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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Modal analysis using a Fourier analyzer, curve-fitting, and modal tuning

The proposed modal test program differs from single-input methods in that preliminary data may be acquired using multiple inputs, and modal tuning procedures may be employed to define closely spaced frquency modes more accurately or to make use of frequency response functions (FRF's) which are based on several input locations. In some respects the proposed modal test proram resembles earlier sine-sweep and sine-dwell testing in that broadband FRF's are acquired using several input locations, and tuning is employed to refine the modal parameter estimates. The major tasks performed in the proposed modal test program are outlined. Data acquisition and FFT processing, curve fitting, and modal tuning phases are described and examples are given to illustrate and evaluate them.

Craig, R. R., Jr.↗
NASA NTRS1983

Experimental modal analysis

Experimental modal analysis is extremely important with regard to verification of analytical models, identification of vibration and acoustic problems, and structural modification and sensitivity analysis. With the expanding access of the testing environment to computational power, the complexity of existing approaches, as well as the development of new approaches, to the estimation of modal parameters has grown tremendously. Currently, the state of the art in experimental modal analysis involves methods that can be grouped in four categories: forced normal mode excitation method, frequency response function method, damped complex exponential function method, and mathematical input-output model methods. The theoretical basis of each of these general approaches, with appropriate references, is reviewed briefly.

Allemang, R. J.↗
NASA NTRS1984

Identification of dynamic coefficients of annular turbulent seals

An identification procedure to determine dynamic coefficients of annular turbulent seals in turbopumps is presented. Measurements were carried out at a built test rig with two symmetrical arranged seals. A rigid rotating shaft is surrounded by an elastically supported housing, which is excited by impact forces. The relative radial motion between the rotating parts and the housing, respectively between the seal surfaces, is measured by displacement pick-ups and from the time signals complex frequency response functions can be calculated. Finally an analytical model, depending on the seal parameters, is fitted to the measured data, to find the dynamic coefficients.

Nordmann, R.↗
NASA NTRS1984

Direct measurement of MPD thrust

An integrally mounted accelerometer has been used to obtain direct thrust measurements during operation of a magnetoplasmadynamic (MPD) thruster. The structural oscillations which can mask the accelerometer output have been ameliorated by stiffening the thruster to increase the lowest resonant frequency above the 1kHz region of interest. In addition, signal conditioning is used to provide a high signal-to-noise accelerometer output waveform. Impulse response measurement of the thruster frequency response function has proven to be an effective means of selecting the signal filter and estimating measurement errors. Comparison of the total impulse derived from the accelerometer output with that from a swinging arm thrust stand indicates 3 percent agreement, providing some confidence in the viability of the accelerometer thrust measurement technique.

Berg, J. M.↗
NASA NTRS1985

Robust failure detection filters

The robustness of detection filters applied to the detection of actuator failures on a free-free beam is analyzed. This analysis is based on computer simulation tests of the detection filters in the presence of different types of model mismatch, and on frequency response functions of the transfers corresponding to the model mismatch. The robustness of detection filters based on a model of the beam containing a large number of structural modes varied dramatically with the placement of some of the filter poles. The dynamics of these filters were very hard to analyze. The design of detection filters with a number of modes equal to the number of sensors was trivial. They can be configured to detect any number of actuator failure events. The dynamics of these filters were very easy to analyze and their robustness properties were much improved. A change of the output transformation allowed the filter to perform satisfactorily with realistic levels of model mismatch.

Sanmartin, A. M.↗
NASA NTRS1985

Some new results concerning the dynamic behavior of annular turbulent seals

The dynamic characteristics of annular turbulent seals applied in high pressure turbopumps can be described by stiffness, damping, and inertia coefficients. An improved procedure is presented for determining these parameters by using measurements made with newly developed test equipment. The dynamic system seal, consisting of the fluid between the cylindrical surfaces of the rotating shaft and the housing, is excited by test forces (input), and the relative motion between the surfaces (output) is measured. Transformation of the input and output time signals into the frequency domain leads to frequency response functions. An analytical model, depending on the seal parameters, is fitted to the measured data in order to identify the dynamic coefficients. Some new results are reported that show the dependencies of these coefficients with respect to the axial and radial Reynolds numbers and the geometrical data of the seal.

Massmann, H.↗
NASA NTRS1985

Multiple-input experimental modal analysis

The development of experimental modal analysis techniques is reviewed. System and excitation assumptions are discussed. The methods examined include the forced normal mode excitation method, the frequency response function method, the damped complex exponential response method, the Ibrahim time domain approach, the polyreference approach, and mathematical input-output model methods. The current trend toward multiple input utilization in the estimation of system parameters is noted.

Allemang, R. J.↗
NASA NTRS1987

Test procedures for detection of in-flight propeller-induced structure-borne noise

A potentially important source of structure-borne interior noise transmission in advanced turboprop aircraft is the impingement of the propeller wake/vortex on downstream aerodynamic surfaces. It can only be safely assumed that this potential source of interior noise may well hold up achievable interior noise levels unless noise control measures are conscientiously incorporated into the aircraft design. Through the use of a laboratory based test apparatus, techniques were developed to estimate the level of in-flight structure-borne noise transmission from combined frequency response function ground testing and in-flight structural response measurements. All phases of the test procedure were simulated in the laboratory and the expected level of accuracy of the procedure is addressed.

Unruh, J. F.↗
NASA NTRS1987

The use of a numerical filter to correct airborne temperature measurements for the effects of sensor lag

A numerical filter for transforming measured temperature signals into a close approximation of the actual temperature signal is described. The filter is derived by minimizing the mean-square error of the system, and assuming a knowledge of the characteristics of the sensing element and its housing. The equation representing the frequency-response function of the numerical filter is given. Input and output spectra for a filter applied to a case with negligible noise and a noise level of 1.5 percent of the total power in the input spectrum are analyzed, and the numerical weights for these two cases are calculated. Phase angle and gain for the entire system are examined. It is noted that the filter can enhance spectral components as high as 8 Hz with little phase and gain degradation over the bandwidth.

Ritter, John A.↗
NASA NTRS1987

Detection of in-flight propeller-induced structure-borne noise

A potentially important source of structure-borne interior noise transmission in advanced turboprop aircraft is the impingement of the propeller wake/vortex on downstream aerodynamic surfaces. It can be safely assumed that this potential source of interior noise may well adversely affect achievable interior noise levels unless noise control measures are conscientiously incorporated into the aircraft design. Through the use of a laboratory-based test apparatus, techniques were developed to estimate the level of in-flight structure-borne noise transmission from combined frequency response function ground testing and in-flight structural response measurements. All phases of the procedure were simulated in the laboratory and the expected level of accuracy of the procedure is addressed.

Unruh, J. F.↗
NASA NTRS1988

Analytical investigation of the dynamics of tethered constellations in earth orbit

This Quarterly Report on Tethering in Earth Orbit deals with three topics: (1) Investigation of the propagation of longitudinal and transverse waves along the upper tether. Specifically, the upper tether is modeled as three massive platforms connected by two perfectly elastic continua (tether segments). The tether attachment point to the station is assumed to vibrate both longitudinally and transversely at a given frequency. Longitudinal and transverse waves propagate along the tethers affecting the acceleration levels at the elevator and at the upper platform. The displacement and acceleration frequency-response functions at the elevator and at the upper platform are computed for both longitudinal and transverse waves. An analysis to optimize the damping time of the longitudinal dampers is also carried out in order to select optimal parameters. The analytical evaluation of the performance of tuned vs. detuned longitudinal dampers is also part of this analysis. (2) The use of the Shuttle primary Reaction Control System (RCS) thrusters for blowing away a recoiling broken tether is discussed. A microcomputer system was set up to support this operation. (3) Most of the effort in the tether plasma physics study was devoted to software development. A particle simulation code has been integrated into the Macintosh II computer system and will be utilized for studying the physics of hollow cathodes.

Lorenzini, Enrico C.↗
NASA NTRS1989

Aircraft propeller induced structure-borne noise

A laboratory-based test apparatus employing components typical of aircraft construction was developed that would allow the study of structure-borne noise transmission due to propeller induced wake/vortex excitation of in-wake structural appendages. The test apparatus was employed to evaluate several aircraft installation effects (power plant placement, engine/nacelle mass loading, and wing/fuselage attachment methods) and several structural response modifications for structure-borne noise control (the use of wing blocking mass/fuel, wing damping treaments, and tuned mechanical dampers). Most important was the development of in-flight structure-borne noise transmission detection techniques using a combination of ground-based frequency response function testing and in-flight structural response measurement. Propeller wake/vortex excitation simulation techniques for improved ground-based testing were also developed to support the in-flight structure-borne noise transmission detection development.

Unruh, James F.↗
NASA NTRS1989

System identification test using active members

A modal test using active members as the excitation source has been performed on the Precision Truss. Using the step sine testing technique, the frequency response functions are obtained and the modal parameters are extracted by the curve-fitting method. Total of 10 global modes and 3 local modes are obtained. The results are compared with those obtained by the conventional external excitation test.

Chen, Jay-Chung↗
NASA NTRS1990

Active control of multi-dimensional random sound in ducts

Previous work has demonstrated how active control may be applied to the control of random noise in ducts. These implementations, however, have been restricted to frequencies where only plane waves are propagating in the duct. In spite of this, the need for this technology at low frequencies has progressed to the point where commercial products that apply these concepts are currently available. Extending the frequency range of this technology requires the extension of current single channel controllers to multi-variate control systems as well as addressing the problems inherent in controlling higher order modes. The application of active control in the multi-dimensional propagation of random noise in waveguides is examined. An adaptive system is implemented using measured system frequency response functions. Experimental results are presented illustrating attained suppressions of 15 to 30 dB for random noise propagating in multiple modes.

Silcox, R. J.↗
NASA NTRS1989

Control of a flexible planar truss using proof mass actuators

A flexible structure was modeled and actively controlled by using a single space realizable linear proof mass actuator. The NASA/UVA/UB actuator was attached to a flexible planar truss structure at an optimal location and it was considered as both passive and active device. The placement of the actuator was specified by examining the eigenvalues of the modified model that included the actuator dynamics, and the frequency response functions of the modified system. The electronic stiffness of the actuator was specified, such that the proof mass actuator system was tuned to the fourth structural mode of the truss by using traditional vibration absorber design. The active control law was limited to velocity feedback by integrating of the signals of two accelerometers attached to the structure. The two lower modes of the closed-loop structure were placed further in the LHS of the complex plane. The theoretically predicted passive and active control law was experimentally verified.

Minas, Constantinos↗
NASA NTRS1990

Structure-borne noise estimates for the PTA aircraft

Estimates of the level of in-flight structure-borne noise transmission in the Propfan Test Assessment Aircraft were carried out for the first three blade passage frequencies. The procedure used combined the frequency response functions of wing strain to cabin sound pressure level (SPL) response obtained during ground test with in-flight measured wing strain response data. The estimated cabin average in-flight structure-borne noise levels varied from 64 to 84 dB, with an average level of 74 dB. The estimates showed little dependence on engine/propeller power, flight altitude, or flight Mach number. In general, the bare cabin noise levels decreased with increasing propeller tone, giving rise to a plausible structure-borne noise transmission problem at the higher blade passage tones. Without knowledge of the effects of a high insertion loss side wall treatment on structure-borne noise transmission, no quantitative conclusions could be made.

Unruh, James F.↗
NASA NTRS1989

On-orbit vibration testing for space structures

A simulated on-orbit modal test using active members as the excitation source has been performed on the Precision Truss. Using the step sine testing technique, the frequency response functions are obtained and the modal parameters are extracted by the curve-fitting method. A total of 10 global modes and 3 local modes are obtained. The results are compared with those obtained by the conventional external excitation test.

Chen, Jay-Chung↗
NASA NTRS1991

Time domain modal identification/estimation of the mini-mast testbed

The Mini-Mast is a 20 meter long 3-dimensional, deployable/retractable truss structure designed to imitate future trusses in space. Presented here are results from a robust (with respect to measurement noise sensitivity), time domain, modal identification technique for identifying the modal properties of the Mini-Mast structure even in the face of noisy environments. Three testing/analysis procedures are considered: sinusoidal excitation near resonant frequencies of the Mini-Mast, frequency response function averaging of several modal tests, and random input excitation with a free response period.

Roemer, Michael J.↗