Abstract
Observations have been made of
u
and
w
, the horizontal and vertical components of turbulent velocity, in a tidal current, at heights of 50 to 175 cm above the bottom. The measuring instrument was an electromagnetic flowmeter, in which the magnetic field was produced by a. c. at 50 c/s and the p. d. induced in the flowing water was measured by two pairs of electrodes. The measuring head of the instrument was 10 cm in diameter, and two such heads were mounted on a tripod which was laid on the sea bed. The observations were made off Red Wharf Bay, Anglesey, in depths of 12 to 22 m, on a fairly flat bottom consisting mainly of firm sand. For mean currents,
U
, in the range 25 to 50 cm/s, the r. m. s. values of
u
were of the order of 10% of
U
, while those of
w
were about 6% of
U
. On a number of records,
u
and
w
were recorded simultaneously, and from these the Reynolds stress —
ρ
[
uw
] was evaluated. At 75 cm above the bottom the values of stress were from 2 to 4 dyn/cm
2
, the corresponding coefficient of correlation between
u
and
w
averaging —0·4. Auto-correlation curves and spectrum functions computed from these records showed that
u
contained considerably more energy in the fluctuations of longer period than
w
did. Other records were of traces of
u
or of
w
at two different heights and showed the smaller vertical scale of
w
compared with that of
u
. In the case of
u
the vertical scale appears to be only about one-third of the scale in the direction of the mean flow.
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