A Cavitation Susceptability Meter with Optical Cavitation Monitoring-Part One: Design Concepts

This work is concerned with the design of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. The operation of the Cavitation Susceptibility Meter is analyzed and the main considerations leading to the proposed design are illustrated and critically discussed. The results of this analysis indicate that the operational range is mainly limited by nuclei interference, flow separation and saturation (choking), and suggest to develop a Cavitation Susceptibility Meter where; (1) the flow possesses a laminar potential core throughout the venturi throat section in all operational conditions; (b) the pressure at the venturi throat is determined from the upstream pressure and the local flow velocity; (c) the detection of cavitation and the measurement of the flow velocity are carried out optically by means of a Laser Doppler Velocimeter; (d) a custom-made electronic Signal Processor incorporating a frequency counter is used for real time data generation and temporary storage; (e) a computerized system performs the final acquisition and reduction of the data

Separation and Surface Nuclei Effects in a Cavitation Susceptibility Meter

This work is concerned with the effects of flow separation and surface nuclei on the operation of a fixed geometry Cavitation Susceptibility Meter (CSM) with laminar flow. Cavitation is induced under controlled conditions at the throat of a glass venturi tube for the measurement of the active nuclei concentration in water samples as a function of the applied tension. Both cavitation and flow velocity are monitored optically by a Laser Doppler Velocimeter. The throat pressure is determined indirectly from the upstream pressure and the local flow velocity. The results show that laminar flow separation and surface nuclei effects are the most stringent operational limitations. Separation in the diffuser increases the minimum attainable throat pressure above the susceptibility of most cavitation nuclei commonly found in technical waters. Surface nuclei can generate extensive sheet or spot cavitation at relatively high tensions even on optically finished glass surfaces. These phenomena are difficult to eliminate and bring therefore into question the practical utility of CSM's with laminar flow and fixed geometry for the measurement of the dependence of the cavitating nuclei concentration over wide ranges of the applied tension, as required for cavitation studies

On the Acoustical Dynamics of Bubble Clouds

Recently, Morch [1,2,3,4] Chahine [5,6] and others have focused attention on the dynamics of a cloud or cluster of cavitating bubbles and have expanded on the work of van Wijngaarden [7,8] and others. Unfortunately, there appear to be a number of inconsistencies in this recent work which will require further study before a coherent body of knowledge on the dynamics of clouds of bubbles is established. For example, Morch and his co-workers [1,2,3] have visualized the collapse of a cloud of cavitating bubbles as involving the inward propagation of a shock wave; it is assumed that the bubbles collapse virtually completely when they encounter the shock. This implies the virtual absense of non-condensable gas in the bubbles and the predominance of vapor. Yet in these circumstances the mixture in the the cloud will not have any real sonic speed. As implied by a negative L.H.S. of equation (9), the fluid motion equations for the mixture would be elliptic not hyperbolic and hence shock wave solutions are inappropriate

A Cavitation Susceptibility Meter with Optical Cavitation Monitoring -- Part Two: Experimental Apparatus and Results

This work in concerned with the development and operation of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. The pressure at the venturi throat is determined from the upstream pressure and the local flow velocity without corrections for viscous effects because the flow possesses a laminar potential core in all operational conditions. The detection of cavitation and the measurement of the flow velocity are carried out optically by means of a Laser Doppler Velocimeter. A custom-made electronic Signal Processor is used for real time data generation and temporary storage and a computerized system for final data acquisition and reduction. The implementation of the whole system is described and the results of the application of the Cavitation Susceptibility Meter to the measurement of the water quality of tap water samples are presented and critically discussed with reference to the current state of knowledge on cavitation inception

The Different Role of Cavitation on Rotordynamic Whirl Forces in Axial Inducers and Centrifugal Impellers

The linearized dynamics of the flow in cavitating axial helical inducers and centrifugal turbopomp impellers is investigated with the purpose of illustrating the impact of the dynamic response of cavitation on the rotordynamic forces exerted by the fluid on the rotors of whirling turbopumps. The flow in the impellers is modeled as a fully-guided, incompressible and inviscid liquid. Cavitation is included through the boundary conditions on the suction sides of the blades, where it is assumed to occur uniformly in a small layer of given thickness and complex acoustic admittance, whose value depends on the void fraction of the vapor phase and the phase-shift damping coefficient used to account for the energy dissipation. Constant boundary conditions for the total pressure are imposed at the inlet and outlet sections of the impeller blade channels. The unsteady governing equations are written in rotating “body fitted” orthogonal coordinates, linearized for small-amplitude whirl perturbations of the mean steady flow, and solved by modal decomposition. In helical turbopump inducers the whirl excitation and the boundary conditions generate internal flow resonances in the blade channels, leading to a complex dependence of the lateral rotordynamic fluid forces on the whirl speed, the dynamic properties of the cavitation region and the flow coefficient of the machine. Multiple subsynchronous and supersynchronous resonances are predicted. At higher levels of cavitation the amplitudes of these resonances decrease and their frequencies approach the rotational speed (synchronous conditions). On the other hand, application of the same approach indicates that no such resonances occur in whirling and cavitating centrifugal impellers and that the rotordynamic fluid forces are almost insensitive to cavitation, consistently with the available experimental evidence. Comparison with the scant data from the literature indicates that the present theory correctly captures the observed features and parametric trends of rotordynamic forces on whirling and cavitating turbopump impellers. Hence there are reasons to believe that it can usefully contribute to shed some light on the main physical phenomena involved and provide practical indications on their dependence on the relevant flow conditions and parameters

Cavitation and flow instabilities in a 3-bladed axial inducer designed by means of a reduced order analytical model

The present paper illustrates the main results of an experimental campaign conducted using the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The tests were carried out on the DAPAMITO inducer, a three-bladed axial pump designed and manufactured by Alta S.p.A. using a simplified analytical model for the prediction of geometry and noncavitating performance of typical space rocket inducers. The transparent inlet section of the facility was instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet and the middle of the axial chord of the blades. At each axial station at least two transducers were mounted with given angular spacing in order to cross-correlate their signals for amplitude, phase and coherence analysis. However, probably because of the high value of the blade tip clearance, very few flow instabilities have been detected on the inducer, including: steady asymmetric cavitation caused by the different extension of the cavitating regions on the blades; cavitation surge at a frequency equal to 0.16 times the inducer rotational frequency; a higher-order axial phenomenon at 7.2 times the rotational frequency.http://deepblue.lib.umich.edu/bitstream/2027.42/84278/1/CAV2009-final92.pd

Cavitation and Rotordynamic Activities at Centrospazio

Centrospazio has long been carrying out an articulated program of experimental, theoretical and numerical research on cavitation and two-phase flow dynamics in connection with liquid propellant rocket fuel feed systems. The areas of involvement dealt with in recent years range from the modeling and simulation of cavitation to the study of rotordynamic fluid forces in whirling and cavitating axial inducers and journal bearings. Specifically, this paper illustrates the development of a new cavitation model accounting in an approximate but physical way for the occurrence of thermal cavitation and liquid quality effects, and the realization of the CPRTF (Cavitating Pump Rotordynamic Test Facility), a water loop for the measurement of rotordynamic fluid forces on whirling and cavitating turbopump impellers, and the TCT (Thermal Cavitation Tunnel), a modified version of the CPRTF for cavitation experiments in fluid dynamic and thermal cavitation similarity. Representative results of the application of cavitation model to journal bearings, hydrofoils and helical inducers and the typical performance of the CPRTF and TCT are presented

Modeling Activities on Cavitating Flows at Centrospazio

The present paper illustrates recent theoretical and numerical activities on cavitating flows carried out at CENTROSPAZIO in the last few years. Specifically, it deals with the development and application of a modified isenthalpic cavitation model that accounts in an approximate but physical way for the occurrence of thermal cavitation effects and the concentration of active cavitation nuclei in the liquid. Expanding on the work of Brennen (1994), the model leads to a quasi-homogeneous barotropic description of cavitating flows, whose sound speed smoothly reduces to that of the liquid in the limit for low void fractions, thus eliminating the inconsistencies of previous formulations. Thermal effects are accounted for by assigning a single parameter expressing the nondimensional thickness of the thermal boundary layer in the liquid surrounding the growing cavities. The value of this parameter is related to the concentration of active cavitation nuclei, whose value can therefore be alternatively used to specify the impact of thermal cavitation effects. Applications to cavitating journal bearings, hydrofoils and helical inducers relevant to space engineering are presented

Self-attraction effect and correction on three absolute gravimeters

The perturbations of the gravitational field due to the mass distribution of an absolute gravimeter have been studied. The so called Self Attraction Effect (SAE) is crucial for the measurement accuracy, especially for the International Comparisons, and for the uncertainty budget evaluation. Three instruments have been analysed: MPG-2, FG5-238 and IMPG-02. The SAE has been calculated using a numerical method based on FEM simulation. The observed effect has been treated as an additional vertical gravity gradient. The correction (SAC) to be applied to the computed g value has been associated with the specific height level, where the measurement result is typically reported. The magnitude of the obtained corrections is of order 1E-8 m/s2.Comment: 14 pages, 8 figures, submitted to Metrologi