平板に衝突する不足膨張噴流から放射される音波の特性

抄録

An underexpanded jet is often used as an assist gas to eliminate molten material during laser processing and as a cooling jet of glass tempering process. When the jet impinges on a flat plate to be processed, a plate shock wave is generated in front of the plate and the jet strongly oscillates depending on the nozzle-plate spacing and the nozzle pressure ratio. Oscillation of jet affects the stagnation point on the plate and, in addition, high-frequency noise called screech is radiated in the flow field, which cause the working efficiency and the processing quality to be worse. In this study, an underexpanded jet issuing from a circular tconverging nozzle and impinging on a flat plate perpendicularly is examined experimentally. The flow field is visualized using schlieren method and the sound radiated from jet is measured to examine the relation between the oscillation of jet and the frequency of noise. As a result, a dominant frequency that seems to be a screech tone appears in the high frequency region. Furthermore, the arrangement of vortices surrounding the jet under such condition is found to be different from that of the free jet. Therefore, these high-frequency sound is shown to be caused by the impingement on the flat plate.