Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session J11: Multiphase Flows: Cavitation and Aerated Flows (8:00am - 8:45am CST)Interactive On Demand
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J11.00001: The transition to aeration in a two-phase turbulent mixing system Lyes Kahouadji, Assen Batchvarov, Cristian R. Constante Amores, Seungwon Shin, Jalel Chergui, Damir Juric, Richard V. Craster, Omar K. Matar We consider the mixing of a viscous fluid by the rotation of a pitched blade turbine inside an open fixed cylindrical tank, with a lighter fluid above, as a model of an industrial mixing environment. The complex impeller induces primary vortices, that arise in many idealised rotating flows, and additionally several secondary vortical structures resembling Kelvin-Helmholtz, vortex breakdown, blade tip vortices, and end-wall corner vortices. As the rotation rate increases we eventually reach an extreme situation, aeration, when the fluid-fluid interface reaches the rotating blades and a mixing bubbly rotating flow occurs; the approach to this aerated state is investigated using numerical simulation. We utilise a highly parallelized numerical implementation, taking advantage of a domain decomposition strategy for parallelization of a hybrid front-tracking/level-set method designed for complex interfacial deformation including rupture and coalescence. [Preview Abstract] |
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J11.00002: Internal flows of ventilated partial cavity. Changchang Wang, Kyungduck Yoon, Guoyu Wang, Jiarong Hong Ventilated partial cavitation is an active air lubrication method used in marine engineering for ship hull drag reduction. This work focuses on understanding the internal flow structures and physical processes that lead to the formation of different cavity regimes and transition across these regimes. Both high resolution flow imaging and large eddy simulation (LES) were conducted for two regimes of ventilated partial cavity, i.e. the open cavity (OC) and two-branch cavity (TBC). The results reveal similar flow patterns for OC and TBC, including shear layer near air-water interface, recirculating region, near-cavitator vortex and internal flow circulation vortex. Specifically, OC internal flow exhibits quasi-2D recirculation region and 3D shear layer with intermittent transverse flow across cavity mid-plane, while TBC internal flow shows quasi-2D shear layer and 3D recirculation region where strong circulation exists at cavity mid-plane and counter-rotating vortex pair on both sides of cavity mid-plane. Remarkably, the onset of three dimensionality in recirculation region is responsible for the regime transition from OC to TBC, in the process of which shear layer transitions from convective to absolute instability. [Preview Abstract] |
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J11.00003: Resolving pseudo-cavitation in the nozzle and its effect on spray formation Rohit Mishra, Dorrin Jarrahbashi Pressure drop in the diesel injector nozzle triggers two simultaneous phase-change phenomena: vaporous cavitation where the local pressure drops below the fuel saturation pressure and pseudo-cavitation that is the precipitation of air bubbles dissolved in the fuel. These phenomena modify the internal nozzle flow and significantly affect the spray characteristics. A multi-fluid Eulerian-Eulerian interface-tracking solver is developed that accounts for vaporous cavitation and de-gassing of the dissolved air. A new de-gassing model is created that relates the solubility of the dissolved gases to the drop in pressure along the nozzle and links the rise in bubble number density to the reduction of the gas solubility as de-gassing evolves. The co-existence of the three fluids namely: fuel, vapor and air and their evolution with time in the nozzle are presented. The development of the spray is examined as the fuel leaves the nozzle region. The role of the vaporous cavitation and pseudo-cavitation on the spray cone angle has been illustrated for a wide range of pressures and nozzle diameters. The predicted dynamic mass flow rates, effective diameter, and cone angles provide a more accurate initial conditions required for setting up the traditional Eulerian-Lagrangian spray simulations. [Preview Abstract] |
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J11.00004: Sheltering of microbubbles in the inner part of boundary layer and non-condensable gas diffusion sustain attached cavitation inception OMRI RAM, Karuna Agarwal, Joseph Katz High-speed imaging and high-resolution PIV are used to study the onset of cavitation on smooth surfaces with sharp pressure minimum, followed by strong adverse pressure gradients. Slightly below the cavitation inception pressure, short-lived isolated cavitation patches form when a free stream nucleus approaches the minimum pressure location, and evolve into traveling bubble cavitation downstream of the minimum pressure point. Numerous residual microbubbles, generated as the attached cavities collapse migrate slowly upstream, against the flow, if the adverse pressure gradient is sufficient to form a low momentum region inside the thickened boundary layer. These microbubbles migrate for 2-20 ms, and grow by 3-4 times in diameter until their size becomes comparable to that of the low momentum region sheltering them from the free stream. The pressure changes along their track cannot account for their growth, and 1D solution for relevant conditions indicates that they grow by non-condensable gas diffusion. At this time, these bubbles are either swept downstream or become nuclei for new attached cavitation patches, which generate new microbubbles, thus creating a self-sustaining cavitation inception mechanism. [Preview Abstract] |
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J11.00005: Role of Swirl in Fuel Injection Systems Sai Darbha, Daniel Duke, David Schmidt Swirl is present in multi-hole fuel injection systems, owing to flow turning a corner upstream of the injector nozzles. The role of swirl in the nozzle internal flow and on the quality of spray atomization is not fully understood. Experimental work in this area is limited by the lack of optical access due to cavitation at the nozzle injector walls. It is difficult to selectively isolate the effect of swirl in the turbulent two-phase flow in the nozzles, owing to a lack of control on the nozzle inlet conditions in an experimental setting. Therefore, computational fluid dynamics simulations are carried out in a canonical fuel injector geometry to determine the influence of swirl on nozzle internal flow. A homogeneous flow assumption is used in an open source finite volume framework for the simulations, along with URANS turbulence models. A controlled degree of swirl is implemented by varying the boundary conditions at the domain inlet. A parametric sweep of the inlet Swirl number (Sw) is carried out to determine what fundamental fluid-mechanical phenomena give rise to the various cavitation regimes such as geometric and string cavitation, and how these are manifested in fuel injection systems. [Preview Abstract] |
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J11.00006: Unsteady cavitation dynamics over a pitching NACA0015 hydrofoil Anubhav Bhatt, Harish Ganesh, S. L. Ceccio Cavitation due to flow unsteadiness is a significant source of unwanted noise and erosion in hydrodynamic applications. Quantifying underlying cavity structure and unsteadiness can help determine the erosive potential and `aggressiveness' of the cavity to make suitable hydrodynamic design changes. This study focuses on the cavitation dynamics over a NACA0015 hydrofoil (165mm chord) subject to pitching motion at different cavitation numbers (1.3 to 3.5), using time-resolved X-ray densitometry and high-speed cinematography. In addition, synchronized unsteady surface pressure and cavity static pressure measurements are also performed The dynamics of the cavity collapse as the foil pitches between pre-set angles of attack (10 to 0deg and 7 to 0deg) is studied at different angular accelerations. The effect of this pitching motion on cavity evolution and collapse mechanisms (re-entrant jet induced and bubbly shockwave driven) is reported. [Preview Abstract] |
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J11.00007: Cavities in Deck Plate for a Rectangular Supersonic Multi-Stream Jet Nozzle Rishov Chatterjee, Seth Kelly, Emma Gist, Mark Glauser Minimizing overall sound pressure levels (OASPL) when dealing with supersonic flow has been a key goal at Skytop Turbulence Laboratory. If the shock structure occurring from supersonic flow is dissipated more rapidly by using surface modifications (cavities), OASPL may decrease as a result. One stream of the jet in the MARS (Multi Aperture Rectangular Single Expansion Ramp Nozzle) is capable of reaching supersonic speeds, with the mainstream reaching speeds of Mach 1.6 and the bypass stream reaching speeds of Mach 1. The streams exit onto a deck plate (to simulate air frame integration) and through particle image velocimetry (PIV), the different shock structures and shear layers can be identified. Current literature suggests cavities on the aft-deck can act as a passive control to alter the shock structure and the acoustics in channel flow. Testing out this form of passive control located in an open loop flow to see if the OASPL can be reduced will build up on the current research. The surface of nominal deck plates were modified using various cavities of different geometries in order to see how the sound pressure level is impacted. To validate its effectiveness, this data is tested against a nominal plate with no surface modifications using near and far field accoustics. [Preview Abstract] |
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J11.00008: A variational finite element formulation for predicting turbulent cavitating flows over flexible marine propellers Suraj Kashyap, Rajeev Jaiman The phenomena of cavitation are omnipresent in nature and engineering applications. We propose a novel variational finite element framework for high Reynolds number cavitating flows in deforming geometries. Goal is to predict accurate velocity and pressure fields in turbulent cavitating flows for use as input in predicting acoustic field. A stabilized finite mass-transfer model for cavitation is coupled in partitioned iterative manner with 3D Navier-Stokes equations in Arbitrary Lagrangian-Eulerian(ALE) framework. A Positivity Preserving Variational(PPV) scheme is applied for monotone, bounded solutions. Proposed framework enables hydrodynamic study of flexible propeller blades in cavitating conditions and structural response to flow excitations. Method is first validated for a collapsing spherical vaporous bubble, benchmarked against analytical solutions of the Rayleigh-Plesset equation. We then apply it with a hybrid RANS-LES model to simulate cavitating flow around a NACA66 hydrofoil section at Re$=$800000; predictions compared with experiment. Finally, proposed framework is extended to study flow over flexible 3D propeller blade and coupled hydroelastic results discussed. [Preview Abstract] |
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J11.00009: How Non Condensable gases modify phase change mass transfer Saikat Mukherjee, Hector Gomez Characterising mass transfer in a fluid from liquid to vapor phase is a classical problem in fluid dynamics which continues to be relevant even today. However, these fluids often contain dissolved non-condensable gases (NCGs), which can dramatically alter the rate of mass transfer from one phase to another. Here, we derive equations to quantify the role of these NCGs using balance laws for mass and momentum transfer. Numerical simulation of the equations point to a dual role of NCGs. We refer to the component primarily driven by the divergence of fluid velocity as hydrodynamic cavitation, which always reduces the rate of mass transfer in the presence of NCGs. On the other hand, there is another component primarily composed of the divergence of mass flux of NCG, which may either promote or impede mass transfer with an increase in the mass fraction of NCGs. We call this component mixing cavitation, and study physical scenarios where it dominates hydrodynamic cavitation. Expressing the rate of phase change in terms of velocity and mass flux of NCGs also opens up avenues to experimentally visualise phase change in multiphase fluids. [Preview Abstract] |
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