This research investigates cavitation around a marine propeller, employing computational fluid dynamic (CFD) solvers, including an incompressible, isothermal compressible, and fully compressible ...flow. The investigation commenced with simulations utilizing an incompressible flow solver, subsequently extending to the two compressible flow solvers. In the compressible flow, there is a close interrelation between density, pressure, and temperature, which significantly influences cavitation dynamics. To verify computational methods, verification tests were conducted for leading-edge cavitating flows over a two-dimensional (2D)-modified NACA66 hydrofoil section at various cavitation numbers. The computational results were validated against the experimental data, with the solvers’ capability to predict cavitation forming the basis for comparison. The results demonstrate consistent predictions among the solvers; however, the fully compressible flow solver demonstrated a superior performance in capturing re-entrant jets and accurately modeling cavity closure regions. Furthermore, the fully compressible flow solver precisely estimated propeller hydrodynamic performance, yielding results closely aligned with experimental observations.
Recent experiments have shown interactions between the cavitation and fluid vortex formation in a hydrodynamic torque converter. This study aimed to clarify the unsteady cavitation trigger mechanism ...and flow-induced vibration caused by turbulence–cavitation interactions. The mass transfer cavitation model and modified Reynolds-averaged Navier–Stokes k–ω model were used with a local density correction for turbulent eddy viscosity to investigate the cavitation structure in a hydrodynamic torque converter under various operating conditions. The model results were then validated against test data. The multi-block structured gridding technique was used to develop an orthogonally structured grid of a three-dimensional full-flow passage as an alternative analysis method for the cavitation flow. The results indicated that the re-entrant jet is the main cause of the shedding cavitation and breaking O-type cavitation. The re-entrant jet is driven by the reverse pressure gradient to move upstream towards the stator nose, and it lifts and splits the attached cavitation, which periodically induces shedding cavitation. When the cavitation was considered, the prediction error of the capacity constant was reduced from 13.23% to <5%. This work provides an insight into the cavitation–vortex interactions in a hydrodynamic torque converter, which can be used to improve the prediction accuracy of the hydrodynamic performance.
Laser-induced forward transfer (LIFT) is a high-resolution direct-write technique, which can print a wide range of liquid materials without a nozzle. In this process, a pulsed laser initiates the ...expulsion of a high-velocity micro-jet of fluid from a thin donor film. LIFT involves a novel regime for impulsively driven free-surface jetting in that viscous forces developed in the thin film become relevant within the jet lifetime. In this work, time-resolved microscopy is used to study the dynamics of the laser-induced ejection process. We consider the influence of thin metal and thick polymer laser-absorbing layers on the flow actuation mechanism and resulting jet dynamics. Both films exhibit a mechanism in which flow is driven by the rapid expansion of a gas bubble within the liquid film. We present high-resolution images of the transient gas cavities, the resulting ejection of high aspect ratio external jets, as well as the first images of re-entrant jets formed during LIFT. These observations are interpreted in the context of similar work on cavitation bubble formation near free surfaces and rigid interfaces. Additionally, by increasing the laser beam size used on the polymer absorbing layer, we observe a transition to an alternate mechanism for jet formation, which is driven by the rapid expansion of a blister on the polymer surface. We compare the dynamics of these blister-actuated jets to those of the gas-actuated mechanism. Finally, we analyze these results in the context of printing sensitive ink materials.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The physics and mechanism of sheet/cloud cavitation in a convergent–divergent channel are investigated using synchronized dynamic surface pressure measurement and high-speed imaging in a water tunnel ...to probe the cavity shedding mechanism. Experiments are conducted at a fixed Reynolds number of
Re
= 7.8 × 10
5
for different values of the cavitation number
σ
between 1.20 and 0.65, ranging from intermittent inception cavitation, sheet cavitation to quasi-periodic cloud cavitation. Two distinct cloud cavitation regimes, i.e. the re-entrant jet and shockwave shedding mechanism, are observed, accompanied by complex flow phenomenon and dynamics, and are examined in detail. An increase in pressure fluctuation intensity at the numbers 3 and 4 transducer locations are captured during the transition from re-entrant jet to shockwave shedding mechanism. The spectral content analysis shows that, in cloud cavitation, several frequency peaks are identified with the dominant frequency caused by the large-scale cavity shedding process and the secondary frequency related to re-entrant jet/shockwave dynamics. Statistical analysis based on defined grey level profiles reveals that, in cloud cavitation, the double-peak behaviours of the probability density functions with negative skewness values are found to be owing to the interactions of the re-entrant jet/shockwave with cavities in the region of 0.25 ~ 0.65 mean cavity length (
L
c
). In addition, multi-scale proper orthogonal decomposition analysis with an emphasis on the flow structures in the region of 0.25 ~ 0.65
L
c
reveals that, under the shockwave shedding mechanism, both the re-entrant jet and shockwave are captured and their interactions are responsible for the dynamics and statistics of cloud shedding process.
Cavitation is a complex multiphase flow phenomenon that is usually involved in marine propulsion systems, and can be simulated with a couple of methods. In this study, three widespread cavitation ...models were compared using experimental data and a new modified simulation method. The accuracy of the three cavitation models was evaluated regarding their steady and unsteady characteristics, such as the flow field, re-entrant jet, vortex-shedding, and so on. Based on the experimental data and numerical results, the applicability of different cavitation models in different conditions was obtained. The Kunz model can accurately capture both the adverse pressure gradient and the action of the re-entrant jet in sheet cavitation, while the full cavitation model (FCM) has an accurate prediction for the flow field structure and the shedding characteristic of cloud cavitation. Through comparing the results, the optimal selection of cavitation models for further study at different conditions was obtained.
To understand the formation mechanism and evolution process of the perpendicular cavitation vortex (PCV) of an axial flow pump for off-design conditions, turbulent cavitating flows were numerically ...investigated using the rotation curvature-corrected shear stress transport (SST-CC) turbulence model and the Zwart–Gerber–Belamri cavitation model. In this work, the origin and evolution of a PCV were analyzed through a high-speed photography experiment and numerical simulation. The results showed that the PCV came from a secondary tip leakage vortex (S-TLV) and was aggregated by the action of the re-entrant jet, combined with the cavitation bubbles driven by the radial flow to form the cavitation vortex (CV). With the joint action of leakage jet lifting and TLV entrainment, the PCV was reoriented and gradually became perpendicular to the chord direction. Then, the PCV and TLV collided, mixed, and entrained, which formed a strong pressure pulsation. The PCV was gradually divided into upper and lower parts. One part was combined with the residual part of the TLV and flowed to the next blade, and the other part flowed out of the impeller area along the axial direction. At the same time, the generation, evolution, and dissipation of the PCV formed high pulsation amplitudes and frequencies in the middle and rear above the blade suction.
Cavitation Passive Control on Immersed Bodies Javadi, Khodayar; Dorostkar, Mohammad Mortezazadeh; Katal, Ali
Journal of marine science and application,
03/2017, Volume:
16, Issue:
1
Journal Article
Peer reviewed
This paper introduces a new idea of controlling cavitation around a hydrofoil through a passive cavitation controller called artificial cavitation bubble generator （ACG）. Cyclic processes, namely, ...growth and implosion of bubbles around an immersed body, are the main reasons for the destruction and erosion of the said body. This paper aims to create a condition in which the cavitation bubbles reach a steady-state situation and prevent the occurrence of the cyclic processes. For this purpose, the ACG is placed on the surface of an immersed body, in particular, the suction surface of a 2D hydrofoil. A simulation was performed with an implicit finite volume scheme based on a SIMPLE algorithm associated with the multiphase and cavitation model. The modified k-ε RNG turbulence model equipped with a modification of the turbulent viscosity was applied to overcome the turbulence closure problem. Numerical simulation of water flow over the hydrofoil equipped with the ACG shows that a low-pressure recirculation area is produced behind the ACG and artificially generates stationary cavitation bubbles. The location, shape, and size of this ACG are the crucial parameters in creating a proper control. Results show that the cavitation bubble is controlled well with a well-designed ACG.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This paper presents a numerical study of periodic cavitating flow around NACA0012 hydrofoil. The cavitation condition is modeled through a bubble dynamics cavitation model. The RNG k–ε turbulence ...model with enhanced wall treatment is used to capture turbulent boundary layer along the hydrofoil surface. The periodic formation of vortex near the trailing edge is simulated and jet of re-entrant flow is visualized in this paper. The pressure fluctuation on the foil surface due to the periodic nature of the vorticity dynamics is observed. The lift and drag coefficients oscillate with a time average value of C¯L=0.697 and C¯D=0.012 with the periodic cavity shedding.
► The cavitating flow is very well predicted by k–ε turbulence model and cavitation model. ► The periodic formation of vortex near the trailing edge is simulated and jet of re-entrant flow is visualized. ► The pressure fluctuation on the foil surface due to the periodic nature of the vorticity dynamics is observed. ► The lift and drag coefficient oscillates with the periodic cavity shedding. ► The computed value of the Strouhal number lies within the range predicted by other researchers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A new unsteady cavitation event tracking model is developed for predicting vapor dynamics occurring in multi-dimensional incompressible flows. The procedure solves incompressible Navier–Stokes ...equations for the liquid phase supplemented with an additional vapor transport equation for the vapor phase. The novel cavitation-induced-momentum-defect (CIMD) correction methodology developed in this study accounts for cavitation inception and collapse events as relevant momentum-source terms in the liquid phase momentum equations. The model tracks cavitation zones and applies compressibility effects, employing homogeneous equilibrium model (HEM) assumptions, in constructing the source term of the vapor transport model. Effects of vapor phase accumulation and diffusion are incorporated by detailed relaxation models. A modified RNG
k–
ε model, including the effects of compressibility in the vapor regions, is employed for modeling turbulence effects. Numerical simulations are carried out using a finite volume methodology available within the framework of commercial CFD software code Fluent v.6.2. Simulation results are in good qualitative agreement with experiments for unsteady cloud cavitation behavior in planar nozzle flows. Multitude of mechanisms such as formation of vortex cavities, vapor cluster shedding and coalescence, cavity pinch off are sharply captured by the CIMD approach. Our results indicate the profound influence of re-entrant jet motion and adverse pressure gradients on the cavitation dynamics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this article, we describe the use of a new dynamic cubic nonlinear model, a new nonlinear subgrid-scale model, for simulating the cavitating flow around an NACA66 series hydrofoil. For comparison, ...the dynamic Smagorinsky model is also used. It is found that the dynamic cubic nonlinear model can capture the turbulence spectrum, while the dynamic Smagorinsky model fails. Both models reproduce the cavity growth/destabilization cycle, but the results of the dynamic cubic nonlinear model are much smoother. The re-entrant jet is clearly captured by the models, and it is shown that the re-entrant jet cuts the cavity into two parts. In general, the dynamic cubic nonlinear model provides improvement over the dynamic Smagorinsky model for the calculation of cavitating flow.
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