Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E4: Multiphase Flows: Cavitation |
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Chair: Casey Harwood, University of Michigan Room: B112 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E4.00001: The Hydroelastic Response of a Flexible Surface-Piercing Strut in Wetted, Ventilated, and Cavitating Flows Casey Harwood, Jacob Ward, Yin Lu Young, Mario Felli, Massimo Falchi, Steven Ceccio High-speed and highly loaded lifting surfaces are prone to ventilation and cavitation. Increasing use of compliant materials (e.g. composites) in such systems necessitates a better understanding of the fluid-structure interactions of lifting surfaces in multiphase flow. Experiments on a flexible surface-piercing hydrofoil have been performed in a towing tank and a free-surface cavitation tunnel. The objectives are (i) to demonstrate the effects of material compliance upon hydrodynamic performance and stability of multiphase flow regimes, and (ii) to quantify the effects of multiphase flow upon the structural response and hydroelastic stability of flexible lifting bodies. A non-optical shape-sensing method is developed, which permits 3D bending and twisting deformations of the hydrofoil to be accurately inferred. The effects of the foil's compliance on hydrodynamic loads, structural motions and flow regimes are discussed. Partial immersion of the hydrofoil causes a mode-dependent change in added-mass that can encourage coalescence of higher modes. At the same time, increasing flow speed and ventilated flow decrease the damping associated with certain modes. Unsteady cavity shedding modulates the system parameters, causing a broadening of the frequency response. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E4.00002: Study of Cavitation Shedding Dynamics on a NACA0015 Hydrofoil Using X-Ray Densitometry Harish Ganesh, Juliana Wu, Steven Ceccio Cavitation dynamics on the NACA0015 hydrofoil at several attack angles are found to be spectrally rich, being multi-modal with abrupt changes in Strouhal number with change in cavitation number. Present study focusses on identifying the physical mechanisms responsible for the change in cavitation dynamics on a NACA0015 hydrofoil in a re-circulating water tunnel using time resolved X-ray densitometry. Time-resolved void fraction flow fields obtained using X-ray densitometry, synchronized with acoustic noise measurements using a hydrophone, are used to identify different flow features and mechanisms that are responsible for the change in the observed spectral behavior. It is shown that under higher cavitation numbers, the shedding mechanism is predominantly re-entrant liquid flow based, but as the cavitation number drops many different processes are at play. At lower cavitation numbers, the shed cavity cloud collapse arrests cavity growth and this results in altered cycle dynamics and hence the Strouhal number. In addition, propagation bubbly shock waves are also found to be a dominant mechanism of shedding for certain conditions. The multi-modal nature of the acoustic pressure signature is explained by presence of different flow features, which could be concurrent or alternating. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E4.00003: Cavitation Dynamics on a Backward Facing Wedge With Streetwise Grooves Juliana Wu, Harish Ganesh, Steven Ceccio Cavitation dynamics in the separated region of backward facing wedges has been studied extensively in the past. From the previous studies it has been found that cavity shedding mechanisms could be due to the presence of a re-entrant liquid flow and/or a propagation bubbly shock wave. Recently, the effect of streamwise grooves on the aft portion of the wedge on the cavity shedding dynamics was explored. In the present study, X-ray densitometry is used to study the effect of streamwise grooves of different sizes and starting locations on the cavitation behavior on the wedge. Using time resolved void fraction flow fields from X-ray densitometry, the effect of the grooves in altering the flow structures and cavity shedding mechanisms are characterized. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E4.00004: Study of Cavitation in Wakes of Circular Cylinders and Symmetric Wedges Using X-ray Densitometry Joachim Koot, Juliana Wu, Harish Ganesh, Steven Ceccio Cavitation in wakes behind canonical objects can exhibit variation in Strouhal number with a reduction in cavitation number. Circular cylinders of two diameters and symmetric wedges with a wedge angle of 15, 30, and 60 degrees are used to study cavitation in their wakes using X-ray densitometry. Using high speed video and X-ray densitometry, the nature of cavitation is studied in near-wake and a part of the far-wake region. In addition, acoustic measurements are also carried out to understand the spectral content of such wake cavities. Based on void fraction flow field and high-speed video measurements, the effect of cavitation on the K\'{a}rm\'{a}n vortex street spacing in the far wake region is studied. The results are the interpreted to explain the physical mechanisms responsible for the observed change in Strouhal number. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E4.00005: The effect of microbubble cavity formation on the breakup of glowing sprays Dennis van der Voort, Nico Dam, Willem van de Water, Rudie Kunnen, Herman Clercx, GertJan van Heijst Spray atomization is affected by an interplay of several physical phenomena. To understand the breakup as a whole, it is essential to understand the contribution of individual phenomena (turbulence,cavitation,etc). This work investigates the contribution of cavity formation inside the nozzle by seeding a system with microbubbles, generating transient cavities. Using transparent nozzles, pressure sensors, and high-speed imaging, we find that the pressure pulses generated by cavity collapse can be used to correlate the time and magnitude of the spray angle increase, valuable for application in non-transparant setups. Laser-induced phosphorescence is used to quantitatively measure liquid dispersion, which excites ('tags') 10-20 nL of fluid at the nozzle exit and tracks the spread of glowing fluid. The dispersion correlated to cavity formation events is compared to the average. We show that the dispersion (which is independent of the starting tagged spray width) does not increase with the formation of a cavity. This indicates that, while the spray angle may increase, the turbulent processes (both liquid and gas phase) that governs the dispersion remains the same, and the cavitation events do not influence this process. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E4.00006: Large eddy simulation of sheet to cloud cavitation Mrugank Bhatt, Krishnan Mahesh Large eddy simulation is used to study sheet to cloud cavitation. A homogeneous mixture model is employed to represent the multiphase mixture of water and water vapor. A novel predictor-corrector method (Gnanaskandan and Mahesh, Int. Journal of Multiphase Flow, 2015, 70:22–34) is used to numerically solve the compressible Navier–Stokes equations for the liquid/vapor mixture along with a transport equation for the vapor mass fraction. The algorithm is implemented on an unstructured grid and a parallel platform, with a fully coupled implicit time advancement of both viscous and advection terms. Simulation of sheet to cloud cavitation over a wedge at a Reynolds number, Re = 200, 000 and cavitation number, $\sigma$ = 2.1 is performed. A propagating condensation shock similar to the one observed in the experiments of Harish et.al. (Harish Ganesh, PhD thesis- University of Michigan, 2015) is observed in the computed flow field. Results will be presented and the flow physics will be discussed. [Preview Abstract] |
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