Search

A novel velocimeter consisting of multiple orthogonal disks fitted with pressure transducers has been developed. Dynamic pressure differences are measured between the center of one disk face and the center of the other face, on each of the disks. While previously-developed anemometers based on dynamic pressure differences (such as yaw or three-hole probes) can only measure velocities with a small range of directions, the new disk probe can measure three components of velocity, even in highly three-dimensional flows where the approximate direction of the flow is not known. Wind tunnel tests have shown the velocimeter to be quite accurate; it can measure velocities to [+ or -]1.4% and wind directions to [+ or -]4 deg. The velocimeter is very robust and therefore can make measurements in environments too harsh for most other velocity transducers.

Sea trials of a 45 ft seine boat, fitted first with a conventional tip, and subsequently with a ducted tip, 36 in. diameter propeller have been conducted. The trials consisted of careful propeller efficiency measurements at a number of advance ratios, and underwater video imaging of the different propellers under Bollard pull conditions. The trials have shown that flow-through ducts installed at the blade tips suppress the tip vortex roll-up, thus resulting in a substantial delay in the onset of tip vortex cavitation. The cavitation inception index for the ducted tip propeller is 50 percent lower than for the conventional propeller. This cavitation improvement comes without any efficiency penalty. In fact, the efficiency of the ducted tip propeller is up to 6 percent higher (at high advance ratios) than the efficiency of the same propeller fitted with conventional tips. The ducted tip propeller was originally thought to be useful only for boats with heavily loaded propellers, such as tug boats and trawlers. The current research has shown that the ducted tips may have much broader application.

Computer simulations of the flow around a pair of two-dimensional, tandem circular cylinders in a flow, for Reynolds numbers in the range 1-40, are described. Cylinder surface-to-surface separations in the range O. 1 < s/D < 400 (D = cylinder diameter) were considered. The computed wake of a single cylinder at these low to moderate Reynolds numbers was in surprisingly good agreement with the laminar wake approximation, and a simple theory is presented to explain this agreement. With tandem cylinders, the drag on the downstream cylinder is a monotonic function of the cylinder separation. The laminar wake approximation can be used to explain reasonably well the variation in drag. The drag on the upstream cylinder is also a monotonic function of separation distance provided that the Reynolds number is less than about 10. For Reynolds numbers between 10 and 40, the upstream cylinder drag first decreases as separation increases up to a few diameters and then increases monotonically with separation distance. [ABSTRACT FROM AUTHOR]

Presents a study that compared flows between hydrofoils with and without ducted tips. Background on the effects of tip vortices on marine propellers; Description of the hydrofoils used in the study; Results of the application of a pressure-based, finite-volume flow solver in the study.