PERIODIC UNSTEADY-FLOW IN AN EXPANDED PIPE

The characteristics of periodic unsteady flow in an expanded pipe in which mass flow rate changes sinusoidally were investigated through flow visualization and numerical analysis. The flow patterns at the expansion were classified into four types; three flow patterns of stable two-dimensional flow and another flow pattern of unstable three-dimensional flow at the transition to turbulence. The effects of frequency and two Reynolds numbers based on time-averaged and amplitude Row rate upon the flow characteristics were discussed. It was found that a specific frequency of 1.5 Hz existed in the present study. This frequency contributed sensitively to flow pat tern and flow stability. The fine structure of the flow at the expansion were elucidated from numerical predictions and flow-visualized photographs. These results clearly showed the details of the occurrence and dissipation processes of separated vortices and vortex-trajectory at the expansion. The relationships of flow structure and flow stability were also discussed.