Experimental investigation of turbulent energy spectra affected by submerged vegetation in shallow open channel flows

In the presence of vegetation in open channel flows, various physical processes, such as sediment transport, may be dominated by large-scale eddies, of which mechanisms are not well understood at present. In this study, we aimed to explore vegetation-affected turbulence from the perspective of energy spectral analysis. First, we conducted a series of laboratory experiments of open channel flows with submerged vegetation by varying the flow rate, water depth, and vegetation density. With flow velocities measured using the particle image velocimetry (PIV) technique, energy spectral analyses were then performed over several representative locations in the flow field. The results show that the Kelvin-Helmholtz (KH) vortices dominate the flow in the surface layer, while the shedding wake controls the flow in the vegetation layer, particularly downstream of individual vegetation stems. The normalized frequency of the KH vortices increases for flows with dense vegetation, of which the peak value, when normalized as the Strouhal number, has an average of 0.21. Furthermore, by applying Taylor's frozen turbulent hypothesis, it is shown that both the scale of the KH vortices and the penetration depth reduce when the vegetation becomes dense. Within the vegetation layer, the minimum of the peak streamwise wavelength is observed to be related to the shedding wake, while its maximum scales with the size of the penetrating KH vortices.