Flow-plant interactions in open-channel flows: a comparative analysis of five freshwater plant species

The paper reports a laboratory study of drag forces exerted on aquatic vegetation and their coupling with flow turbulence. The experiments were conducted in a 12.5 m long and 0.30 m wide flume, using five different freshwater plant species (Ranunculus penicillatus, Fontinalis antipyretica, Myriophyllum alterniflorum, Glyceria fluitans, and Callitriche stagnalis). Velocity components and drag forces were measured using two acoustic Doppler velocimeters (ADV) and a specially designed drag measurement device. Drag fluctuations exerted on the plants appeared to be closely related to the ambient turbulence, especially to large-scale turbulent structures. The enhanced turbulent energy downstream from the plants was associated with wake generation, with its magnitude controlled by plant morphology reconfiguration. Frequency spectra of drag fluctuations reveal a frequency range where they follow a power law that can be parameterized using plant and flow properties. The rescaled spectral functions are shown to match the collected data reasonably well. In addition, drag-velocity cross-correlation analyses enabled the estimation of the location of the resultant drag force, which can be interpreted as an integral measure of plant adaptation to the flow. The results revealed different degrees of adaptation of the plants to flowing water, with Fontinalis and Glyceria showing the best hydrodynamic performances, i.e., lower levels of turbulence and drag generation.