01. Experimental/Computational Fluid Dynamics

Strong tonal sounds are generated by self-excited flow oscillation when a flow passes over cavities. These cavity tones pose a problem in many engineering applications, such as transportation machinery and plant equipment, thus promoting the development of control techniques. This study developed a novel passive control method using protrusions installed on the cavity’s rear wall. To understand how these protrusions affect cavity flow and sound and to investigate the effective conditions of protrusions for sound control, aeroacoustic simulations were conducted. This study focused on a rectangular cavity with a length-to-depth ratio of 2.0 and a laminar upstream boundary layer. The Reynolds number, which is based on the cavity length, was 15,000, and the freestream Mach number was 0.044. Protrusions with different heights, widths, and lengths, were considered to elucidate their effects on sound control. Three-dimensional aeroacoustic simulations were conducted using the lattice Boltzmann method. A high-resolution computational grid with 370 million points was used to directly simulate flow and sound fields. The results suggested that the flow field within the cavity, influenced by the protrusions, might be affecting the cavity tone. When L/θ was less than or equal to 87, the cavity tone significantly decreased by the proposed passive control method using protrusions. The sound reduction was attributed to the suppression of the circulation flow in the cavity by the protrusions. Furthermore, we elucidated that there were optimal values for the length and height of the protrusions. However, as L/θ increased, the protrusions complicated the flow structure near the trailing edge of the cavity and led to another tonal sound.