A novel boundary condition for the complete description of the acoustic behaviour of perforated wall linings with grazing flow

• Applicant: Dr.-Ing. Anita Schulz
• Subject Area: Hydraulic and Turbo Engines and Piston Engines ; Acoustics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 433582255

The calculation of the sound propagation, especially the sound attenuation, in flow ducts is of great technical interest in many practical applications. In order to suppress unwanted sound propagation through the duct, acoustic liners are applied at the duct walls, i.e. the walls are made acoustically permeable by perforations and the space behind is filled, for example, with sound-absorbing material. The main parameter that determines the sound propagation in the duct is - at least in the no-flow case - the complex-valued wall impedance. With a simple design of the acoustic lining the wall impedance can be predicted theoretically. In more complex cases it must be determined experimentally. If a flow is present, the optimization of the wall lining with respect to the sound spectrum to be attenuated is challenging, because the wall impedance is not only changed due to the flow, but it cannot longer be considered as the sole parameter to describe the influence of the acoustic lining on the sound propagation. To complete the boundary condition, the effect of the momentum transfer between the grazing flow and the wall must be considered. This transfer is caused by the deceleration of the flowing medium penetrating into the openings. This causes the force exerted by the wall on the flowing medium to have not only a normal wall component but also a tangential component. In the previous work this effect was either completely neglected or described by the no-slip condition, which is questioned here. The applicant therefore assumes that the momentum transfer between the wall and the flow has to be taken into account by an independent parameter of the acoustic boundary condition. Therefore, the momentum transfer impedance – the relationship between the acoustic wall shear stress and the wall normal velocity – is introduced as an additional parameter of the boundary condition. Its investigation is the main subject of the planned research project. So far, the theoretical description of the momentum transfer impedance is far from satisfactory and its experimental determination encounters considerable difficulties with the previous measuring methods due to the high accuracy requirements. Within the framework of this research project, a new experimental method is to be developed and applied which avoids these difficulties. Apart from the dependencies on the flow parameters and the frequency, dependencies on the geometry of the openings of the perforated wall are to be expected. These relations should provide the basis for a deeper physical understanding. Models should be formed in order to explain and predict the momentum transfer impedance and its effect on the sound field. The completed acoustic boundary condition finally allows a more accurate optimization of sound absorbers used in flows.

• DFG Programme: Research Grants
• Co-Investigators: Dr.-Ing. Friedrich Bake; Professor Dr. Lars Enghardt