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Published Articles

The Volume 11, No 3, September 2006

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Vibroacoustical Damping Diagnostics: Complex Frequency Response Function versus its Magnitude

L. Gelman, P. Jenkin, I. Petrunin, M. J. Crocker


In this paper new generic approach is proposed for damping diagnostics based on a forced oscillation method with sinusoidal and random white noise excitation. The main contribution of the paper is the development of a novel diagnostic technique. The proposed new approach and the traditional magnitude approach are compared and it is shown that the proposed approach provides an essential effectiveness gain over that of the magnitude approach.

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An Experimental and Numerical Study of Sound Propagation from a Supersonic Jet Passing through a Rigid-Walled Duct with a J-Deflector

Max Kandula


The generation, propagation, and radiation of sound (the acoustical characteristics) from a perfectly expanded Mach 2.5 cold supersonic jet of 25.4 mm exit diameter flowing through an enclosed rigid-walled duct with an upstream J-deflector have been studied experimentally and numerically. In the experiments, the nozzle is mounted vertically, with the nozzle exit plane at a height of 73 jet diameters above ground level. Relative to the nozzle exit plane, the location of the duct inlet is varied at 10, 5, and ?1 jet diameters. Far-field sound pressure levels were obtained at 54 jet diameters above ground with the aid of acoustical sensors equally spaced around a circular arc of radius equal to 80 jet diameters. Data on the acoustic field were obtained with and without the duct. The numerical simulations were carried out with the help of the OVERFLOW Navier-Stokes computational fluid dynamics (CFD) code in conjunction with a one-equation turbulence model. While the near-field sound sources were computed by the CFD code, the far-field sound was evaluated by the Kirchhoff surface integral formulation. Predictions of the far-field directivity of the overall sound pressure level (OASPL) agree satisfactorily with the experimental data. CFD calculations also suggest that there was significant entrainment of air into the duct, with the mass flow rate of entrained air being about three times the jet exit mass flow rate.

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Dynamic Characterisation of Base-Isolated Structures Using Analytical Shear-Beam Model

Vasant A. Matsagar, R. S. Jangid


An analytical investigation of the dynamic characteristics of a shear beam structure supported on linear laminated rubber bearings is carried out. The superstructure is modelled as a continuum shear beam. The base-raft forming part of the base-isolated building is modelled as mass lumped at the lower end of the shear beam, while the effective linear stiffness is assumed for the bearings. In addition to the base-isolated shear beam, closed-form solutions for two auxiliary shear beams with free-spring and unconstrained end conditions are obtained for comparison. The frequencies of the base-isolated shear beam are found to fall into a range bounded by those of the two auxiliary shear beams, forming two limiting cases of base-isolated shear beams. It is seen that the higher modes of the base-isolated beam are practically identical with those of the unconstrained shear beam, and therefore, orthogonal to the horizontal ground motion. The contribution of higher modes towards the response of superstructure is therefore negligible.

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Detection of a Helicopter Input Pinion Bearing Fault Using Interstitial Envelope Analysis

Qiao Sun, Sally Anne McInerny, William Hardman


Data recorded in an endurance test sequence during which the starboard pinion support bearing failed are analysed. Fault indicators, previously developed for this bearing, fail to detect its degradation until after the first chip light. Further analyses show that a cage fault is detectable in the earliest recorded dataset when the acceleration signal is bandpass filtered between the first and second GM frequencies, an interstitial frequency band. The starboard pinion bearing cage fault can also be seen in the earliest recorded data at the port main location, indicating that the distorted cage affects the power transmitted from the starboard main pinion to the main bevel gear and can, therefore, be detected on the other side of the gear box. Impact generated high frequency energy, as reflected in the kurtosis of the signal in a high frequency bandpass range, is not detectable until after the first chip light. The initial bearing fault, perhaps a cage distortion, may have led to the eventual spalling of the rollers. The results presented here can be used to develop prognostics that may safely permit extended operation of a main gearbox after the removal of a pinion with a spalled integral raceway.

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Autoregressive Order Selection for Rotating Machinery

Suguna Thanagasundram and Fernando Soares Schlindwein


This paper provides a practical rule for determining the minimum model order for Autoregressive (AR) based spectrum analysis of data from rotating machinery. The use of parametric methods for spectral estimation, though having superior frequency resolution than Fast Fourier Transform (FFT) based methods, has remained less favoured mainly because of the difficulties in estimating the model order. The minimum model order pmin required is the ratio of the sampling rate and the rotating speed of the machine. This is the number of samples in one shaft revolution. Traditional model order selection criteria, Akaike Information Criterion (AIC), Final Prediction Error (FPE), Minimum Description Length (MDL), Criterion Autoregressive Transfer-function (CAT), and Finite Information Criterion (FIC) are used to estimate the optimal order. These asymptotic criteria for model order estimation are functions of the prediction error and the optimal order of an AR model is chosen as the minimum of this function. Experimental results, using vibration data taken from a dry vacuum pump at different sampling rates and rotating speeds, show that at pmin there is a marked reduction in the prediction error. For low speed rotating machinery, the optimal order is pmin. As the speed of the rotating machine increases, there is some advantage in using twice or thrice pmin, to produce more accurate frequency estimates. The Box- Jenkins method of order determination using autocorrelation and partial autocorrelations plots are also used for justification of the selection of this minimal order.

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