We present the results of an experimental study of isodense displacement of a yield stress viscoplastic fluid by a miscible Newtonian one in a horizontal pipe. A central type displacement flow develops for the configuration given. Three distinct flow types belonging to this central displacement are identified namely corrugated, wavy and smooth depending on the level of the residual layer variation along the pipe. The transition between these flow regimes is found to be a function of the Reynolds number defined as the ratio of the inertial stress to the characteristic viscous stress of the viscoplastic fluid. (C) 2016 Elsevier B.V. All rights reserved.