Flow patterns for an annular flow over an axisymmetric sudden expansion

In this paper, an experimental investigation is described for a concentric annular flow over an axisymmetric sudden expansion by using both flow visualization and laser-Doppler anemometry (LDA) techniques. Depending upon the value of the Reynolds number and whether the Reynolds number was increased or decreased, four typical flow patterns were classified according to the characteristics of the central and corner recirculation zones. The flow patterns are open annular flow, closed annular flow, vortex shedding, and stable central flow. Bifurcation for this flow occurred when 230 < Re < 440, which was verified by observing the variation of the reattachment length. The spatial growth of velocity fluctuations from the measurements demonstrated a tendency that shedding vortices behind the centrebody more strongly affect the reattachment length than flows without a centrebody.