Three-Dimensional Structure of Laminar Flow through a Square Sudden Expansion Channel (Effect of Reynolds Number)

Abstract

Three-dimensional numerical predictions are presented for a laminar flow through a square channel with a sudden expansion of area ratio 4 : 1 in the range of Reynolds numbers Re = 20 to 1,900, using an Open Source CFD Toolbox (OpenFOAM). In particular, the effect of Reynolds number variation on the bifurcation of flow structure downstream of the sudden expansion and the evolution of unsteadiness was investigated. The results showed that the whole flow field behind the sudden expansion could be classified into three different regions as Reynolds number varies. The steady flow region was in the range of Re ≤ 900, the unstable steady flow or unsteady disordered motion flow appeared in the range of 900 < Re ≤ 1,300, and the unsteady disordered motion flow was stable in the range of 1,300 < Re ≤ 1,900. The observation area of disordered motion could be determined from RMS value of streamwise fluctuating velocity on the flow center line. As the Reynolds number increased, the observation area moved to upstream direction and the maximum RMS value increased. The high speed flow cluster near the centerline was diffused toward the wall by the disordered motion. So that, the vortex structure in the recirculation zone was disturbed by the high speed flow and the reattachment points were moved to upstream direction. The development of the separating shear layer depended on the initial position of the disturbance that added to the diagonal corner or the wall centerline in the upstream flow region of sudden expansion.