MEASUREMENTS IN TURBULENT SWIRLING FLOW THROUGH AN ABRUPT AXISYMMETRIC EXPANSION

Experimental data are presented for both axial and tangential velocity components in turbulent swirling flow downstream of an abrupt 1:2 expansion. Measurements of mean and rms velocities were performed in a water flow with a laser Doppler anemometer. In the upstream tube, the Reynolds number was varied from 30,000 to 100,000 and the swirl number from zero to 1. 2. For low swirl levels, as the core flow passed through the expansion, it departed the axis of symmetry and precessed about that axis at frequencies on the order of 1 Hz. As swirl was increased to moderate levels, the flow became axisymmetric with on-axis recirculation marking the onset of vortex breakdown. At the highest swirl levels, flow on the tube centerline was in the same direction as the mean flow, with reverse flow occurring just off-axis. Turbulence intensities at the highest swirl levels were found to reach 60 percent. As the swirl was increased from zero to its maximum value, the flow reattachment point moved upstream from 9 to 2 step heights.