Simplified Approximate Expressions for the Boundary Layer Flow in Cylindrical Sections in Plankton Nets and Trawls

Svein Helge Gjøsund

SINTEF Fisheries and Aquaculture, Trondheim, Norway.

DOI: 10.4236/ojms.2012.22009                

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.

Keywords

Plankton Nets; Trawls; Non-Tapered Sections; Flow; Boundary Layer; Filtration; Drag

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	S. H. Gj?sund and B. Enerhaug, “Flow through Nets and Trawls of Low Porosity,” Ocean Engineering, Vol. 37, No. 4, 2010, pp. 345-354. doi:10.1016/j.oceaneng.2010.01.003
[2]	H. Schlichting, “Boundary Layer Theory,” 7th Edition, McGraw-Hill, Boston, 1979.
[3]	F. M. White, “Viscous Fluid Flow,” 2nd Edition, Mc- Graw-Hill, Boston, 1991.
[4]	B. Vincent and D. Marichal, “Computation of the Flow Field in the Codend,” Fishing Technology and Fish Behaviour Working Group Meet-ing, Woods Hole, 5-18 April 1996.