Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session GE: Multiphase Flows: Cavitation |
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Chair: Roger Arndt, University of Minnesota Room: Hilton Chicago Continental B |
Monday, November 21, 2005 10:34AM - 10:47AM |
GE.00001: Ventilated Partially Cavitating Hydrofoils in Steady Flow and in Periodic Gusts James Kopriva, Roger Arndt, Eduard Amromin, Martin Wosnik Experiments with the partially cavitating, low-drag hydrofoil OK-2003 have been carried out in the SAFL high speed water tunnel to study the ventilated hydrofoil behavior under steady and periodic conditions. The periodic conditions are created by a two-dimensional gust generated by two NACA hydrofoil-flaps oscillating upstream of the tested cavitating hydrofoil. The designed system allows +/-10 degrees oscillations of the flap angle of attack. A flywheel and the ability to adjust the servo-motor's PID gains allow the system to run very smoothly, with a motor RPM error as low as 0.5 percent. The motor's ability to operate up to 3500 RPM and the water tunnel's flow speed capabilities give a wide range of attainable Strouhal numbers, allowing the ability to model the desired sea wave impact. LDV has been used to analyze the generated oscillating flow showing a nearly perfect gust that can be represented with a cosine function with frequency equal to the upstream perturbation. Measurements of the lift and drag of the ventilated hydrofoil OK-2003 and its ventilation flow-rate under impact of the gust of various magnitudes and frequencies have been made and are compared with their values in steady incoming flow. [Preview Abstract] |
Monday, November 21, 2005 10:47AM - 11:00AM |
GE.00002: An experimental investigation of cavitation in JP8 jet fuel. Muhammad Iqbal, Flint Thomas, Patrick Dunn, Michael Davis In this experimental study flow induced cavitation in JP8 jet fuel is investigated for the first time. In internal fuel handling machinery cavitation is associated with sharp turns in the flow path. It is well known that cavitation bubble collapse leads to damage and eventual failure of pumps and valves. In this study we consider a generic flow problem that is relevant to a wide number of fuel handling applications. This consists of an axisymmetric jet of JP8 fuel that emerges from the center of a disk. The jet impinges on a second disk suspended just above the first and consequently forces the flow to turn suddenly and flow radially into a thin gap between the disks. In this manner, the initially axial flow must turn rapidly into the radial direction. The large radial dilatation acts on nuclei present to give rise to cavitation bubble formation. Detailed experiments are performed in a closed loop facility that allows the dissolved gas and contaminant particle concentration in the fuel to be controlled. The cavitation inception process is studied through high resolution imaging, pressure measurement and with a phase Doppler particle analyzer. [Preview Abstract] |
Monday, November 21, 2005 11:00AM - 11:13AM |
GE.00003: Identification of Large Scale Structures in the Wake of Cavitating Hydrofoils Using LES and TR-PIV Martin Wosnik, Qiao Qin, Roger E.A. Arndt Large-scale three-dimensional cavitating structures exist in the wake of two-dimensional hydrofoils, as a result of sheet/cloud cavitation. This type of cavitation produces unsteady lift on most hydrofoils -- including the NACA 0015 studied here -- but is sufficiently periodic to have potential for control. A Large Eddy Simulation (LES) based on a virtual single-phase, fully compressible cavitation model captures the complex dynamical features of this highly unsteady cavitating flow well. The LES results are compared to Time-Resolved Particle Image Velocimetry (TR-PIV, recorded at 2000Hz) in the region immediately downstream of the hydrofoil, with particular attention to the predicted vortex shedding mechanisms. With a careful choice of photometric parameters and adaptive masking, the large, vortical, cavitating structures are identified quantitatively. The existence of the primary vortex pair predicted by the LES is confirmed by TR-PIV. This vortex pair produces large cross-stream velocities, with a general ejection direction of $3/4 \pi$ to the free stream. However, the shedding pattern as recorded with TR-PIV is not nearly as regular as in the LES, due to the limited number of spanwise grid points in the simulation and the highly three- dimensional nature of cloud cavitation shedding in the experiment. [Preview Abstract] |
Monday, November 21, 2005 11:13AM - 11:26AM |
GE.00004: Investigation of a Cavitating Jet Emanating From a Crown-Shaped Nozzle Stephane Poussou, Raul Sanchez, Michael Plesniak Cavitation in submerged jets is used in applications such as material cutting or underwater surface cleaning. Incipient conditions have been found to depend on the flow structure in the near-jet region. The developing shear layer can be passively controlled at the nozzle exit by modifying the geometry of the edge, which affects the three-dimensionality of the vortical structures. Recent investigation using PIV and high-speed shadowgraphs showed that a crown-shaped geometry alters significantly the near-jet flow field by introducing streamwise vorticity. Substantial deviations from axisymmetry were observed in the magnitudes of the normal Reynolds stresses and spanwise vorticity at particular azimuthal locations. In the present study, a 3D Large Eddy Simulation (LES) is carried out to investigate the turbulent jet emanating from a crown-shaped nozzle at typical Reynolds number (based on diameter) of the order of 300,000. Numerical and experimental data are compared. The effects of the nozzle on the Reynolds stresses and cavitation inception are investigated at different flow conditions. [Preview Abstract] |
Monday, November 21, 2005 11:26AM - 11:39AM |
GE.00005: Control of Cavitation in a Separated Flow using Vortex Generators Hailing An, Michael Plesniak In this study, cavitation inception and evolution in a mildly adverse pressure gradient region downstream of a backward facing step flow was studied. A flow separation control scheme employing streamwise vortex generators to modify the turbulent flow structures and thus potentially to modify cavitation inception and development was evaluated. PIV and LDV measurements were made at selected planes for baseline and controlled cases. Cavitation inception and development was also visualized using a high speed camera (2000 frames/s). The flow field measurements indicate that the controlled case should have earlier cavitation inception because of the changes in the separation and reattachment phenomena and corresponding pressure field, although direct verification is still necessary. In the (vortex generator) controlled case, cavitation is more developed, the noise level is lower, and the cloud shedding structures are less energetic and contain smaller scales and higher frequency. Cavitation does not significantly alter the overall flow field on the selected measurement planes, but it does weaken the introduced streamwise vortices and increases turbulence levels with respect to the non-cavitating case. [Preview Abstract] |
Monday, November 21, 2005 11:39AM - 11:52AM |
GE.00006: Cavitation in hydrofoils at large angle of attack Kartikeya Mahalatkar, Karman Ghia, Urmila Ghia Hydrofoils are used in surface and underwater applications to develop large lift for maneuvering, stabilization, etc. They often experience cavitation at high-Reynolds number Re ($>$ 20x10$^{6})$ and large angles of attack $\alpha $ (up to 25$^{\circ})$, and this results in periodic loss of lift. Prediction of the dynamics of these lift fluctuations requires accurate simulation of the flow physics of cavitation. Most experimental or numerical studies presently available have been carried out for Re $<$ 5x10$^{6}$ and at $\alpha <$ 10$^{\circ}$. A 2-D cavitating flow over a NACA0015 hydrofoil is simulated numerically, for $\alpha $ and Re varying from 0-28 degrees and 5x10$^{6}$-45x10$^{6}$, respectively. The Fluent solver, with a second-order accurate scheme, is used for the analysis. A validation study is carried out at $\alpha $ = 8$^{\circ}$, and showed good agreement with existing numerical and experimental studies. The final presentation will include detailed analysis of dynamics of lift variation, frequency of shedding of cavitation clouds, and the cavity vortex formation and its effect on lift. [Preview Abstract] |
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