Abstract
This paper presents an oscillatory phenomenon of the underexpanded jet which issues from a convergent nozzle and impinges on a flat plate perpendicularly. As the characteristics of the generating noise deeply depend on the oscillation of the impinging jet, the flow field is visualized using the shadowgraph method and the pressure on the plate is measured to classify the oscillation modes for different parameters such as the nozzle pressure ratio and the distance between the nozzle and the plate. For the comparison, the jet is numerically simulated using Total Variation Diminishing (TVD) scheme under the assumption of the non-viscous flow. Simulating the impinging main jet and the radial wall jet on the plate separately, the sound waves propagating around the jet are shown and the frequency of the oscillation for each jet is tested. The wall-jet oscillation which is caused by the small vortices from the main jet occurs at high frequency when the nozzle-plate distance is comparatively small, while the main-jet oscillation induced by the grown vortices are shown to generate at large nozzle-plate spacing and the frequencies under such condition have well-known saw-tooth characteristics. With the correlation of pressure on the plate, three types of the oscillation – axisymmetric mode, whirling mode and non-regular pattern – are shown.
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