Velocity-based analysis of mainstream direction effects on low-speed region formation and oscillation in dilute polymer solution around triangular pillars in microchannel

Abstract

This paper reports a velocity-based analysis of flow behavior in the low-speed regions of a dilute hydrolyzed polyacrylamide solution formed around triangular pillars installed in a microchannel. We found that the size and shape of the low-speed regions around the pillars change depending on the Wissenberg number (Wi) and the mainstream direction in the channel. Additionally, the low-speed regions exhibit oscillatory behavior in the spanwise direction, which varies with the main flow direction. This difference suggests that elastic flow, due to the strong contraction flow between the edges of the triangular pillars and the channel sidewalls, is dominant in forward flow cases, while shear flow is dominant in backward flow cases. A time series of the velocity distribution inside the low-speed region was obtained using particle image velocimetry. The cross-correlation coefficient and time delay of the flow behavior in neighboring low-speed regions were determined, suggesting that the flow behavior propagates between neighboring low-speed regions and can become synchronized at large Wi. It also suggests that the low-speed regions’ flow behavior affects the mainstream flow in the channel depending on the flow direction. Consequently, the results of this study provide insight into the complex flow regime occurring between triangular pillars, which are sometimes utilized as flow rectifiers or channel-supporting structures in particle-sorting devices.