Motivated by the need to predict vortex cavitation inception, a study has been conducted to investigate bubble capture by a concentrated line vortex of core size r c and circulation ⌫ 0 under noncavitating and cavitating conditions. Direct numerical simulations that solve simultaneously for the two phase flow field, as well as a simpler one-way coupled point-particle-tracking model ͑PTM͒ were used to investigate the capture process. The capture times were compared to experimental observations. It was found that the point-particle-tracking model can successfully predict the capture of noncavitating small nuclei by a line vortex released far from the vortex axis. The nucleus grows very slowly during capture until the late stages of the process, where bubble/vortex interaction and bubble deformation become important. Consequently, PTM can be used to study the capture of cavitating nuclei by dividing the process into the noncavitating capture of the nucleus, and then the growth of the nucleus in the low-pressure core region. Bubble growth and deformation act to speed up the capture process.

A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the nearzero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11 m. The test model spanned the 3.05 m width of the LCC test section and had an overall length of 12.9 m. Skin-friction drag was measured with six floating-plate force balances at downstream-distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20 m s −1 . For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 < y + < ∼ 10 3 ) were examined at three locations downstream of the injector using near-wall planar laserinduced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditional K-factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in the K-scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flowinduced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with downstream distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the nearwall polymer concentrationis correlated to the measured drag reduction for the † Email address for correspondence: ceccio@umich.edu 260 E. S. Winkel and others three polymer molecular weights in the form of a proposed empirical drag-reduction curve.

The sound produced by the collapse of discrete cavitation bubbles was examined. Laser-generated cavitation bubbles were produced in both a quiescent and a vortical flow. The sound produced by the collapse of the cavitation bubbles was recorded, and its spectral content was determined. It was found that the risetime of the sound pulse produced by the collapse of single, spherical cavitation bubbles in quiescent fluid exceeded that of the slew rate of the hydrophone, which is consistent with previously published results. It was found that, as collapsing bubbles were deformed by the vortical flow, the acoustic impulse of the bubbles was reduced. Collapsing nonspherical bubbles often created a sound pulse with a risetime that exceeded that of the hydrophone slew rate, although the acoustic impulse created by the bubbles was influenced largely by the degree to which the bubbles became nonspherical before collapse. The noise produced by the slow growth of cavitation bubbles in the vortex core was not detectable. These results have implications for the interpretation of hydrodynamic cavitation noise produced by vortex cavitation.

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An extensive experimental investigation was carried out to examine the tip-leakage flow on ducted propulsors. The flow field around three-bladed, ducted rotors operating in uniform inflow was measured in detail with three-dimensional laser Doppler velocimetry and planar particle imaging velocimetry. Two geometrically similar, ducted rotors were tested over a Reynolds number range from 0.7×106 to 9.2×106 in order to determine how the tip-leakage flow varied with Reynolds number. An identification procedure was used to discern and quantify regions of concentrated vorticity in instantaneous flow fields. Multiple vortices were identified in the wake of the blade tip, with the largest vortex being associated with the tip-leakage flow, and the secondary vortices being associated with the trailing edge vortex and other blade-wake vortices. The evolution of identified vortex quantities with downstream distance is examined. It was found that the strength and core size of the vortices are weakly dependent on Reynolds number, but there are indications that they are affected by variations in the inflowing wall boundary layer on the duct. The core size of the tip-leakage vortex does not vary strongly with varying boundary layer thickness on the blades. Instead, its dimension is on the order of the tip clearance. There is significant flow variability for all Reynolds numbers and rotor configurations. Scaled velocity fluctuations near the axis of the primary vortex increase significantly with downstream distance, suggesting the presence of spatially uncorrelated fine scale secondary vortices and the possible existence of three-dimensional vortex-vortex interactions.