An uncertainty investigation for liner impedance eduction methods

Impedance eduction methods have been developed for decades to meet the increasing need for high-quality impedance data in the design and optimization of acoustic liners.To this end,it is important to fully investigate the uncertainty problem,to which only limited attention has been devoted so far.This paper considers the possibility of acoustically-induced structural vibration as a nonnegligible uncertainty or error source in impedance eduction experiments.As the frequency moves away from the resonant frequency,with the increase in the value of cavity reactance,the acoustic particle velocity inside liner orifices possibly decreases to the extent comparable to the vibration velocity of liner facing sheet.Thus,the acoustically-induced vibration,although generally being weak except at the inherent structural frequencies,may considerably affect the impedance eduction results near the anti-resonant frequency where the liner has poor absorption.To demonstrate the effect of structural vibration,the vibration velocity of liner facing sheet is estimated from the experimentally educed admittance of the liner samples whose orifices are sealed with tape.Further,a three-dimensional numerical model is set up,in which normal particle velocity is introduced over the solid portion of liner facing sheet to imitate structural vibration,rather than directly solving the acoustic-structural coupling problem.As shown by the results,the vibration of liner facing sheet,whose velocity is as small as estimated by the experiment,can result in anomalous deviation of the educed impedance from the impedance model near the anti-resonant frequency.The trend that the anomalous deviation varies with frequency is numerically captured.

Investigation of straightforward impedance eduction method on single-degree-of-freedom acoustic liners

In order to address the current aircraft noise problem, the knowledge of impedance of acoustic liners subjected to high-intensity sound and grazing flow is of crucial importance to the design of high-efficiency acoustic nacelles. To this end, the present study is twofold. Firstly, the StraightForward impedance eduction Method(SFM) is evaluated by the strategy that the impedance of a liner specimen is firstly experimentally educed on a flow duct using the SFM, and then its accuracy is checked by comparing the numerical prediction with the measured wall sound pressure of the flow duct. Secondly, the effects of grazing flow and high-intensity sound on the impedance behavior of two single-layer liners are investigated based on comparisons between educed impedance and predictions by three impedance models. The performance of the SFM is validated by showing that the educed impedance leads to excellent agreement between the simulation and the measured wall sound pressure for different grazing flow Mach numbers and Sound Pressure Levels(SPLs) and over a frequency range from 3000 Hz down to 500 Hz. The grazing flow effect generally has the tendency that the acoustic resistance exhibits a slight decrease before it increases linearly with an increase in Mach,predicted successfully by the sound-vortex interaction theoretical model and the Kooi semi-empirical impedance model. However, the Goodrich semi-empirical impedance model gives only a simple linear relation of acoustic resistance starting from Mach zero. Additionally, when the SPL increases from 110 to 140 d B in the present investigation, the acoustic resistance exhibits a significant increase at all frequencies in the absence of flow; however, the resistance decreases slightly under a grazing flow of Mach 0.117. It indicates that the SPL effect can be greatly inhibited when flow is present,and the grazing flow effect can be reduced partly as well at a relatively high SPL.

绘本妙用妙无穷

本文主要探讨如何运用绘本解决日用平常中的困扰问题。包括灾区学校之生命重建、病情告知、防范性侵害、临终关怀及悲伤辅导等。并讨论其延伸的效果、均举实例,以落实其功效。

Geochemical Analysis on Educts in the Drainage Corridor of a Big Dam over Yangtze River

The analysis of educts has become an important aspect for the security evaluation of hy-draulic construction. Brown, white-based colloidal educts appeared in the left drainage corridors of the dam over the Yangtze River after its 5-year operation, which did not happen in the initial investigation. In this article, the physics characteristics of the educts were affirmed, and the analysis results of the major and trace elements among educts and the corresponding location of water samples, core samples, indicate that educts are the products from the chemical potential erosion of the dam foundation rock. Brown educts are uncertainties Fe and Mn-based materials, mainly derived from the corrosion of the chlorite and biotite, which may be caused by the changes of the oxidizing condition. White educts mainly with Si and Ca are derived from the corrosion of the calcite, feldspar, and partly invalidation of foundation curtain. Additionally, similarities of the chondrite normalized rare earth element （REE） patterns between educts and bedrock further illustrate homologous, and Ce and Eu anomalies relative to chondrite reflect changes of the water environment condition.