The impact of twine/mesh ratio on the flow dynamics through a porous cylinder

Abstract

The impact of the twine/mesh ratio on the flow through a porous hollow cylinder of diameter D has been experimentally investigated at Reynolds number Re = 800 with a surface porosity varying from 0.67 to 0.90. Our porous cylinder models are inspired by aquaculture pens in that they have similar geometries, and porosities, to those nets commonly used within the aquaculture industry. We show that the surface porosity alone is not the key parameter determining the flow topology of the model, but rather a non-dimensional parameter \(\alpha =t^{0.5}D^{0.5}/m\) (based upon twine thickness t, mesh void m and cylinder diameter D) effectively collapses first-order moments. Three different wake regimes are observed in the flow for different twine/mesh ratios: a laminar flow regime where streamlines pass through the model without significant deformation; a partially occluded flow, where the mean flow is decelerated, and a flow with a fully developed recirculation zone exhibiting a von Karman vortex street similar to that produced in the wake of a solid cylinder. Our observation that the flow structure depends upon the parameter \(\alpha \), rather than simply the surface porosity, is supported by calculated dispersion times of virtual particles released both inside the model and within the wake. The particle distributions display three distinct dispersion behaviours, from nearly linear to a logarithmic decay slower than that of a solid cylinder, thus emphasising the existence of multiple flow regimes and the importance of the relative twine/mesh ratio.