Untersuchung und Auslegung periodisch durchströmter Schalldämpfer

Kurzfassung

The jet engine is one of the dominant sources of aircraft noise. With increasing air traffic and rigorous requirements regarding noise emission, a demand for effective damping devices arises. Conventional dampers, so-called liners, work according to the Helmholtz or the quarter-wave principle and express a dependency on the flow grazing the perforated walls of the liners. Hence, conventional liners can only fully deploy their damping capabilities at one specific operating point. To overcome this limitation, the so-called Zero-Mass-Flow-Concept has been introduced. As part of the concept, a periodic bias flow is utilized to adapt the impedance of the liner to a certain grazing flow velocity and optimize its damping capabilities. No physical analysis of the mechanism of periodic bias flow and its implications on the impedance of orifices and perforates has been conducted. Therefore, in this work, the physical mechanism of periodic bias flow is studied theoretically and experimentally with regard to applying it to damping devices according to the Zero-Mass-Flow-Concept. In the experimental part of this work, scale laws are derived and several flow regimes are identified based on a dimensional analysis. In general, the results show that, while the resistive part of the impedance is increasing with the periodic bias flow velocity, the reactive part is decreasing. The increase of the resistance is connected to vortex shedding at the edges of the orifices and the decrease of reactance is related to a consequent loss of inertial mass in the orifices as the fluid is convected away by the vorticity. The shedding of vortices due to the periodic bias flow constitutes the dissipation of acoustic energy by turbulence. The experimental findings and the analytic approach are combined into a semiempirical model describing the change of impedance due to periodic bias flow. To design a Zero-Mass-Flow-Liner, an universally applicable optimization strategy is developed on the basis of a genetic algorithm and a numerical simulation, wherein the impedance of the liner is used as a boundary condition. Applying the method yields a set of optimized liner parameters, which are used accordingly to manufacture a prototype. The impedance calculated by the model in combination with the numeric simulation predicts the measured dissipation of the Zero-Mass-Flow-Liner accurately. The evaluation of the prototype demonstrates, that the concept can, on principle, be used to adapt the impedance of the liner to certain grazing flow conditions. The Zero-Mass-Flow-Concept thereby exhibits increased broadband dissipation characteristics compared to classic Single Degree of Freedom Liners. As the periodic bias flow is suppressed under grazing flow, powerful actuators are required in order to apply the concept to high grazing flow speeds.