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2 pages, 168 KiB

Open AccessEditorial

Editorial for Special Issue: New Advances of Cavitation Instabilities

by Florent Ravelet

Appl. Sci. 2021, 11(12), 5313; https://0-doi-org.brum.beds.ac.uk/10.3390/app11125313 - 8 Jun 2021

Viewed by 1638

Abstract

This editorial presents the main articles published in the Special Issue: New Advances of Cavitation Instabilities. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

Research

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13 pages, 4579 KiB

Open AccessArticle

Peening Natural Aging of Aluminum Alloy by Ultra-High-Temperature and High-Pressure Cavitation

by Toshihiko Yoshimura, Masayoshi Iwamoto, Takayuki Ogi, Fumihiro Kato, Masataka Ijiri and Shoichi Kikuchi

Appl. Sci. 2021, 11(7), 2894; https://0-doi-org.brum.beds.ac.uk/10.3390/app11072894 - 24 Mar 2021

Cited by 5 | Viewed by 1731

Abstract

The peening solution treatment was performed on AC4CH aluminum alloy by ultra-high-temperature and high-pressure cavitation (UTPC) processing, and the peening natural aging was examined. Furthermore, peening artificial aging treatment by low-temperature and low-pressure cavitation (LTPC) was performed, and the time course of peening [...] Read more.

The peening solution treatment was performed on AC4CH aluminum alloy by ultra-high-temperature and high-pressure cavitation (UTPC) processing, and the peening natural aging was examined. Furthermore, peening artificial aging treatment by low-temperature and low-pressure cavitation (LTPC) was performed, and the time course of peening natural aging and peening artificial aging were compared and investigated. It was found that when the AC4CH alloy is processed for an appropriate time by UTPC processing, compressive residual stress is applied and natural aging occurs. In addition, the UTPC processing conditions for peening natural aging treatment with high compressive residual stress and surface hardness were clarified. After peening artificial aging by LTPC processing, the compressive residual stress decreases slightly over time, but the compression residual stress becomes constant by peening natural aging through UTPC treatment. In contrast, it was found that neither natural nor artificial peening natural aging occurs after processing for a short time. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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12 pages, 2853 KiB

Open AccessArticle

Numerical Insight into the Kelvin-Helmholtz Instability Appearance in Cavitating Flow

by Peter Pipp, Marko Hočevar and Matevž Dular

Appl. Sci. 2021, 11(6), 2644; https://0-doi-org.brum.beds.ac.uk/10.3390/app11062644 - 16 Mar 2021

Cited by 10 | Viewed by 2682

Abstract

Recently the development of Kelvin-Helmholtz instability in cavitating flow in Venturi microchannels was discovered. Its importance is not negligible, as it destabilizes the shear layer and promotes instabilities and turbulent eddies formation in the vapor region, having low density and momentum. In the [...] Read more.

Recently the development of Kelvin-Helmholtz instability in cavitating flow in Venturi microchannels was discovered. Its importance is not negligible, as it destabilizes the shear layer and promotes instabilities and turbulent eddies formation in the vapor region, having low density and momentum. In the present paper, we give a very brief summary of the experimental findings and in the following, we use a computational fluid dynamics (CFD) study to peek deeper into the onset of the Kelvin-Helmholtz instability and its effect on the dynamics of the cavitation cloud shedding. Finally, it is shown that Kelvin-Helmholtz instability is beside the re-entrant jet and the condensation shock wave the third mechanism of cavitation cloud shedding in Venturi microchannels. The shedding process is quasi-periodic. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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18 pages, 7008 KiB

Open AccessArticle

Analysis of Bulb Turbine Hydrofoil Cavitation

by Andrej Podnar, Marko Hočevar, Lovrenc Novak and Matevž Dular

Appl. Sci. 2021, 11(6), 2639; https://doi.org/10.3390/app11062639 - 16 Mar 2021

Cited by 9 | Viewed by 3429

Abstract

The influence of a bulb runner blade hydrofoil shape on flow characteristics around the blade was studied. Experimental work was performed on a bulb turbine measuring station and a single hydrofoil in a cavitating tunnel. In the cavitation tunnel, flow visualization was performed [...] Read more.

The influence of a bulb runner blade hydrofoil shape on flow characteristics around the blade was studied. Experimental work was performed on a bulb turbine measuring station and a single hydrofoil in a cavitating tunnel. In the cavitation tunnel, flow visualization was performed on the hydrofoil’s suction side. Cavitation structures were observed for several cavitation numbers. Cavitation was less intense on the modified hydrofoil than on the original hydrofoil, delaying the cavitation onset by several tenths in cavitation number. The results of the visualization in the cavitation tunnel show that modifying the existing hydrofoil design parameters played a key role in reducing the cavitation inception and development, as well as the size of the cavitation structures. A regression model was produced for cavitation cloud length. The results of the regression model show that cavitation length is dependent on Reynolds’s number and the cavitation number. The coefficients of determination for both the existing and modified hydrofoils were reasonably high, with R² values above 0.95. The results of the cavitation length regression model also confirm that the modified hydrofoil exhibits improved the cavitation properties. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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14 pages, 2249 KiB

Open AccessArticle

Cavity Formation during Asymmetric Water Entry of Rigid Bodies

by Riccardo Panciroli and Giangiacomo Minak

Appl. Sci. 2021, 11(5), 2029; https://0-doi-org.brum.beds.ac.uk/10.3390/app11052029 - 25 Feb 2021

Cited by 5 | Viewed by 1549

Abstract

This work numerically evaluates the role of advancing velocity on the water entry of rigid wedges, highlighting its influence on the development of underpressure at the fluid–structure interface, which can eventually lead to fluid detachment or cavity formation, depending on the geometry. A [...] Read more.

This work numerically evaluates the role of advancing velocity on the water entry of rigid wedges, highlighting its influence on the development of underpressure at the fluid–structure interface, which can eventually lead to fluid detachment or cavity formation, depending on the geometry. A coupled FEM–SPH numerical model is implemented within LS-DYNA, and three types of asymmetric impacts are treated: (I) symmetric wedges with horizontal velocity component, (II) asymmetric wedges with a pure vertical velocity component, and (III) asymmetric wedges with a horizontal velocity component. Particular attention is given to the evolution of the pressure at the fluid–structure interface and the onset of fluid detachment at the wedge tip and their effect on the rigid body dynamics. Results concerning the tilting moment generated during the water entry are presented, varying entry depth, asymmetry, and entry velocity. The presented results are important for the evaluation of the stability of the body during asymmetric slamming events. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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13 pages, 4972 KiB

Open AccessArticle

Investigation of the Time Resolution Set Up Used to Compute the Full Load Vortex Rope in a Francis Turbine

by Jean Decaix, Andres Müller, Arthur Favrel, François Avellan and Cécile Münch-Alligné

Appl. Sci. 2021, 11(3), 1168; https://0-doi-org.brum.beds.ac.uk/10.3390/app11031168 - 27 Jan 2021

Cited by 3 | Viewed by 1942

Abstract

The flow in a Francis turbine at full load is characterised by the development of an axial vortex rope in the draft tube. The vortex rope often promotes cavitation if the turbine is operated at a sufficiently low Thoma number. Furthermore, the vortex [...] Read more.

The flow in a Francis turbine at full load is characterised by the development of an axial vortex rope in the draft tube. The vortex rope often promotes cavitation if the turbine is operated at a sufficiently low Thoma number. Furthermore, the vortex rope can evolve from a stable to an unstable behaviour. For CFD, such a flow is a challenge since it requires solving an unsteady cavitating flow including rotor/stator interfaces. Usually, the numerical investigations focus on the cavitation model or the turbulence model. In the present works, attention is paid to the strategy used for the time integration. The vortex rope considered is an unstable cavitating one that develops downstream the runner. The vortex rope shows a periodic behaviour characterized by the development of the vortex rope followed by a strong collapse leading to the shedding of bubbles from the runner area. Three unsteady RANS simulations are performed using the ANSYS CFX 17.2 software. The turbulence and cavitation models are, respectively, the SST and Zwart models. Regarding the time integration, a second order backward scheme is used excepted for the transport equation for the liquid volume fraction, for which a first order backward scheme is used. The simulations differ by the time step and the number of internal loops per time step. One simulation is carried out with a time step equal to one degree of revolution per time step and five internal loops. A second simulation used the same time step but 15 internal loops. The third simulations used three internal loops and an adaptive time step computed based on a maximum CFL lower than 2. The results show an influence of the time integration strategy on the cavitation volume time history both in the runner and in the draft tube with a risk of divergence of the solution if a standard set up is used. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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15 pages, 6964 KiB

Open AccessFeature PaperArticle

Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation

by Florent Ravelet, Amélie Danlos, Farid Bakir, Kilian Croci, Sofiane Khelladi and Christophe Sarraf

Appl. Sci. 2020, 10(20), 7350; https://0-doi-org.brum.beds.ac.uk/10.3390/app10207350 - 20 Oct 2020

Cited by 2 | Viewed by 2055

Abstract

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from [...] Read more.

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from

R e = 800

to

R e = 2000

. Special care has been taken to extract the maximum amount of dissolved air. At the lowest Reynolds numbers the cavities are steady and grow regularly with decreasing ambient pressure. A transition takes place between

R e = 1200

and

R e = 1400

for which different dynamical regimes are identified: a steady regime for tiny cavities, a periodical regime of attached cavity shrinking characterized by a very small Strouhal number for cavities of intermediate sizes, the bursting of aperiodical cavitational vortices which further lower the pressure, and finally steady super-cavitating sheets observed at the lowest of pressures. The growth of the cavity with the decrease of the cavitation number also becomes steeper. This scenario is then well established and similar for Reynolds numbers between

R e = 1400

and

R e = 2000

. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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23 pages, 32382 KiB

Open AccessArticle

Compressible Two-Phase Viscous Flow Investigations of Cavitation Dynamics for the ITTC Standard Cavitator

by Ville M. Viitanen, Tuomas Sipilä, Antonio Sánchez-Caja and Timo Siikonen

Appl. Sci. 2020, 10(19), 6985; https://0-doi-org.brum.beds.ac.uk/10.3390/app10196985 - 7 Oct 2020

Cited by 9 | Viewed by 2456

Abstract

In this paper, the ITTC Standard Cavitator is numerically investigated in a cavitation tunnel. Simulations at different cavitation numbers are compared against experiments conducted in the cavitation tunnel of SVA Potsdam. The focus is placed on the numerical prediction of sheet-cavitation dynamics and [...] Read more.

In this paper, the ITTC Standard Cavitator is numerically investigated in a cavitation tunnel. Simulations at different cavitation numbers are compared against experiments conducted in the cavitation tunnel of SVA Potsdam. The focus is placed on the numerical prediction of sheet-cavitation dynamics and the analysis of transient phenomena. A compressible two-phase flow model is used for the flow solution, and two turbulence closures are employed: a two-equation unsteady RANS model, and a hybrid RANS/LES model. A homogeneous mixture model is used for the two phases. Detailed analysis of the cavitation shedding mechanism confirms that the dynamics of the sheet cavitation are dictated by the re-entrant jet. The break-off cycle is relatively periodic in both investigated cases with approximately constant shedding frequency. The CFD predicted sheet-cavitation shedding frequencies can be observed also in the acoustic measurements. The Strouhal numbers lie within the usual ranges reported in the literature for sheet-cavitation shedding. We furthermore demonstrate that the vortical flow structures can in certain cases develop striking cavitating toroidal vortices, as well as pressure wave fronts associated with a cavity cloud collapse event. To our knowledge, our numerical analyses are the first reported for the ITTC standard cavitator. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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13 pages, 4844 KiB

Open AccessArticle

Numerical Investigation of Methodologies for Cavitation Suppression Inside Globe Valves

by Jun-ye Li, Zhi-xin Gao, Hui Wu and Zhi-jiang Jin

Appl. Sci. 2020, 10(16), 5541; https://0-doi-org.brum.beds.ac.uk/10.3390/app10165541 - 11 Aug 2020

Cited by 18 | Viewed by 3669

Abstract

Cavitation inside globe valves, which is a common phenomenon if there is a high-pressure drop, is numerically investigated in this study. Firstly, the cavitation phenomenon in globe valves with a different number of cages is compared. When there is no valve cage, cavitation [...] Read more.

Cavitation inside globe valves, which is a common phenomenon if there is a high-pressure drop, is numerically investigated in this study. Firstly, the cavitation phenomenon in globe valves with a different number of cages is compared. When there is no valve cage, cavitation mainly appears at the valve seat, the bottom of the valve core, and the downstream pipelines. By installing a valve cage, cavitation bubbles can be restricted around the valve cage protecting the valve body from being damaged. Secondly, the effects of the outlet pressure, the working temperature, and the installation angle of two valve cages in a two-cage globe valve are studied to find out the best method to suppress cavitation, and cavitation number is utilized to evaluate cavitation intensity. Results show that cavitation intensity inside globe valves can be reduced by increasing the valve outlet pressure, decreasing the working temperature, or increasing the installation angle. Results suggest that increasing the outlet pressure is the most efficient way to suppress cavitation intensity in a globe valve, and the working temperature has a minimal effect on cavitation intensity. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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Review

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27 pages, 14156 KiB

Open AccessFeature PaperReview

Cavitating Jet: A Review

by Hitoshi Soyama

Appl. Sci. 2020, 10(20), 7280; https://0-doi-org.brum.beds.ac.uk/10.3390/app10207280 - 17 Oct 2020

Cited by 49 | Viewed by 8989

Abstract

When a high-speed water jet is injected into water through a nozzle, cavitation is generated in the nozzle and/or shear layer around the jet. A jet with cavitation is called a “cavitating jet”. When the cavitating jet is injected into a surface, cavitation [...] Read more.

When a high-speed water jet is injected into water through a nozzle, cavitation is generated in the nozzle and/or shear layer around the jet. A jet with cavitation is called a “cavitating jet”. When the cavitating jet is injected into a surface, cavitation is collapsed, producing impacts. Although cavitation impacts are harmful to hydraulic machinery, impacts produced by cavitating jets are utilized for cleaning, drilling and cavitation peening, which is a mechanical surface treatment to improve the fatigue strength of metallic materials in the same way as shot peening. When a cavitating jet is optimized, the peening intensity of the cavitating jet is larger than that of water jet peening, in which water column impacts are used. In order to optimize the cavitating jet, an understanding of the instabilities of the cavitating jet is required. In the present review, the unsteady behavior of vortex cavitation is visualized, and key parameters such as injection pressure, cavitation number and sound velocity in cavitating flow field are discussed, then the estimation methods of the aggressive intensity of the jet are summarized. Full article

(This article belongs to the Special Issue New Advances of Cavitation Instabilities)

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