Abstract: To rule out span-wise flow structures, a rig with a small aspect ratio (AR=0.125) was built. The rig was similar to the pure two-dimensional Hele-Shaw Cell. The flow was confined between two parallel plexiglas plates with a distance of 5 mm. The step height was 40 mm, and the expansion ratio was 2:3. A sensor array composed of 16 microphones was installed along the stream-wise direction in the downstream center to collect the wall pressure fluctuations. The microphones were placed between 20 mm and 320 mm from the step, and the distance between adjacent microphones was 20 mm. The income flow velocity was adjustable and coherent from 9 to 26 m/s. The distribution of pressure fluctuation, the spectrum and the correlation and coherence coefficients at different locations were obtained. The results suggest that the flapping phenomenon still exists when three dimensional flow structures are confined. In addition, the transition point of free stream Reynolds number (Re=U₀H/ν) was found, and two different characteristics were distinguished.