Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A3: Multiphase Flows: Cavitation |
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Chair: Martin Wosnik, University of New Hampshire Room: 23B |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A3.00001: Velocimetry in both phases of a cavitating flow by fast X-ray imaging Olivier Coutier-Delgosha, Ilyass Khlifa, Marko Hocevar, Sylvie Fuzier, Alexandre Vabre, Kamel Fezzaa A promising method to measure velocity fields in a cavitating flow is presented. Dynamics of the liquid phase and of the bubbles are both investigated. The measurements are based on ultra fast X-ray imaging performed at the APS (Advanced Photon Source) of the Argonne National Laboratory. The experimental device consists of a millimetric Venturi test section associated with a transportable hydraulic loop. Various configurations of velocity, pressure, and temperature have been investigated. Radio-opaque particles are used as tracers for the liquid phase, in association with a multi-pixels sensor to record the successive positions of the particles. The use of X-rays instead of light solves the problem of light reflection and dispersion on phase boundaries, since X-rays penetrate a gas/liquid flow in straight lines. Images contain simultaneously the information related to the particles (for PIV analysis in the liquid), to the vapor bubbles (for PIV in the gas). The slip velocity between vapor and liquid is calculated everywhere both velocities can be obtained. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A3.00002: Time Resolved 2D X-ray Densitometry of a Cavitating Wedge Simo Makiharju, Harish Ganesh, Steven Ceccio A canonical cavitating two dimensional wedge experiment was setup to provide quantitative validation quality data for CFD through the use of time resolved x-ray densitometry. The x-ray system was used to the measure the spatial distribution of void fraction of the cavitating flow. The water tunnel's dissolved gas content was controlled and dissolved oxygen content measured. The flow was nominally two dimensional, and time resolved 2D projections of the void fraction distribution were measured with resolution of the order of 1/100$^{th}$ of the cavity length. FFT of void fraction revealed the frequencies inherent to the cavitating wedge at Strouhal number on the order of 0.3. The pressure in the region of cavitation was recorded simultaneously with the x-ray measurement to compare the detected frequencies. As ongoing work, we are incorporating a 2D electrode array onto the surface of the model to have a second measure of the void fraction within $\sim $1 mm of the surface, thus providing a consistency check for the x-ray densitometry and revealing any near surface 3D features of this nominally 2D flow. The data covered the incipient sheet cavity, intermittent cloud shedding and fully shedding periodic cavity regions for a range of cavitation numbers. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A3.00003: Large eddy simulation of cavitating mixing layer: the effect of splitter plate Kameswararao Anupindi, Dinesh Shetty, Steven Frankel The accurate prediction and validation of various cavitation mechanisms such as internal jets, cavity extent etc., play an important role in furthering the reliability of simulations. Previous simulation studies\footnote{Bensow et al. J. Fluids Engg., \textbf{132}, 041302 (2010)} point out a need for high resolution solver. Therefore, in the present study we perform high order large eddy simulation (LES) study on a turbulent incompressible mixing layer operating under cavitating and non-cavitating conditions. The solver is based on incompressible Navier-Stokes formulation, in which a single fluid, two-phase mixture is solved with a separate liquid volume fraction ($\alpha_l$) equation coupled with a mass-transfer model. We first validate our solver by making comparisons to experimental results\footnote{Aeschlimann et al. Phys. Fluids, \textbf{23}, 055101 (2011)}. Various quantities such as similarity velocity profiles, mean vapor fraction ratio profiles, and the evolution of vorticity thickness are compared. Having validated the solver, we include the splitter plate in the simulations through immersed boundary method approach and study the effect of the wake created by it on the cavitating mixing layer dynamics. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A3.00004: Towards DNS/LES of cavitating flows in complex geometries Aswin Gnanaskandan, Krishnan Mahesh We are developing a numerical method for DNS/LES of turbulent cavitating flows in complex geometries. The multiphase medium is represented using a homogeneous equilibrium model that assumes thermal equilibrium between the liquid and the vapor phase. The governing equations are the compressible Navier Stokes equations for the liquid/vapor mixture along with a transport equation for the vapor mass fraction. A separate total energy equation is solved, as opposed to assuming isothermal flow. The unstructured compressible algorithm in (Park $\&$ Mahesh, AIAA Paper 2007-0722) has been extended to solve for multiphase flows. A characteristic filter based shock capturing scheme, extended to handle non-ideal gases and mixtures, is applied in a predictor-corrector approach, ensuring that the shock-capturing is active only in the regions of discontinuity. A segregated implicit method is used to address the stiffness of the system. We discuss our numerical method, validation using benchmark problems and its application to study cavitation behind a circular cylinder for three different cavitation numbers $\sigma = $ 2.0, 1.0 and 0.7. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A3.00005: Cavitation Characteristics of a NACA 63-424 Hydrofoil and Performance Comparison with a Bidirectional Version of the Foil Ivaylo Nedyalkov, Martin Wosnik A NACA 63-424 hydrofoil with a 75mm chord and a 152mm span was tested in the recently renovated 6-inch high-speed water tunnel at the University of New Hampshire. The NACA 63-424 foil is being considered for use on rotors of marine hydrokinetic turbines, including the US Department of Energy Reference Horizontal Axis Turbine (RHAT) for tidal and ocean current applications. For various angles of attack, the foil was tested at speeds ranging from 2m/s to 12m/s. Pressure in the test section was varied independently. For each angle, speed and pressure setting, high speed videos were recorded (at 3600 frames per second and above). Cavitation inception and desinance were obtained. Lift and drag were measured using a new 2-component force balance. In tidal turbines applications, bidirectional foils do not require pitch control, hence the experiments were repeated for a bidirectional version of the NACA 63-424 foil and the characteristics of the two foils were compared. The results can be used to predict cavitation inception and performance of marine hydrokinetic turbines, for a given site, deployment depth and and tip speed ratio. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A3.00006: Large Eddy Simulation of cavitation in turbulence Sergei Chumakov, David Cook, Frank Ham, Uwe Iben Large Eddy Simulation of a turbulent cavitating flow has been performed using the explicit spatially-filtered compressible Navier-Stokes solver. The unstructured finite volume method uses a blended central-upwind scheme in single-phase regions to minimize artificial damping of resolvable turbulence scales. In the areas with discontinuities such as phase change, the method switches to a lower-order reconstruction (WENO and first order) and an approximate Riemann solver. Time discretization is performed with an explicit third order Runge-Kutta scheme. Comparison of our results for several cases to simulations and experiments from the current literature is presented. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A3.00007: Axisymmetric, Ventilated Supercavitation in Unsteady, Horizontal Flow Ellison Kawakami, Seung-Jae Lee, Roger Arndt Drag reduction and/or speed augmentation of marine vehicles by means of supercavitation is a topic of great interest. During the initial launch of a supercavitating vehicle, an artificial supercavity is required until the vehicle can reach conditions at which a natural supercavity can be sustained. Previous studies at Saint Anthony Falls Laboratory (SAFL) focused on the behavior of ventilated supercavities in steady horizontal flows. In open waters, vehicles can encounter unsteady flows, especially when traveling under waves. A study has been carried out at SAFL to investigate the effects of unsteady flow on axisymmetric supercavities. An attempt is made to duplicate sea states seen in open waters. In an effort to track cavity dimensions throughout a wave cycle, an automated cavity tracking script has been developed. Using a high speed camera and the proper software, it is possible to synchronize cavity dimensions with pressure measurements taken inside the cavity. Results regarding supercavity shape, ventilation demand, cavitation parameters and closure methods are presented. It was found that flow unsteadiness caused a decrease in the overall length of the supercavity while having only a minimal effect on the maximum diameter. The supercavity volume varied with cavitation number and a possible relationship between the two is being explored. (Supported by ONR) [Preview Abstract] |
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