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Published Articles

The Volume 5, No 3, September 2000

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Non-Stationary Responses of Cables with Slowly Varying Length

Stefan Kaczmarczyk, Wieslaw Ostachowicz


In this paper the longitudinal dynamics of hoisting cables with time-varying length is investigated. The overall response of a hoisting cable system to inertial load due to transport motion acceleration/deceleration and to external harmonic excitation is studied. A mathematical model of the system is formulated, and the response is simulated numerically. Due to the time-varying length of the cable, the natural frequencies of the system vary slowly, and a transient resonance occurs when one of the frequencies coincides with the frequency of the excitation at some critical time. Near the resonance region the resonant mode is dominant and a single-mode approximation of the system response in this region is formulated. A combined perturbation and numerical technique is applied to predict the response during passage through the main resonance. A numerical study is presented in which the dynamic responses of cables in a deep mine hoisting installation are examined.

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A Study of the Effects of Rotational Terms in the Power Transmission through Vibration Isolation Systems on Beam-like Structures

Ho-Jung Lee and Kwang-Joon Kim


The performance of a multi-dimensional vibration isolation system can be evaluated in several ways, one of which is to employ the concept of vibration power to define the isolation efficiency. It is especially useful in the high frequency range where the radiation of noise from the receiver structure is concerned. The idea is basically simple to understand but rather complicated to formulate and apply in practice. For an accurate estimation of power flow especially over a high frequency range, it is well known that, in principle, rotational motions should be taken into consideration together with translational motions at the isolator locations. In reality, however, power transmissions related to rotational terms are often neglected mainly due to difficulties in the instrumentation. In this paper, the effects of neglect of rotational terms in the power estimation are investigated for several different approximation methods; the mobility approach, impedance approach, and force and velocity approach. Since it is not possible to derive formulas analytically of the vibration power for structures of finite dimension, numerical simulations for a beam-like structure are presented to investigate the characteristics of the distortions in the power estimation induced by neglect of the rotational terms for each approximation approach.

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Ground and Meteorological Effects on Sound Propagation in the Atmosphere - Predictions and Measurements

Y.W. Lam


Sound propagation in the atmosphere is inevitably affected by the meteorological conditions. Most existing standard calculation schemes do not have the ability to account for meteorological effects over non-flat grounds. Numerical methods such as that based on the Parabolic Equation can deal with complex meteorological conditions but are limited to slowly varying grounds. A heuristic ray tracing model can be used to approximate the meteorological effects but the validity of using a linear sound speed profile to represent complex real life meteorological conditions is still debatable. In this paper, predictions of sound propagation under strong meteorological influences are investigated. Predictions from the standard calculation schemes of ISO and CONCAWE, the heuristic ray tracing model and the Parabolic Equation model are compared with reallife measurements over smooth, flat ground as well as over complex grounds. The situations include the presence of rough, non-flat ground with an intervening hill and considerable meteorological conditions with wind speeds up to 10 m/s. As expected, the more sophisticated numerical models produced better predictions but there are also indications that the meteorological conditions changes significantly over the propagation path over a complex ground and the usual practice of monitoring the meteorological conditions at two heights at a single locations may not be sufficient.

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Machine Condition Monitoring Based on Transient Vibration Signal Analysis

Zhidong Chen and C.K. Mechefske


Most existing signal analysis methods are only suitable for stationary processes while transient vibration signals, due to their non-stationary nature, have been studied somewhat less. This paper seeks to find a method for the analysis of transient vibration signals and attempts to apply such a method to Machine Condition Monitoring. The Prony method is a modelling method that fits a linear combination of a series of exponential functions to sampled data. Because of its exponential base functions, this method is inherently suitable for analysing transient signals. The intention of this paper is to explore the application of the Prony method to the study of transient vibration signals and the potential of this method in Machine Condition Monitoring. This paper describes the preliminary work done to investigate this procedure as the basis of a useful machine condition monitoring and diagnostic technique. Also presented are preliminary numerical results of the Pronys model based frequency analysis of computer-generated, simulated vibration signals as well as experimentally-derived vibration signals.

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Experimental Determination of the Speed of Sound in Viscoelastic Pipes

Matjaz Prek


Vibro-acoustic energy travels not only as sound waves through a fluid medium, but also as longitudinal and flexural waves through the pipe walls in a hydraulic system. Longitudinal waves in the pipe wall are coupled to the sound waves inside according to Poisson?s ratio. One of the most common examples of this form of interaction is observed in water-filled pipes in which the flexibility of the walls significantly alters the speed of propagation of acoustic disturbances along the pipe. This interaction is most evident in viscoelastic pipes, which takes into account the effect of the viscoelasticity of the pipe wall material on the wave propagation. In order to determine the speed of sound in water-filled pipes, the pressure transfer function between points along the pipe is experimentally determined. Pressure transducers (hydrophones) are used to measure the fluid-borne sound pressure. The equivalent wavespeed and frequency-dependent attenuation factor is determined from this measured data. With this data, the standard impedance or transfer matrix can be derived directly to analyse the resonance conditions in hydraulic lines. Experimental results for four different pipe wall materials, with particular applications in water supply installations, are presented.

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