Visualization of a jet stream group influencing the noise increase phenomena in generated by airflow through a perforated metal plate

The perforated plate is made by a punching process. Because of this manufacturing process, the cross section of holes in a perforated plate varies in the direction of the thickness of the plate. When an airflow passes through a group of holes in a perforated plate, the noise characteristics change depending on the direction of passage. In the case where the cross-sectional area of holes increases in the downstream direction, the aerodynamic noise generated by airflow through holes in perforated plates increases. The cause of the noise increase is considered to be the lock-in phenomenon between the vibration of the plates and the outflow of the vortex. For this to occur, at least seven closely spaced groups of holes are required. In this study, flow visualization using the fog method was performed to clarify the behavior of the jet stream group during the noise increase. The results showed that the vortex outflows at each hole were in opposite phase when the lock-in phenomenon did not occur, but that the vortex outflows at adjacent holes were synchronized when the lock-in phenomenon occurred.