Michael J. Moeny

Combined polymer and microbubble drag reduction on a large flat plate

Drag-reduction experiments with combined injection of high-molecular-weight long-chained polymers and microbubbles were conducted on a 3.1 m long flat plate model in the 1.22 m diameter water tunnel at the Applied Research Laboratory of the Pennsylvania State University. Combined gas injection upstream of polymer injection produced, over a wide range of test conditions, higher levels of drag reduction than those obtained from the independent injection of polymer or microbubbles alone. These increased levels of drag reduction with combined injection were often greater than the product of the drag reductions obtained by the independent constituents, defined as synergy. We speculate that the synergy is a result of the gas-layer-induced extension of the polymer-alone initial diffusion zone in combination with the increased drag reduction by microbubbles. This increased length of the initial zone layer, consistent with high drag reduction, can significantly increase the persistence of the drag reduction and may improve the outlook for practical application.

Microbubble Drag Reduction in Rough Walled Turbulent Boundary Layers

Experiments were conducted in the 12-inch diameter tunnel at ARL/PSU using the tunnel wall boundary layer facility to determine the influence of surface roughness on microbubble drag reduction. To accomplish this, carbon dioxide was injected through a slot at rates of 0.001 m3 /s to 0.011 m3 /s, and the resulting skin friction drag measured on a 317.5 mm long by 152.4 mm span balance. In addition to the hydrodynamically smooth balance plate, additional plates were covered with roughly 75, 150 and 300 micron grit. Over the speed range tested of 7.6, 10.7 and 13.7 m/s, the roughness ranged from smooth to fully rough. Not only was microbubble drag reduction achieved over the rough surfaces, but the percentage drag reduction at a given gas flow rate was larger for larger roughness. A new scaling parameter that collapses all of the data is also introduced.Copyright

Noise generated by ventilated supercavities

The noise generated by ventilated supercavities has been explored experimentally in a water tunnel facility. The most prominent acoustical characteristic is the monopole behavior exhibited by a ventilated supercavity in its pulsating closure regime. The interior cavity pressure and near-field radiated sound are monotonic with a frequency that is related to the speed and length of waves propagating on the supercavity gas/water interface. The cavity interior pressure spectrum level is shown to be related to the near-field and far-field noise spectrum level through spherical spreading of the sound waves from the supercavity interface. As a result, the cavity interior pressure can be used as a measure of the radiated noise. The noise radiated by a pulsating supercavity at the pulsation frequency is at least 40 dB above that radiated by comparable re-entrant jet and twin vortex cavities.

Jet-Supercavity Interaction: Insights from Experiments

An experimental study was performed to evaluate some of the claims of Paryshev (2006) regarding changes to ventilated cavity behavior caused by the interaction of a jet with the cavity closure region. The experiments, conducted in the 1.22m dia. Garfield Thomas Water Tunnel, were performed for EDD to tunnel diameter of 0.022, Fr = 14.5 and 26.2. The model consisted of a converging-section nozzle mounted to the base of a 27.9mm 37° cone cavitator placed on the tunnel centerline at the end of a 138.4mm long streamlined strut. A ventilated cavity was formed over the model, then an air jet, issuing from a converging nozzle, was initiated. Changes to cavity behavior were quantified in terms of cavitation number, thrust-to- drag ratio, and stagnation pressure ratio at the jet nozzle. The results show that, while the overall trends predicted by Paryshev were observed, the data did not fully collapse, suggesting that many of the effects neglected by Paryshevs model have measureable effect.

Jet-Supercavity Interaction: Insights from Physics Analysis

Various closure conditions of a ventilated cavity enveloping all or part of a high-speed underwater body are introduced, including those involving a propulsion jet. The flow regimes for the latter are described based on Efros-Paryshev theory, which is extended to estimate the efficiency and fundamental limitations of a rocket-type propulsor.

Finite Volume Computation of the Mitigation of Cavity Pulsation

Finite volume based modeling of ventilated supercavity pulsation and its mitigation via a priori modulation of ventilation flow was investigated. Simulated pulsation was numerically achieved, as was mitigation of pulsation via sinusoidal modulation of the ventilation flow. In addition to confirmation that the numerical approach is sufficient to capture mitigation, it was found that modulated ventilation, without altering the mean ventilation mass flow rate, results in altered cavity size, pressure, and closure condition.

Jet-Supercavity Interaction: Insights from CFD

In this work, the interaction between a ventilated supercavity and a jet are examined using computational fluid dynamics (CFD). The CFD model is validated using experimental data, and shows to capture the correct trend in the bulk cavity behavior (qualitatively and quantitatively). Using these models, a number of novel insights into the physical characteristics of the interaction are developed. These interactions are described by: (1) the jet gas and ventilation gas poorly mix within the cavity, (2) the jet appears to cause additional gas leakage by transitioning the cavity from a recirculating flow to an axial flow, (3) the jet has the ability to lengthen the cavity, and (4) the jet invokes wake instabilities that drive cavity pulsation. These phenomena are documented and discussed in the following paper.

A Method of Mitigating Pulsation in Ventilated Supercavities

A method for mitigating ventilated supercavity pulsation is presented. The method, which has its roots in parametric oscillators, shifts the supercavity resonance frequency by modulating its gas ventilation rate. When appropriately modulated, the supercavity is driven off resonance by the waves on the gas/water interface (that remain unchanged) and pulsation is, therefore, suppressed. Initial experimental results indicate that the gas ventilation rate modulation frequency must be sufficiently different from the supercavity resonance frequency to mitigate pulsation. If the modulation frequency is not sufficiently different from the supercavity resonance frequency, pulsation is simply shifted in frequency with a corresponding small reduction in the supercavity interior pressure spectrum level and radiated noise.

Synergy Mechanism in Combined Polymer and Microbubble Drag Reduction: Decreased Polymer Diffusion

Drag reduction (DR) experiments with combined micro-bubble and polymer solution injection were conducted in a water tunnel at the ARL/Penn State. Previous research has shown that gas injected upstream of polymer produced higher levels of DR than expected based on DR levels observed with the independent injection of micro-bubbles or polymer alone. This synergy between the two injection processes was speculated to have resulted from a decrease in the diffusion rate of injected polymer away from the surface by the effects of the micro-bubbles. The slowed polymer diffusion should extend the length of the zone where large polymer DR occurs. In the current work, a confocal-style laser induced fluorescence based probe was developed and used to measure the wall concentration of injected polymer solutions and injected water with and without upstream micro-bubble injection. The local wall concentrations were increased with gas injection by more than an order of magnitude in many cases. These results show that synergistic drag reduction occurs with combined injection as a result of increased polymer wall concentrations.© 2006 ASME