An Experimental Study of Unsteady Partial Cavitation

2004 ◽  
Vol 126 (1) ◽  
pp. 94-101 ◽  
Author(s):  
Jean-Baptiste Leroux ◽  
Jacques Andre´ Astolfi ◽  
Jean Yves Billard
Keyword(s):  
Cavity Length ◽  
Pressure Fluctuations ◽  
Cavity Growth ◽  
Pressure Measurements ◽  
Cloud Cavitation ◽  
Hydraulic Machinery ◽  
Partial Cavitation ◽  
Basic Physics ◽  
Vapor Cloud ◽  
Cavity Closure

Unsteady partial cavitation can cause damage to hydraulic machinery and understanding it requires knowledge of the basic physics involved. This paper presents the main results of a research program based on wall-pressure measurements aimed at studying unsteadiness in partial cavitation. Several features have been pointed out. For cavity lengths that did not exceed half the foil chord the cavity was stated to be stable. At the cavity closure a peak of pressure fluctuations was recorded originating from local cavity unsteadiness in the closure region at a frequency depending on the cavity length. Conversely, cavities larger than half the foil chord were stated to be unstable. They were characterized by a cavity growth/destabilization cycle settled at a frequency lower than the previous ones. During cavity growth, the closure region fluctuated more and pressure fluctuations traveling in the cavity wake were detected. When the cavity was half the foil chord, cavity growth was slowed down and counterbalanced by large vapor cloud shedding. When the cavity length was maximum (l/c∼0.7–0.8), it was strongly destabilized. The reason for such destabilization is discussed at the end of the paper. It is widely believed that the cavity instability originates from a process involving the shedding of vapor clouds during cavity growth, a re-entrant jet, and a shock wave phenomenon due to the collapse of a large cloud cavitation.

A New Unsteady Model for Dense Cloud Cavitation

2005 ◽  
Author(s):  
A. Hosangadi ◽  
V. Ahuja
Keyword(s):  
Volume Fraction ◽  
Pressure Fluctuations ◽  
Cavitation Model ◽  
Cloud Cavitation ◽  
Dense Cloud ◽  
Vapor Cloud ◽  
History Of ◽  
Liquid Rocket ◽  
Multi Phase ◽  
Dense Vapor

A new unsteady, cavitation model for dense cloud cavitation is presented wherein the phase change process (bubble growth/collapse) is coupled to the acoustic propagation in a multi-phase fluid. This cavitation model predicts the number density and radius of bubbles in vapor clouds by tracking both the aggregate surface area and volume fraction of the cloud. Hence, formulations for the dynamics of individual bubbles (e.g. Rayleigh-Plesset equation) may be integrated within the macroscopic context of a dense vapor cloud i.e. a cloud that occupies a significant fraction of available volume and contains numerous bubbles. This formulation has been implemented within the CRUNCH CFD, which has a compressible “real” fluid formulation, a multi-element, unstructured grid framework, and has been validated extensively for liquid rocket turbopump inducers. Rigorous validation of the formulation is presented for various cases including unsteady simulations of a cavitating NACA0015 airfoil where the frequency of pressure fluctuations and time-averaged mean cavity lengths were compared with experimental data. The model also provides the spatial and temporal history of the bubble size distribution in the vapor clouds that are shed, an important physical parameter that is difficult to measure experimentally and is a significant advancement in the modeling of dense cloud cavitation.

Numerical simulation and analysis of condensation shocks in cavitating flow

2018 ◽  
Vol 838 ◽  
pp. 759-813 ◽  
Author(s):  
Bernd Budich ◽  
S. J. Schmidt ◽  
N. A. Adams
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Volume Fraction ◽  
Cavity Growth ◽  
Pressure Rise ◽  
Shock Propagation ◽  
Two Phase ◽  
Cloud Cavitation ◽  
Cavity Collapse ◽  
Partial Cavitation

We analyse unsteady cavity dynamics, cavitation patterns and instability mechanisms governing partial cavitation in the flow past a sharp convergent–divergent wedge. Reproducing a recent reference experiment by numerical simulation, the investigated flow regime is characterised by large-scale cloud cavitation. In agreement with the experiments, we find that cloud shedding is dominated by the periodic occurrence of condensation shocks, propagating through the two-phase medium. The physical model is based on the homogeneous mixture approach, the assumption of thermodynamic equilibrium, and a closed-form barotropic equation of state. Compressibility of water and water vapour is taken into account. We deliberately suppress effects of molecular viscosity, in order to demonstrate that inertial effects dominate the flow evolution. We qualify the flow predictions, and validate the numerical approach by comparison with experiments. In agreement with the experiments, the vapour volume fraction within the partial cavity reaches values ${>}80\,\%$ for its spanwise average. Very good agreement is further obtained for the shedding Strouhal number, the cavity growth and collapse velocities, and for typical coherent flow structures. In accordance with the experiments, the simulations reproduce a condensation shock forming at the trailing part of the partial cavity. It is demonstrated that it satisfies locally Rankine–Hugoniot jump relations. Estimation of the shock propagation Mach number shows that the flow is supersonic. With a magnitude of only a few kPa, the pressure rise across the shock is much lower than for typical cavity collapse events. It is thus far too weak to cause cavitation erosion directly. However, by affecting the dynamics of the cavity, the flow aggressiveness can be significantly altered. Our results indicate that, in addition to classically observed re-entrant jets, condensation shocks feed an intrinsic instability mechanism of partial cavitation.

Relationship Between Unsteady Flow, Pressure Fluctuations, and Noise in a Centrifugal Pump—Part B: Effects of Blade-Tongue Interactions

1995 ◽  
Vol 117 (1) ◽  
pp. 30-35 ◽  
Author(s):  
S. Chu ◽  
R. Dong ◽  
J. Katz
Keyword(s):  
Unsteady Flow ◽  
Centrifugal Pump ◽  
Pressure Difference ◽  
Pressure Fluctuations ◽  
Primary Sources ◽  
Pressure Measurements ◽  
Local Pressure ◽  
Pressure Distributions ◽  
Flow Pressure ◽  
Tip Of The Tongue

Maps of pressure distributions computed using PDV data, combined with noise and local pressure measurements, are used for identifying primary sources of noise in a centrifugal pump. In the vicinity of the impeller pressure minima occur around the blade and near a vortex train generated as a result of non-uniform outflux from the impeller. The pressure everywhere also varies depending on the orientation of the impeller relative to the tongue. Noise peaks are generated when the pressure difference across the tongue is maximum, probably due to tongue oscillations, and when the wake impinges on the tip of the tongue.

Cavitation Experiments on Turbopump Inducers and Hydrofoils at Alta/Centrospazio: Overview and Future Activities

2005 ◽  
Author(s):  
Angelo Cervone ◽  
Cristina Bramanti ◽  
Emilio Rapposelli ◽  
Luca d’Agostino
Keyword(s):  
Thermal Effects ◽  
Cavity Length ◽  
Rocket Engine ◽  
Pressure Coefficient ◽  
Suction Side ◽  
Test Facility ◽  
Final Part ◽  
Cloud Cavitation ◽  
Naca 0015

The aim of the present paper is to provide some highlights about the most interesting experimental activities carried out during the years 2000–2004 through the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Centrospazio/Alta S.p.A. After a brief description of the facility, the experimental activities carried out on a NACA 0015 hydrofoil for the characterization of the pressure coefficient on the suction side and evaluation the cavity length and oscillations are presented. Then, the results obtained to characterize the performance and the cavitation instabilities on three different axial inducers are showed: in particular, a commercial three-bladed inducer, the four-bladed inducer installed in the LOX turbopump of the Ariane Vulcain MK1 rocket engine and the “FAST2”, a two-bladed one manufactured by Avio S.p.A. using the criteria followed for the VINCI180 LOX inducer. The most interesting results are related to the effects of the temperature on the cavitation instabilities on hydrofoils and inducers. Experiments showed that some instabilities, like the cloud cavitation on hydrofoils and the surge on inducers, are strongly affected by the temperature, while others seem not to be influenced by the thermal effects. In the final part of this paper, some indications of the main experimental activities scheduled for the next future are provided.

Wall-Pressure Fluctuations Inside Attached Cavitation

2021 ◽  
Author(s):  
Changchang Wang ◽  
Guoyu Wang ◽  
Mindi Zhang ◽  
Qin Wu
Keyword(s):  
Shock Wave ◽  
Shear Layer ◽  
High Frequency ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Cloud Cavitation ◽  
Unsteady Pressure ◽  
Wall Pressure Fluctuations ◽  
Kurtosis Factor ◽  
Skewness And Kurtosis

Abstract This study experimentally investigates the statistics of wall-pressure fluctuations and their source inside attached cavitation under different cavity regimes. Experiments were conducted in the divergent section of a convergent-divergent channel at a constant Reynolds number of Re = 7.8 × 105 based on throat height, and different cavitation numbers σ = 1.18, 0.92, 0.82 and 0.78. Four high-frequency unsteady pressure transducers were flushed-mounted in the divergent section downstream the throat where cavitation develops to sample the unsteady pressure signals induced by cavity behaviors. Flow visualization and wall-pressure measurement in high frequency on the order of MHz were employed using a synchronizing sampling technique. Results are presented for sheet/cloud cavitating flows. Specifically, sheet cavitation with both inception shear layer and fully cavitated shear layer and cloud cavitation under re-entrant jet dominated shedding and shock wave dominated shedding are studied. Compared with re-entrant jet, the interactions between shock wave and cavity could induce pressure peaks with high magnitude within cavity, which will collapse the local vapor along its propagating path and reduce local void fraction. Furthermore, statistics analysis shows that within the cavity, wall-pressure fluctuations increase with the distance to cavity leading edge increase in the first half of cavity length, and the moments of the probability density distribution skewness and kurtosis factor decrease, indicating the asymmetry and intermittency of wall-pressure fluctuation signals decrease. In shock wave dominated cavity shedding condition, the skewness and kurtosis factor increase. These results can provide data to improve the accuracy of turbulence modeling in numerical simulation of turbulent cavitating flow.

Experimental Study on Pressure Fluctuations in the Boundary Layer of an Axisymmetric Body

2011 ◽  
Vol 66-68 ◽  
pp. 1488-1493
Author(s):  
Hong Xiao ◽  
Chao Gao ◽  
Zhen Kun Ma
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Mach Number ◽  
Dynamic Pressure ◽  
Pressure Fluctuations ◽  
Axisymmetric Body ◽  
Pressure Measurements ◽  
Fluctuating Pressure ◽  
Frequency Domains ◽  
Dynamic Pressure Measurements

The characteristics of the fluctuating pressure in the boundary layer of an axisymmetric body have been investigated experimentally using dynamic pressure measurements and Schlieren photograghs. Data were acquired at subsonic and super-sonic Mach numbers. The angles of attack ranged from 0° to 5°. Pressure signals were measured simultaneously in several positions along the model and were analyzed both in the time and frequency domains. The Mach number shows the relevant influence on . Furthermore, the pressure fluctuations’ level decreases with the increasing of Mach number except M=1.15. And it is shown that, the location along the axis of the model and the angles of attack have small effect on pressure fluctuations.

scholarly journals Cavitation in Turbopumps—Part 2

1962 ◽  
Vol 84 (3) ◽  
pp. 339-349 ◽  
Author(s):  
L. B. Stripling
Keyword(s):  
Correlation Coefficients ◽  
Performance Data ◽  
Inlet Pressure ◽  
Cavitation Performance ◽  
Partial Cavitation ◽  
Cavitation Region ◽  
Cavity Closure ◽  
Correlation Factors ◽  
Mixing Losses

Cavitation performance data of several helical inducers for various flow coefficients are correlated with existing theory. For complete head breakdown conditions, the method employs semiempirical correlation coefficients which supplement the idealized free-streamline solutions obtained by various investigators. Considerations are given to the partial cavitation region utilizing the free-streamline wake solution. The model clearly illustrates the influence of the mixing losses, downstream of the cavity closure, on the inducer’s developed head as the inlet pressure is reduced. With the use of the above semiempirical correlation factors the theory forms a useful basis for design.

scholarly journals Dynamics and Intensity of Erosive Partial Cavitation

2007 ◽  
Vol 129 (7) ◽  
pp. 886-893 ◽  
Author(s):  
Xavier Escaler ◽  
Mohamed Farhat ◽  
Eduard Egusquiza ◽  
François Avellan
Keyword(s):  
Free Stream ◽  
Stream Velocity ◽  
Cloud Cavitation ◽  
Sheet Cavitation ◽  
A Value ◽  
Partial Cavitation ◽  
Cavitation Tunnel ◽  
Shedding Frequency ◽  
Testing Condition ◽  
Collapse Process

An experimental work has been carried out to investigate the dynamic behavior and the intensity of erosive partial cavitation on a 2-D hydrofoil. Both sheet (stable) and cloud (unstable) cavitation have been tested in a cavitation tunnel for various free stream velocities. Special attention has been given to validate the use of acceleration transducers for studying the physical process. In particular, the modulation in amplitude of the cavitation induced vibrations in a high frequency band has allowed us to determine the shedding frequency and the relative intensity of the collapse process for each testing condition. Regarding the cavity dynamics, a typical Strouhal value based on its length of about 0.28 has been found for cloud cavitation; meanwhile, for sheet cavitation, it presents a value of about 0.16. Furthermore, the level of the vibration modulation in the band from 45kHz to 50kHz for cloud cavitation shows a power law dependency on the free stream velocity as well as a good correlation with the pitting rate measured on stainless steel samples mounted on the hydrofoil.

Thermodynamic Effect on a Cavitating Inducer—Part I: Geometrical Similarity of Leading Edge Cavities and Cavitation Instabilities

2010 ◽  
Vol 132 (2) ◽  
Author(s):  
Jean-Pierre Franc ◽  
Guillaume Boitel ◽  
Michel Riondet ◽  
Éric Janson ◽  
Pierre Ramina ◽  
...  
Keyword(s):  
Transit Time ◽  
Cavity Length ◽  
Cold Water ◽  
Rotation Speed ◽  
Leading Edge ◽  
Cavitation Number ◽  
Thermodynamic Effect ◽  
Cavity Closure ◽  
Cavity Development ◽  
Onset Of Cavitation

The thermodynamic effect on a cavitating inducer is investigated from joint experiments in cold water and Refrigerant 114. The analysis is focused on leading edge cavitation and cavitation instabilities, especially on alternate blade cavitation and supersynchronous rotating cavitation. The cavity length along cylindrical cuts at different radii between the hub and casing is analyzed with respect to the local cavitation number and angle of attack. The similarity in shape of the cavity closure line between water and R114 is examined and deviation caused by thermodynamic effect is clarified. The influence of rotation speed on cavity length is investigated in both fluids and analyzed on the basis of a comparison of characteristic times, namely, the transit time and a thermal time. Thermodynamic delay in the development of leading edge cavities is determined and temperature depressions within the cavities are estimated. Thresholds for the onset of cavitation instabilities are determined for both fluids. The occurrence of cavitation instabilities is discussed with respect to the extent of leading edge cavitation. The thermodynamic delay affecting the occurrence of cavitation instabilities is estimated and compared with the delay on cavity development.

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.

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