The average flow field inside and around the bottom of porous cylinders in a uniform flow is explored using particle image velocimetry (PIV). Tests were conducted on six cylinders with porosities of 0%, 30%, 60%, 75%, 82%, and 90% in a flume tank where the flow field inside and around the models is time averaged over 180 s. The models had a height-to-diameter ratio of 3 and were made from metal mesh. The Reynolds numbers ranged from 5000 to 20,000 based on the diameter of the models and from 75 to 300 based on the diameter of individual strands of the mesh, which corresponds to the Reynolds numbers occurring at salmon fish cage netting used along the Norwegian coast. The porosities of 82%, 75%, and 60% correspond to those of a fish cage netting in Norwegian salmon farming with no, light, and heavy biofouling, respectively. The results from this study are discussed with respect to the instantaneous flow field in and around the same cylinders at identical Reynolds numbers. The focus is on the effect of porosity on the ventilation inside the cages and the vertical transports within the near wake. It is shown that heavy fouling of aquacultural netting can lead to internal circulation inside fish cages and, therefore, has the potential to dramatically reduce the ventilation of the net pens. The description of the time-averaged flow field inside and around porous cylinders can be used as benchmarks to validate and adjust numerical models of the flow past porous cylinders. The results from this study can also be valuable for the fish farming industry, since bio-fouling and the reduced porosity of fish cages can be monitored and controlled directly by fish farmers.

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