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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

On damping mechanisms in beams

A partial differential equation model of a cantilevered beam with a tip mass at its free end is used to study damping in a composite. Four separate damping mechanisms consisting of air damping, strain rate damping, spatial hysteresis and time hysteresis are considered experimentally. Dynamic tests were performed to produce time histories. The time history data is then used along with an approximate model to form a sequence of least squares problems. The solution of the least squares problem yields the estimated damping coefficients. The resulting experimentally determined analytical model is compared with the time histories via numerical simulation of the dynamic response. The procedure suggested here is compared with a standard modal damping ratio model commonly used in experimental modal analysis.

Banks, H. T.↗
NASA NTRS1992

Active member bridge feedback control for damping augmentation

An active damping augmentation approach using active members in a structural system is described. The problem of maximizing the vibration damping in a lightly damped structural system is considered using the analogy of impedance matching between the load and source impedances in an electrical network. The proposed active damping augmentation approach therefore consists of finding the desired active member impedances that maximize the vibration damping, and designing a feedback control in order to achieve desired active member impedances. This study uses a bridge feedback concept that feeds back a combination of signals from sensors of the axial force and relative velocity across the active member to realize the desired active member impedance. The proposed active damping augmentation approach and bridge feedback concept were demonstrated on a three-longeron softly suspended truss structure.

Chen, Gun-Shing↗
NASA NTRS1991

Damping characterization in large structures

This research project has as its main goal the development of methods for selecting the damping characteristics of components of a large structure or multibody system, in such a way as to produce some desired system damping characteristics. The main need for such an analytical device is in the simulation of the dynamics of multibody systems consisting, at least partially, of flexible components. The reason for this need is that all existing simulation codes for multibody systems require component-by-component characterization of complex systems, whereas requirements (including damping) often appear at the overall system level. The main goal was met in large part by the development of a method that will in fact synthesize component damping matrices from a given system damping matrix. The restrictions to the method are that the desired system damping matrix must be diagonal (which is almost always the case) and that interbody connections must be by simple hinges. In addition to the technical outcome, this project contributed positively to the educational and research infrastructure of Tuskegee University - a Historically Black Institution.

Eke, Fidelis O.↗
NASA NTRS1993

Dynamic characteristics of specialty composite structures with embedded damping layers

Damping mechanics for simulating the damped dynamic characteristics in specialty composite structures with compliant interlaminar damping layers are presented. Finite-element based mechanics incorporating a discrete layer (or layer-wise) laminate damping theory are utilized to represent general laminate configurations in terms of lay-up and fiber orientation angles, cross-sectional thickness, shape, and boundary conditions. Evaluations of the method with exact solutions and experimental data illustrate the accuracy of the method. Additional applications investigate the potential for significant damping enhancement in angle-ply composite laminates with cocured interlaminar damping layers.

Saravanos, D. A.↗
NASA NTRS1993

Analysis of passive damping in thick composite structures

Computational mechanics for the prediction of damping and other dynamic characteristics in composite structures of general thicknesses and laminations are presented. Discrete layer damping mechanics that account for the representation of interlaminar shear effects in the material are summarized. Finite element based structural mechanics for the analysis of damping are described, and a specialty finite element is developed. Applications illustrate the quality of the discrete layer damping mechanics in predicting the damped dynamic characteristics of composite structures with thicker sections and/or laminate configurations that induce interlaminar shear. The results also illustrate and quantify the significance of interlaminar shear damping in such composite structures.

Saravanos, D. A.↗
NASA NTRS1993

Human-in-the-loop evaluation of RMS Active Damping Augmentation

Active Damping Augmentation is the insertion of Controls-Structures Integration Technology to benefit the on-orbit performance of the Space Shuttle Remote Manipulator System. The goal is to reduce the vibration decay time of the Remote Manipulator System following normal payload maneuvers and operations. Simulation of Active Damping Augmentation was conducted in the realtime human-in-the-loop Systems Engineering Simulator at the NASA Johnson Space Center. The objective of this study was to obtain a qualitative measure of operational performance improvement from astronaut operators and to obtain supporting quantitative performance data. Sensing of vibratory motions was simulated using a three-axis accelerometer mounted at the end of the lower boom of the Remote Manipulator System. The sensed motions were used in a feedback control law to generate commands to the joint servo mechanisms which reduced the unwanted oscillations. Active damping of the Remote Manipulator System with an attached 3990 lb. payload was successfully demonstrated. Six astronaut operators examined the performance of an Active Damping Augmentation control law following single-joint and coordinated six-joint translational and rotational maneuvers. Active Damping Augmentation disturbance rejection of Orbiter thruster firings was also evaluated. Significant reductions in the dynamic response of the 3990 lb. payload were observed. Astronaut operators recommended investigation of Active Damping Augmentation benefits to heavier payloads where oscillations are a bigger problem (e.g. Space Station Freedom assembly operators).

Demeo, Martha E.↗
NASA NTRS1956

Determination of the structural damping coefficients of six full-scale helicopter rotor blades of different materials and methods of construction

Results of an experimental investigation of the structural damping of six full-scale helicopter rotor blades, made to determine the variation of structural damping with materials and methods of construction, are presented. The damping of the blades was determined for the first three flapwise bending modes, first chordwise bending mode, and first torsion mode. The contribution of structural damping to the total damping of the blades is discussed for several aerodynamic conditions in order to point out situations where structural damping is significant.

WINGS, ROTATING - EXPERIMENTAL STUDIES↗
NASA NTRS1951

Damping in a Roll of a Missile Configuration with a Modified Triangular Wing and a Cruciform Tail at a Mach Number of 1.52

The damping-in-Toll stability derivatives of a missile configuration and its components were determined both experimentally and theoretically. The tests were conducted at a Mach number of 1.52 and at a Reynolds number, based on the mean aerodynamic chord of the wing, of 0.82 x 10(exp 6). The experimental damping derivative of the wing-body combination was 67 percent of the theoretical value. The difference is believed to have resulted mainly from the fact that the theory is not strictly applicable when the Mach number normal to the leading edge is almost unity, which was the case in the present investigation. For the tail-body combination the damping derivative was 86 percent of the theoretical value. In this case, the difference is believed to have been caused partially by mutual interference between the tail surfaces and partially by the low Reynolds number of the flow over the tail. It was found that the damping of the complete configuration was not equal to the sum of the damping derivatives of the components because of the effect of the wing downwash on the damping of the tail.

Scherrer, Richard↗
NASA NTRS1994

Structural damping studies at cryogenic temperatures

Results of an engineering study to measure changes in structural damping properties of two cryogenic wind tunnel model systems and two metallic test specimens at cryogenic temperatures are presented. Data are presented which indicate overall, a trend toward reduced structural damping at cryogenic temperatures (-250 degrees F) when compared with room temperature damping properties. The study was focused on structures and materials used for model systems tested in the National Transonic Facility (NTF). The study suggests that the significant reductions in damping at extremely cold temperatures are most likely associated with changes in mechanical joint compliance damping rather than changes in material (solid) damping.

Young, Clarence P., Jr.↗
NASA NTRS1994

Weakly damped modes in star clusters and galaxies

A perturber may excite a coherent mode in a star cluster or galaxy. If the stellar system is stable, it is commonly assumed that such a mode will be strongly damped and therefore of little practical consequence other than redistributing momentum and energy deposited by the perturber. This paper demonstrates that this assumption is false; weakly damped modes exist and may persist long enough to have observable consequences. To do this, a method for investigating the dispersion relation for spherical stellar systems and for locating weakly damped modes in particular is developed and applied to King models of varying concentration. This leads to a following remarkable result: King models exhibit very weakly damped m = 1 modes over a wide range of concentration (0.67 less than or equal to c less than or equal to 1.5 have been examined). The predicted damping time is tens of hundreds of crossing times. This mode causes the peak density to shift from and slowly revolve about the initial center. The existence of the mode is supported by n-body simulation. Higher order modes and possible astronomical consequences are discussed. Weakly damped modes, for example, may provide a neutral explanation for observed discrepancies between density and kinematic centers in galaxies, off-center nuclei, the location of velocity cusps due to massive black holes, and both m = 1 and barlike disturbances of disks enbedded in massive halos or spheroids. Gravitational shocking may excite the m = 1 mode in globular clusters, which could modify their subsequent evolution and displace the positions of exotic remnants.

Weinberg, Martin D.↗
NASA NTRS1994

Landau damping of auroral hiss

Auroral hiss is observed to propagate over distances comparable to an Earth radius from its source in the auroral oval. The role of Landau damping is investigated for upward propagating auroral hiss. By using a ray tracing code and a simplified model of the distribution function, the effect of Landau damping is calculated for auroral hiss propagation through the environment around the auroral oval. Landau damping is found to be the likely mechanism for explaining some of the one-sided auroral hiss funnels observed by Dynamics Explorer 1. It is also found that Landau damping puts a lower limit on the wavelength of auroral hiss. Poleward of the auroral oval, Landau damping is found in a typical case to limit omega/k(sub parallel) to values of 3.4 x 10(exp 4) km/s or greater, corresponding to resonance energies of 3.2 keV or greater and wavelengths of 2 km or greater. For equatorward propagation, omega/k(sub parallel) is limited to values greater than 6.8 x 10(exp 4) km/s, corresponding to resonance energies greater than 13 keV and wavelengths greater than 3 km. Independent estimates based on measured ratios of the magnetic to electric field intensity also show that omega/k(sub parallel) corresponds to resonance energies greater than 1 keV and wavelengths greater than 1 km. These results lead to the difficulty that upgoing electron beams sufficiently energetic to directly generate auroral hiss of the inferred wavelength are not usually observed. A partial transmission mechanism utilizing density discontinuities oblique to the magnetic field is proposed for converting auroral hiss to wavelengths long enough to avoid damping of the wave over long distances. Numerous reflections of the wave in an upwardly flared density cavity could convert waves to significantly increased wavelengths and resonance velocities.

Morgan, D. D.↗
NASA NTRS1995

A Faraday rotation search for magnetic fields in quasar damped Ly alpha absorption systems

We present the results of a Faraday rotation survey of 61 radio-bright QSOs conducted at the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA). The Galactic contribution to the Faraday rotation is estimated and subtracted to determine the extragalactic rotation measure (RRM) for each source. Eleven of these QSOs are known to exhibit damped Ly alpha absorption. The rate of incidence of significant Faraday rotation of these 11 sources is compared to the remaining 50 and is found to be higher at the 99.8% confidence level. However, as this is based upon only two detections of Faraday rotation in the damped Ly alpha sample, the result is only tentative. If the two detections in the damped Ly alpha sample are dug to the absorbing systems, then the inferred rotation measure induced by these systems is roughly 250 rad/sq m. The two detections were for the two lowest redshift absorbers in the sample. We find that a rotation measure of 250 rad/sq m would have gone undetected for any other absorber in the damped Ly alpha sample due to the 1/(1 + 2) squared dilution of the observed RRM with redshift. Thus the data are consistent with, but do not prove, the hypothesis that Faraday rotation is a generic property of damped Ly alpha absorbers. We do not confirm the suggestion that the amplitude of RRMs increases with redshift. Rather, the data are consistent with no redshift evolution. We find that the uncertainty in the estimation of the Galactic rotation measure (GRM) is a more serious problem than previously realized for extra-galactic Faraday rotation studies of QSO absorbers. A careful analysis of current methods for estimating GRM indicate that it can be determined to an accuracy of about 15 - 20 rad/sq m. Previous studies underestimated this uncertainty by more than a factor of 2. Due to this uncertainty, rotation measures such as we suspect are associated with damped Ly alpha absorption systems can only be detected at redshifts less than z approximately equal 1.

Oren, Abraham L.↗
NASA NTRS1996

Elastic Moduli and Damping of Vibrational Modes of Aluminum/Silicon Carbide Composite Beams

Elastic and shear moduli were determined for two aluminum matrix composites containing 20 and 40 volume percent discontinuous silicon carbide, respectively, using transverse, longitudinal, and torsional vibrational modes of specimens prepared as thin beams. These moduli are consistent with those determined from stress-strain measurements. The damping factors for these modes were also determined. Thermal properties are used to show that part of the damping of transverse modes is caused by the transverse thermal currents discussed by C. Zener (thermo-elastic damping); this damping is frequency-dependent with a maximum damping factor of approximately 0.002. The remaining damping is frequency-independent, and has roughly similar values in transverse, longitudinal, and torsional modes: approximately 0.0001.

Leidecker, Henning↗
NASA NTRS1997

Hybrid Damping System for an Electronic Equipment Mounting Shelf

The objective of this study was to design and construct a vibration control system for an electronic equipment shelf to be evaluated in the NASA Dryden FTF-11. The vibration control system was a hybrid system which included passive and active damping techniques. Passive damping was fabricated into the equipment shelf using ScothDamp(trademark) damping film and aluminum constraining layers. Active damping was achieved using a two channel active control circuit employing QuickPack(trademark) sensors and actuators. Preliminary Chirp test results indicated passive damping smoothed the frequency response while active damping reduced amplitudes of the frequency response for most frequencies below 500Hz.

Voracek, David↗
NASA NTRS1959

Some Effects of Yaw Damping on Airplane Motions and Vertical-Tail Loads in Turbulent Air

Results of analytical and flight studies are presented to indicate the effect of yaw damping on the airplane motions and the vertical-tail loads in rough air. The analytical studied indicate a rapid reduction in loads on the vertical tail as the damping is increased up to the point of damping the lateral motions to 1/2 amplitude in one cycle. Little reduction in load is obtained by increasing the lateral damping beyond that point. Flight measurements made in rough air at 5,000 and 35,000 feet on a large swept-wing bomber equipped with a yaw damper show that the yaw damper decreased the loads on the vertical tail by about 50 percent at 35,000 feet. The reduction in load at 5,000 feet was not nearly as great. Measurements of the pilot's ability to damp the lateral motions showed that the pilot could provide a significant amount of damping but that manual control was not as effective as a yaw damper in reducing the loads.

Funk, Jack↗
NASA NTRS1959

A Flight Investigation to Determine the Lateral Oscillatory Damping Acceptable for an Airplane in the Landing Approach

An F-86E airplane, in which servo actuation of the ailerons and rudder provides artificial variation of the important lateral and directional aerodynamic stability parameters, has been flown by test pilots of the NASA, U.S. Air Force, and one aircraft manufacturer to determine satisfactory and acceptable levels of lateral oscillatory damping in the landing approach. In addition to normal operational use, particular consideration was given to the emergency condition of failure of stability-augmentation equipment. In this study, the pilots' opinions of the airplane dynamic stability and control characteristics in smooth and simulated rough air have been recorded according to a numerical rating scale. The results are presented in the form of boundaries in terms of cycles to damp to half amplitude, 1/C(sub 1/2), or time to damp to half amplitude, 1/T(1/2) and bank-to-sideslip ratio, and are discussed in relation to existing flying-qualities criteria. Though the present results, which were obtained at 170 knots indicated airspeed and 10,000-feet altitude, indicated that increased damping is required with increased bank-to-sideslip ratio (as found in previous work), consideration of the dampers-failed condition indicated a great reduction in the minimum acceptable damping. At moderate values of bank-to-sideslip ratio, effects of lateral-oscillation period on pilot-opinion variation with damping appeared to be taken into account by use of the parameter 1/T(sub 1/2).

McNeill, Walter E.↗
NASA NTRS1997

Hybrid Damping System for an Electronic Equipment Mounting Shelf

The objective of this study was to design and construct a vibration control system for an electronic equipment shelf to be evaluated in the NASA Dryden FTF-II. The vibration control system was a hybrid system which included passive and active damping techniques. Passive damping was fabricated into the equipment shelf using ScothDamp(trademark) damping film and aluminum constraining layers. Active damping was achieved using a two channel active control circuit employing QuickPack(trademark) sensors and actuators. Preliminary Chirp test results indicated passive damping smoothed the frequency response while active damping reduced amplitudes of the frequency response for most frequencies below 500Hz.

Voracek, David↗
NASA NTRS1961

An Experimental Investigation of the Damping of Liquid Oscillations in an Oblate Spheroidal Tank With and Without Baffles

An experimental investigation was conducted to determine the damping of the fundamental antisymmetric mode of oscillation of liquids contained in an oblate spheroidal tank. The decay of the fundamental mode was studied for a range of liquid depths in tanks with and without baffles. In the investigation of baffle effects, ring and cruciform baffles of various sizes were fixed at different locations within the tank. Data presented show the variation of the damping factor with tank fullness and with baffle type, width, location, and orientation as well as the effects of the amplitude of the liquid oscillations and of small variations in the liquid kinematic viscosity on the damping factor. The results of the investigation indicate that the addition of ring baffles to the tank results in an increase in the available effective damping when the baffle plane is in a region near the equilibrium liquid surface, and that cruciform baffles are effective in the damping of the fundamental mode in the near-empty tank. No apparent changes in damping for the tanks having ring baffles were observed as the kinematic viscosity of the liquid was varied over a small range.

Stephens, David G.↗