Abstract
The underexpanded jet is well known as one of the supersonic jets, and it is formed when the pressure ratio across a convergent nozzle from which the jet is exhausted is more than the critical value. The underexpanded jet has a characteristic barrel-shaped cellular structure. When the underexpanded jet impinges on a flat plate, a detached shock wave called “plate shock” is formed in front of the flat plate. Such flow fields are utilized in industrial applications such as laser processing and tempered glass manufacturing processes. The impinging jet oscillates and induces the high frequency sound called “screech tone”. The present study focused on the screech tone generated from the underexpanded jet impinging on the flat plate and aimed to examine a relation between the high-frequency sound and the jet structure. In the experiment, the jet impinged on a flat plate or a mesh, and the flow characteristics of each case were compared. The pressure ratio was kept constant at 3.0 and the distance between the nozzle and the object being impinged by the jet was changed. Visualization of the flow field and acoustic measurement were carried out under each condition. As a result, integral multiples of the fundamental frequency in the impingement on the mesh agreed with the frequency of the screech tone generated in the impingement on the flat plate. Since the screech tone frequency is based on the feedback loop between the nozzle and the flat plate, this agreement indicates the screech tone is caused by the agreement of the harmonic frequency of the feedback loop between the nozzle and the plate shock with that between the nozzle and the flat plate.
