Suddenly expanding recirculating and non-recirculating viscoplastic non-newtonian flows

Abstract

Viscoplastic non-Newtonian flows through axisymmetric 1:5 and 1:2 sudden expansions have been studied numerically by solving the mass and momentum conservation equations and the Bingham constitutive equation. Two distinct viscoplastic flow regimes are observed. For a given Reynolds number, increasing the yield number, a non-dimensional yield stress results in transitioning from a recirculating to a non-recirculating viscoplastic flow regime. For Newtonian fluids, a typical separating and reattaching flow field with a large corner recirculation region and a reattachment point is observed. For a low yield number viscoplastic non-Newtonian flow, the incoming flow separates, creating a weak recirculation region, and unyielded attached-to-the wall, corner and impingement regions. For a high yield number, a non-separating viscoplastic flow field forms, resulting in the elimination of flow recirculation and the formation of significant unyielded region behind the expansion plane. For Newtonian and low yield number viscoplastic flows, axisymmetric 1:5 expansion flows result in more intense recirculation, larger reattachment, and flow redevelopment lengths in comparison with axisymmetric 1:2 expansion flows.