Noise reduction potential of flow permeable materials for jet-flap interaction noise

Abstract

Jet-flap interaction (JFI) noise plays a particularly important role during take-offs and landings, i.e. when the aircraft is in the immediate vicinity of inhabited areas. The interaction of the jet with the deployed flaps causes a substantial increase of low-frequent aircraft noise. Noise reduction technologies (NRTs) were studied for their potential to diminish the JFI effect: The wing was equipped with various permeable or porous (flap) trailing edges which cover the last 10% of the clean chord length. The first test was conducted at the Aeroacoustic Wind tunnel in Braunschweig (AWB) on a cruise wing model, i.e. the engine was not installed. Beneath the testing of various materials and concepts, a design parameter study was conducted about the minimally needed treated chord length. The four most promising materials were down-selected for an installed engine test at the jet noise test facility JExTRA in Berlin: Two NRTs are designed as hollow flaps: perforated or slotted metal sheets are fixed onto a rib structure. The other two NRTs are porous materials (fine/coarse size). The tests were conducted for static operations between Mj=0.45 and 0.75. The engine integration was mainly defined by a height of H=0.6Dj (close integration) and a length of L=2Dj. Remarkable noise reduction was measured especially for low jet Mach numbers and up to Mj=0.7. At high Mach number, some NRTs produce tones which follow a Helmholtz analogy. The only tested NRT which is immune to this, uses a cover mesh which introduces another trailing edge. The tests with flow permable flap trailing edges show the great potential of reducing jet-flap interaction noise. Open questions regard (1) the deepening of the physical understanding of noise reduction mechanisms, (2) a proof of performance under flight conditions and (3) a design optimization which balances a small aerodynamic lift penalty vs. the acoustic benefit.