A NUMERICAL AND EXPERIMENTAL-STUDY OF TRANSITION PROCESSES IN AN OBSTRUCTED CHANNEL FLOW

Incompressible Newtonian flow in a two-dimensional channel with period ically placed sharp edged baffles has been studied both by numerical s imulation and by experimental flow visualization. The flow was observe d to be steady and symmetric at low Reynolds numbers, with recirculati ng eddies downstream of each baffle. At a critical Reynolds number (ba sed on channel width and cross-sectional mean velocity) of approximate ly 100 the flow became asymmetric and unsteady. This transition to uns teadiness led to an eddy shedding regime, with eddies formed and shed successively from each baffle. A stability study suggested that the me chanism for transition to unsteady flow is a Kelvin-Helmholtz instabil ity associated with the shear layer formed downstream of the sharp edg ed baffles. The frequency of the unsteadiness is, however, dependent o n the full flow field, and not only the shear layer characteristics. E xperimental observations show that the instability is followed by a se condary transition to three-dimensional disordered flow. Experimentall y observed flows in the two-dimensional regime were found to be in clo se agreement with the numerical simulation for both the steady and uns teady flows.