Author(s): Qiang Zhong
Linked Author(s):
Keywords: Open channel flows; Free surface; Spanwise vortices; Large-Scale motions; Super streamwise vortices
Abstract: This paper reports particle image velocimetry (PIV) measurements of the instantaneous velocity vector fields in smooth open channel flows. The friction Reynolds number of the flow ranges from 560 to 1353. The instantaneous velocity fields were averaged to evaluate mean velocity, turbulent intensity, and Reynolds stress. Spanwise vortices were extracted from background turbulence to quantify population density and advection velocity. The fluctuating velocity fields were further analyzed using proper orthogonal decomposition (POD) to reveal the prominent energetic structures. Evaluation of the turbulence statistics highlights several distinct features of open channel flows in the near-surface region, e. g., reduced wake strength and redistribution of turbulent kinetic energy. These features are closely related to the spanwise vortices which travel at a lower speed than the local flow. The advection lag together with wall-normal dependence of the vortex population density partially explains the reduction in wake strength and redistribution in turbulent kinetic energy. The POD results reveal the existence of coherent structures with different sizes and energy levels. Current evidence supports the view that hairpin vortices constitute a major form of coherent structures in wall turbulence. It appears that hairpin packets can develop into the outer layer and impinge onto the free surface as boils. Intensive alternating super Q4 and Q2 events in the flow indicate that super streamwise vortices exist and play an organizational role in the entire turbulent structures. Based on the experimental evidence, a closed-loop feedback cycle has been proposed for describing the development of super streamwise vortices and the alignment of hairpin packets.
Year: 2013