Abstract
Trawls and plankton nets are basically made up of conical and cylindrical net sections. In conical sections the flow will pass through the inclined net wall with a noticeable angle of attack, and then the flow, filtration and drag can be suitably modelled e.g. by a pressure drop approach [1]. In cylindrical and other non-tapered net sections, such as foreparts and extension pieces in trawls and plankton nets, the flow is directed along the net wall and is best considered in terms of a boundary layer. Boundary layer theory and turbulence models can be used to describe such flow, but this requires extensive numerical modelling and computational effort. Simplified approximate formulas providing a qualitative description of the flow with some quantitative accuracy are therefore also useful. This work presents simplified parametric expressions for boundary layer flow in cylindrical net sections, including the boundary layer thickness and growth rate along the net, the filtration velocity out of the net wall, the decrease in mass flux through the net due to the growing boundary layer, and the effect of twine thickness, flow (towing) velocity and the dimensions of the net. These expressions may be useful for assessing the existence and extension of a boundary layer, for appropriate scaling of boundary layer effects in model tests, for proper placement of velocity measurement probes, for assessing the influence on filtration and clogging of plankton net sections, and more.
Highlights
When a cylindrical net is towed through water a boundary layer develops and grows in thickness along the inside and outside of the net wall
In cylindrical and other non-tapered net sections, such as foreparts and extension pieces in trawls and plankton nets, the flow is directed along the net wall and is best considered in terms of a boundary layer
In the following we assume that roughness has a stronger relative influence on the boundary layer thickness than on the shape of the boundary layer velocity profile, and that Prandtl’s powerlaw can be used as an approximation for rough walls if the boundary layer thickness is corrected for roughness
Summary
When a cylindrical net is towed through water a boundary layer develops and grows in thickness along the inside and outside of the net wall. The boundary layer can be assumed to be turbulent all along the net, and classic turbulent boundary layer results may be used given that the velocity across the wall is small compared to the flow velocity outside the boundary layer, i.e. v/U < 0.01 [2]. The classic result for the turbulent boundary layer along a rough wall is the logarithmic law [3], but this is cumbersome to use directly. In the following we assume that roughness has a stronger relative influence on the boundary layer thickness than on the shape of the boundary layer velocity profile, and that Prandtl’s powerlaw can be used as an approximation for rough walls if the boundary layer thickness is corrected for roughness
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