Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H02: Multiphase Flows: Cavitation and Aerated Flows |
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Chair: Harish Ganesh, U Michigan Room: 2B |
Monday, November 25, 2019 8:00AM - 8:13AM |
H02.00001: On the mechanism that sustains intermittent attached cavitation inception in a boundary layer without flow separation. Omri Ram, Karuna Agrawal, Joseph Katz Attached cavitation inception on a curved body typically occurs when the minimum pressure is reduced below the vapor pressure. There is only limited qualitative understanding of the mechanisms involved with the attachment of free stream nuclei to the surface, and it is not clear how the inception is sustained intermittently once it starts. Water tunnel experiments involving high-speed microscopic imaging focus on these processes for a flow with an attached boundary layer downstream of the minimum point. Once a freestream nucleus grows and attaches to the surface, it collapses in milliseconds, leaving a cloud of microbubbles small than 30 $\mu $m downstream of the minimum pressure point. Some of these bubbles migrate randomly near the surface, as confirmed by statistical analysis, presumably under the balancing influence of drag that pushes them downstream and local adverse pressure gradient that pulls them upstream. Hence, the bubbles are maintained near the surface, even without flow separation. Of those that migrate upstream, a fraction grows to form another intermittent cavity that generates new microbubbles. Hence, once the initial attachment occurs, subsequent attached cavities appear at a frequency that is much higher than that associated with the freestream nuclei. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H02.00002: Role of Compressibility on the Shedding Dynamics of a Cavitating Wake Harish Ganesh, Juliana Wu, Daniel Knister, Steven Ceccio Cavitation dynamics in wakes behind bluff bodies is known to be strongly dependent on the inlet cavitation number, for a given Reynold's number. The cavity shedding frequency of a cavitating wake increases with a decrease in inlet cavitation number, to attain a peak, and decrease again with a further reduction in inlet cavitation number. The physical mechanism of this observed trend in the shedding frequency is yet to be fully understood. High-void fraction bubbly cavitating flows observed in separated liquid flow regions can have low speed of sound within the bubbly mixture. The effect of compressibility of such a bubbly mixture on the underlying cavity dynamics can be important and is explored in the current study. By performing Proper Orthogonal Decomposition (POD) of X-ray densitometry-based time-resolved void fraction flow fields of the cavitating wake, it is shown that the type of the mode observed and its energy content depends on the inlet cavitation number. At higher cavitation numbers, a sinusoidal mode is observed as a dominant mode, while a pulsating mode, in addition to the sinuous mode, becomes increasingly dominant when approaching the peak shedding frequency. The appearance of this observed pulsating mode is similar to that observed on single-phase compressible wakes reported in literature. Upon non-condensable gas injection into the base flow it is verified that the observed modal patterns are strongly dependent on the compressibility of the mixture. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H02.00003: Computational Modeling of Air Cavities under Solid Bodies in Water Flow Konstantin Matveev Significant drag decrease of marine vessels can be achieved by forming thin air cavities on underwater hull surfaces. Modeling of air-ventilated cavities in water flows under solid bodies is rather complex, since a variety of factors are important, including viscosity, turbulence, surface tension, gravity, and periodic air shedding. A finite-volume solver, employing the volume-of-fluid method for the air-water interface and RANS turbulence models, has been applied to model air-ventilated water flows in experimental setups previously tested at moderate and large Reynolds numbers. During computational studies it was found that sufficient refinement of the numerical mesh near the cavity re-attachment to the solid body and sharpening treatment in the interface-capturing method must be incorporated into modeling to achieve reasonable agreement with test data. The solution verification on numerical grids of different mesh density has been carried out. In addition, it is demonstrated how cavity shapes can be changed by modifying body geometry. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H02.00004: Cavitation dynamics in the wake of a backward facing step Anubhav Bhatt, Harish Ganesh, Steven L. Ceccio The flow over a backward facing step is often used as a benchmark for experimental studies on separation and reattachment in hydrodynamic applications. The flow has a separating and reattaching shear layer forming a recirculating~region. These shear flows are susceptible to cavitation, and can experience different dynamics depending upon the extent of the cavitation formed in the shear flow and contained within the recirculation bubble. Beginning with cavitation inception, a reduction in cavitation number results in different cavitation regimes such as fully developed cavitation, self-sustained cloud shedding and super-cavitation. This study focuses on quantifying the cavitation dynamics of the developed cavitation, at three Reynolds numbers (8.5*1E4, 10.6*1E4 and 12.7*1E4) using time-resolved X-Ray densitometry and high speed videography. In addition, static and unsteady pressure measurements are performed to understand the change in dynamics within the region of flow separation. The effect of compressibility of the vapor-liquid mixture is assessed by estimating the speed of sound based on the static pressure and void-fraction measurements at different cavitation conditions. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H02.00005: Investigation of turbulent cavitating flows in a small Venturi by fast X-ray imaging Guangjian Zhang, Mingming Ge, Ilyass Khlifa, Olivier Coutier-Delgosha The cavitating flows created in a small Venturi nozzle are investigated based on ultra-fast x-ray imaging. The instantaneous velocities of the liquid and vapor are measured simultaneously by tracking seeding particles and vapor structures respectively while the vapor volume fraction is derived from the different x-ray attenuation. Wavelet decomposition with appropriate thresholds is used to separate seeding particles from vapor structures, so that image cross-correlations could be applied on the two phases separately. This study presents data on mean velocity and void ratio field, statistical turbulent quantities in three different cavitation levels with the same reference velocity. A type of cavitation associated with a weak but persistent re-entrant jet is described. The comparison between the cavitation and the non cavitating flow shows that the averaged flow field is significantly altered by the presence of cavitation and the vapor formation near the throat area is observed to suppress velocity fluctuations. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H02.00006: Thermodynamic effects on Venturi cavitation characteristics Zhigang Zuo, Haochen Zhang, Knud Aage Mørch, Shuhong Liu In studies using cold water as the working liquid, the thermodynamic effect of cavitation is usually ignored. However, in cryogenic liquids, refrigerants and high temperature water the thermodynamic effect is significant, and it suppresses the development of cavitation by reducing the temperature at the boundary of expanding cavitation bubbles. In this paper the thermodynamic effect is systematically studied by Venturi cavitation in a blow-down type tunnel for the first time, using water at temperatures up to relatively high levels, and at controlled dissolved gas content in the supply reservoir (measured by dissolved oxygen, DO). The cavitation characteristics are analyzed from the experiments, and the mean cavitation length is chosen as the quantity suited to reveal the thermodynamic effect. With an increase of the thermodynamic parameter, a decrease of cavitation length is observed, which is consistent with suppression of cavitation by the thermodynamic effect. Within the range of DO content tested, the DO content has little influence on the mean cavitation length and the unsteady cavitation characteristics, which is in contrast to the effect of gas content on cavitation nuclei generally. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H02.00007: Thermal effects in cavitating flows Martin Petkovsek, Drew Jacobs, Mingming Ge, Olivier Coutier-Delgosha The effects of temperature on hydrodynamic cavitation in water is investigated. Temperature is varied between 20\textdegree C and 85\textdegree C in a cavitating flow generated in a small-scale venturi type section of 1 mm characteristic dimension. The effects of the temperature on the large-scale periodical cloud shedding is investigated, as well as the evolution of the flow structure and dynamics. The measurements are based on high speed visualizations and 2D2C PIV. A competition between two different effects, namely the Reynolds number change and the cavitation delay due to the so-called thermal effect, is observed. A critical temperature in the range 50\textdegree C-60\textdegree C is found and especially investigated. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H02.00008: Control of cavity bubble in water entry using laser-induced cavitation Kyuseong Choi, Nayoung Kim, Guwon Seon, Wontae Hwang, Hyungmin Park We investigate how the cavity attached to the metallic sphere in water entry changes when laser is irradiated. The sphere (radius, R=1, 2mm) is roughened (0.1-1$\mu$m in size) to generate a cavity even at a relatively low impact speed (Uo=1.5-3.3m/s). By varying the height of dropping position and irradiation time, that is speed and surface temperature (To=110-350$^{\circ}$C) at the impact instant, we measure the cavity dynamics with a high-speed camera (the water is at room temperature). In the case of a shallow seal (R=1mm, Uo=1.5m/s), we classify two regimes of cavity growth (To=170-240$^{\circ}$C) and destruction (To$>$240$^{\circ}$C). In the destruction regime, microbubble emission boiling happens, so the cavity bubble is destructed to numerous microbubbles. In the case of a deep seal (R=1mm, Uo=3.3m/s), the slight cavity growth occurs at To=130-150$^{\circ}$C and considerable destruction of cavity bubble at To$>$170$^{\circ}$C. At a transient of To=150-170$^{\circ}$C, the deep seal changes to shallow seal with a slight destruction of cavity. As a change of cavity dynamics, the forces acting on the sphere is varied, which is estimated from measured cavity geometry and sphere trajectory. Finally, we suggest a mechanism of cavity growth and destruction according to Uo and To. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H02.00009: Air entrainment mechanisms of a forced plunging jet Sophia Relph, Ken Kiger, Akash Dhruv, Elias Balaras Plunging jets play a major role in the quality of cast metal parts, as the pouring process can entrain metal oxides in much the same way as pouring water captures bubbles. These air and oxide pockets interfere with the metal’s crystal structure and can compromise strength and fatigue life. The mitigation of such defects is of great interest to foundries. Most research on plunging jets relevant to metal casting considers either smooth or passively “disturbed” jets that result from a turbulent nozzle state, with little characterization beyond the variance of the velocity. We know that at higher velocities, jet disturbances play a large role in air entrainment, but the literature is inconclusive on the mechanism by which this occurs. The current work examines the role of carefully controlled forced disturbances on both a plunging jet and the pool surface, allowing us to correlate surface disturbance properties with air entrainment behavior. The current effort is focused on determining the size (wavelength and amplitude) of the disturbances, as well as the relative phasing, on controlling the inception of air entrainment and the volume of the resulting air entrainment events. Results from a laboratory-scale air/water experiment and corresponding DNS will be presented. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H02.00010: Application of a Piecewise Barotropic Equation of State in a Homogeneous Equilibrium Mixture (HEM) Cavitation Model Joshua Brinkerhoff, Saeed Rahbarimanesh, Ioannis Karathanassis, Manolis Gavaises The commonly-used homogeneous equilibrium mixture (HEM) cavitation models lack accurate treatment of two phase flow properties at mixture saturation states, which leads to miscalculation of pressure and density in computational fluid dynamics (CFD) simulations. This issue can be addressed by employing a reliable barotropic equation of state (EOS) in the cavitation model. The current study presents a piecewise EOS that can accurately capture flow properties at liquid, vapor, and transition states. Implementing the proposed EOS in a HEM solver to simulate the cavitating flow of diesel in a throttle shows a better match against mass flow rates measured in experiments than a single-step EOS approach. The improvement is due to the enhanced functionality of the solver in relaxing sharp pressure and density gradients during condensation and vaporization processes. The lack of such capability in the single-step model seems to cause additional numerical diffusion in shear layers and interface regions, particularly in areas with stronger condensation, resulting in poorly predicted cavitation. The work is a step towards the main objective of improving HEM cavitation modeling in CFD of cryogenic fluids. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H02.00011: Numerical study of cavitating flows around an immersed solid body. Gihun Son, Seongjin Hong A numerical approach is presented for cavitating flows around an immersed solid body. The single-fluid model for cavitating flows, which is based on a barotropic relation between fluid density and pressure variations in the liquid-vapor phase-change region, is coupled to a fictitious-domain (FD) method, where the immersed solid region is assumed to be filled with the surrounding fluid with a high viscosity. The FD method can be efficiently applied to complex body geometries without generating body-fitted meshes and does not need an explicit calculation of forces and toques acting on the solid boundary unlike immersed boundary methods. The conservation equations of mass and momentum with the compressibility effect in the cavitation region are solved by employing a projection method and a semi-implicit pressure correction method to avoid the serous time step restriction in weakly compressible flows. The present numerical method combined with a non-equilibrium k-$\varepsilon $ turbulence model is tested through computations of cavitating flows around a hemispherical body and a wedge-shaped body, whose numerical results or experimental data are available in the literature. The numerical method is extended for cavitating flows around a moving solid body. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H02.00012: Gelatin cavity dynamics in the wake of high-speed solid sphere impact Akihito Kiyama, Mohammad Mansoor, Nathan Speirs, Yoshiyuki Tagawa, Tadd Truscott We investigate the impact and penetration of a small solid sphere onto gelatin at speeds greater than 100 m/s. High-speed videography allows us to capture the cavity dynamics in the wake of the sphere. Varying the gelatin concentrations affects the elastic response of the medium. The high-speed videography reveals several unique features of the cavity dynamics in gelatin when compared to water (e.g., the appearance of the texture on the wall of gelatin cavity, the attenuation of the vertical jet upon the pinch-off of gelatin cavity). We present a phase diagram that classifies the cavity pinch-off type using the elastic Froude (inertia vs. elasticity) and the elastic Grashof (gravity vs. elasticity) numbers (Akers {\&} Belmonte, J. Non-Newtonian Fluid Mech., 2006), similar to the Weber and Bond numbers used for water entry (Aristoff {\&} Bush, J. Fluid Mech, 2009). We also discuss the detailed dynamics of each cavity type based on high-speed images. [Preview Abstract] |
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