Miniature drag sphere velocity probe.

The development and testing of a miniature drag sphere velocity probe is described. Impetus for its development arose from a need to investigate the flow structure and turbulence beneath breaking surface water waves, i.e., a requirement for a fast-response small velocity probe which is useable in oscillatory flows. However, the presence of both drag (velocity) and inertial (acceleration) forces limits the measurable fluctuations to those having scales at least an order of magnitude larger than the sphere diameter, while the sphere diameter is itself limited by the requirement that the drag coefficient be independent of velocity. The resulting probe, with a sphere diameter of 0.43 cm, while not free of temperature drifts, pressure sensitivity, and cross-channel talk, yields encouraging results.