Some observations of turbulence near the sea bed in a tidal current

Abstract

AbstractOceanographers are interested in turbulence in the flow of water in the sea or in estuaries because of its influence on the dynamics of water movements and mixing. These processes are more usually studied by their large‐scale effects, and detailed investigation of the turbulent fluctuations has been attempted less frequently in oceanography than on the model scale or in the atmosphere. This paper deals with fluctuations in the velocity components. Earlier observations were reviewed by Bowden and Proudman (1949), who also described experiments in the Mersey estuary, using the Doodson current meter. Further observations on similar lines were described by Bowden and Fairbairn (1952a). An attempt to measure the vertical fluctuations as well as the horizontal fluctuations was made by Francis, Stommel, Farmer and Parson (1953) in the Kennebec estuary, Maine.The observations described in this paper were made using an electromagnetic flow meter, a new type of instrument for this purpose. It was possible to measure the vertical component w, as well as the longitudinal component u, of the fluctuations, and hence the correlation between them. The instrument was designed at the National Institute of Oceanography, and uses alternating current at 50 c/s, passed through a coil, to produce the magnetic field. Two pairs of electrodes are used to measure the two components of flow in the plane perpendicular to the direction of the magnetic field. The measuring unit is only 4 in. in diameter and two such units can be mounted in various positions on a tripod, 6 ft high, which is laid on the sea bed. The potential difference across the electrodes is approximately 50 μv per kt, and is amplified by electronic circuits after induced p.d.'s, other than those due to the flow of water, have been balanced out. The output is recorded photographically.The observations at sea were made off Red Wharf Bay, Anglesey, in an area from 12 m to 20 m deep, where measurements of tidal currents and bottom friction had been made previously (Bowden and Fairbairn 1952b). The tidal currents at springs reach a velocity of about 2 kt at the surface. The technique employed was to lower the tripod from an anchored boat, a canvas fin keeping it orientated in the direction of the mean current while being lowered. The tripod with the measuring units remained stationary on the bottom while the records were taken and was raised again before the tidal current changed direction.