Normally, tip vortex cavitation (TVC) is first observed at a certain location behind the tips of propeller blades. Therefore, TVC is important for naval ships and research vessels that require raising the cavitation inception speed to maximum possible values. The concepts for alleviating the tip vortex are summarized by Platzer and Souders (1979), who carried out a thorough literature survey. Active control of TVC involves the injection of a polymer or water from the blade tip. The main effect of such mass injection (both water and polymer solutions) into the vortex core is an increase in the core radius, consequently delaying TVC inception. However, the location of the injection port needs to be selected with great care to ensure that the mass injection is effective in delaying TVC inception. In the present study, we propose a semi-active control scheme that is achieved by attaching a thread at the propeller tip. The main idea of a semi-active control is that because of its flexibility, the attached thread can be sucked into the low-pressure region closer to the vortex core center. An experimental study using a scale model was carried out in the cavitation tunnel at the Seoul National University. It was found that a flexible thread can effectively suppress the occurrence of TVC under the design condition for a model propeller.

An experimental investigation was carried out to test the possibility of improving the propulsive performance of waterjets by the introduction of polymeric macromolecules in water solution. Four pumps were tested either in closed and open circulating loops or in a static waterjet plant. It was clearly demonstrated that polymeric dilute solutions of Polyox WSR-301 always produced an increase of pump head, static thrust and efficiency, together with shaft power reduction. Improvements up until 20 percent could be obtained. On the basis of the distribution and of the weight of the losses inside the pump, an interpretation of the effect is given together with a suggestion for a more simple method to obtain it.

Introduction - It is well known that the injection of dilute polymer solutions into the boundary layer on two-dimensional hydrofoils produces changes in both the drag and lift of the foils, with the changes being dependent on the polymer [1], 3 its concentration [2], the injection technique [8], the location of the injection slit [1], and the rate of injection [1, 4, 5]. Previous tests [4] have also shown that the observed drag reduction does not necessarily increase linearly with the rate of injection of the polymer. Indeed, beyond a certain injection rate, further increases lead to little or no drag reduction and, in some cases, actually lead to a drag increase. Thus, one question that is of considerable practical interest is the effect of multiple injections of a given amount of polymer from several chordwise locations as compared with that due to the injection from a single location. The present study investigates the effects on lift and drag of a 20.16-cm (8 in) chord NACA 684–010 two-dimensional hydrofoil due to multiple injections of 200-ppm Polyox WSR 301 solution from several chordwise locations. The results indicate that, for a given flux of polymer injection, multiple injections from properly selected locations as compared with the injection from a single location result in a larger drag reduction without adversely affecting the foil lift.

The diffusion of a thin tangential jet of an aqueous solution of drag-reducing polymer injected into the water-turbulent boundary layer of a flat plate at a freestream Reynolds number, 3.6 × 107, and the accompanying drag reduction are investigated for a variety of initial concentrations and ratios of injection to freestream velocities. The concentration distribution along the wall is found to be mainly represented by two regions. In the first region the wall concentration is practically constant and equal to the injected one; in the second region the concentration varies approximately as the inverse of the distance from the injection slit. The length of the first region is significantly increased by the polymer solution injection as compared with the pure solvent injection. The drag-reduction effect associated with the polymer injection depends on the trailing-edge concentration achieved as a result of the diffusion process.

The effect of a boundary-layer injection of drag-reducing additive solutions on the lift and drag of a 10-cm chord, NACA 63A020 symmetrical two-dimensional hydrofoil was investigated for various freestream and injection velocities, foil incidences, and additive concentrations ranging from 50 to 400 ppm of POLYOX WSR 301. The experimental results demonstrate that the lift of the hydrofoil can either increase or decrease depending upon whether the polymer injection is made on the suction or pressure side of the foil surface, respectively. In both cases, however, the drag is reduced. The net result of the injection of the drag-reducing agent is an augmentation of the lift-drag ratio. The magnitude of this augmentation and its dependence on the freestream velocity, the injection velocity, the concentration of polymer, and the incidence of the foil are analyzed.

For a flat plate moving in a dilute polymer solution, effects on boundary-layer characteristics, shear stress, drag reduction and maximum drag reduction are considered for polymers which satisfy the Meyer-Elata law. Results are derived from a model in which the velocity profiles satisfy the law-of-the-wall and a velocity-defect law, and the polymer has no effect in the range in which the latter law is valid. It is also assumed that the polymer affects the law of variation of the mixing length, and a family of velocity profiles representing this effect is adopted. This model then yields a curve of maximum drag reduction as well as a two-parameter family of curves of drag reduction, consequences of the nonoverlapping or overlapping of the two velocity-profile laws. The results are compared with those of Granville for drag reduction, and with the predicted curve of Virk-Granville and an experimental result of Levy and Davis for maximum drag reduction.

The resisting torque of disks rotating in an unbounded fluid is analyzed on the basis of three-dimensional boundary-layer theory. Smooth and rough surfaces in ordinary fluids and in drag-reducing polymer solutions are considered. A general logarithmic relation is derived for the torque as a function of Reynolds number for arbitrary roughness and arbitrary drag reduction. Special formulas are obtained for smooth surfaces, fully rough surfaces, polymer solutions with a linear logarithmic drag-reduction characterization, and polymer solutions with maximum drag reduction. Relations are also obtained for boundary-layer parameters such as thickness and wall shearing stress. The computed results are in excellent agreement with experimental data available in the literature.

A method is developed for prediction of frictional-drag reduction in high Reynolds number flows past smooth flat plates with polymer injection near the leading edge. Numerical results are given for water-Polyox WSR 301 solutions with either uniform concentration or injection.

The drag reduction due to polymer being emitted from a slot is analytically treated for the fourth stage for which the concentration boundary layer coincides with the momentum boundary layer.

The velocity similarity laws of shear flows are developed for drag-reducing dilute polymer solutions. Relations for boundary-layer parameters and frictional-resistance formulas of the logarithmic type are derived for pipe flow and for flat plates in parallel flow.