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35 pages, 23170 KiB

Open AccessArticle

Multidimensional Vibro-Acoustical Diagnostics of Cavitation: Theory and Illustration on a Kaplan Turbine

by Branko Bajic, Simon Weissenberger and Markus Keller

Fluids 2022, 7(6), 193; https://doi.org/10.3390/fluids7060193 - 2 Jun 2022

Cited by 1 | Viewed by 1882

Korto’s multidimensional method for vibro-acoustical diagnostics and monitoring of turbine cavitation is based on a high number of spatially distributed sensors and the signal and data processing that systematically utilises three data dimensions: spatial, temporal, and operational. The method delivers unbiased data on [...] Read more.

Korto’s multidimensional method for vibro-acoustical diagnostics and monitoring of turbine cavitation is based on a high number of spatially distributed sensors and the signal and data processing that systematically utilises three data dimensions: spatial, temporal, and operational. The method delivers unbiased data on cavitation intensity and rich diagnostical data on cavitation mechanisms. It is applicable on Kaplan, Francis, bulb, and reversible pump turbines, as well as pumps. In this paper, the theory of the method is introduced, and its application is illustrated on a prototype and three models of a Kaplan turbine. In the considered case, two distinct cavitation mechanisms responsible for the two erosion patches found in an overhaul are vibro-acoustically identified, quantified, and analysed. The cavitation quality of the models is compared. Cavitation as a source of vibration is discussed. Full article

(This article belongs to the Special Issue Cavitating Flows)

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23 pages, 6232 KiB

Open AccessArticle

Numerical Investigation of Flow and Heat Transfer over a Shallow Cavity: Effect of Cavity Height Ratio

by Salem S. Abdel Aziz and Abdel-Halim Saber Salem Said

Fluids 2021, 6(7), 244; https://doi.org/10.3390/fluids6070244 - 3 Jul 2021

Cited by 7 | Viewed by 2827

Abstract

Flow over shallow cavities is used to model the flow field and heat transfer in a solar collector and a variety of engineering applications. Many studies have been conducted to demonstrate the effect of cavity aspect ratio (AR), but very few studies have [...] Read more.

Flow over shallow cavities is used to model the flow field and heat transfer in a solar collector and a variety of engineering applications. Many studies have been conducted to demonstrate the effect of cavity aspect ratio (AR), but very few studies have been carried out to investigate the effect of cavity height ratio (HR) on shallow cavity flow behavior. In this paper, flow field structure and heat transfer within the 3-D shallow cavity are obtained numerically for two height ratio categories: HR = 0.0, 0.25, 0.5, 0.75, and 1.0 and HR = 1.25, 1.5, 1.75, 2.0, 2.25, and 2.5. The governing equations, continuity, momentum, and energy are solved numerically and using the standard (K-ε) turbulence model. ANSYS FLUENT 14 CFD code is used to perform the numerical simulation based on the finite volume method. In this study, the cavity aspect ratio, AR = 5.0, and Reynolds number, Re = 3 × 10⁵, parameters are fixed. The cavity’s bottom wall is heated with a constant and uniform heat flux (q = 740 W/m²), while the other walls are assumed to be adiabatic. For the current Reynolds number and cavity geometry, a single vortex structure (recirculation region) is formed and occupies most of the cavity volume. The shape and location of the vortex differ according to the height ratio. A reverse velocity profile across the recirculation region near the cavity’s bottom wall is shown at all cavity height ratios. Streamlines and temperature contours on the plane of symmetry and cavity bottom wall are displayed. Local static pressure coefficient and Nusselt number profiles are obtained along the cavity’s bottom wall, and the average Nusselt number for various height ratios is established. The cavity height ratio (HR) is an important geometry parameter in shallow cavities, and it plays a significant role in the cavity flow behavior and heat transfer characteristics. The results indicate interesting flow dynamics based on height ratio (HR), which includes a minimal value in average Nusselt number for HR ≈ 1.75 and spatial transitions in local Nusselt number distribution along the bottom wall for different HRs. Full article

(This article belongs to the Special Issue Cavitating Flows)

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14 pages, 1479 KiB

Open AccessArticle

An Analysis of Acoustic Cavitation Thresholds of Water Based on the Incubation Time Criterion Approach

by Ivan Smirnov and Natalia Mikhailova

Fluids 2021, 6(4), 134; https://doi.org/10.3390/fluids6040134 - 1 Apr 2021

Cited by 3 | Viewed by 2734

Abstract

Researchers are still working on the development of models that facilitate the accurate estimation of acoustic cavitation threshold. In this paper, we have analyzed the possibility of using the incubation time criterion to calculate the threshold of the onset of acoustic cavitation depending [...] Read more.

Researchers are still working on the development of models that facilitate the accurate estimation of acoustic cavitation threshold. In this paper, we have analyzed the possibility of using the incubation time criterion to calculate the threshold of the onset of acoustic cavitation depending on the ultrasound frequency, hydrostatic pressure, and temperature of a liquid. This criterion has been successfully used by earlier studies to calculate the dynamic strength of solids and has recently been proposed in an adapted version for calculating the cavitation threshold. The analysis is carried out for various experimental data for water presented in the literature. Although the criterion assumes the use of macroparameters of a liquid, we also considered the possibility of taking into account the size of cavitation nuclei and its influence on the calculation result. We compared the results of cavitation threshold calculations done using the incubation time criterion of cavitation and the classical nucleation theory. Our results showed that the incubation time criterion more qualitatively models the results of experiments using only three parameters of the liquid. We then discussed a possible relationship between the parameters of the two approaches. The results of our study showed that the criterion under consideration has a good potential and can be conveniently used for applications where there are special requirements for ultrasound parameters, maximum negative pressure, and liquid temperature. Full article

(This article belongs to the Special Issue Cavitating Flows)

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14 pages, 5129 KiB

Open AccessArticle

Erosion Mechanism of a Cavitating Jet on Groove Roughness

by Nobuyuki Fujisawa, Takayuki Yamagata, Ryotaro Seki and Motofumi Ohki

Fluids 2021, 6(1), 6; https://doi.org/10.3390/fluids6010006 - 26 Dec 2020

Viewed by 1986

Abstract

The erosion behavior of a cavitating jet on groove roughness was investigated experimentally using mass-loss characteristics, scanning electron microscopy (SEM) observation, time-resolved shadowgraph, and schlieren flow visualizations. The wall morphology of the cavitating-jet erosion on the groove roughness indicated an increased mass loss, [...] Read more.

The erosion behavior of a cavitating jet on groove roughness was investigated experimentally using mass-loss characteristics, scanning electron microscopy (SEM) observation, time-resolved shadowgraph, and schlieren flow visualizations. The wall morphology of the cavitating-jet erosion on the groove roughness indicated an increased mass loss, which was highly increased along the groove rather than across the groove. Furthermore, increased erosion pits were observed on the groove bottom along the grooves. The shadowgraph imaging of the cavitating jet on the rough wall showed noncircular cavitation bubble distributions along and across the grooves, which corresponds to the increased number of cavitation bubbles along the grooves and the decreased number of bubbles across the grooves. This result is consistent with the erosion morphology of the groove roughness. Schlieren imaging indicated that the frequency and intensity fluctuation of the shockwave formation did not change significantly on the groove roughness along and across the grooves. The findings in the study show that the increased erosion mechanism on groove roughness is caused by the increased number of impulsive forces and the shockwave focusing effect on the groove bottom. Full article

(This article belongs to the Special Issue Cavitating Flows)

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18 pages, 22304 KiB

Open AccessArticle

Impact of Leading Edge Roughness in Cavitation Simulations around a Twisted Foil

by Abolfazl Asnaghi and Rickard E. Bensow

Fluids 2020, 5(4), 243; https://doi.org/10.3390/fluids5040243 - 14 Dec 2020

Cited by 7 | Viewed by 1843

Abstract

The simulation of fully turbulent, three-dimensional, cavitating flow over Delft twisted foil is conducted by an implicit large eddy simulation (LES) approach in both smooth and tripped conditions, the latter by including leading-edge roughness. The analysis investigates the importance of representing the roughness [...] Read more.

The simulation of fully turbulent, three-dimensional, cavitating flow over Delft twisted foil is conducted by an implicit large eddy simulation (LES) approach in both smooth and tripped conditions, the latter by including leading-edge roughness. The analysis investigates the importance of representing the roughness elements on the flow structures in the cavitation prediction. The results include detailed comparisons of cavitation pattern, vorticity distribution, and force predictions with the experimental measurements. It is noted that the presence of roughness generates very small cavitating vortical structures which interact with the main sheet cavity developing over the foil to later form a cloud cavity. Very similar to the experimental observation, these interactions create a streaky sheet cavity interface which cannot be captured in the smooth condition, influencing both the richness of structures in the detached cloudy cavitation as well as the extent and transport of vapour. It is further found to have a direct impact on the pressure distribution, especially in the mid-chord region where the shed cloud cavity collapses. Full article

(This article belongs to the Special Issue Cavitating Flows)

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18 pages, 7082 KiB

Open AccessArticle

Numerical Investigation of the Cavitation Effects on the Vortex Shedding from a Hydrofoil with Blunt Trailing Edge

by Jian Chen, Linlin Geng and Xavier Escaler

Fluids 2020, 5(4), 218; https://doi.org/10.3390/fluids5040218 - 21 Nov 2020

Cited by 3 | Viewed by 2446

Abstract

Vortex cavitation can appear in the wake flow of hydrofoils, inducing unwanted consequences such as vibrations or unstable behaviors in hydraulic machinery and systems. To investigate the cavitation effects on hydrofoil vortex shedding, a numerical investigation of the flow around a 2D NACA0009 [...] Read more.

Vortex cavitation can appear in the wake flow of hydrofoils, inducing unwanted consequences such as vibrations or unstable behaviors in hydraulic machinery and systems. To investigate the cavitation effects on hydrofoil vortex shedding, a numerical investigation of the flow around a 2D NACA0009 with a blunt trailing edge at free caviation conditions and at two degrees of cavitation developments has been carried out by means of the Zwart cavitation model and the LES WALE turbulence model which permits predicting the laminar to turbulent transition of the boundary layers. To analyze the dynamic behavior of the vortex shedding process and the coherent structures, two identification methods based on the Eulerian and Lagrangian reference frames have been applied to the simulated unsteady flow field. It is found that the cavitation occurrence in the wake significantly changes the main vortex shedding characteristics including the morphology of the vortices, the vortex formation length, the effective height of the near wake flow and the shedding frequency. The numerical results predict that the circular shape of the vortices changes to an elliptical one and that the vortex shedding frequency is significantly increased under cavitation conditions. The main reason for the frequency increase seems to be the reduction in the transverse separation between the upper and lower rows of vortices induced by the increase in the vortex formation length. Full article

(This article belongs to the Special Issue Cavitating Flows)

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21 pages, 8305 KiB

Open AccessArticle

Bubble Dynamics in a Narrow Gap Flow under the Influence of Pressure Gradient and Shear Flow

by Peter Reinke, Jan Ahlrichs, Tom Beckmann and Marcus Schmidt

Fluids 2020, 5(4), 208; https://doi.org/10.3390/fluids5040208 - 16 Nov 2020

Cited by 3 | Viewed by 2481

Abstract

The volume-of-flow method combined with the Rayleigh–Plesset equation is well established for the computation of cavitation, i.e., the generation and transportation of vapor bubbles inside a liquid flow resulting in cloud, sheet or streamline cavitation. There are, however, limitations, if this method is [...] Read more.

The volume-of-flow method combined with the Rayleigh–Plesset equation is well established for the computation of cavitation, i.e., the generation and transportation of vapor bubbles inside a liquid flow resulting in cloud, sheet or streamline cavitation. There are, however, limitations, if this method is applied to a restricted flow between two adjacent walls and the bubbles’ size is of the same magnitude as that of the clearance between the walls. This work presents experimental and numerical results of the bubble generation and its transportation in a Couette-type flow under the influence of shear and a strong pressure gradient which are typical for journal bearings or hydraulic seals. Under the impact of variations of the film thickness, the VoF method produces reliable results if bubble diameters are less than half the clearance between the walls. For larger bubbles, the wall contact becomes significant and the bubbles adopt an elliptical shape forced by the shear flow and under the influence of a strong pressure gradient. Moreover, transient changes in the pressure result in transient cavitation, which is captured by high-speed imaging providing material to evaluate transient, three-dimensional computations of a two-phase flow. Full article

(This article belongs to the Special Issue Cavitating Flows)

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13 pages, 7730 KiB

Open AccessArticle

An Experimental Investigation of Coherent Structures and Induced Noise Characteristics of the Partial Cavitating Flow on a Two-Dimensional Hydrofoil

by Byoung-Kwon Ahn, So-Won Jeong, Cheol-Soo Park and Gun-Do Kim

Fluids 2020, 5(4), 198; https://doi.org/10.3390/fluids5040198 - 3 Nov 2020

Cited by 6 | Viewed by 1898

Abstract

In many practical submerged objects, various types of cavitation such as bubble, sheet, and cloud cavitation occur according to flow conditions. In spite of numerous theoretical, numerical, and experimental studies, there are still many problems to be solved such as induced noise and [...] Read more.

In many practical submerged objects, various types of cavitation such as bubble, sheet, and cloud cavitation occur according to flow conditions. In spite of numerous theoretical, numerical, and experimental studies, there are still many problems to be solved such as induced noise and damage potential due to cavitation. In this paper, an experimental investigation on coherent structures and induced noise characteristics of partial cavitation on a two-dimensional hydrofoil is presented. Experiments that focused on the dynamics of cavitation clouds were conducted in a cavitation tunnel. Using high-speed visualization, the series process consisting of inception, growth, and desinence of the partial cavity was investigated. The noise generated during the process was also measured, and the correlation with the cavity pattern was examined. The results show that the periodic behavior of cavitation clouds is directly reflected in the noise characteristics. In addition, the visualization of coherent structures within the sheet and cloud cavity provides a qualitative understanding of hairpin vortices and their packets, which play a dominant role in turbulent cavitating flows. Full article

(This article belongs to the Special Issue Cavitating Flows)

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20 pages, 1270 KiB

Open AccessArticle

The Influence of Inflow Swirl on Cavitating and Mixing Processes in a Venturi Tube

by Hongbo Shi and Petr Nikrityuk

Fluids 2020, 5(4), 170; https://doi.org/10.3390/fluids5040170 - 30 Sep 2020

Cited by 5 | Viewed by 4787

Abstract

A study of the mixing flows (Schmidt number = 10

^{3}

) in a cavitating Venturi tube that feature linear and swirling flows is presented in this paper. The Large Eddy Simulation (LES) turbulence model, the Schnerr–Sauer cavitation model, and the mixture multiphase [...] Read more.

A study of the mixing flows (Schmidt number = 10

^{3}

) in a cavitating Venturi tube that feature linear and swirling flows is presented in this paper. The Large Eddy Simulation (LES) turbulence model, the Schnerr–Sauer cavitation model, and the mixture multiphase model, as implemented in the commercial CFD ANSYS FLUENT 16.2, were employed. The main emphasis is spending on the influence of different inlet swirling ratios on the generation of cavitation and mixing behaviors in a Venturi tube. Four different inflow regimes were investigated for the Reynolds number Re = 19,044, 19,250, 19,622, 21,276: zero swirl, 15% swirl, 25% swirl and 50% swirl velocity relative to the transverse inflow velocity, respectively. The computed velocity and pressure profiles were shown in good agreement with the experiment data from the literature. The predicted results indicate that the imposed swirl flow moves the cavitation bubbles away from throat surfaces toward the throat axis. The rapid mixing between two volumetric components is promoted in the divergent section when the intense swirl is introduced. Additionally, the increase in the swirl ratio from 0.15 to 0.5 leads to a linear increase in the static pressure drop and a nonlinear increase in the vapor production. The reduction in the fluid viscosity ratio from

\frac{μ_{2}}{μ_{1}} = 10

to

\frac{μ_{2}}{μ_{1}} = 1

generates a high cavitation intensity in the throat of the Venturi tube. However, the changes in the pressure drop and vapor volume fraction are significantly small of pure water flow. Full article

(This article belongs to the Special Issue Cavitating Flows)

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24 pages, 4913 KiB

Open AccessEditor’s ChoiceArticle

Assessment of Cavitation Models for Compressible Flows Inside a Nozzle

by Aishvarya Kumar, Ali Ghobadian and Jamshid M. Nouri

Fluids 2020, 5(3), 134; https://doi.org/10.3390/fluids5030134 - 13 Aug 2020

Cited by 28 | Viewed by 5006

Abstract

This study assessed two cavitation models for compressible cavitating flows within a single hole nozzle. The models evaluated were SS (Schnerr and Sauer) and ZGB (Zwart-Gerber-Belamri) using realizable k-epsilon turbulent model, which was found to be the most appropriate model to use for [...] Read more.

This study assessed two cavitation models for compressible cavitating flows within a single hole nozzle. The models evaluated were SS (Schnerr and Sauer) and ZGB (Zwart-Gerber-Belamri) using realizable k-epsilon turbulent model, which was found to be the most appropriate model to use for this flow. The liquid compressibility was modeled using the Tait equation, and the vapor compressibility was modeled using the ideal gas law. Compressible flow simulation results showed that the SS model failed to capture the flow physics with a weak agreement with experimental data, while the ZGB model predicted the flow much better. Modeling vapor compressibility improved the distribution of the cavitating vapor across the nozzle with an increase in vapor volume compared to that of the incompressible assumption, particularly in the core region which resulted in a much better quantitative and qualitative agreement with the experimental data. The results also showed the prediction of a normal shockwave downstream of the cavitation region where the local flow transforms from supersonic to subsonic because of an increase in the local pressure. Full article

(This article belongs to the Special Issue Cavitating Flows)

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