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Title: Computational aeroacoustic-structural interaction in internal flow with CE/SE method
Authors: Fan, Ka Heng
Degree: Ph.D.
Issue Date: 2018
Abstract: In many engineering applications, such as ventilation systems and aircraft bodies, flexible thin structures in contact with the unsteady flow are used. Acoustic radiation will be generated if the structural vibration is excited by flow disturbances or acoustic waves. It can propagate back and in turn modify the flow process or vibration that generated it. Such kinds of problem involve a complex interaction between acoustics, flow and structural dynamics, known as aeroacoustic-structural interaction, and it is a major consideration in engineering design. Therefore, accurate prediction of the interaction is an important task. Motivated by the needs for better understanding of the aeroacoustic-structural interaction in advance silencer design, an effective yet accurate numerical methodology has been developed to study the interaction in both inviscid and viscous internal flows. Two fluid-structure coupling approaches are presented and validated theoretically and experimentally for different problems in this thesis. The partitioned approach is used for the inviscid problem due to its satisfactory accuracy and flexibility in development. The aeroacoustic model is governed by the two-dimensional compressible Euler equations together with equation of state and solved by a direct aeroacoustic simulation solver based on the conservation element and solution element (CE/SE) method. The panel dynamic model is governed by the nonlinear one-dimension plate equation and solved by standard finite-difference procedure with an iterative coupling scheme to achieve the communication between two media. The numerical methodology is validated with a theoretical study of a single frequency grazing incident acoustic excited flexible panel vibration problem in a duct. The acoustic and structural responses are discussed in detail. To study the effect of a mean flow, a uniform flow is introduced to the duct. A subsonic flow results in the suppression of transmission loss significantly. Higher-order modes and oblique shock waves emerge in the sonic and the supersonic flow. Besides, the bimodal pattern of panel response is observed that incompressible theory is not able to predict. The phase speed of both upstream and downstream travelling bending wave are changed by the effect of fluid inertia. The subsonic and supersonic panel responses and nearfield fluid response with broadband excitation are observed and discussed. The monolithic approach is applied for the viscous problem because of the failure of the partitioned one in a flow-induced structural instability problem. The governing equations of both fluid (Navier-Stokes equations) and flexible panel are combined and solved through the Newton iteration procedure. It shows higher accuracy and better time efficiency than the partitioned approach. The numerical methodology is validated with two experimental studies on a broadband grazing incident acoustic excited flexible panel with a low subsonic flow problem and a grazing flow-induced structural instability problem. Both aeroacoustic and structural responses are captured correctly by the numerical methodology in these two cases. The importance of including the viscous effect is demonstrated. The effect of cavities is also discussed. In the acoustic-induced vibration problem, it can amplify and attenuate the upstream and downstream travelling bending waves and changes the effective silencing frequency range. In the flow-induced vibration problem, it is independent of the occurrence of structural instability but can modify the dominant vibration mode and the amount of energy radiation and induce higher fluid inertia loading.
Subjects: Hong Kong Polytechnic University -- Dissertations
Air ducts -- Noise
Fluid dynamics
Pages: xxiii,150 pages : color illustrations
Appears in Collections:Thesis

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