Compared to non-cavitating flow, cavitating flow is much complex owing to the numerical difficulties caused by cavity generation and collapse. In this paper, the cavitating flow around a NACA66 ...hydrofoil is studied numerically with particular emphasis on understanding the cavitation structures and the shedding dynamics. Large Eddy Simulation (LES) was coupled with a homogeneous cavitation model to calculate the pressure, velocity, vapor volume fraction and vorticity around the hydrofoil. The predicted cavitation shedding dynamics behavior, including the cavity growth, break-off and collapse downstream, agrees fairly well with experiment. Some fundamental issues such as the transition of a cavitating flow structure from 2D to 3D associated with cavitation-vortex interaction are discussed using the vorticity transport equation for variable density flow. A simplified one-dimensional model for the present configuration is adopted and calibrated against the LES results to better clarify the physical mechanism for the cavitation induced pressure fluctuations. The results verify the relationship between pressure fluctuations and the cavity shedding process (e.g. the variations of the flow rate and cavity volume) and demonstrate that the cavity volume acceleration is the main source of the pressure fluctuations around the cavitating hydrofoil. This research provides a better understanding of the mechanism driving the cavitation excited pressure pulsations, which will facilitate development of engineering designs to control these vibrations.
Recent experiments showed that there is an interaction between the fluid vortex formation and cavitation, but the mechanism is still an open problem. In the present paper, the structure of the ...cavitating flow around a twisted hydrofoil was investigated numerically using the mass transfer cavitation model and the modified RNG k-ε model with a local density correction for turbulent eddy viscosity. The predicted three dimensional cavity structures and the shedding frequency agree fairly well with experimental observations. Three types of flow behavior along the suction side of the twisted hydrofoil are discussed. Further analysis of the flow field reveals that cavitation promotes vortex production and increases the boundary layer thickness with local separation and the flow unsteadiness. Finally, the influence of cavitation on the vorticity distribution is illustrated using the vorticity transport equation in a variable density flow and is demonstrated by the contribution of vortex stretching, vortex dilatation and baroclinic torque terms.
•3D cavitating turbulent structure around a twisted hydrofoil is simulated.•Three types of flow behavior along the hydrofoil suction side are illustrated.•The mechanism of cavitation–vortex interaction is discussed.
•Experimental study for ventilated supercavitation in unsteady flows is presented.•Flow unsteadiness leads to a cyclical variation in instantaneous cavitation number.•The variation of cavitation ...number is accompanied by the change in cavity closure modes.•The change of closure modes is independent of the presence of a body inside the supercavity.•Control volume analysis suggests the relation between cavitation number and closure modes.
The present work reports some interesting experimental results for ventilated supercavitation in an unsteady flow. Our experiments have shown that incoming flow unsteadiness does not only affect supercavity shape but also leads to a change in supercavity closure, irrespective of the presence of a body inside the supercavity. Synchronized high-speed imaging and pressure measurements have ascertained the dependence of supercavity closure on instantaneous cavitation number under unsteady flow conditions. Further, control-volume analysis at the closure shows the intricate relation among the cavitation number, pressure difference occurring at the supercavity closure and the obtained closure mechanisms.
•A systematic comparison of supercavitation is conducted across two water tunnel facilities.•Mismatch of ventilation demand for supercavity formation occurs across the facilities.•Both facilities ...show similar choking phenomena in natural supercavitation experiments.•The overall geometry of supercavity differs across the facilities and supercavitation modes.•Discrepancies are attributed to the difference in cavitators and test section pressure distribution across the facilities.
Despite half a century of experimental investigation into both natural and ventilated supercavitation, there are still significant discrepancies among the results, in terms of supercavity geometry and ventilation demand, etc., under approximately similar conditions from different water tunnel facilities. To understand the influences of the flow facilities on the supercavitation experiments, a systematic comparison is conducted using the results from two closed-wall water tunnels, i.e. the Saint Anthony Falls high-speed water tunnel and the Chuangnam National University Closed Tunnel. For both ventilated and natural supercavitation, the experimental conditions from the two facilities are designed to match over a wide range of Froude number and blockage ratio, etc. For the ventilated supercavitation, the cavitation number for generating a ventilated supercavity and the hysteresis process for sustaining a supercavity show a proper match across the two facilities while holding the Froude number and blockage ratio constant. However, the ventilation demand to form a supercavity shows a noteworthy difference across the facilities even under the same Froude number and blockage ratio. Such a difference in the ventilation requirement is attributed to the mismatch of Reynolds number, the detailed geometry of the cavitator models as well as the test section which influences the pressure distribution along the span of the supercavity. Similarly, for natural supercavitation, both facilities yield a similar vaporous cavitation number for the supercavity formation under the same Froude number and blockage ratio, as well as similar choking behavior, i.e. cavitation number stays constant despite the decrease of test section pressure once a natural supercavity forms. The theoretical analysis of the choking phenomenon explains the trend of cavitation number under choking and its dependence on cavitator geometry, Froude number as well as the pressure loss in the water tunnel. A geometry comparison is conducted for both natural and ventilated supercavities in the two facilities under the same Froude number, blockage ratio and cavitation number. The comparison results show differences in the normalized cavity total length across different facilities as well as supercavity types despite the similarities in the supercavity maximum diameter and half-length. These differences were attributed to the variance in the pressure and flow distributions from the different facilities and across ventilated and natural supercavitation. The natural supercavities from the two facilities are further compared with the estimated natural supercavitation in unbounded flow under the same cavitation conditions. The comparison result highlights the limitation of the conventional theory in capturing the cavity geometries in actual experiments.
The use of aerating hydroturbines to mitigate the problem of low dissolved oxygen in the discharge of hydroelectric power plants has recently attracted a lot of attention. The design of a ventilated ...hydroturbine requires a precise understanding of the dependence of the operating conditions (viz. liquid velocity, air ventilation rate, hydrofoil configuration, etc.) on the bubble size distribution generated in the bubbly wake and the consequent rise in dissolved oxygen in the downstream water. In the current research, experiments are conducted in the wake of a ventilated NACA0015 hydrofoil by systematic variation of hydrodynamic conditions allowing for quantitative analysis of aeration statistics and capabilities for turbine blade hydrofoil designs. The data concerning bubble velocity distributions, bubble locations and size distribution, void fraction, etc. are reported for a chosen reference case. In addition, trends in the variation of bubbly wake are explored particularly in the light of wake physics. It is found out that an increase in Reynolds number (Re) led to greater breakup, while an increase in normalized air ventilation rate (CQ) favored greater coalescence events in the wake. Further, the PDF(d̃) of the normalized bubble size d̃=d/d32, where d32 represents Sauter mean size distribution, is found to have a universally similar shape independent of either Re, CQ or hydrofoil angle of attack. Finally, a numerical formulation is proposed for the bubble sizes in the hydrofoil wake. This rich dataset will also contribute to the development of a numerical turbulence model, to investigate turbulence effects on bubble size distribution and predict the rate of air entrainment and the oxygen transfer occurring in the wake at different hydrodynamic conditions.
•An experimental investigation on bubbly wake of a ventilated hydrofoil is conducted.•Bubble statistics is measured at different liquid velocity and ventilation air flow rates.•Distinct breakup and ...coalescence dominant regimes are identified in the wake.•Analytical expressions are derived for maximum stable bubble size in breakup regime.•High-speed imaging provides insights on mechanisms of bubble coalescence and breakup.
Auto-venting turbines have been proposed as a promising solution to the problem of low oxygen content in the discharged downstream water of an electric power plant. The current design of these turbines relies primarily on computational simulation. The experimental studies that focus on the physical processes occurring in turbulent bubbly wake are urgently needed to improve the performance of these simulations in predicting the bubble size distribution behind auto-venting turbines. Therefore, in the current study, we conducted detailed experimental investigations into the bubble size distributions in the wake of a ventilated hydrofoil. The mean bubble statistics is measured at different liquid velocities and air entrainment rates, and then the variation in mean bubble statistics is studied at different downstream locations in the wake. The bubble size distributions at different downstream locations have revealed the presence of distinct coalescence-dominant and breakup-dominant regimes. Analytical expressions are derived for the prediction of maximum stable diameter and Sauter mean diameter of bubbles, in the breakup and coalescence regimes, respectively. The observations from high speed imaging provide support for the measurements of bubble statistics, and physical insights into different mechanisms of bubble breakup and coalescence in turbulent wake. It is hoped that these insights will aid in developing generic model of bubble size distribution, and will help researchers improve bubbly flow simulations for auto-venting turbines.
•The deformation of supercavity shapes is explored in a cavitation tunnel.•Maximum diameter is affected less than cavity length by experimental conditions.•A new convention can exclude the effects of ...tunnels walls on cavity deformation.•These findings have important implications for translating model experiments.
In this study, the deformation of supercavity shapes due to tunnel-wall blockage effects was explored in the medium-sized high-speed cavitation tunnel at the Korea Research Institute of Ships and Ocean Engineering (KRISO). It was observed that maximum cavity diameter is affected less than cavity length by experimental conditions. We adopt a new convention for specifying the cavity dimensions; namely, the cavity diameter at the first peak on the lower cavity outline is defined as the maximum cavity diameter. This enables one to exclude the shape deformations caused by tunnel walls when extrapolating experimental cavity dimensions to the equivalent cavities that would occur in unbounded flows.
A systematic approach to parameter-dependent control synthesis of a high-speed supercavitation vehicle (HSSV) is presented. The aim of the control design is to provide robust reference tracking ...across a large flight envelope, while directly accounting for the interaction of liquid and gas phases with the vehicle. A nonlinear dynamic HSSV model is presented and discussed relative to the actual vehicle. A linear, parameter-varying (LPV) controller is synthesized for angle rate tracking in the presence of model uncertainty. The control design takes advantage of coupling in the governing equations to achieve improved performance. Multiple LPV controllers synthesized for smaller overlapping regions of the parameter space are blended together, providing a single controller for the full flight envelope. Time-domain simulations implemented on high-fidelity simulations, provide insight into the performance and robustness of the proposed scheme.
Aerating hydroturbines have recently been proposed as an effective way to mitigate the problem of low dissolved oxygen in the discharge of hydroelectric power plants. The design of such a ...hydroturbine requires a precise understanding of the dependence of the generated bubble size distribution upon the operating conditions (viz. liquid velocity, air ventilation rate, hydrofoil configuration, etc.) and the consequent rise in dissolved oxygen in the downstream water. The purpose of the current research is to investigate the effect of location of air injection on the resulting bubble size distribution, thus leading to a quantitative analysis of aeration statistics and capabilities for two turbine blade hydrofoil designs. The two blade designs differed in their location of air injection. Extensive sets of experiments were conducted by varying the liquid velocity, aeration rate and the hydrofoil angle of attack, to characterize the resulting bubble size distribution. Using a shadow imaging technique to capture the bubble images in the wake and an in-house developed image analysis algorithm, it was found that the hydrofoil with leading edge ventilation produced smaller size bubbles as compared to the hydrofoil being ventilated at the trailing edge. KCI Citation Count: 0
Aerating hydroturbines have recently been proposed as an effective way to mitigate the problem of low dissolved oxygen in the discharge of hydroelectric power plants. The design of such a ...hydroturbine requires a precise understanding of the dependence of the generated bubble size distribution upon the operating conditions (viz. liquid velocity, air ventilation rate, hydrofoil configuration, etc.) and the consequent rise in dissolved oxygen in the downstream water. The purpose of the current research is to investigate the effect of location of air injection on the resulting bubble size distribution, thus leading to a quantitative analysis of aeration statistics and capabilities for two turbine blade hydrofoil designs. The two blade designs differed in their location of air injection. Extensive sets of experiments were conducted by varying the liquid velocity, aeration rate and the hydrofoil angle of attack, to characterize the resulting bubble size distribution. Using a shadow imaging technique to capture the bubble images in the wake and an in-house developed image analysis algorithm, it was found that the hydrofoil with leading edge ventilation produced smaller size bubbles as compared to the hydrofoil being ventilated at the trailing edge.
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