Author: Liu, Yang
Title: Flow induced vibration and noise control with flow
Degree: Ph.D.
Year: 2011
Subject: Vibration.
Fluid dynamics.
Ventilation -- Noise.
Noise control.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xx, 170 leaves : ill. (some col.) ; 30 cm.
Language: English
Abstract: The objective of this project is twofold. One is to understand the features and mechanism of the instability phenomena of the duct with flexible wall, and the other is to use the tensioned membrane to control noise source with flow such as fan noise in the duct. In pursuing noise and wave control with minimal aerodynamic or hydrodynamic effect, a tensioned membrane backed by a cavity, namely the drum-like silencer, is used to partly line on the rigid duct wall. The membrane segment vibrates in response to the grazing incident sound and the vibration of the membrane acts as reflector to reflect the sound towards the noise source. The reflection of sound can be maximized at low-to-medium frequency range when the axial tensile force on the membrane is high to support the dominance of the first and second modes of vibration. The device has been tested successfully to achieve a good performance of transmission loss without flow and with mean flow at very low speeds. The effect of the flow on this device in practical usage has not been extensively investigated. In case of high flow speeds, tensile force on the membrane is highly increased in order to compensate and maintain the same amount of sound radiated energy. As a result, flow induced vibration is also observed at moderately high flow speeds and the unstable vibration of the membrane also generates much noise itself. Firstly, to characterize the instability of vibration, direct measurement of the wall pressure fluctuation in the boundary layer is conducted as a preliminary to evaluate the characteristic and distribution of both aerodynamic and acoustic loadings on the membrane by using two-microphone with multiple measurement points. The experimental results revealed that the vibration of the membrane depends on the aerodynamic fluid loading rather than the acoustic loading. In order to allow for the free vibration at the lateral edge of the membrane in drum-like silencer for achieving effective sound radiation, there is an extremely small gap along the lateral edge which leads to the flow leakage through the gap. This is very crucial to the occurrence of vibration instability. In this regard, the aerodynamic effect on the vibration instability is investigated in details in case of the axial-flow and cross-flow directions. The instability phenomenon is found at the moderately high flow speed and it tends to disappear as the axial tensile force on the membrane is increased. Through a series of experiments, it is found that the vibration mode of membrane under the cross-flow condition will experience a three-stage process at different flow speeds: 1st mode of vibration at low flow speed, 1st and 2nd coupled modes of vibration at critical flow speed, and higher order modes of vibration at high supercritical flow speed. These findings are beneficial for the design of the membrane typed device and the method of clamping membrane in case of flow practically such as drum-like silencer so that the instability of the vibration can be avoided.
On the other hand, the configuration of the membrane with cavity can be used to reduce the strength of noise source directly instead of sound propagating path. This may share the same features of drum-like silencer but the working principle is different. The noise source is preferred to be dipole in nature and it can be an axial fan which is commonly used in the duct system. The noise suppression of it was successfully demonstrated both numerically and experimentally. The axial fan put at the middle of the membrane along the axial direction is regarded as the dipole source, which can induce the second mode of vibration of the membrane. The radiated sound from the membrane propagates towards upstream and downstream and is cancelled with the dipole source due to the full couplings between the vibration of the membrane and acoustic field inside the cavity. The two-dimensional simulation model is constructed to explore the sound and structure coupling as well as to understand the mechanism of the sound propagations, sound cancellation and the response of the membrane. The effects of several controlling parameters on the performance such as membrane length, the structure to air mass ratio and the tensile forces on the membrane are also investigated. From the view point of practical usage and installation of the fan, the performance of using membrane with cavity to cancel the axial flow fan noise is explored in a comprehensive three-dimensional simulation model. Optimization is conducted in searching the optimal shape of cavity for a given volume and for the optimal structural properties of the membrane. It is found that the optimal insertion loss can achieve more than 20dB over the frequency range of interest when the tension applied is low. It is much better than that by using expansion chamber with the same expansion ratio to control the dipole source. Besides, the experimental result agrees fairly well with the numerical prediction, showing the effectiveness and reliability of the numerical model. Apart from the effective control of noise source, the membrane lined on the wall can eliminate the pressure drop in the flow through duct and hence no extra power consumption of the engine is required and energy can be saved.
Rights: All rights reserved
Access: open access

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