heat treatment temperature and exposure time.

Grady, Joseph E.↗

Damping and vibration considerations for the design of optical systems in a launch/space environment

Engineering philosophies for the design of optical systems launched into space and operating in a vacuum or cryovacuum environment are reviewed. Particular attention is given to sources of energy dissipation which are usually lumped under a single modal parameter denoted as the equivalent viscous damping coefficient. Caging and/or damping system components or application of viscoelastic materials and/or dry friction devices are considered to be alternative methods for stabilizing instruments sensitive to motion.

Richard, Ralph M.↗

The effects of time-dependent excitation on the oscillatory motion of viscously damped laminated composite flat panels

Analytical estimates are used to describe the effects of transverse shear deformation, transverse normal stress, damping, and heterogeneity on the dynamic responses of laminated composite structures. The higher-order plate model is delineated with attention given to applications for viscously damped cross-ply symmetrically laminated plates subject to different time-history excitations. These effects are considered in the context of sonic-boom and blast loadings, and comparisons are made between the results of higher-order plate theory, first-order transverse-shear deformation, and classical theory. The results of the present higher-order model are found to be immune to the changes incurred by adding transverse viscous into the analysis. The responses of pulse-loadings due to symmetrical sonic boom and a triangular blast are found to be similar for certain times and pressure fields. It is shown that only the higher-order plate theory permits the accurate evaluation of frequency spectrum and time-history structural response of composite laminates.

Nosier, A.↗

Theory Of Damping In Composite Laminates

Integrated mathematical model predicts vibration-damping properties of laminated fiber/matrix composite material from micromechanical level to laminate level. Model accounts simultaneously for wider, more realistic variety of laminate configurations and physical phenomena, including asymmetrically oriented plies, anisotropic fiber damping properties, hygral and thermal effects, and coupling among hygral, thermal, and mechanical effects.

Saravanos, D. A.↗

Assigning Damping Parameters To Component Dynamical Modes

Damping parameters found by solving nonlinear optimization problem. Paper discusses assignment of matrices of damping parameters to mathematical models of vibrations and rotations of interconnected flexible components of large, flexible structure. Such models truncated at component level to reduce amount of computation necessary to simulate dynamics of structure numerically at both component and system levels.

Lee, Allan Y.↗