Measurements of Turbulence near the Sea Bed in a Tidal Current

The turbulent fluctuations of velocity have been measured, using an electromagnetic flowmeter, at heights up to 1.75 meters above the sea bed. The area of observations was off Anglesey, North Wales, in depths of 12 to 22 meters, on a firm, sandy bottom and during neutrally stable conditions. A number of records of the longitudinal (u) and vertical (w) components of velocity were obtained, and a few records included the lateral (v) component. From these the intensities, autocorrelations, and cross correlations for various time lags were computed and the corresponding u, v, w, and uw spectra were determined. Sufficient numbers of records have been obtained to enable mean values to be derived for a given set of physical conditions. METHOD OF OBSERVATION In the series of observations described below, the turbulent fluctuations in the velocity of the flow of water were recorded with an electromagnetic flowmeter. A brief description of this instrument has been given by Bowden and Fairbairn [1956]. The measuring unit has an over-all diameter of 10 cm, and the magnetic field is produced by a circular coil through which an alternating current at 50 cps is passed. Two pairs of electrodes mounted on this unit enable the potential gradients due to two components of the flow of water past the unit to be measured. With the plane of the coil vertical, the longitudinal component u and the vertical component w of the flow may be recorded. By mounting the unit with the plane of the coil horizontal, it is possible to record the transverse component v with the longitudinal component u. Using two units, the corresponding component, u for example, at two points may be recorded simultaneously. The over-all response of the equipment was fiat for periods greater than I sec, falling to 50 per cent at a period of 0.25 sec. When the flowmeter was used at sea, the measuring units were mounted on a tripod laid • Based on a paper presented at the International Symposium on Fundamental Problems in Turbulence and Their Relation to Geophysics sponsored by the International Union of Geodesy and Geophysics and the International Union of Theoretical and Applied Mechanics, held September 4-9, 1961, in Marseilles, France. on the sea bed. A triangular fin kept the units aligned to the mean flow while the tripod was being lowered. Bowden and Fairbairn [1956] gave an account of a number of records obtained with this equipment in Red Wharf Bay, off Anglesey, North Wales, in 1954. Since then a considerably larger number of records covering a wider range of conditions has been obtained during further observations in the same area. This area was considered suitable, as the bottom is fairly fiat and is composed mainly of firm sand. The tidal currents are approximately rectilinear, and near the bottom they reach velocities up to about 50 cm/sec. Since the recording was limited to two channels, in addition to the time trace, only two components could be recorded simultaneously. By using two measuring units and suitably switching the inputs to the amplifiers, the two channels could be used for making a variety of recordings: u and w, or u and v, at one position; u at two positions with a given vertical or transverse separation; or w at two positions in a similar way. The height of a measuring unit varied from 50 to 175 cm above the bottom, giving vertical separations between the two units up to 125 cm. ANALYSIS OF I•ECORDS Usually the duration of an individual record was 10 min; the first stage in the analysis was to read the two traces on the record at 1-sec