Base Pressure and Noise Produced by the Abrupt Expansion of Air in a Cylindrical Duct

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Abstract
In the investigation described, the base pressure resulting from the abrupt expansion of an air jet from a circular nozzle into a concentric cylindrical duct or shroud has been measured. Stagnation pressure ratios of the forcing jet to atmospheric of up to six were used with shrouds of various lengths and diameters. As the primary or forcing jet pressure is increased and then decreased, the jet flow attaches and separates from the shroud wall and a hysteresis effect is exhibited by the pressure at the base of the shroud. With an attached flow, the base pressure attains a minimum value which depends mainly on the duct to nozzle area ratio and on the geometry of the nozzle, lower base pressures being obtained with convergent-divergent nozzles. When the jet pressure was increased beyond that required to attain the minimum value of the base pressure, it was observed that the ratio of the forcing jet pressure to base pressure remained constant. Noise measurements of the flow from the nozzle and shroud are presented. The plot of overall noise showed a minimum value of noise at a jet pressure approximately equal to that required to produce the minimum base pressure. As the pressure was increased beyond this value, the noise from the shroud was essentially that of a subsonic jet until the flow at the shroud exit became choked. The excessive overall noise observed at low forcing pressures was due to the emission of powerful discrete tones from an organ-pipe resonance within the ducts.

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