Scale Effects on the Cavitation Development in Fluid Machinery

2011 ◽  
Author(s):  
Weiping Yu ◽  
Xianwu Luo ◽  
Yao Zhang ◽  
Bin Ji ◽  
Hongyuan Xu
Keyword(s):  
Turbulence Model ◽  
High Speed ◽  
Scale Effects ◽  
Design Procedure ◽  
Leading Edge ◽  
Characteristic Size ◽  
Cavitation Model ◽  
Fluid Machinery ◽  
Cloud Cavitation ◽  
Sheet Cavitation

The prediction of cavitation in a design procedure is very important for fluid machinery. However, the behaviors of cavitation development in the flow passage are believed to be much different due to scale effects, when the characteristic size varies greatly for fluid machines such as pumps, turbines and propellers. In order to understand the differences in cavitation development, the evolution of cavity pattern in two hydro foils were recorded by high-speed video apparatus. Both foils have the same section profile, and their chord lengths are 70mm and 14mm respectively. For comparison, the cavitating flows around two foils were numerically simulated using a cavitation model based on Rayleigh-Plesset equation and SST k-ω turbulence model. The experiments depicted that for both hydro foils, there was attached sheet cavitation near the leading edge, which separated from the rear part of the cavity and collapsed near the foil trailing edge. There was clear cloud cavitation in the case of the mini foil. The results also indicated that the numerical simulation captured the cavitation evolution for the ordinary foil quite well compared with the experiments, but could hardly predict the cloud cavitation for the mini foil. Thus, it is believed that both the cavitation model and the turbulence model should be carefully treated for the scale effect on cavitation development in fluid machinery.

Investigation of Unsteady Sheet Cavitation and Cloud Cavitation Mechanisms

1999 ◽  
Vol 121 (2) ◽  
pp. 289-296 ◽  
Author(s):  
T. M. Pham ◽  
F. Larrarte ◽  
D. H. Fruman
Keyword(s):  
High Speed ◽  
Leading Edge ◽  
Pressure Measurements ◽  
Foil Surface ◽  
Mean Velocity ◽  
Cloud Cavitation ◽  
Fluctuating Pressure ◽  
Sheet Cavitation ◽  
Cavitation Instability ◽  
Unsteady Characteristics

Sheet cavitation on a foil section and, in particular, its unsteady characteristics leading to cloud cavitation, were experimentally investigated using high-speed visualizations and fluctuating pressure measurements. Two sources of sheet cavitation instability were evidenced, the re-entrant jet and small interfacial waves. The dynamics of the re-entrant jet was studied using surface electrical probes. Its mean velocity at different distances from the leading edge was determined and its role in promoting the unsteadiness of the sheet cavitation and generating large cloud shedding was demonstrated. The effect of gravity on the dynamics of the re-entrant jet and the development of interfacial perturbations were examined and interpreted. Finally, control of cloud cavitation using various means, such as positioning a tiny obstacle (barrier) on the foil surface or performing air injection through a slit situated in the vicinity of the leading edge, was investigated. It was shown that these were very effective methods for decreasing the amplitude of the instabilities and even eliminating them.

scholarly journals Detached eddy simulation of unsteady cavitation and pressure fluctuation around 3-D NACA66 hydrofoil

2015 ◽  
Vol 19 (4) ◽  
pp. 1231-1234 ◽  
Author(s):  
De-Sheng Zhang ◽  
Hai-Yu Wang ◽  
Lin-Lin Geng ◽  
Wei-Dong Shi
Keyword(s):  
Turbulence Model ◽  
Pressure Fluctuation ◽  
Cavitating Flow ◽  
Numerical Results ◽  
Detached Eddy Simulation ◽  
Cavitation Model ◽  
Cloud Cavitation ◽  
Sheet Cavitation ◽  
Eddy Simulation ◽  
Unsteady Cavitating Flow

The unsteady cavitating flow and pressure fluctuation around the 3-D NACA66 hydrofoil were simulated and validated based on detached eddy simulation turbulence model and a homogeneous cavitation model. Numerical results show that detached eddy simulation can predict the evolution of cavity inception, sheet cavitation growth, cloud cavitation shedding, and breakup, as well as the pressure fluctuation on the surface of hydrofoil. The sheet cavitation growth, detachment, cloud cavitation shedding are responsible for the features of the pressure fluctuation.

A Detailed Observation of Hydrofoil Cavitation and a Proposal for Improving Cavitation Model

2010 ◽  
Author(s):  
Motohiko Nohmi ◽  
Naoya Ochiai ◽  
Yuka Iga ◽  
Toshiaki Ikohagi
Keyword(s):  
Free Surface ◽  
High Speed ◽  
Bubble Dynamics ◽  
Leading Edge ◽  
Free Surface Flow ◽  
Surface Flow ◽  
Experimental Results ◽  
Cavitation Model ◽  
Suction Surface ◽  
Cloud Cavitation

Cavitation of a hydrofoil is observed in detail by using a high speed video camera. A paint removal test is also carried out in order to evaluate cavitation aggressiveness for erosion. 2D hydrofoil profile is Clark Y 11.7% and its angle of attack is seven degrees. Cavitation number is σ = 1.08. The experimental results are compared with cavitation CFD. Numerous features of unsteady cavitation are observed such as cyclic fluctuation of the sheet cavity, existence of the glassy cones on a sheet cavity, generation of the cloud cavitation from the sheet cavity and the isolated bubbles traveling over the suction surface of the blade. The isolated traveling bubbles and their collapses are thought to be one of the main causes of the severe paint removals. The isolated traveling bubbles are derived from the flowing cavitation nucleus or from abrupt onset at the leading edge of the blade. For computing these complicated phenomena, combination of grid scale bubbles (GSB) and sub grid scale bubble model (SGSB) are proposed. GSB shall be computed by using the computational scheme for the free surface with phase change model. SGSB can be computed with conventional cavitation model. The breakup of GSB generates SGSB, and the coalescence of SGSB makes GSB. Upper limit of void fraction of SGSB is estimated in the range of five or ten percent from the simple speculation of the structure of packed spheres. The two types of cavitation bubble inception model are also discussed based on the generation of the isolated bubbles observed in the experiments. To verify the proposed concepts of cavitation model, a traveling air bubble over a hydrofoil is computed by using the free surface flow scheme of Volume of Fluid (VOF) approach. Cavitation on the hydrofoil is also computed by VOF approach with boiling model concerning the heat transfer. Both the computed results show qualitatively similar characteristics of the bubble dynamics to those in experimental results.

scholarly journals Experimental Study on the Relationship Between Cavitation and Lift Fluctuations of S-Shaped Hydrofoil

2021 ◽  
Vol 9 ◽  
Author(s):  
Haiyu Liu ◽  
Pengcheng Lin ◽  
Fangping Tang ◽  
Ye Chen ◽  
Wenpeng Zhang ◽  
...  
Keyword(s):  
High Speed ◽  
Developmental Process ◽  
Angle Of Attack ◽  
Cavitation Number ◽  
High Speed Photography ◽  
Cloud Cavitation ◽  
Cavitation Inception ◽  
Sheet Cavitation ◽  
Hydraulic Machinery ◽  
Stall Angle

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.

Effects of Diffuser Length on Cloud Cavitation in an Axisymmetrical Convergent-Divergent Nozzle

2015 ◽  
Author(s):  
Keiichi Sato ◽  
Naoya Takahashi ◽  
Yasuhiro Sugimoto
Keyword(s):  
High Speed ◽  
Pressure Effect ◽  
Water Jet ◽  
Video Observation ◽  
Fluid Machinery ◽  
Cloud Cavitation ◽  
Frame Difference ◽  
Upstream Pressure ◽  
Cavitating Hydrofoil ◽  
Unsteady Behavior

Unsteady behavior of periodic cloud cavitation is typically observed in the field of fluid machinery under a high speed liquid flow such as a cavitating hydrofoil as well as cavitating water jet. The instability of cloud cavitation remains to be completely solved though it has been confirmed that there are two instabilities which is an intrinsic instability of cavitation and a system instability. Sato, et al. have found through previous investigations that the pressure wave at the collapse of shedding clouds can make a trigger to cause a reentrant motion. In the present study, the authors focus on a cavitating water jet to investigate the cavitation aspects in an axisymmetrical convergent-divergent nozzle and examine an unsteady behavior of cloud cavitation through high speed video observation and image analysis based on the frame difference method. Especially, the authors study the effect of nozzle divergent part (diffuser) as well as the upstream pressure effect on cloud cavitation in the nozzle. As a result the authors have found that there are two kinds in the shedding pattern and the reentrant motion pattern for cloud cavitation depending on the nozzle diffuser length.

scholarly journals Investigation of Cavitation Noise in Cavitating Flows around an NACA0015 Hydrofoil

2019 ◽  
Vol 9 (18) ◽  
pp. 3736
Author(s):  
An Yu ◽  
Xincheng Wang ◽  
Zhipeng Zou ◽  
Qinghong Tang ◽  
Huixiang Chen ◽  
...  
Keyword(s):  
Spectral Density ◽  
Power Spectral Density ◽  
Leading Edge ◽  
Acoustic Power ◽  
Cavitation Number ◽  
Acoustic Analogy ◽  
Cavitation Noise ◽  
Cloud Cavitation ◽  
Sheet Cavitation ◽  
Power Spectral

To provide theoretical basis for cavitation noise control, the cavitation evolution around a hydrofoil and its induced noise were numerically investigated. A modified turbulence model and Zwart cavitation model were employed to calculate the flow field and predict the cavitation phenomenon accurately. Then, the acoustic analogy method based on the Ffowcs Williams-Hawking (FW-H) equation was applied to analyze the cavitation-induced noise. Seven cavitation numbers were selected for analysis. Acoustic power spectral density (PSD) and acoustic pressure were investigated to establish the relationship between cavitation number and their acoustic characteristics. It was indicated that as cavitation number decreases, cavitation cycle length gets shorter and the magnitude of acoustic power spectral density increases dramatically. One peak value of acoustic power spectral density induced by the extending and retracting of leading-edge cavitation can be obtained under sheet cavitation conditions, while under cloud cavitation, two peak values of acoustic power spectral density can be obtained and are induced by superposition from leading-edge cavitation and trailing vortex.

Study on Vortex Generators for Control of Attached Cavitation

2017 ◽  
Author(s):  
Bangxiang Che ◽  
Dazhuan Wu
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Main Flow ◽  
Vortex Generators ◽  
Streamwise Vortices ◽  
Fluid Machinery ◽  
Cavitation Inception ◽  
Sheet Cavitation ◽  
Cavitation Phenomenon

Attached cavitation is a type of common cavitation phenomenon in fluid machinery. It is important to develop methods to control its generation. From the view of cavitation inception, the generation of attached cavitation is greatly influenced by the separated boundary layer upstream of cavitation detachment. In this research, a row of microscopic delta-shaped counter-rotating vortex generators (VGs) was applied on the leading edge of the NACA0015 hydrofoil in order to suppress the boundary layer separation and then suppress the generation of attached cavitation. The application of VGs fixed the position of cavitation inception on hydrofoil thus the sheet cavitation became more stable and the cloud cavity shed from hydrofoil with trim trailing edge more regularly. It was found that cavitation inception always appeared adjacent to VGs due to the low pressure in the corner of streamwise vortices induced by VGs. Hydrofoil with VGs showed an entirely different cavitation morphology on the leading edge. A row of separate microscopic vortex cavitation was induced by the counter-rotating vortices firstly. With the lower the height of VGs, the longer the length of these vortex cavitation due to the weaker interaction between vortices and main flow. Following the vortex cavitation, the attached cavitation was developing, but without typical “finger” structure anymore.

Numerical Investigations of Scale Effect on Cavitation Around Hydrofoils

2014 ◽  
Author(s):  
Haiyu Wang ◽  
Weidong Shi ◽  
Desheng Zhang ◽  
Ling Zhou ◽  
Dazhi Pan
Keyword(s):  
Scale Effects ◽  
Homogeneous Model ◽  
Critical Role ◽  
Three Dimensional ◽  
Water Mixture ◽  
Detached Eddy Simulation ◽  
Hexahedral Mesh ◽  
Numerical Work ◽  
Cloud Cavitation ◽  
Sheet Cavitation

Although scale effects on cavitation have been studied, there seems to be no experimental or numerical work has been done on scale effects on cavitation regimes for hydrofoils. The present study was motivated by the prediction uncertainty of cavitation performance in scaling an axial flow pump, which is also an indispensable step in the design and application of hydraulic machineries. Hydrofoil NACA66 was adopted in this paper to represent the general characteristics of hydrofoils. Three hydrofoils with similar boundary conditions were simulated and analyzed, i.e. the initial hydrofoil, the hydrofoil models scaled down 0.5 and 0.25 times, respectively. High quality hexahedral mesh was established based on three-dimensional geometry of the hydrofoils. The monitoring points were arranged at the same locations relative to the boundary in each case. Computations were conducted on these three-dimensional hydrofoils, based on Detached Eddy Simulation (DES) turbulence model and Zwart cavitation model, which is a homogeneous model of cavitation, considering vapor/water mixture as one phase. In order to validate the practicability of numerical method and configuration employed in this paper, the numerical calculation of the initial hydrofoil was compared with experimental results provided by previous researchers, including the evolution of cavitation and pressure fluctuation on suction surface of the hydrofoil. According to the comparisons of the simulation results of the initial case and the other two scaled down models, we found the boundary layer suppresses the reentrant jet, which plays a critical role in cavitation detachment. Consequently, it influences the evolution of cavitation from the initial bubble, sheet cavitation, cloud cavitation and bubble breakup. Meantime, cavity evolution, cavity lengths, as well as cloud shedding periods were analyzed and discussed.

Mechanism and Control of Cloud Cavitation

1997 ◽  
Vol 119 (4) ◽  
pp. 788-794 ◽  
Author(s):  
Y. Kawanami ◽  
H. Kato ◽  
H. Yamaguchi ◽  
M. Tanimura ◽  
Y. Tagaya
Keyword(s):  
High Speed ◽  
Leading Edge ◽  
Generation Mechanism ◽  
Pressure Measurements ◽  
Foil Surface ◽  
Cavitation Noise ◽  
Cloud Cavitation ◽  
High Speed Video ◽  
And Control ◽  
Small Obstacle

Generation mechanism of cloud cavitation on a hydrofoil section was investigated in a sequence of experiments through observation of cloud cavitation by high-speed video and high-speed photo as well as pressure measurements by pressure pick-ups and a hydrophone. The mechanism was also investigated by controlling cloud cavitation with an obstacle fitted on the foil surface. From the results of these experiments, it was found that the collapse of a sheet cavity is triggered by a re-entrant jet rushing from the trailing edge to the leading edge of the sheet cavity, and consequently, the sheet cavity is shed in the vicinity of its leading edge and thrown downstream as a cluster of bubbles called cloud cavity. In other words, the re-entrant jet gives rise to cloud cavitation. Moreover, cloud cavitation could be controlled effectively by a small obstacle placed on the foil. It resulted in reduction of foil drag and cavitation noise.

scholarly journals Numerical Simulation of Cavitation Erosion Aggressiveness Induced by Unsteady Cloud Cavitation

2020 ◽  
Vol 10 (15) ◽  
pp. 5184
Author(s):  
Linlin Geng ◽  
Jian Chen ◽  
Oscar De La Torre ◽  
Xavier Escaler
Keyword(s):  
Flow Velocity ◽  
Stream Flow ◽  
Cavitation Erosion ◽  
Free Stream ◽  
Temporal Distribution ◽  
Leading Edge ◽  
Average Pressure ◽  
Cavitation Model ◽  
Cloud Cavitation ◽  
Energy Balance Approach

A numerical investigation of the erosion aggressiveness of leading edge unsteady cloud cavitation based on the energy balance approach has been carried out to ascertain the main damaging mechanisms and the influence of the free stream flow velocity. A systematic approach has permitted the determination of the influence of several parameters on the spatial and temporal distribution of the erosion results comprising the selection of the cavitation model and the collapse driving pressure. In particular, the Zwart, Sauer and Kunz cavitation models have been compared as well as the use of instantaneous versus average pressure values. The numerical results have been compared against a series of experimental results obtained from pitting tests on copper and stainless steel specimens. Several cavitation erosion indicators have been defined and their accuracy to predict the experimental observations has been assessed and confirmed when using a material-dependent damaging threshold level. In summary, the use of the average pressure levels during a sufficient number of simulated shedding cycles combined with the Sauer cavitation model are the recommended parameters to achieve reliable results that reproduce the main erosion mechanisms found in cloud cavitation. Moreover, the proposed erosion indicators follow a power law as a function of the free stream flow velocity with exponents ranging from 3 to 5 depending on their definition.

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