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The turbulent flow in a duct with an abrupt increase in its cross-sectional area is very unstable and exhibits strong oscillations. Such a flow is investigated experimentally using a simple device — a pipe–collar nozzle, composed of a convergent nozzle and a pipe lengthened by a collar with an enlarged cross-section. The oscillating flow characteristics are considered over a wide range of flow velocities including both subsonic and transonic flow regimes. Two modes of symmetric and one of asymmetric oscillations are distinguished. For the first mode of symmetric oscillation the jet is separated in each phase of the oscillation cycle, whereas for the second the jet alternately separates and reattaches to the collar wall. Both modes of symmetric oscillations are controlled by shear-layer instability of the collar flow. For low velocities and short collars the oscillation is also affected by the organ-pipe resonance in the preceding pipe. The asymmetric oscillation appears at relatively large flow Mach numbers. This mode exhibits a radial resonance of the jet.

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