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.

Effects of Free Stream Turbulence on Intermittent Boundary Layer Flows

1995 ◽  
Author(s):  
Anestis I. Kalfas ◽  
Robin L. Elder
Keyword(s):  
Boundary Layer ◽  
Stream Flow ◽  
Free Stream ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Flow Transition ◽  
Flow Conditions ◽  
Boundary Layer Flows ◽  
Laminar Separation ◽  
Free Stream Turbulence

This paper considers the effects of free stream turbulence intensity on intermittent boundary layer flows related to turbomachinery. The present experimental investigation has been undertaken under free stream flow conditions dominated by grid generated turbulence and Reynolds numbers appropriate for turbomachinery applications. Unseparated flow transition in the boundary layer has been considered using a flat plate with the C4 leading edge which has been designed to avoid laminar separation. This configuration provided the opportunity to study the effect of a realistic turbomachinery leading edge shape on transition. Boundary layer type hot-wire probes have been used in order to acquire detailed information about the effect of the free stream conditions and the leading edge configuration on the structure of the boundary layer. Furthermore, information about the intermittency distribution throughout the boundary layer has been obtained using statistical analysis of the velocity record of the flow field.

Method for Cavitation Erosion Prediction: Model Development

2005 ◽  
Author(s):  
Matevzˇ Dular ◽  
Bernd Stoffel ◽  
Brane Sˇirok
Keyword(s):  
Flow Velocity ◽  
Cavitation Erosion ◽  
Model Development ◽  
Three Dimensional ◽  
Leading Edge ◽  
Gas Content ◽  
Physical Description ◽  
System Pressure ◽  
Jet Impact ◽  
Radial Pump

A study of visual and erosion effects of cavitation on simple single hydrofoil configurations in a cavitation tunnel was made. A two-dimensional hydrofoil with circular leading edge was used for the experiments. In addition, the hydrofoil geometry was modified to obtain some three-dimensional cavitation effects. A thin copper foil, applied to the surface of the hydrofoil, was used as an erosion sensor. The cavitation phenomenon on hydrofoils at different flow conditions (system pressure, water gas content, flow velocity) was observed. Images of vapour cavities from above and from a side view were taken. Plausible results that showed a significant relationship between cavitation erosion and the visual effects of cavitation made it possible to use these information to develop a cavitation erosion model. The model is based on the physical description of different phenomena, which are involved in the process of pit formation — pressure wave emission and its attenuation, micro-jet formation, the jet impact to the solid surface and pit creation. The model is capable to predict the influence of significant parameters as flow velocity and gas content of water. The model that was developed on the basis of measurements of cavitation on a single hydrofoil was later tested on an actual hydraulic machine in the form of a radial pump. The predicted magnitude and distribution of cavitation damage relates well to the experimentally measured one.

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.

Numerical Simulation of Cloud Cavitation in Hydrofoil and Orifice Flows With Analysis of Viscous and Nonviscous Separation

2018 ◽  
Vol 140 (11) ◽  
Author(s):  
Phillip Limbach ◽  
Karoline Kowalski ◽  
Jeanette Hussong ◽  
Romuald Skoda
Keyword(s):  
Flow Separation ◽  
Measurement Data ◽  
Leading Edge ◽  
Viscosity Reduction ◽  
Model Parameters ◽  
Cavitation Model ◽  
Cloud Cavitation ◽  
Model Interaction ◽  
Wall Functions ◽  
Wide Range

Three-dimensional (3D) numerical flow simulations with a mass transfer cavitation model are performed to analyze cloud cavitation at two different flow configurations, i.e., hydrofoil and orifice flows, focusing on the turbulence and cavitation model interaction, including a mixture eddy viscosity reduction and cavitation model parameter modification. For the cavitating flow around the hydrofoil with circular leading edge, a good agreement to the measured shedding frequencies as well as local cavitation structures is obtained over a wide range of operation points, even with a moderate boundary layer resolution, i.e., utilizing wall functions (WF), which are found to be adequate to capture the re-entrant jet reasonably in the absence of viscous separation. Simulations of the orifice flow, that exhibit significant viscous single-phase (SP) flow separation, are analyzed concerning the prediction of choking and cloud cavitation. A low-Reynolds number turbulence approach in the orifice wall vicinity is suggested to capture equally the mass flow rate, flow separation, and cloud shedding with satisfying accuracy in comparison to in-house measurements. Local cavitation structures are analyzed in a time-averaged manner for both cases, revealing a reasonable prediction of the spatial extent of the cavitation zones. However, different cavitation model parameters are utilized at hydrofoil and orifice for best agreement with measurement data.

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.

Aspects of Shear Layer Unsteadiness in a Three-Dimensional Supersonic Wake

2005 ◽  
Vol 127 (6) ◽  
pp. 1085-1094 ◽  
Author(s):  
Alan L. Kastengren ◽  
J. Craig Dutton
Keyword(s):  
Shear Layer ◽  
Stream Flow ◽  
Free Stream ◽  
Angle Of Attack ◽  
Three Dimensional ◽  
Base Flow ◽  
Recirculation Region ◽  
Side View ◽  
Near Wake ◽  
Supersonic Wake

The near wake of a blunt-base cylinder at 10° angle-of-attack to a Mach 2.46 free-stream flow is visualized at several locations to study unsteady aspects of its structure. In both side-view and end-view images, the shear layer flapping grows monotonically as the shear layer develops, similar to the trends seen in a corresponding axisymmetric supersonic base flow. The interface convolution, a measure of the tortuousness of the shear layer, peaks for side-view and end-view images during recompression. The high convolution for a septum of fluid seen in the middle of the wake indicates that the septum actively entrains fluid from the recirculation region, which helps to explain the low base pressure for this wake compared to that for a corresponding axisymmetric wake.

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