16:00
Cavitation and cavitating flows II
Chair: Jun Ando
16:00
30 mins
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EXPERIMENT ON CAVITATING FLOW AROUND AN AXISYMMETRIC PROJECTILE NEAR THE FREE SURFACE
Xiaocui Wu, Yiwei Wang, Chenguang Huang
Abstract: The typical experiment on the cloud cavitating flow around an axisymmetric projectile near the free surface is carried out in a Split-Hopkinson pressure bar (SHPB) launching system in this paper. An axisymmetric projectile with flat head is tested. The trajectory and cavitations’ features are obtained with different distances from the free surface and the projectile. The development of the cavity length and the effect of free surface are discussed. The free surface can delay the shed process of the cavity on the upper side, which will cause the length differce between the upper and down side. With the decrement of distance from the free surface and the projectile, the cavity length on the upper side increase.
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16:30
30 mins
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A COMPARISON STUDY OF DIFFERENT CAVITATION MODELS ON THE DEVELOPMENT OF 2D CAVITATING FLOWS
Shuo-Ting Chiang, Jiahn-Horng Chen
Abstract: Flow cavitation has been a vital, yet complicated, phenomenon in many engineering applications. Various cavitation models have been proposed in the past few decades. Roughly speaking, they can be divided into three main categories: interface tracking models, density-pressure coupling models, and transport models for liquid/mass fraction [1]. Among them, the type of models in the last category appears to be the most popular one in numerical simulations since the first one is difficult to take care of the detached cavity and the second one does not have the capability to couple with the transport phenomenon.
In the present study, three mixture models are employed for computing the cavitating flow phenomena in several 2-D hydrofoils. The three models were developed by Singhal et al. [2], Zwart el al. [3], and Schnerr & Sauer[4], respectively. the first model accounts for various first-order effects including phase change, bubble dynamics, turbulent pressure fluctuations, and noncondensable gases [2]; the second one proposes that the total interphase mass transfer rate per unit volume is determined by the bubble density numbers and the mass change rate of a single bubble [3]; the last one suggests a new formulation of mass transfer rate and is able to simulate the cyclic formation of cavitation cloud, the formation of re-entrant jet, and so on [4].
In the study, some 2-D typical hydrofoil sections are employed for investigation. For each case, the cavitating flow is carefully computed. Then, the cavitation characteristics, such as its size, location, and physical properties, are then compared. The vortical flow development is also examined. Finally, some features on the cavitation simulation using these models are concluded.
REFERENCES
[1] C.-C Tseng, Modeling of Turbulent Cavitating Flows, Ph.D. dissertation, University of Michigan, 2010.
[2] A.K. Singhal, M.M. Athavale, H. Li and Y. Jiang, “Mathematical basis and validation of the full cavitation model,” J. Fluids. Eng., Vol. 124, pp. 617-624 (2002).
[3] P.J. Zwart, A.G. Gerber, and T. Belamri, “A two-phase flow model for predicting cavitation dynamics,” 5th Int. Conf. Multiphase Flow, Yokohama, Japan (2004).
[4] G.H. Schnerr and J. Sauer, “Physical and numerical modeling of unsteady cavitation dynamics,” 4th Int. Conf. on Multiphase Flow, New Orleans, USA (2001).
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17:00
30 mins
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STUDY OF MICROBUBBLES FOR SKIN-FRICTION DRAG REDUCTION
Yan Yao, Jin-ling Luo, Kun Zhu, Hai-bo He, Rui Wu, Shi-jie Qin
Abstract: In this paper, the numerical simulation method and experimental test are applied to research and analyze the drag reduction performance of microbubbles. It presents numerical simulations of external flow over a flat porous surface with blowing crossflow that result in significant drag reduction at the interface between the boundary layer and the solid wall. Using the gas-liquid two-phase flow model to simulate the drag reduction efficiency. Through parametric numerical experiments it is demonstrated that it is fundamentally possible to achieve a reduction in the shear force on the wall. The influences between the drag reduction performance and factors of gas velocity, free-stream velocity, downstream distance from the injector are analyzed. And the streamwise velocity developing tendency and bubbles distribution of boundary layer are discussed. During the experimental testing, the equipment is tested in the water tunnel. The experimental results mutually coincide with the simulation analysis. It shows that the drag reduction technology of the microbubbles can significantly reduce frictional resistance of the solid wall. The results showed that when the gas velocity was increased, the friction resistance reduction increased.
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