Cavitation control using Cylindrical Cavitating-bubble Generators (CCGs): Experiments on a benchmark CAV2003 hydrofoil
Introduction
Cavitation often causes undesirable effects on the performance of hydromechanical systems, such as marine propellers, pumps, hydraulic turbines and other machinery, which leads to an enhancement of deleterious noise, vibration and erosion (Fortes Patella et al., 2013, Franc and Michel, 2005, Leroux et al., 2005). At present, along with the optimization of the shape and configuration of operating elements of modern hydraulic equipment (Fujii et al., 2007, Garg et al., 2015, Sun, 2012; Zobeiri et al., 2012), the operating efficiency of hydroengineering systems is further enhanced by applying and developing various means of flow management. Selection of the most appropriate control method and its parameters are, in turn, based on an in-depth understanding of the physics of occurring phenomena. That is why comprehensive studies of the dynamics of unsteady turbulent flows, especially with cavitation, and different approaches to manipulate them are of a high importance.
Among all existing techniques of flow control, passive methods are preferred for using since they are easy for implementation in real operating conditions of hydraulic machines and do not require additional energy supply into a hydraulic system, although do not allow an effective flow management in a wide range of flow regimes as compared to active approaches. To date, various passive methods have been tested and reported in literature, including the ones based on using hydrophilic or hydrophobic materials (Ivanov, 1980; Tassin Leger and Ceccio, 1998), applying flexible coatings (Akcabay et al., 2014, Wu et al., 2015, Zarruk et al., 2014), transforming the wall texture by irregular (Churkin et al., 2016, Coutier-Delgosha et al., 2005) or distributed roughness (Danlos et al., 2014, Kawanami et al., 1997) and installing vortex generators on the surface of a test object (Kadivar et al., 2019b, Che et al., 2019).
With respect to unsteady cloud cavitation, vortex generators (VGs) probably present one of the best prospects to improve the design of ship propellers and rudders, stationary and rotary guide vanes of hydroturbines, blades of axial pumps and impellers and other machinery. This method was previously investigated by several research groups. An and Plesniak, 2008 considered the effect of the height of a backward-facing step on cavitation inception in a downstream region in a Venturi nozzle and found that it influences both the inception location and cavitation development due to different flow structures. Javadi et al., 2017 placed a wedge-type VG near the leading edge of a 2D hydrofoil, where a separation bubble was expected to form, and showed that a low-pressure recirculation area was produced behind the VG and a stationary cavitating separation bubble can be formed, which allowed a well-predictable control of cavitation.
In the papers by Kadivar and Javadi (2017) and Kadivar and el Moctar (2018), a wedge-type VG was used to mitigate unsteady cloud cavitation on a 3D hydrofoil. It was proved that wall-pressure peaks and turbulent velocity fluctuations over the hydrofoil surface could be reduced with this control technique. Che et al., 2019 installed separated micro vortex generators (MVGs) within the boundary layer on a NACA0015 hydrofoil close to its leading edge to induce streamwise counter-rotating vortices and, instead of a stable cavity interface along the laminar separation line, observed vortex cavitation. These cavitating vortices broke down into bubbly structures that eventually accumulated in the attached cavity region. Reducing the height of MVGs, the authors could enhance the control effect on cavitation dynamics by increasing the momentum of near-wall liquid. Moreover, owing to MVGs the near-wall flow became less susceptible to disturbances from the outer flow.
In the recent research by Kadivar et al., 2019a, the effect of cylindrical VGs referred to as Cylindrical Cavitating-bubble Generators (CCGs) on the mechanism of unsteady cloud cavitation was studied numerically. CCGs were mounted near the leading edge and close to the midsection of a benchmark CAV2003 hydrofoil. Varying the geometrical parameters and disposition of CCGs, it was demonstrated that, for certain conditions, the unsteady cavity structure could be made quasi-stable. Using this control approach, the unsteady cloud cavitation was mitigated and only small-scale cavities shed from the closure region of an attached cavity. As a result, a remarkable reduction of the amplitude of cavitation-induced vibrations and high wall-pressure peaks on the solid surface of the hydrofoil was registered, proving once again that VGs are an efficient instrument for cavitation management.
Despite a great number of studies on unsteady cloud cavitation and a variety of passive techniques of flow control, comprehensive data on destructive effects of unsteady flow dynamics linked with the transition to cloud cavitation conditions and detailed information on the main parameters of governing elements used to manipulate cavitating flow are still lacking. This research is aimed at an experimental investigation of the passive method of cavitating flow control based on using miniature vortex generators of a cylindrical type (Cylindrical Cavitating-bubble Generators – CCGs) on the surface of a benchmark CAV2003 hydrofoil. In the paper, we focus on the effects of CCGs on cavitation evolution, spatial structure and dynamics of partial cavities, distributions of the mean and turbulent characteristics in the flow around the test model and amplitude-frequency spectra of the pressure pulsations associated with attached cavity length oscillations under unsteady flow conditions.
In the study, we employed a high-speed imaging to capture snapshots of instantaneous cavitation patterns, a PIV technique to measure instantaneous velocity fields over and behind the hydrofoil and a hydroacoustic pressure transducer to record local pressure pulsations in the hydrofoil wake. The layout of this article is as follows. In Section 2, we introduce the methodology of cavitation passive control with CCGs. The experimental rig and measurement/visualization techniques along with the test object and flow conditions are described in detail in Section 3. Section 4 is the main one where we document the effects of CCGs on cavitation dynamics and flow structure for different flow regimes, starting from the cavitation inception, including quasi-steady partial cavities and finishing by unsteady cloud cavitation. Finally, in Section 5, we summarize our findings and draw the concluding remarks.
Section snippets
Methodology of cavitation passive control
In this research, the passive control is implemented, employing discrete miniature vortex generators (MVGs) that are one of the most effective tools to variate characteristics and to govern separation of a boundary layer by promoting its transition to turbulence on airfoils in aerospace engineering applications (Gad-el-Hak, 1996). The laminar-to-turbulent transition of a boundary layer induced by miniature vortex generators occurs through generation of streamwise vortices and, therefore,
Test objects, control device and experimental conditions
The research was performed in the cavitation tunnel in Kutateladze Institute of Thermophysics SB RAS (Fig. 2). The test section of the experimental rig is a 1.3 m long channel with a rectangular inlet cross-section of m equipped with transparent plexiglass windows to enable cavitation visualization and velocity measurements by optical techniques. The maximum flow rate is 1147 m3/h, which corresponds to the maximum free flow velocity (without a test body) of 15.93 m/s. The initial
Unsteady cavitating flow without the passive control
We firstly analyzed the dynamics of quasi-steady partial cavitation and the mechanism of unsteady cloud cavitation using high-speed visualization for the hydrofoil without the control element. It is visible in Fig. 6 that, in the quasi-steady partial cavitation regime, small-scale cloud cavities are regularly shed from the closure region of an attached cavity.
Sequential images in Fig. 7 show four primary stages of a period of unsteady cloud cavitation which are the growth of an attached cavity,
Conclusions
A passive method of flow control based on vortex generators (cavitation controllers) – Cylindrical Cavitating-bubble Generators (CCGs) that were installed on the surface of a benchmark CAV2003 hydrofoil – was investigated experimentally to manage the cavitation inception, quasi-steady partial cavities and unsteady cloud cavitation. The research was carried out using a high-speed imaging to analyze instantaneous cavitation patterns, a PIV technique to measure velocity fields over and behind the
Acknowledgments
The experiment was carried out with a financial support from the Russian Foundation for Basic Research (Project Nos. 17-08-01199 and 18-38-20167) and the government of Novosibirsk region of the Russian Federation (Project No. 18-48-543022). The methods of automatization of data acquisition and processing used in the study were elaborated under the state contract with IT SB RAS (AAAA-A19-119052190039-8). The authors also appreciate useful discussions with Dr. Javadi and are grateful to student
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