Flow parameters in an unpressurized circular right-angle elbow under
secondary flow propagation
Abstract
The secondary flow is deflected under pressure and superimposed on the
main flows. This research investigated its characteristics, including
velocity gradient, vorticity, shear stress, and Reynolds stress in
unpressurized circular pipes, through physical experiments and
Computational Fluid Dynamics numerical simulations. Combining numerical
simulations with the physical experiments under three flow rates (30 m
3/h, 35 m 3/h, and 40 m
3/h) and width–perimeter ratios (
Wr = 0.43, 0.4, and 0.35), the experimental data
demonstrate the secondary-flow propagation in unpressurized circular
pipes. The secondary flows manifest as deviations of velocities,
substitutions of secondary vortices, and flips of shear stresses, which
present decay tendencies and are negligible at 52 times the pipe
diameter. The secondary flows are driven by the velocity gradient or
Reynolds stress, the dominance of which shifts with the increase of
diffusion distance. The secondary flow turbulence is reduced and
smoothing when the width–perimeter ratio reaches a threshold
(approximately Wr = 0.40) because of Dean
vortices collisional depletion.