Flow structure optimization in key regions of a combined fishway under varying side-weir lengths

This study investigated side-weir-length effects in a right-angle fold weir–vertical slot–bottom orifice fishway (RAFW-VS-BO). Four side-weir-length configurations (350, 500, 650, and 800 mm) were examined using computational fluid dynamics (CFD) combined with the Q -criterion vortex identification method. Velocity distribution, turbulent kinetic energy (TKE), and three-dimensional vortex structures were analyzed, and the numerical results were validated by physical model experiments using Acoustic Doppler Velocimeter (ADV) measurements. The results showed that the overall flow pattern was similar among the four configurations, whereas local mainstream intensity, low-velocity zone extent, and TKE distribution differed significantly. With increasing side-weir length, the 800 mm configuration showed a reduction of about 6% in maximum velocity and an increase of about 5% in the area proportion of the low-velocity zone ( U < 0.4 m/s) at the side-weir-center section. Local high-TKE regions weakened, and vortex structures evolved from locally concentrated strong vortices to weaker vortices distributed along the side weir.