Flow structure and turbulence characteristics downstream of a spanwise suspended linear canopy through laboratory experiments
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Laboratory experiments were conducted to explore the flow structure and turbulence properties downstream of a spanwise suspended linear canopy in a two-dimensional open channel flow using the Particle Tracking Velocimetry (PTV) technique. This canopy was employed to simulate a long-line structure in mussel farms. Four experimental scenarios were under investigation in this study with the following approach velocities: 50 mm/s, 80 mm/s, 110 mm/s and 140 mm/s. All of the experimental scenarios had a depth ratio (the canopy height over water depth) of 0.36, and canopy porosity (the ratio of the openings in-between the canopy elements over the area of the whole cross-section of the canopy) of 0.75. Three sub-layers formed downstream of the canopy. First, an internal canopy layer formed within the canopy, where the time-averaged velocity decreases linearly with increasing distance downstream. Second, an external canopy layer formed under the canopy. The external canopy layer had enhanced horizontal convection caused by higher velocity under the canopy which may bring nutrients or oxygen from the local ambient environment into this layer, which is of interest for a long-line structure in mussel farms. Third, a canopy mixing layer formed between the internal and external layers, which increased in vertical extent into both internal and external layers with increasing distance downstream of the canopy. As the canopy mixing layer develops, it experiences two regions characterized by different growth rates downstream of the canopy. The momentum transport in the canopy mixing layer caused by the canopy turbulence has opposite directionalities in the two canopy mixing layer regions. The canopy turbulence results in upward momentum transport in the first region and create downward momentum transport in the second region. In addition, for scenarios with relatively lower approach velocity 50 mm/s and 80 mm/s, the wake turbulence (in the internal canopy layer) results in upward momentum transport as well. The broader goal of this study is to offer guidelines for the design and site selection of more productive mussel farms. The results suggest that 0.6 times the height of an individual long-line dropper might be the maximum distance interval between the parallel long-lines in a mussel farm, for the sake of bringing more nutrients into the canopy. Also, the offshore areas characterized by relatively lower current velocities (50 mm/s to 80 mm/s) may be better locations for mussel farms because the near wake inside the canopy in these scenarios can result in more nutrients transport into the farms.