Incipient Cavitation Number Research Articles

A Note on Measurements of Cavitation Inception.

Some factors are experimentally investigated concerning the method of cavitation inception measurement. They are definition of cavitation inception, effect of air content in tested water, effect of methods of pressure change in the cavitation tunnel and its rate of change. The test body is a circular cylinder in the range of flow of subcritical Reynolds number. In this flow condition, the cavitation bubbles occur in the separated shear layer behind the cylinder with large scatter of incipient cavitation numbers, according to past investigations. One of the most effective definitions is proposed for cavitation inception in the present work, where the main results are obtained using the proposed definition : (1) the inception values can be greatly reduced under water with small air content of less than a critical value, (2) the inception point can be affected by pressure change rate under the condition of variable flow velocity. At the same time, the measurement uncertainty on cavitation inception is also shown through a large number of measurements in the present experiment.

Basic Study of Coupled Damage Caused by Silt Abrasion and Cavitation Erosion. 2nd Report. Experiments with Water Channel.

Damage due to the coupled action of silts abrasion and cavitation erosion was experimentally studied using water channel in which artificial silt (Al2O3) were included. It was confirmed that the incipient cavitation number in silt laden water increased by 10-15% compared with that in tap water. It was shown that the magnitude of impact pressure measured by the pressure detecting films and casing vibration-acceleration had good correlation with the cavitation erosion intensity but not with the abrasive wear intensity. The damage patterns on aluminum specimens were greatly affected by the cavitation occurrence, but the amount of damaged volume was almost constant with and without cavitation. A ploughing type of abrasive wear was mainly observed on the damaged surface by scanning electron microscope.

Cavitation Performance of Cylindrical Choke. Critical Cavitation Number in unsteady Flow.

This paper deals with an experimental study on the unsteady performance in the cylindrical choke of the oil hydraulic. Experiments are carried out under an unsteady flow in which the outlet pressure p2 of the cylindrical choke takes the form of a trapezoid varying with time, keeping the inlet pressure p1 constant. It is found that as the absolute value of the rates of pressure change dp2/dt increases, the incipient cavitation number kci shows a tendency to decrease, and the desinent one, kcd, shows a tendency to increase under two geometric parameters: diameter and length of the choke. As the larger absolute values of dp2/dt further increase, the hysterisis between kci and kcd becomes larger for each combination of the cylindrical choke.

The cavitation performance of an oil hydraulic poppet valve. (Continued. The critical cavitation number for various rates of pressure change in a valve).

This paper deals with an experimental study on the unsteady cavitation performance in the convergent flow of an oil hydraulic poppet valve. Experiments are carried out under two kinds of flow conditions : one is an unsteady flow in which the valve inlet pressure, p1, takes the form of a trapezoid varying with time and keeping the outlet pressure, p2, constant ; the other is the flow in which the former p1 and p2 are interchanged, respectively. It is found that as the absolute value of the rate of pressure change dp2/dt increases, the incipient cavitation number, kci, shows a tendency to decrease, and the desinent one, kcd, shows a tendency to increase under all geometric parameters in this test. As the absolute value of dp2/dt further increases, the hysteresis between kci and kcd becomes larger for each combination of the valve and valve seat.

Cavitation performance of an oil hydraulic poppet valve. (Critical cavitation number for various rates of pressure change in valve).

This paper deals with an experimental study on the unsteady cavitation performance in an oil hydraulic poppet valve. Experiments are carried out under two kinds of flow conditions : one is an unsteady flow in which the valve inlet pressure p1 takes the form of trapezoid varying with time keeping the outlet pressure p2 constant ; and the other is the flow in which the former p1 and p2 are interchanged, respectively. It is found that as absolute values of the rates of pressure change dp1/dt and dp2/dt increase the incipient cavitation number kci shows a tendency to decrease, and the desinent one, kcd, shows a tendency to increase under all geometrical parameters in this test. As the absolute values of dp1/dt and dp2/dt further increase, the hysteresis between kci becomes larger for each combination of the valve and valve seat.

Cavitation characteristics of right-angled branch ducts with rounded corners.

To make clear the effects of curvature on cavitation characteristics of right-angled branch ducts, the cavitation aspects and the associated change in the branch loss coefficient were carefully and systematically investigated in several kinds of right-angled branch ducts with square cross-sections, for several radii of curvature on the corners, cavitation numbers and flow ratios. Clearly the cavitation aspects and the incipient cavitation numbers and significantly affected by the curvature, while the branch loss is scarecely affected by cavitation for any curvature in the near-subcavitating range defined from the inception range to the transient one.

Cavitation in flows past circular cylinders with splitter plates.

Cavitation which occurs in the wake of circular cylinder with splitter plate is investigated experimentally, especially in respect of the inception. The incipient cavitation number as well as the appearance of cavitation bubbles depends on Reynolds number and the length of splitter plate. The characteristics of the change largely when Reynolds number exceeds the critical value. The occurence and development of cavitation are controlled by the splitter plate and the extent of the effect depends on Reynolds number and air content. Especially, the larger effect is obtained at smaller Reynolds number. The scaling of Critical cavitation number in respect of the inception can be obtained by choosing the base pressure and the velocity of free streamline as the representative values.

The Effect of a Slat on Boundary Layer Characteristics and Cavitation of the Main Foil

It is known that the cavitation is dependent upon the boundary layer characteristics. Therefore, it is considered that cavitation can be controlled by controlling the boundary layer.This paper deals with a SLAT-FOIL combination, main foil and upstream slat. The slat is one of the boundary layer control methods for foil sections.Firstly, the authors designed the SLAT-FOILs to increase the pressure at the laminar separation point as high as possible, and verified the cavitation suppression effect by cavitation tunnel tests.Secondly, the authors studied the interaction between the wake of the slat and the boundary layer on the main foil, and found that when the wake of the slat merged into the boundary layer on the main foil, it accelerated turbulent transition. If the pressure of the turbulent transition point was high enough, the cavitation was remarkably suppressed.In conclusion, two methods of the cavitation control (suppression) can be suggested, those are as follows.1. A slat can change the pressure distribution on the foil. As the result of increase of the pressure at the laminar separation point, the incipient cavitation number of sheet type cavitation can be decreased.2. When a slat is near the foil, the wake of the slat is mixed into the boundary layer on the main foil, which accelerates turbulent transition. If the pressure on the transition point is high enough, the incipient cavitation number is remarkably decreased.

Effect of polymeric additives on cavitation and radiated noise in water flowing past a circular cylinder

The present study is concerned with the effect of high molecular weight polymers on the cavitating flow around a cylinder. A decrease of the incipient cavitation number and disappearance of the transient cavitation regime are observed when polymers are added to the flow. Simultaneously, the radiated noise and the drag on the cylinder are substantially reduced for σ values above critical.

Cavitation Inception in Spool Valves

Cavitation has been investigated in directional control valves in order to identify damage mechanisms characteristic of components of aircraft hydraulic systems. Tests have been conducted in a representative metal spool valve and in a model three times larger. Data taken under noncavitating conditions with both valves showed that the position of the high-velocity annular jet shifts orientation, depending upon valve opening and Reynolds number. By means of high-frequency response pressure transducers strategically placed in the valve chamber cavitation could be sensed by the correlation of noise with a cavitation index. The onset of cavitation can be detected by comparing energy spectra for a fixed valve opening and a constant discharge. Another sensitive indicator of cavitation inception is the ratio of cavitating to noncavitating spectral densities. The incipient cavitation number as defined in this investigation is correlated with the Reynolds number for both valves.

The Effect of Nuclei on the Inception of Bubble and Sheet Cavitation on Axisymmetric Bodies

Hydrogen bubbles were generated as cavitation nuclei and their distributions were measured. The number and size distribution of bubble cavities generated on axisymmetric bodies was calculated and compared with experimental results. The measured size distribution of bubble cavities agreed qualitatively with the calculated value, but the total number of cavities was about one half of the calculation. The role of stream nuclei on the inception of sheet cavity was investigated experimentally. Without added nuclei, the value of the incipient cavitation number σi showed a large scattering, whereas with added nuclei the scattering became fairly small and σi converged to the upper limit of that when no nuclei were added. σi with added nuclei also coincided with the desinent cavitation number σd, and σd remained unchanged by adding nuclei.

Study on Cavitation Inception (2 nd Report)

The role of cavitation nuclei on sheet cavitation inception was investigated experimentally for three axisymmetric test bodies, by using a nuclei generator.With no added nuclei, the incipient cavitation number σi showed a large scattering, whereas, when nuclei were added, the scattering became fairly small and σi converged to the upper limit of σi with no added nuclei. It also coincided with the desinent cavitation number σd.By using Laser Doppler Velocimeter, the velocity distribution on axisymmetric bodies were measured, and good agreement was obtained with calculated results. The pressure coefficient at separation point cps was also measured by LDV and was compared with σi, which confirmed the result obtained by Casey that σi is equal to, or slightly less than (-cps).A hypothesis for the mechanism of sheet cavity inception was made, taking account of the surface tension of nuclei and the pressure fluctuation at the reattachment zone of a separated flow. It explained, at least qualitatively, the dependence of σi on u∞ and the relation between σi and (-cps).By controlling the amount of added nuclei, the radius of nuclei which contribute to the sheet cavity inception was estimated to be about 20μm.

On the Transient Cavitation

In research on cavitation, it is well known that the incipient cavitation number σinc is less than or equal to the desinent cavitation number σdes. This condition is called “Cavitation Histeresis”. Authors observed how this condition could be affected by the time-dependent change of the system pressure. This test is named “the transient cavitation”. The delay time of cavitation after stepwise pressure drop, is concidered mainly affected by gas nuclei. From the experiments which is made under sinusoidal oscillation of the system pressure, it is revealed that in the region of lower velocity and higher frequency the incipient cavitation number was larger than the desinent cavitation number.However, this rather strange result is only tentative and quantitative, clearly indicating the need of further careful experimentation and analysis.

Cavitation Suppression and Stress Effects in High-Speed Flows of Water With Dilute Macromolecule Additives

Experiments were carried out to study the effect of dilute aqueous polymer solutions on flow-generated cavitation inception. The cavitation inception on 1/4 and 1/2-in-dia models with a hemispherical nose in a blowdown water tunnel was definitely inhibited at Reynolds numbers from 7 × 104 to 3.1 × 105. Reduction of the incipient cavitation number to 30 percent of its value for tap water was observed, depending on the kind of polymer used, its concentration, and the Reynolds number. Flow velocity field visualization was achieved by optical techniques, which measured the flow velocities and was used to correct values computed from venturi pressure drop. For guar gum (300 ppm) tests, the computed velocities were approximately 8 percent low.

Effects of Cavitation on Periodic Wakes Behind Symmetric Wedges

Measurements of the shedding frequency and spacing of cavitating vortex street wakes have been made for four wedge models having apex angles of 15, 30, 60, and 90 deg. The wake pattern made visible by the presence of cavitation bubbles enabled measurements of the shedding frequency and vortex street spacing to be taken. It was found that the cavitation produced negligible effects on the shedding frequency with decreasing cavitation number down to as low as half the incipient cavitation number. Further decrease in the cavitation number produced an increase in the shedding frequency to a maximum value followed by a decrease with a general decay to random shedding. The analysis of high-speed motion pictures demonstrated a general decrease in the cross-stream spacing of the two rows of vortices with decreasing cavitation number. The longitudinal spacing of vortices in the same row also decreased with decreasing cavitation number followed by a rapid increase at cavitation numbers lower than that producing the peak frequency. In a simplified analysis, it was hypothesized that the cavitation in the vortex cores caused a general sink effect in the shed wake. Modeling the cavitation with cylindrical cavitation bubbles, some agreement with the experimental results was obtained for the cross-stream spacing with decreasing cavitation number.

Experimental Investigations of Incipient and Desinent Cavitation

Experiments with water in a high speed recirculating water tunnel were undertaken to measure the pressures at which incipient and desinent cavitation occurred. Incipient cavitation is defined as the onset of cavitation; desinent cavitation is defined as the cessation of cavitation. The two lucite test sections used formed the minimum area region of the nozzle in the water tunnel. One test section had a smoothly changing internal contour and the other had an abrupt contour change at the entrance to the cylindrical throat region. Cavitation in the abrupt contour occurred at the throat entrance at higher pressures than the cavitation pressures in the smooth contour. The cavitation in the smooth contour occurred at the entrance to the diffuser part of the nozzle. It was concluded that the cavitation pressures and cavitation numbers increased with velocity, the increase being greater for the abrupt contour with the exception of minimums indicated at incipient conditions in the abrupt contour at throat velocities near 88 ft/sec. A notable difference between the incipient and desinent cavitation numbers and pressures occurred for the abrupt contour, but not for the smooth contour using the techniques described for identifying the incipient and desinent cavitation regimes.

Theory of Scale Effects on Cavitation Inception on Axially Symmetric Bodies

On the assumption that the motion of incipient cavitation bubbles on geometrically similar bodies is dynamically similar, the relation between incipient cavitation number kdi and Reynolds number Re has been obtained from the dynamical similarity law deduced from the equation of the motion of spherical bubbles. A comparison of calculated values based on this theory with experimental data obtained by R. W. Kermeen and others shows good agreement at values of Reynolds number greater than the critical Reynolds number. Also, by comparing this theory with the formula by R. T. Knapp, concerning scale effects on cavitation inception, it is shown that Knapp’s formula is a special case of the present theory.

Approximation of Axisymmetric Body Forms for Specified Pressure Distributions

The complex relationship between body profile and pressure distribution is important in the design of high-speed undersea bodies. For slender axisymmetric bodies integration of the equations for irrotational flow has been accomplished, the coordinates of the body profile being related to the pressure distribution through coefficients of Legendre polynomials. The limitations and applications of this approximate analysis have been demonstrated through comparisons with preassigned conditions and with the results of experiments. Computed values closely approach those preassigned or measured throughout the central portion of the body if the maximum diameter does not exceed three-tenths of the length. The analysis is accordingly useful in designing bodies with low coefficients of drag and low incipient cavitation numbers.