Fluctuation of Force Acting on Flat Plate by Underexpanded-Jet Impingement

When an underexpanded jet impinges on a flat plate, the force acts on the flat plate. The force is fluctuated due to the oscillation of the jet. A high-frequency sound called a screech tone is emitted from the underexpanded impinging jet. The screech frequency variation with \(l/D\) was examined, where \(l\) is the distance between the nozzle and the plates and \(D\) is the nozzle diameter. The screech frequency variation showed the saw-tooth characteristics. In the range from \(l/D\) = 3.7 to 4.2, the sound pressure of screech tone decreased. In order to clarify the relation between the force acting on the flat plate and the jet behavior, the strain of the flat plate was measured and the flow field was visualized. A thin disk was used as the flat plate. 8 strain gages were glued to the reverse side of the thin disk at equal intervals on the same circumference. The measurement of strain showed that the amplitude of the strain increased at the nozzle-plate distance where the frequency of the screech tone jumped to the high value. Visualization revealed that the jump in screech tone was closely related with the position of the plate shock and occurred when the plate shock was located near the cell node or near the boundary between the expansion region and the compression region of cell. Thus, the position of the plate shock affects the fluctuation of the force on the flat plate on which the jet impinges.