Drag Measurements of Porous Plate Acoustic Liners

HE reduction of aircraft propulsion system noise is an area of great research interest. Communities around airports demand that noise levels be reduced to their lowest practical level, and governments have responded with more stringent noise requirements. Through past research and development efforts, the aeronautics community has reduced noise levels of propulsion systems significantly. However, with regulated noise levels being continually reduced, we need to make propulsion systems of the future even quieter. One of the many approaches being used to address this need is the use of acoustic liners in the noise producing regions of aircraft engines. While the meaning of the term "acoustic liner" varies somewhat, for our purposes, an acoustic liner is a passive noise damping surface in a flow passage. In propulsion systems, these liners are commonly used in the fan and nozzle sections of the flow path. Liners usually consist of an acoustic absorbing material covered by a perforated face sheet. The face sheet provides a relatively smooth surface over which the fluid flow may pass. The absorbing material may be divided up into small cells, but in this experiment, the material was in bulk sheets. The existing literature contains a large number of studies of turbulent boundary layers over porous walls with injection or suction, but the number of studies without injection or suction is much smaller. These differ from the current work in two significant ways: 1. most featured significantly lower porosity than in the current work, and 2. most were for a perforated sheet over an open cavity, such as would be used for cooling, instead of the perforated sheet over absorber material used in the current work. Roberts 1 obtained drag measurements from pitot pressure surveys for eleven different perforated plates of