The instability of a partial cavity induced by the development of a re-entrant jet is investigated on the basis of experiments conducted on a diverging step. Detailed visualizations of the cavity behaviour allowed us to identify the domain of the re-entrant jet instability which leads to classical cloud cavitation. The surrounding regimes are also investigated, in particular the special case of thin cavities which do not oscillate in length but surprisingly exhibit a re-entrant jet of periodical behaviour. The velocity of the re-entrant jet is measured from visualizations, in the case of both cloud cavitation and thin cavities. The limits of the domain of the re-entrant jet instability are corroborated by velocity fluctuation measurements. By varying the divergence and the confinement of the channel, it is shown that the extent of the auto-oscillation domain primarily depends upon the average adverse pressure gradient in the channel. This conclusion is corroborated by the determination of the pressure gradient on the basis of LDV measurements which shows a good correlation between the domain of the cloud cavitation instability and the region of high adverse pressure gradient. A simple phenomenological model of the development of the re-entrant jet in an adverse pressure gradient confirms the strong influence of the pressure gradient on the development of the re-entrant jet and particularly on its thickness. An ultrasonic technique is developed to measure the re-entrant jet thickness, which allowed us to compare it with the cavity thickness. By considering an estimate of the characteristic height of the perturbations developing on the interface of the cavity and of the re-entrant jet, it is shown that cloud cavitation requires negligible interaction between both interfaces, i.e. a thick enough cavity. In the case of thin cavities, this interaction becomes predominant; the cavity interface breaks at many points, giving birth to small-scale vapour structures unlike the large-scale clouds which are periodically shed in the case of cloud cavitation. The low-frequency content of the cloud cavitation instability is investigated using spectral analysis of wall pressure signals. It is shown that the characteristic frequency of cloud cavitation corresponds to a Strouhal number of about 0.2 whatever the operating conditions and the cavity length may be, provided the Strouhal number is computed on the basis of the maximum cavity length. For long enough cavities, another peak is observed in the spectra, at lower frequency, which is interpreted as a surge-type instability. The present investigations give insight into the instabilities that a partial cavity may undergo, and particularly the re-entrant jet instability. Two parameters are shown to be of most importance in the analysis of the re-entrant jet instability: the adverse pressure gradient and the cavity thickness compared to the re-entrant jet thickness. The present results allowed us to conduct a qualitative phenomenological analysis of the stability of partial cavities on cavitating hydrofoils. It is conjectured that cloud cavitation should occur for short enough cavities, of the order of half the chordlength, whereas the instability often observed at the limit between partial cavitation and super-cavitation is here interpreted as a cavitation surge-type instability.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/2819429B69AFDF15C7C3ECFEAEB3A69D/S0022112001005420a.pdf/div-class-title-the-cavitation-instability-induced-by-the-development-of-a-re-entrant-jet-div.pdf

The cavitation instability induced by the development of a re-entrant jet

This paper mainly summarizes the recent progresses for the cavitation study in the hydraulic machinery including turbo-pumps, hydro turbines, etc.. Especially, the newly developed numerical methods for simulating cavitating turbulent flows and the achievements with regard to the complicated flow features revealed by using advanced optical techniques as well as cavitation simulation are introduced so as to make a better understanding of the cavitating flow mechanism for hydraulic machinery. Since cavitation instabilities are also vital issue and rather harmful for the operation safety of hydro machines, we present the 1-D analysis method, which is identified to be very useful for engineering applications regarding the cavitating flows in inducers, turbine draft tubes, etc. Though both cavitation and hydraulic machinery are extensively discussed in literatures, one should be aware that a few problems still remains and are open for solution, such as the comprehensive understanding of cavitating turbulent flows especially inside hydro turbines, the unneglectable discrepancies between the numerical and experimental data, etc.. To further promote the study of cavitation in hydraulic machinery, some advanced topics such as a Density-Based solver suitable for highly compressible cavitating turbulent flows, a virtual cavitation tunnel, etc. are addressed for the future works.

A review of cavitation in hydraulic machinery

Numerical and Experimental Analysis of Flow Phenomena in a Centrifugal Pump Operating Under Low Flow Rates

Research on rotating cavitation progressed during the development of a liquid oxygen (LOX) turbopump for the LE-7 engine of the H-II rocket In some ranges of cavitation numbers, supersynchronous shaft vibrations were observed in the LE-7 LOX main pump inducer From a comparison with the results of our previous studies it was concluded that such shaft vibrations were caused by rotating cavitation in the inducer A simple modie cation of the inducer upstream housing almost completely extinguished such shaft vibrations Some characteristics of rotating cavitation have been fairly well elucidated However, we were not able to fully explain the mechanism of the ine uence of the simple modie cation of the inducer upstream housing on the rotating cavitation We thus commenced further experimental studies to investigate rotating cavitation in more detail In the present study, some visual observations of rotating cavitation were conducted using the same inducer test facility as that used in our previous work

Experimental Study on Rotating Cavitation of Rocket Propellant Pump Inducers

Experimental Investigation of the Role of Large Scale Cavitating Vortical Structures in Performance Breakdown of an Axial Waterjet Pump

Research on rotating cavitation progressed during the development of a liquid oxygen (LOX) turbopump for the LE-7 engine of the H-II rocket. In some ranges of cavitation numbers, supersynchronous shaft vibrations were observed in the LE-7 LOX main pump inducer. From a comparison with the results of our previous studies it was concluded that such shaft vibrations were caused by rotating cavitation in the inducer. A simple modie cation of the inducer upstream housing almost completely extinguished such shaft vibrations. Some characteristics of rotating cavitation have been fairly well elucidated. However, we were not able to fully explain the mechanism of the ine uence of the simple modie cation of the inducer upstream housing on the rotating cavitation. We thus commenced further experimental studies to investigate rotating cavitation in more detail. In the present study, some visual observations of rotating cavitation were conducted using the same inducer test facility as that used in our previous work.

Experimental study on rotating cavitation of rocket propellant pump inducers

This paper presents the results of experiments on the instability of a cavitating inducer, which is considered to be closely related with the POGO phenomena in liquid propellant rockets. The present experiment employed a high speed camera for visual observations. The dynamic pressure of both the inducer upstream and downstream was also measured. The difference between rotating cavitation and cavitation induced low cycle oscillations was examined. The cavities moved around the periphery of the inducer inlet with a constant rotating speed in rotating cavitation. All cavities oscillated almost in unison in the cavitation induced low cycle oscillation. This suggests that the low cycle oscillation is a system oscillation. The high speed movie films showed the rotating cavitation process and a comparatively high rotating cavitation zone velocity. Rotating cavitation, with very low cycle cavity oscillation for each blade and steady asymmetric cavitation was also observed during the testing. In addition, the present paper illustrates the effect of inducer inlet pressure on the cavity formation.

A Visual Observation of Cavitating Inducer Instability

Observation of Backflow and Prewhirl at Rocket Propellant Pump Inducer

Cavitation is reduced in a rotary pump while the pump performance is maintained by utilizing a low-temperature fluid source already present in the pump system. The fluid from the low-temperature fluid source receives heat from the fluid flowing to the pump, thereby lowering its temperature and the saturated vapor pressure, which increases the allowable margin for a decrease in fluid pressure and reduces the occurrence of cavitation. The pump system may be used for a liquid rocket engine. The fluid velocity of the fluid directed to the pump is low before releasing heat, so there is only a slight pressure loss at the pump. Accordingly, the temperature is lowered and the occurrence of cavitation is reduced within the pump.

System for reducing pump cavitation

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