Sound Absorption
Sound absorbing wall elements, acoustic liners, are studied and improved for the application in aero engines and stationary gas turbines. Various types of liners are available for very different assignments. Our work is focused on bias flow liners and Helmholtz resonator liners.
Bias Flow Liner
Bias flow liners are applied to combustion chambers of aero engines and stationary gas turbines to suppress thermoacoustic instabilities. The liner allows to extend the stable operating range and to implement modern combustion concepts forlow emissions. The figure demonstrates a generic design. The liner consists of a cylindrical tube with a defined perforation. The sound absorpion is significantly improved when a bias flow through the perforation is applied. This increase in absorption is the based on the interaction of the acoustic wave and the fluid dynamic jet, resulting in periodic shedding of vorticity. However, the insight into the physical process is still insufficient. Our goal is to enhance the physical understanding and to improve bias flow liners for their technical application.
Helmholtz Resonator Liner
Helmholtz resonator liners are commonly applied to the inlet and bypass ducts of aero engines. Their main target are the tonal noise components, for example the blade-passing-frequency of the fan. They contribute significantly to the reduction of the overall sound radiation. The figure illustrates the setup with many cavities covered by a perforated facesheet. The dynamics of such a system can be described by the mass-spring analogy. The air within the cavity represents the spring, while the mass of air within the small holes is oscillating due to the acoustic excitation. Sound energy is absorbed by fricton losses within the holes, compression and expansion of the cavity volume, as well as shedding of vorticity from the edges of the hole.
Experimental Methods
The liner in the inlet of an aero engine is subjected to high grazing flow velocities and high sound pressure levels. The prediction of the liners performance under these conditions is a major goal of todays research and accurate experimental data is essential. DLR operates several wind tunnels that are optimized for acoustic measurements, i. e. DUCTC-R/-C and HAT, where the influence of various acoustic, fluid dynamic, or geometric parameters is studied. The Hot Acoustic Test Rig (HAT) is a unique test facility that allows high quality acoustic measurements at elevated pressure and temperature, for example for hot stream or combustion chamber applications. Our work includes many local and international coorporations with academia and industrial partners.
Test Rigs: