Abstract: Occurrence of secondary flows in an open channel greatly affects its distributions of flow velocity and energy, and its aspect ratio has a direct impact on the secondary flows. This study uses a high-resolution particle image velocimetry to investigate the behaviors of secondary flows in a straight channel with different aspect ratios, through measuring flow parameters at the sections of one half of the flume by considering flow symmetry. We focus on the influence of aspect ratio on the structure and strength of secondary flows under supercritical conditions; and conduct experimental tests on a narrow, deep chute, using the channel aspect ratio in the range of 3.75 - 7.5. The results indicate that (1) several secondary cells exist at the cross sections with their total number increasing with aspect ratio, and aspect ratio exerts a great influence on the number and strength of secondary flow. (2) Turbulence intensity and velocity distribution are closely related to secondary flow behaviors. (3) A dip phenomenon occurs in the distribution of streamwise velocity, and the location of velocity peak depends on aspect ratio and the distance to the sidewall. (4) Reynolds stress distribution around the cross section center shows no significant difference when the aspect ratio is changed, but it is affected by the gradient in streamwise velocity.

Key words: open channel turbulent flow, high resolution PIV, secondary flow, aspect ratio, Reynolds stress