In order to understand the mechanism of cavity shedding and evolution, turbulent cavitating flows of the twisted hydrofoil were numerically investigated using the k-? turbulence model and the ZGB ...cavitation model. The results of the numerical calculation and the experimental method are basically consistent, which confirms the feasibility of the numerical calculation model. This study has obtained the following conclusions. Firstly, the cavity shedding can be summarized into six stages, and the cavity shape, pressure and velocity field at different stages are displayed, analyzed and compared in detail. Secondly, the shedding of cavity and its evolution are mainly caused by the re-entrant jet and side-entrant jet, in which the former provides the kinetic energy and the latter plays the role of guiding the direction. Thirdly, under the convective shearing action of the re-entrant jet and the main flow, a strong vortex located in the mid-back edge of the hydrofoil is formed, which promotes the transformation of the cavity shape into a U-shaped structure.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The objective of this work is to investigate experimentally controlling cavitating flow over NACA66 (MOD) hydrofoils by means of an active water injection along its suction surface. The continuous ...water vertically jets out of the chamber inside the hydrofoil through evenly distributed surface holes. Experiments were carried out in cavitation water tunnel, using high-speed visualization technology and the particle image velocimetry (PIV) system to study the sheet/cloud cavity behaviors. We studied the effects of this active control on cavity evolution with four kinds of jet flow at two different jet positions. We analyzed the effect of water injection on the mechanism of the cavitating flow control. The results were all compared with that for the original hydrofoil without jet and show that the active jet can effectively suppress the sheet/cloud cavitation characterized by shrinking the attached cavity size and breaking the large-scaled cloud shedding vortex cavity into small-scaled ones. The optimum effectiveness of cavitation suppression is affected by the jet flow rates and jet positions. The water injection at flow rate coefficient 0.0245 with the jet position of 0.45
C
reduces the maximum sheet cavity length by 79.4
%
and the cavity shedding is diminished completely, which gives the most superior effect of sheet cavitation suppression. The jet blocks the re-entrant jet moving upstream and weakens the power of re-entrant jet and thus restrains the cavitation development effectively and stabilizes the flow field.
Graphic Abstract
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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
The objective of this study was to understand better the ventilated cavitation flow structure around an underwater ventilated vehicle. A high-speed camera system was used to observe the cavity ...evolution of unsteady cavitation flow, and a dynamic pressure measurement system was used to measure the instantaneous pressure during cavity growth. The numerical simulation is presented using the secondary development of computational fluid dynamics code CFX with a filter-based turbulence model. The results indicate that the ventilated flow rate of the gas influences the development of ventilated cavitation, and the pressure fluctuation is suppressed remarkably by the ventilated cavity evolution. The results also indicate that the proposed method can effectively capture the unsteady cavitation structure in accordance with the quantitative features observed in the experiment. It can therefore be concluded that the pressure fluctuations are induced by the vortex because of its periodic shedding toward downstream. The vortex shedding causes changes in the pressure distribution on the vehicle surface. Some secondary pressure oscillations can be observed that are attributable to the shedding of secondary vortex structures near the vehicle surface. These findings provide an important basis for facilitating the better understanding of the unsteady ventilated cavitation flows.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
An experimental investigation was conducted to study the cavitating flow around cylinders with different chamfer angles vertically inserted in a circular pipe. The experiments were performed in a ...variable-pressure closed-loop cavitation rig at a constant Reynolds number of Re=1.2×105 with cavitation numbers corresponding from inception to supercavitation. In practical hydraulic systems, numerous structures can be modeled as vertically inserted chamfered cylinders and the cavitation phenomenon can occur around such structures with serious side effects. This work is an attempt to understand the cavity dynamics and the influence of the chamfer angle θ on the characteristics of cavitating flows, with the aim of providing guidelines for designing relevant structures. The low- and high-speed images of the cavitating flow in different cavitation regimes were analyzed to investigate the dynamics of the cavitating flow around the cylinder with chamfer angle θ=60°. From the space–time diagram obtained from the high-speed series, it is found that for cavitation number σ>0.80, the shedding of the small vapor filaments is induced by interfacial instabilities; for cavitation number σ<0.75, the re-entrant jet accounts for the periodic shedding of the vapor clouds. In the transition region (0.75<σ<0.80), the shedding is caused by a complex combination of both mechanisms. The dominant shedding frequency, expressed as Strouhal number St, drops with the decrease of the cavitation number σ in the regime dominated by interfacial instabilities; while in re-entrant jet regime, it is almost insensitive to the change of the cavitation number σ. In addition, the influences of the chamfer angle θ on the inception condition, cavity length, pressure drop, and pressure fluctuation are also discussed in the present work. With the increase of θ, the cavitation resistance becomes better and the pressure drop through the test section decreases.
•Cavitation over different chamfered cylinders inserted in a circular pipe is studied.•Two distinct cavity shedding mechanisms are identified.•Spectral content of cavity shedding is investigated by wavelet analysis.•Large-scale shedding dominated by re-entrant jet leads to severe pressure drop.•Chamfer angle significantly influences the cavitation behavior.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•We investigate the mechanism responsible for periodic cavitation shedding in nozzles.•The re-entrant jet motion is more complex than previously presented in the literature.•A liquid sublayer exists ...throughout all periods of the cavitation process.•Periodic shedding is caused by a travelling wave disturbance.•Liquid sublayer velocities are significantly lower than the speed of the wave disturbance.
Periodic shedding of cloud cavitation is a common form of cavitation instability. The motion of a re-entrant liquid jet is central to this process but the mechanism which drives the phenomenon remains unclear, particularly for cavitation in cylindrical orifices. The current work describes an experimental investigation of the re-entrant jet mechanism for periodic cloud shedding in a large-scale (8.25mm) cylindrical acrylic orifice. Refractive index matching and high-speed visualisation reveal in detail the motion of the re-entrant jet and indicate a complex mechanism causing the instability. Unabated optical access to the near-wall region of the orifice revealed a constant presence of liquid throughout the shedding cycle. The mechanism causing the periodic shedding was shown to be a combination of a traveling wave style deformation of the cavity interface and a translational pulse, each with distinctly different velocities.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•The supercavitating flow over a disk-shaped cavitator is simulated at extremely low cavitation numbers.•Fairly good agreement is observed between predicted and experimental results for a ...hemispherical headform body.•Presence of a body inside the cavity causes the maximum length of the cavity decreases.•Adding a concentric hole to the disk-cavitator causes the cavity length decreases for large values of the hole diameter.
Supercavitation has been recently proposed as an effective method for drag reduction of underwater vehicles. For this purpose, a cavitator is used to generate a large continuous bubble to cover the vehicle. Extremely low cavitation numbers are required in these applications to provide a sufficiently voluminous cavity. In this study, the effects of body presence inside the cavity of a disk-shaped cavitator are studied numerically. An element based finite volume approach is used for solving Reynolds-averaged Navier-Stokes equations. In order to validate the numerical scheme, turbulence and cavitation models, the cavitating flow over a hemispherical nosed cylinder body and also behind a disk-shaped cavitator has been simulated and the numerical results are validated against existing analytical and experimental data. Next, the effects of body presence inside the cavity of the disk-cavitator on the cavity characteristics are investigated. Results show that the cavity length is slightly smaller when it closes on the body, in comparison with the freely-closing cavity. Furthermore, the effects of adding a concentric hole with various diameters to the disk-cavitator on the cavity features are examined. The degree of drag reduction and changes of the cavity dimensions are determined at various cavitation numbers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In order to study the energy loss of bi-directional hydraulic machinery under cavitation conditions, this paper uses high-speed photography combined with six-axis force and torque sensors to collect ...cavitating flow images and lift signals of S-shaped hydrofoils simultaneously in a cavitation tunnel. The experimental results show that the stall angle of attack of the S-shaped hydrofoil is at ±12° and that the lift characteristics are almost symmetrical about +1°. Choosing
α
= +6° and
α
= −4° with almost equal average lift for comparison, it was found that both cavitation inception and cloud cavitation inception were earlier at
α
= −4° than at
α
= +6°, and that the cavitation length at
α
= −4° grew significantly faster than at
α
= +6°. When
α
= +6°, the cavity around the S-shaped hydrofoil undergoes a typical cavitation stage as the cavitation number decreases: from incipient cavitation to sheet cavitation to cloud cavitation. However, when
α
= −4°, as the cavitation number decreases, the cavitation phase goes through a developmental process from incipient cavitation to sheet cavitation to cloud cavitation to sheet cavitation to cloud cavitation, mainly because the shape of the S-shaped hydrofoil at the negative angle of attack affects the flow of the cavity tails, which is not sufficient to form re-entrant jets that cuts off the sheet cavitation. The formation mechanism of cloud cavitation at the two different angles of attack (α = +6°、−4°) is the same, both being due to the movement of the re-entrant jet leading to the unstable shedding of sheet cavity. The fast Fourier analysis reveals that the fluctuations of the lift signals under cloud cavitation are significantly higher than those under non-cavitation, and the main frequencies of the lift signals under cloud cavitation were all twice the frequency of the cloud cavitation shedding.
Re-entrant jet causes cloud cavitation shedding, and cavitating vortical flow results in flow field instability. In the present work, a method of water injection is proposed to hinder re-entrant jet ...and suppress vortex in cloud cavitating flow of a NACA66 (MOD) hydrofoil (Re = 5.1 × 105, σ = 0.83). A combination of filter-based density corrected turbulence model (FBDCM) with the Zwart–Gerber–Belamri cavitation model (ZGB) is adopted to obtain the transient flow characteristics while vortex structures are identified by Q criterion & λ2 criterion. Results demonstrate that the injected water flow reduces the range of the low-pressure zone below 1940 Pa on the suction surface by 54.76%. Vortex structures are observed both inside the attached and shedding cavitation, and the water injection shrinks the vortex region. The water injection successfully blocks the re-entrant jet by generating a favorable pressure gradient (FPG) and effectively weakens the re-entrant jet intensity by 46.98%. The water injection shrinks the vortex distribution area near the hydrofoil suction surface, which makes the flow in the boundary layer more stable. From an energy transfer perspective, the water injection supplies energy to the near-wall flow, and hence keeps the steadiness of the flow field.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Quasi-periodical evolutions such as shedding and collapsing of unsteady cloud cavitating flow, induce strong pressure fluctuations, what may deteriorate maneuvering stability and corrode surfaces of ...underwater vehicles. This paper analyzed effects on cavitation stability of a trip bar arranged on high-speed underwater projectile. Small scale water tank experiment and large eddy simulation using the open source software Open FOAM were used, and the results agree well with each other. Results also indicate that trip bar can obstruct downstream re-entrant jet and pressure wave propagation caused by collapse, resulting in a relatively stable sheet cavity between trip bar and shoulder of projectiles.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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