Study of Junctions in 1-D & 3-D Simulation for Steady and Unsteady Flow

In this work a comparative study between 1-D and 3-D calculations has been performed on different junctions. The geometries are a 90° T-junction with an area ratio of unity and a 45° junction with an area ratio of 1.78 between the main pipe and the side branch. The latter case had an offset between the centerlines of the main and the branched pipe. The 3-D modeling framework uses the Reynolds Averaged Navier-Stokes (RANS) equations with the k-ε model both for the steady and the unsteady flow cases. The comparison is made both under steady and pulsating flow conditions. The aim has been to assess the 1-D/3-D differences in terms of measures for flow losses.

For the steady flow cases it is shown that there is a large difference between the 1-D and 3-D computed losses for both junction geometries. The differences are largest in the junction and right downstream of it. For the 90° case the 1-D computed loss in isentropic power is 10% higher as compared to the 3-D results for the loss computed in the junction plus two pipe diameters into the outlet pipe. The corresponding 1-D/3-D discrepancy for the 45° junction is 115%. The 1-D/3-D comparison for the pulsating flow case is a discrepancy of 30% for the 90° junction and as much as 365% for the 45° junction. The 1-D losses are the larger of the two.