Three-dimensional velocity fields measurement of bulge structure observed in a cavity via particle tracking velocimetry

Abstract

The viscoelastic flow of a surfactant solution in a continuous contraction-expansion flow channel exhibits three types of characteristic flows based on Reynolds numbers. At low Reynolds numbers, the Barus effect is observed in a cavity of the channel. At high Reynolds number, the separation flow, where the main flow separates from the fluids in the cavity, is observed, and the flow does not penetrate the cavity. At moderate Reynolds numbers, the fluid penetrates the cavity at the cavity midsection, changes the flow direction to the opposite direction of the main flow, returns to the forward direction near the upstream wall of the cavity, and flows out of the cavity. The flow regime is called the bulge structure. The bulge structure is an interesting flow regime observed in the cavity only when the surfactant solution exhibits high viscoelasticity. Three-dimensional velocity fields in the cavity were measured using particle tracking velocimetry (PTV) to elucidate the mechanism of the bulge structure appearance. From the three-dimensional velocity measurements, the unique velocity fields of the bulge structure were obtained. In particular, the spanwise velocity of the bulge structure was much higher at the cavity inlet and outlet than that at the Barus effect. This indicates that the expansion and contraction flow in the spanwise direction result in the bulge structure. A high spanwise flow was observed at the cavity inlet and outlet, which may have resulted from the expansion flow. Thus, the expansion flow, not only in the flow direction but also in the spanwise direction, generates the bulge structure in the cavity. In this study, the formation mechanism of the bulge structure was elucidated.