Flow through nets and trawls of low porosity

Commercial Exploitation of Zooplankton in the Norwegian Sea

“…Hence the filtration efficiency of plankton nets depends strongly on the net parameters and towing velocity. Gjøsund and Enerhaug (2010) compare the theoretical model with flume tank measurements with finemeshed net cones, and demonstrate among other things how the filtration efficiency for typical plankton nets increases with increasing towing velocity, and decreases with decreasing velocity (Figure 3). This is contrary to common belief, as noted also by Tranter and Heron (1967); there is a widespread and persistent, but incorrect perception that filtration efficiency generally decreases as towing velocity increases.…”

“…Schubauer et al (1950) found that Eq. 5 describes the pressure drop well for ≥ 45°, and Gjøsund and Enerhaug (2010) argue that it applies to lower taper angles also if the Reynolds number dependency in K 0 is properly accounted for. A suitable model for T f is not available, and Gjøsund and Enerhaug use a constant value T f = 0.02.…”

“…A theoretical model for the flow through fine-meshed nets and trawls is presented in Gjøsund and Enerhaug (2010). They derive basic relations for the flow through and forces on inclined net sections, based on pressure drop and streamline deflection through porous screens, and present parametric expressions for the filtration efficiency and drag on conical nets, cf.…”

“…A suitable model for T f is not available, and Gjøsund and Enerhaug use a constant value T f = 0.02. A number of other models for K 0 and C N exist, see Gjøsund (2006) and Gjøsund and Enerhaug (2010), and may yield equally good or better predictions is some cases. Gjøsund and Enerhaug (2010) conclude that the filtration efficiency is well predicted by Eq.…”

“…A key element in Gjøsund and Enerhaug (2010) is the importance of viscous effects at very low Reynolds numbers, i.e. as local dimensions and velocities become very small.…”