Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L7: Multiphase Flows: Bubbly flows, Cavitation and VentilationBubbles Multiphase
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Chair: Harish Ganesh, University of Michigan Room: 407 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L7.00001: Experimental investigation of heat transport enhancement in bubbly flows Biljana Gvozdic, Elise Almeras, Varghese Mathai, Dennis van Gils, Chao Sun, Detlef Lohse Bubble injection into a carrier fluid can enhance the convective heat transfer. The exact mechanism behind this phenomenon is still unclear since most of the heat transport measurements in bubbly flows are limited to time-averaged global quantities. In this study we measure the statistical properties of the local temperature fluctuations along with global heat flux measurements in a rectangular bubble column heated from one sidewall and cooled from the opposite wall. We varied the Rayleigh number from $10^9$ to $10^{11}$, and the gas volume fraction from 0.5 to $5{\%}$. Due to bubble injection, the Nusselt number is increased up to 20 times as compared to the single-phase case. Surprisingly, we find that the Nusselt number is nearly independent on the Rayleigh number in two-phase flows for each studied gas volume fraction. Furthermore, the Nusselt number is found to be proportional to the square root of the gas volume fraction, which is suggestive of a diffusive process. Local measurements of the bulk temperature fluctuations show that not only are the fluctuations increased up to 100 times due to bubble injection, but also that mixing is present at shorter time scales, which is reflected in the power spectrum of the temperature fluctuations. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L7.00002: Gas-liquid two-phase flows in an upward square pipe with sudden expansion Yewon Kim, Hyungmin Park The bubble dynamics and consequent changes in the liquid-phase flow characteristics in an upward bubbly square pipe with sudden expansion (expansion ratio of 2.0) are experimentally studied in this work. The experiments are conducted under two Reynolds numbers of 600 (laminar) and 6600 (turbulent), respectively, based on the inlet bulk velocities of the single-phase (without bubbles) flow. The mean volume void fraction and averaged bubble size considered are $1\%$ and $3.5$~mm, respectively, and we use the high-speed two-phase particle image velocimetry and the shadowgraphy to measure the gas and liquid phases simultaneously. In addition, the particle tracking velocimetry is performed using two cameras to track the three-dimensional paths of each bubble. It is observed that lateral void fraction distribution change to core peak from wall peak after sudden expansion and peak at near the wall again after 3 times of inlet pipe width. Also, the reattachment length in the two-phase flow decreases compared to that of a single-phase flow, while smaller bubbles tend to migrate into the recirculation region and being trapped. Further discussions on the turbulence statistics and Reynolds number effects will be given. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L7.00003: Separation dynamics of dense dispersions in laminar pipe flows: An experimental and numerical study Victor Voulgaropoulos, Rashid Jamshidi, M.I.I. Zainal Abidin, Panagiota Angeli The physical mechanisms governing the separation of dense liquid dispersed flows in pipes are not well understood. In this work, both experiments and numerical simulations are performed to investigate these mechanisms. Liquid-liquid dispersions are generated using a static mixer and their evolution is studied along a horizontal pipe (26mm ID) at laminar flow and input dispersed phase volume fractions up to 50{\%}. To conduct optical measurements (PLIF and PIV) in the dense dispersions, the refractive index of both liquids is matched. Measurements are carried out at two axial locations downstream the mixer (15D and 135D, where D is the pipe diameter). Homogeneous dispersions, observed at 15D, segregate at 135D. The packing of the drops results in asymmetric velocity profiles and high slip velocities. The mixture approach is used in the numerical simulations, including gravity and shear-induced diffusion of drops. The predictions on separation and on velocity fields agree well with the experiments. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L7.00004: Numerical Simulation of Air Entrainment and Bubbles in Wave Breaking Qiang Gao, Lian Shen Bubbles generated by breaking waves play an important role in air-sea interactions, environmental sciences, and ocean engineering. Air entrainment, void fraction distribution, and bubble size spectrum are dominant factors for the bubble effects in breaking waves. In this study, we perform numerical simulations for wave breaking and bubbly flows using a new simulation method that computes resolved bubbles and subgrid-scale bubbles dynamically. Bubbles larger than the grid size are directly captured by a coupled level-set and volume of fluid method. Subgrid-scale bubbles are modeled using a four-way coupled polydispersed two-fluid model. By analyzing the data from our simulation results, we investigate the air entrainment, void fraction, bubble size spectrum, and bubble cloud in breaking waves. We also study the behaviors of different sizes of bubbles. The results show that our numerical method can capture the wave breaking and air entrainment processes accurately. Behaviors of bubble cloud decay consistent with experimental observations have been obtained from our simulation data. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L7.00005: (Yet Another) Bubble-Induced Turbulence Closure Relation for Multiphase CFD Ben Magolan, Emilio Baglietto Modeling generally complex two-phase flows remains a formidable challenge for Eulerian-Eulerian (E-E) multiphase Computational Fluid Dynamics (M-CFD). Of particular interest and modeling difficulty is the effective bubble-induced turbulence (BIT) closure relation, which manifests as additional production and dissipation source terms in typical two-equation turbulence model formulations. The primary challenge is three-pronged and comprises (1) identifying the dominant multiphase turbulence mechanisms, (2) synthesizing them into a model compatible with the E-E framework, and (3) ensuring model extensibility to a broad array of geometries and flow configurations. Here we present a new BIT model that has been developed via analysis of a comprehensive parameter study of bubbly flow Direct Numerical Simulation (DNS) data. This new BIT model is shown to deliver reliable predictions for the DNS data from which it was constructed, demonstrating close agreement with the mean, turbulent, and energy budget profiles. More importantly, the model extends well to other geometries and flow conditions, as evidenced by simulation and presentation of selected flow cases. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L7.00006: DNS study of speed of sound in two-phase flows with phase change. Kai Fu, Xiaolong Deng Heat transfer through pipe flow is important for the safety of thermal power plants. Normally it is considered incompressible. However, in some conditions compressibility effects could deteriorate the heat transfer efficiency and even result in pipe rupture, especially when there is obvious phase change, due to the much lower sound speed in liquid-gas mixture flows. Based on the stratified multiphase flow model (Chang and Liou, JCP 2007), we present a new approach to simulate the sound speed in 3-D compressible two-phase dispersed flows, in which each face is divided into gas-gas, gas-liquid, and liquid-liquid parts via reconstruction by volume fraction, and fluxes are calculated correspondingly. Applying it to well-distributed air-water bubbly flows, comparing with the experiment measurements in air water mixture (Karplus, JASA 1957), the effects of adiabaticity, viscosity, and isothermality are examined. Under viscous and isothermal condition, the simulation results match the experimental ones very well, showing the DNS study with current method is an effective way for the sound speed of complex two-phase dispersed flows. Including the two-phase Riemann solver with phase change (Fechter et al., JCP 2017), more complex problems can be numerically studied. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L7.00007: Examination of Wake Cavitation Dynamics Using Time-Resolved X-Ray Densitometry Harish Ganesh, Lisa Deijlen, Juliana Wu, Anubhav Bhatt, Steven Ceccio Cavitation in the wakes of bluff bodies is known to affect the wake shedding frequency and the properties of the resulting far wake. In particular, as the cavitation forms in the wake, with decreasing cavitation number the wake shedding Strouhal number will increase, reaching a peak value before decreasing as the wake forms a super cavity. further. The physical mechanism responsible for this observed change in shedding dynamics is yet to be fully understood. In the current study, we employed time resolved X-ray densitometry, high-speed videography, to study the cavitation dynamics in the wake of a triangular, nominally two-dimensional wedge in a re-circulating water tunnel to understand the underlying mechanisms responsible for cavity formation and shedding. Void fraction flow fields revealed the presence of bubbly shocks in the cavitating vortical region around the conditions of peak Strouhal number. Using average static pressure and dynamic pressure measurements at the base of the wedge at different cavitation numbers, a physical mechanism responsible for the observed change in dynamics is proposed. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L7.00008: Bubbly Shock Waves in Multi-modal Cavitation Shedding Dynamics on a NACA0015 Hydrofoil Juliana Wu, Harish Ganesh, Steven Ceccio Cavitation dynamics on the NACA0015 hydrofoil is known to be multi-modal with abrupt changes in Strouhal number with change in cavitation number at several attack angles. In one of our previous studies we found that cavity collapse can arrest cavity growth abruptly thereby altering the shedding frequency. In addition, occurrence of propagating bubbly shocks that cause leading edge pinch-off is another process that can have an effect on changing the dynamics. In the current study, we obtain time-resolved X-ray densitometry measurements on larger model scale to resolve the processes involved in cloud collapse induced growth-arrest. Furthermore, time-resolved void fraction flow fields measurements obtained using X-ray densitometry, synchronized both with acoustic noise measurements using a hydrophone and dynamic pressure measurements from flush mounted pressure transducers on the model, are used to observe the role of shock waves in causing the abrupt change in cavitation dynamics. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L7.00009: Ventilated supercavitation around a moving body in a still fluid Jaeho Chung, Yeunwoo Cho Present experimental study examines ventilated supercavity formation in an unbounded or free-surface bounded environment where the body is in motion and the fluid is at rest. The experiments were conducted in an open water tank where a high-speed towing system (max. 10m/s) is adopted to move an underwater axisymmetric ellipsoidal body with a certain speed. The body has a disk-type cavitator on its nose and compressed air is ventilated radially between the nose and the cavitator. Various steady-state supercavity formations are observed according to relevant Froude numbers, the air entrainment coefficients, and the cavitation numbers; twin-vortex supercavity (TV), reentrant-jet supercavity (RJ), partial supercavity with foamy cavity downstream (PSF), partial supercavity with shedding of continuous vortex rings downstream (PSV), double-layer supercavity (RJ inside {\&} TV outside, TV inside {\&} TV outside, RJ inside {\&} RJ outside). Connected with this behavioral observation, the body-frontal-area based drag coefficient for a moving ellipsoidal body with a supercavity is measured to be on the order of 0.1 while that for a cavitator-free moving body without supercavity is on the order of 0.4. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L7.00010: Large eddy simulation of hydrodynamic cavitation Mrugank Bhatt, Krishnan Mahesh Large eddy simulation is used to study sheet to cloud cavitation over a wedge. The mixture of water and water vapor is represented using a homogeneous mixture model. Compressible Navier--Stokes equations for mixture quantities along with transport equation for vapor mass fraction employing finite rate mass transfer between the two phases, are solved using the numerical method of Gnanaskandan and Mahesh (International Journal of Multiphase Flows, 2015, 70:22--34). The method is implemented on unstructured grid with parallel MPI capabilities. Flow over a wedge is simulated at $Re = 200,000$ and the performance of the homogeneous mixture model is analyzed in predicting different regimes of sheet to cloud cavitation; namely, incipient, transitory and periodic, as observed in the experimental investigation of Harish $\it et. al.$ (Journal of Fluid Mechanics, 2016, 802:37--78). [Preview Abstract] |
Monday, November 20, 2017 6:15PM - 6:28PM |
L7.00011: Numerical Investigation of a Cavitating Mixing Layer of Liquefied Natural Gas (LNG) Behind a Flat Plate Splitter Saeed Rahbarimanesh, Joshua Brinkerhoff The mutual interaction of shear layer instabilities and phase change in a two-dimensional cryogenic cavitating mixing layer is investigated using a numerical model. The developed model employs the homogeneous equilibrium mixture (HEM) approach in a density-based framework to compute the temperature-dependent cavitation field for liquefied natural gas (LNG). Thermal and baroclinic effects are captured via iterative coupled solution of the governing equations with dynamic thermophysical models that accurately capture the properties of LNG. The mixing layer is simulated for vorticity-thickness Reynolds numbers of 44 to 215 and cavitation numbers of 0.1 to 1.1. Attached cavity structures develop on the splitter plate followed by roll-up of the separated shear layer via the well-known Kelvin-Helmholtz mode, leading to streamwise accumulation of vorticity and eventual shedding of discrete vortices. Cavitation occurs as vapor cavities nucleate and grow from the low-pressure cores in the rolled-up vortices. Thermal effects and baroclinic vorticity production are found to have significant impacts on the mixing layer instability and cavitation processes. [Preview Abstract] |
Monday, November 20, 2017 6:28PM - 6:41PM |
L7.00012: Simulation of the ultrasound-induced growth and collapse of a near-wall bubble Bradley Boyd, Sid Becker In this study, we consider the acoustically driven growth and collapse of a cavitation bubble in a fluid medium exposed to an ultrasound field. The bubble dynamics are modelled using a compressible, inviscid, multiphase model. The numerical scheme consists of a conservative interface capturing scheme which uses the fifth-order WENO reconstruction with a maximum-principle-satisfying and positivity-preserving limiter, and the HLLC approximate Riemann flux. To model the ultrasound input, a moving boundary oscillates through a fixed grid of finite-volume cells. The growth phase of the simulation shows the rapid non-spherical growth of the near-wall bubble. Once the bubble reaches its maximum size and the collapse phase begins, the simulation shows the formation of a jet which penetrates the bubble towards the wall at the later stages of the collapse. For a bubble with an initial radius of 50 $\mu m$ and an ultrasound pressure amplitude of 200 kPa, the pressure experienced by the wall increased rapidly nearing the end of the collapse, reaching a peak pressure of 13 MPa. This model is an important development in the field as it represents the physics of acoustic cavitation in more detail than before. [Preview Abstract] |
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