Experimental insights into separation zone and recirculation flows in open channel bifurcation

Abstract

Open-channel bifurcations exhibit complex three-dimensional flow structures, such as separation zones and recirculating motions. While traditional representations of the recirculation flow in bifurcation are two-dimensional and closed, recent studies suggest the presence of open three-dimensional (3-D) helical recirculation flows. The present work addresses this question by employing particle tracking velocimetry to measure the time-averaged 3-D velocity field in a simplified bifurcation geometry. The measurements reveal that all examined flow configurations exhibit an open separation zone along the lateral branch’s inner wall, with no evidence of closed, two-dimensional recirculation. Instead, two distinct helical recirculation patterns emerge, governed by the width-to-depth aspect ratio

. In shallow flows (

upper B divided by h greater than B / h > $B/h \gt $

4), a vertical-axis helix conveys the recirculating flow from the bed to the free surface. In deeper flows (

upper B divided by h less than B / h < $B/h \lt $

4), the recirculation axis transitions from vertical upstream to streamwise downstream, generating a more complex motion. A key finding is the presence of an ‘entrance gate’ in both patterns: a spatially confined near-bed region through which flow enters the separation zone near the intersection, before escaping further downstream near the surface or at mid-depth. This study challenges conventional closed-zone representations, instead revealing two types of open, three-dimensional helical recirculation patterns with finite but prolonged residence times.