Analysis of turbulent flow structures in the straight rectangular open channel with floating vegetated islands.

Abstract

Floating vegetated islands (FVIs) are extensively implemented in various river ecology restoration projects, given their capability of decontaminating pollutants. The fluid dynamical behaviors of turbulence through FVIs are studied in the flume by using the SonTek Acoustic Doppler Velocimetry. Through conventional spectral and quadrant analyses, flow characteristics, such as energy content and turbulent momentum exchange, are investigated as the flow encountered a series of root canopies. A shear layer with corresponding coherent vortex structures at the bottom of root canopies occurred, which is generated by Kelvin-Helmholtz instabilities. These instabilities are usually derived from velocity differences between root canopy and gap region. Shear- and stem-scale vortices are identified by using spectral analysis. The power spectral density function on measured vertical velocity fluctuations in the flow direction near the bottom of root canopies from the leading edge of FVIs is computed. Given the flow developing downstream, a series of the spectral curves has gradually showed one dominant dimensionless frequency at 0.046. The sweep and ejection events have contributed prominently to the Reynolds stress in whole vertical direction. Momentum flux carried by sweeps outweighs its counterpart carried by ejections inside root canopies. However, the situation is different outside root canopies. The sweep-ejection contributions are brief but crucial to the total turbulent momentum exchange, which is in good agreement with considerable studies on turbulent flow through canopies.