A three component drag probe has been built, calibrated, and
used to measure velocities beneath deep water ocean waves and
turbulence in a tidal channel. Simple variable inductance devices
which may be submerged in water were used as displacement transducers
and the associated electronics provided voltage outputs which
were proportional to the three components of force that were exerted
on a small 5 cm diameter sphere. The force components were due to
both the water drag force and the water inertial force in an accelerating
flow field. Techniques are described for interpreting measurements
made with the drag probe and for obtaining the three velocity
components from the measured force components. From the drag
probe calibration and its use in the field, it is concluded that the drag
probe is a suitable instrument for the measurement of wave velocities
and turbulence. Modifications are suggested to improve the performance
of the drag probe.
For the wave velocity measurements, the experimental results
indicate that linear wave theory is adequate to describe the relations
between the wave pressure and the wave velocity components. At
frequencies higher than the predominant wave frequency the velocity
spectra are roughly proportional to f⁻³ where f is the frequency
in Hz. The wave velocity components were used to obtain an estimate
of the directional energy spectrum.
From the measurements in a tidal channel, it appears that the
instrument is suitable to measure turbulent fluctuations with scale
sizes larger than about 20 cm. If the turbulence were isotropic the
velocity spectra would be proportional to f[⁻⁵/³]. Due to the influence
of boundaries, the flow was not isotropic but the results appear
to be consistent with other observations that turbulent velocity spectra
usually show a f⁻¹ to f⁻² behavior and are quite different from
wave velocity spectra
