Experiments on filtration in plankton nets

Abstract

Experiments were carried out in wind tunnels, in a test tank, and at sea, to investigate the interaction between the water column and a towed plankton net.

Flow patterns about stationary nets held in a stream of air were observed and photographed. The velocity and turbulence of the stream near the net, and the drag upon the net, were measured. Experimental tows were made in the laboratory and in the field to test the effect of various factors on "filtration efficiency". This interaction between the discharge and the flow around the net leads to enhanced filtration. The effect is accentuated by a flared mouth which increases the velocity differential. The effect is reduced by encasing the net, which separates the interacting streams and probably causes the stream from the net surface to lose momentum.

Filtration efficiency is independent of towing velocity, except for a gradual decline at velocities less than 1-2 kt. Filtration efficiency is also independent of filtering area in nets of usual length, but declines sharply in nets shorter than twice their mouth diameter. There is a zone of turbulence and reduced velocity in the centre of a net with bridles. At approximately one-quarter of a radius from the ring, the stream is flowing at mean mouth velocity. This seems to be the most suitable site for a flowmeter.