The effects of a passive control boundary layer applied around the throat on a transonic diffuser were investigated experimentally by wall static pressure measurements and by schlieren optical observations. The experiments were conducted using three kinds of the diffuser walls, one was a solid wall and the other two were porous with a 30 mm or 50 mm-long cavity underneath enabling the flow around the shock wave to circulate through the porous wall. The results show that the Mach waves normal to the flow were observed when diffusers were almost choked and that the pressure fluctuations in a transonic diffuser were greatly reduced by passive control. According to the frequency analysis, the frequency range attenuated by passive control is between about 700 Hz and l kHz regardless of the length of the cavities.
