Acoustic velocity of pressure wave propagating in fluid-filled pipeline depends on the fluidand the mechanical properties of the pipeline. The mechanical properties of the pipe wallmaterial determine the pressure response of a fluid within the pipe. For metal or concretepipes the models based on linear-elastic behavior relatively accurate describe pressurewave propagation. However, these models show significant discrepancies in the case ofplastic pipes, which exhibit viscoelastic behavior that influences the pressure response ofthe pipe system. The viscoelastic behavior is characterized by an instantaneous strainfollowed by gradual retarded strain. This retardation of the pipe wall causes a significantattenuation of pressure oscillations and increases the dispersion of pressure wave. In thispaper a wavelet transform of non-stationary sound signal was used to identify thefrequency-dependent fluid sound speed. Measurement and analysis of non-stationarysignals with the use of time-frequency method provides a view to frequency dependenttransfer characteristics of fluid – pipe coupled system. The instantaneous characteristicsare obtained from the ridges and skeletons of the wave transform. The essentialinformation is contained in the skeleton of maxima lines and ridges. From the ridges themodal parameters can be extracted and the signal can be reconstructed. The so called fluidmode and pipe mode resonant frequencies are evident and the impact of different pipe wallmaterial properties is shown. The results also showed that, in the case of propagating smalldisturbances (such as acoustic waves), the pipe wall inertance has a minor influence on thewave propagation characteristics.
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