THE AEROACOUSTICS AND AERODYNAMICS OF HIGH-SPEED COANDA DEVICES, PART2 - EFFECTS OF MODIFICATIONS FOR FLOW-CONTROL AND NOISE-REDUCTION

The paper describes two studies of the effects of flow control devices on the aerodynamics and aeroacoustics of a high speed Coanda flow tha t is formed when a supersonic jet issues from a radial nozzle and adhe res to a tulip-shaped body of revolution. Shadowgraphy and other flow- visualization techniques are used to reveal the various features of th e complex flow fields. The acoustic characteristics are obtained from far- and near-field measurements with an array of microphones in an an echoic chamber. First the effects of incorporating a step between the annular exit slot and the Coanda surface are investigated. The step is incorporated to ensure that the breakaway pressure is raised to a lev el well above the maximum operating pressure. It substantially increas es the complexity of the flow field and acoustic characteristics. In p articular, it promotes the generation of two groups of discrete tones. A theoretical model based on a self-generated feedback loop is propos ed to explain how these tones are generated. The second study investig ates the effects of replacing the annular exit slot with a saw-toothed one with the ain of eliminating the discrete tones and thereby substa ntially reducing the level of noise generated. (C) 1997 Academic Press Limited.