NUMERICAL STUDY OF A PISTON-DRIVEN UNSTEADY-FLOW IN A PIPE WITH SUDDEN EXPANSION

This paper presents results of the numerical study of a piston-driven unsteady flow in a pipe with sudden expansion. The piston closes the l arger-diameter pipe and moves between two limiting positions with stro ng acceleration or deceleration at the beginning and end of each strok e and constant velocity in between. The piston velocity in the exhaust stroke is about four times higher than in the intake stroke. Periodic piston movement in this fashion creates a complex unsteady flow betwe en the piston head and the plane of sudden expansion. The numerical me thod is implicit and of finite volume type, using a moving grid and a collocated arrangement of variables. Second-order spatial discretizati on, fine grids and a multigrid solution method were used to ensure hig h accuracy and good efficiency. Spatial and temporal discretization er rors were of the order of 1% and 0.1% respectively. The features of th e flow are discussed and the velocity profiles are compared with exper imental data, showing good qualitative and quantitative agreement.