239 research outputs found
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Nuceli Effects on Tip Vortex Cavitation Scaling
No full textA cavitation susceptibility meter has been used by the US Navy for 16 years to measure the tensile strength of water in the NSWCCD water tunnels, Lake Pend Oreille in Idaho, Exuma Sound in the Bahama Islands, the Pacific Ocean along the US West Coast, and the North Atlantic Ocean. This meter measures the variation of cavitation nuclei with water tension. Using the bubble stability equation, these tensions are related to equivalent size bubbles to produce a spectrum of bubble concentration versus bubble size. This paper compares these nuclei sizes in the lake and ocean waters. Results of bubble dynamics calculations show a relationship for estimating the nuclei effect on scaling that is separate from the viscous effects on the pressure fields. The influence of these nuclei variations on cavitation inception scaling are discussed for a hypothetical propeller tip vortex cavity. The scaling of two model inception conditions are studied for nuclei variations in the model environment as well as variations in the full-scale environment. The sizes of the nuclei in the two environments differ, but the resulting effects on inception are predicted to be minor, and this trend may be globally true for nuclei variations within natural water bodies. The variations illustrated may be minor compared to the nuclei effect for scaling water tunnel model tests to full-scale
Numerical Study of Sheet Cavitation Break-off Phenomenon on a Cascade Hydrofoil
No full text2-D unsteady cavity flows through hydrofoils in cascade which is the most fundamental element of turbomachinery are numerically calculated. In particular, attention was paid to instability phenomena of the sheet cavity in transient cavitation condition and the mechanism of break-off phenomenon was examined. A TVD MacCormack's scheme employing a locally homogeneous model of compressible gas-liquid two-phase media was applied to analyze above cavity flows. The present method permits us to treat the whole cavitating/noncavitating unsteady flow field. By analyzing numerical results in detail, it became clear that there are at least two mechanisms in the break-off phenomena of sheet cavity; one is that re-entrant jets play a dominant role in such a break-off phenomenon, and the other is that pressure waves propagating inside the cavity bring about an another type of break-off phenomenon accompanied with cavity surface waves
Rarefaction Waves and Bubbly Cavitation in Real Liquid
No full textThe paper presents the short review of two stages of cavitating liquid fracture at the explosive loading. The
problems of the real liquid state (with view point of its inhomogeneity) and limit tensile stress, as well as
the mechanics of the cavitation development excited by intense rarefaction waves and the dynamic feature
of breaking of a spherical liquid drop under the action of ultra-short shock wave are considered
Experimental Estimate of Helical Inducer Blade Force in Cavitating Surge Condition
No full textAn attachment of inducer is a powerful method to improve the cavitation performance of a turbo-pump. Cavitation surge phenomena, occurring under the severe suction pressure at a partial flow rate, is focused in the present paper. Flow measurements were carried out at the inlet and outlet sections of a flat-plate helical inducer with the solidity of 2.0 and tip blade angle of 11° by using a total-head yaw-meter with a phase locked sampling method in one period of the shaft rotation or the cavitation oscillation. Time variation of the flow distributions during the oscillation is clarified with observed cavitation behaviors. After these results are compared with those in conditions just before and after the oscillations, the fluctuating blade forces are estimated from the blade to blade distributions of casing wall pressures measured in these conditions
A Velocity Based Boundary Element Method with Modified Trailing Edge for Prediction of the Partial Cavities on the Wings and Propeller Blades
No full textThis paper presents a new velocity based boundary element method for calculation of the partial cavities on the wings and propeller blades. The fully correct mathematical formulation of the problem about cavitating bodies is characteristic feature of the developed method and numerical algorithm. The special algorithm of Modified Trailing Edge (MTE) is used for definition of the lifting (circulation) part of the flow. MTE tool allows the approximate accounting for viscous effects on lift and pressure distribution. The miscellaneous cavity closure model is recommended for calculation. The prediction of the cavity patterns is performed using Iterative Cavity Alignment (ICA) procedure with Free Cavity Length (FCL). The results of the test calculations and comparisons with experimental data are given in the paper
An Experimental Investigation of Partial Cavitation on a Two - Dimensional Hydrofoil
No full textAn investigation of leading edge partial cavitation was performed including the cavitation inception conditions, the cavitation patterns together with cavity length measurements. The investigation was enhanced by wall-pressure measurements using an instrumented hydrofoil equipped with seventeen wall-pressure transducers. Several experimental results are presented in the paper. A peak of pressure fluctuations was recorded at the cavity closure. The peak magnitude was seen to increase with the cavity length. For cavity lengths that did not exceed about half the foil chord, the peak originated from pressure fluctuations at a Strouhal number based on the cavity length close to 0.2. For longer cavities, the cavity began to pulsate at a lower frequency with the cavity length varying from the vicinity of the leading edge up to the trailing edge. The reason for such a phenomena is discussed
Numerical and Experimental Investigations on the Cavitating Flow in a Cascade of Hydrofoils
No full textThe cavitating flow in a cascade of three hydrofoils was investigated by experimental means and numerical simulation. Experiments on the 2D-hydrofoils cascade were carried out at Darmstadt University of Technology in a rectangular test section of a cavitation tunnel. A numerical model developed at LEGI (Grenoble) to describe the unsteady behaviour of cavitation including the shedding of vapour structures was applied to the hydrofoils cascade geometry. Results of both experimental and numerical studies show a strong interaction between the cavities of each flow channel besides the typical self-oscillation of cloud cavitation. A detailed comparison of the results allows proposing an interpretation of the interaction mechanisms
Cavitating Inducer Instabilities: Experimental Analysis and 2D Numerical Simulation of Unsteady Flow in Blade Cascade
No full textCavitating flows in rocket engine turbo-pump inducers are studied by experimental and numerical ways. The experimental analysis focused on the cavitating structures at the periphery of the inducer, the quantification of the vapour volume at inducer inlet, its relation with the backflow and the obstruction effect on the main flow. A 2D numerical model of unsteady cavitation was applied to a blade cascade drawn from the inducer geometry. Unsteady behaviour of sheet cavities attached to the blade suction side depends on the flow rate and cavitation number. In certain conditions, a rotating cavitation phenomenon is observed, in good agreement with experimental observations
On the History of Propeller Cavitation and Cavitation Tunnels
No full textA condensed survey on the history of propeller cavitation and its understanding as well the development of cavitation
tunnels is given. Propeller cavitation effects ship performance by thrust breakdown, erosion, vibration and noise. Each
of these detrime ntal conseq uences will be shortly dealt w ith. Following the similarity laws for cavitation, a limited
number of full scale investigations on propellers will be looked at which form one basis for the cavitation and propeller
excited pressure fluctuation correlation between model and full scale
Observations of Oscillating Cavitation on a Flat Plate Hydrofoil
No full textAn experimental investigation was made to clarify the characteristics of oscillating cavitation on a flat plate hydrofoil in a water tunnel. Dynamic the behavior of oscillating cavitation is discussed from the unsteady pressure measurements at the upstream of the blade and the visual observations of cavitation phenomena using high-speed video recording. It was found that the mean cavity length characterizes the fundamental characteristics of cavity oscillation. The cavity oscillations are categorized into two types, i.e. the transitional cavity oscillation and the partial cavity oscillation