Flume Experiments on the Hydrodynamics of the Lowestoft High-speed Plankton Samplers: I

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Abstract
Experiments on the hydrodynamics of a high-speed encased plankton net (50·8 cm internal tube diameter; net 24·6 meshes per cm, porosity 46%) with two nose cones (apertures 35·6 and 20·3 cm diameter, R = 6·2 and 19·0 respectively) were made in the circulating water channel of the National Physical Laboratory's Ship Division. Measurements of velocity profiles inside the sampler indicated a complicated three dimensional flow pattern with large fluctuations in linear velocity. The encased net had a funnelling effect and flow accelerated along its central axis to the apex. Velocity and pressure gradients extended to a maximum of 80 cm ahead of the sampler at 5 knots. Calibration showed linear relationships between channel speed and (I) flowmeter rev/sec and (2) volume accepted by the sampler both in free flow and with a net in position. With the larger nose cone aperture (35·6 cm diam, R = 6·2), the net reduced the volume accepted by 9% from the free flow value, but with the smaller nose cone aperture (20·3 cm diam, R = 19·0) it had no effect; equivalent filtering efficiencies were 91% and 100%. Estimates of volume accepted by the sampler based on the product of mouth area (A) and distance towed (D) greatly exceeded the measured value. It was concluded that high speed encased samplers must be calibrated for the volume of water accepted. The calibration of the flowmeter against flume speed was not in itself sufficient.

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