Determination of Cavitation Noise Limiting Value

In this paper. the cause of cavitation noise limiting value is analyzed. and the de- termining method of cavitation noise limiting value of body os giving.

THE INFLUENCE OF BODY SPEED ON CAVITATION NOISE

The present paper analyzes the relationship between the body speed and the cavitation noise. The results indicate that there is the limiting value of cavitation noise.

Numerical investigation of the cavitation noise around a marine propeller focusing on the influence of ventilation

Cavitation noise around propellers has many adverse effects.It is still very limited nowadays to inhibit propeller cavitation noise in engineering.In this study,the cavitation noise around a PPTC propeller is simulated using the large eddy simulation(LES)coupled with the porous Ffowcs Williams-Hawkings(PFW-H)equation.The investigation aims to find a strategy to suppress cavitation noise and analyze the noise suppression mechanism.The predicted hydrodynamic results agree well with the experimental data and are utilized in the hydroacoustic analysis.The hydroacoustic results indicate that the pseudo-thickness noise dominates the dominant frequency component of the total cavitation noise due to the effect of cavity evolution,which is one of the reasons why the pseudo-thickness noise dominates the total cavitation noise.A method is found to weaken the cavitation noise through ventilation at the generation location of the sheet cavity(SC).It is worth noting that ventilation inhibits the generation and development of SC by changing the pressure distribution on the suction surface of the blade and pushing away the cavities around the ventilation holes.Moreover,cavity evolution noise dominates the fluid volume evolution noise under the ventilated cavitating condition.Ventilation significantly attenuates the vapor volume pulsation and thus the cavity evolution noise,which leads to a reduction in pseudo-thickness noise and total cavitation noise.The ventilation mainly reduces noises at the dominant frequency of the pseudo-thickness noise and the total cavitation noise.

EXPERIMENTAL INVESTIGATION OF CAVITATION NOISE

This paper is a final report of the experimental investigation of the cavitation noise.In the paper is described the basic characteristics of the cavitation noise and the significant experiment results for the similarity the limiting value and the control of the cavitation noise

THE MATHEMATICAL MODEL OF BUBBLE GROUP NOISE

In this paper the mathematical model of bubble group noise are introduced under the arbitrary conditions by using the method of Euler. The calculation indicates that the simulation results consist with the measured value.

Scale Effects on Propeller Cavitating Hydrodynamic and Hydroacoustic Performances with Non-uniform Inflow

Considering the lack of theoretical models and ingredients necessary to explain the scaling of the results of propeller cavitation inception and cavitating hydroacoustics from model tests to full scale currently, and the insufficient reflection of the nuclei effects on cavitation in the numerical methods, the cavitating hydrodynamics and cavitation low frequency noise spectrum of three geometrically similar 7-bladed highly skewed propellers with non-uniform inflow are addressed. In this process, a numerical bridge from the multiphase viscous simulation of propeller cavitation hydrodynamics to its hydro-acoustics is built, and the scale effects on performances and the applicability of exist scaling law are analyzed. The effects of non-condensable gas（NCG） on cavitation inception are involved explicitly in the improved Sauer＇s cavitation model, and the cavity volume acceleration related to its characteristic length is used to produce the noise spectrum. Results show that, with the same cavitation number, the cavity extension on propeller blades increases with diameter associated with an earlier shift of the beginning point of thrust decline induced by cavitation, while the three decline slopes of thrust breakdown curves are found to be nearly the same. The power of the scaling law based on local Reynolds number around 0.9R section is determined as 0.11. As for the smallest propeller, the predominant tonal noise is located at blade passing frequency（BPF）, whereas 2BPF for the middle and both 2BPF and 3BPF for the largest, which shows the cavitating line spectrum is fully related to the interaction between non-uniform inflow and fluctuated cavity volume. The predicted spectrum level exceedance from the middle to the large propeller is 6.65 dB at BPF and 5.94 dB at 2BPF. Since it just differs less than 2 dB to the increment obtained by empirical scaling law, it is inferred that the scale effects on them are acceptable with a sufficient model scale, and so do the scaling law. The numerical implementation of cavitating hydrodynamics and hydro-acoustics prediction of propeller in big scale in wake has been completed.

Dynamics of Vapor Bubble in a Variable Pressure Field

This paper presents analytical and numerical results of vapor bubble dynamics and acoustics in a variable pressure field.First,a classical model problem of bubble collapse due to sudden pressure increase is introduced.In this problem,the Rayleigh–Plesset equation is treated considering gas content,surface tension,and viscosity,displaying possible multiple expansion–compression cycles.Second,a similar investigation is conducted for the case when the bubble originates near the rounded leading edge of a thin and slightly curved foil at a small angle of attack.Mathematically the flow field around the foil is constructed using the method of matched asymptotic expansions.The outer flow past the hydrofoil is described by linear(small perturbations)theory,which furnishes closed-form solutions for any analytical foil.By stretching local coordinates inversely proportionally to the radius of curvature of the rounded leading edge,the inner flow problem is derived as that past a semi-infinite osculating parabola for any analytical foil with a rounded leading edge.Assuming that the pressure outside the bubble at any moment of time is equal to that at the corresponding point of the streamline,the dynamics problem of a vapor bubble is reduced to solving the Rayleigh-Plesset equation for the spherical bubble evolution in a time-dependent pressure field.For the case of bubble collapse in an adverse pressure field,the spectral parameters of the induced acoustic pressure impulses are determined similarly to equivalent triangular ones.The present analysis can be extended to 3D flows around wings and screw propellers.In this case,the outer expansion of the solution corresponds to a linear lifting surface theory,and the local inner flow remains quasi-2D in the planes normal to the planform contour of the leading edge of the wing(or screw propeller blade).Note that a typical bubble contraction time,ending up with its collapse,is very small compared to typical time of any variation in the flow.Therefore,the approach can also be applied to unsteady flow problems.

SIMULATION AND EXPERIMENT OF BUBBLY FLOW INSIDE THROTTLING GROOVE

The relationship between pressure distribution and cavitation （noise） inside throttling groove is investigated by numerical simulation and experimental method. A valve pocket with several transducers is performed to detect the pressure distributions inside the valve chamber, and the results fit quite well with the computational fluid dynamics（CFD） analysis. High-speed imaging techniques are employed to investigate the cavitation mechanisms, in particular bubble inception and cluster formation near the throttling groove. A spectrum analyzer is used to measure the sound pressure level of noise generated by the bubble flow. It is found that the pressure distributions inside the groove are sensitive to the valve port configuration and back pressure. The pressure distribution determines the bubble size and number passing through the valve grooves and the sound pressure level of noise induced by collapsing bubbles. The inlet pressure mainly affects the saturation degree of bubbly flow inside the groove and the intensity of sound nressure level accordingly.

Propeller cavitation viscous simulation and low-frequency noise prediction with non-uniform inflow

Aiming at predicting ship propeller＇s cavitation low-frequency noise spectrum, a hy- brid method combining the cavitation multi-phase flow unsteady simulation with the pulsating spherical bubble radiated noise theory was proposed. Then, both of the NSRDC4383 5-bladed propeller and a 7-bladed highly-skewed propeller＇s cavitation low-frequency noise spectrum sub- jected to the full appended SUBOFF submarine＇s nominal wake were investigated. The effects of thrust loading and cavity extension on the discrete line spectrum frequency and its spectrum source level were analyzed. The improved Sauer cavitation model and modified shear stress transport turbulence models were adopted to simulate the propeller sheet cavitation along with integrated verification. The cavity volume acceleration related to the characteristic length rep- resenting the unsteady sheet cavitation extension, which was more reasonable than the spherical cavity hypothesis, was used to the cavitation low-frequency noise spectrum prediction. Results show that the 7-bladed propeller truly appreciates the advantages of smaller loads, latter cav- itation inception and lower cavitating tonal noise comparing to that of the 5 blades. Under the same cavitation index based on ship speed, the interaction of wake inflow and blades will induce significantly low frequency line spectrums and strengthen their source level. Given the submarine wake, cavitation index and rotating speed condition, the thrust, torque and cavity area of blades will decrease with the decreasing load, but the fluctuated acceleration amplitude of cavity volume and the tonal noise spectrum level increases, and the discrete line spectrum components shift mainly to the even times of the BPF harmonics from the odd. If the cavita- tion extension lightens, the BPF harmonics line spectrums will be depressed, and the spectrum level at 1 kHz reduces 2.54 dB. The numerical method above constructs a numerical system to measure the cavitating hydrodynamics and noise performances of ship propellers, which can be productive for the numerical design of wake adapted low noise submarine propeller.

ON CORRELATION OF CAVITATION EROSION AND NOISE ON A FOIL SECTION

Tests with two different sized models were conducted in the Marine Propeller Cavitation Tunnel at the Univer- sity of Tokyo.At two angles of attack the developed sheet-cloud and bubble cavitation occurred.Erosion pattern was recorded by pure indium specimens and noise was measured with a hydrophone mounted inside the tunnel.The broadband SPL was mainly used for correlation of noise with pit counting results from erosion.In case of sheet- cloud cavitation power law relations between velocity and SPL have been found and there existed a linear relation- ship between SPL values and IgPR values(PR-pitting rate).However in case of bubble cavitation such power law relation does not exist between pitting rate and velocity,and neither between SPL and velocity.It was noticed that the different trends of SPL for these two different types of cavitation could be qualitatively linked to their pitting rates by using some SPL values at high center frequencies.

Numerical study of the acoustic spectrum of bubble clusters

This study delved into the acoustic spectrum of bubble clusters,each consisting of 352 vapor bubbles across volume fractions ranging from 0.005%to 40%.The clusters,organized in five distinct layers,were modeled using the volume of fluid(VOF)method to capture the bubble interfaces,and the Ffowcs Williams-Hawkings(FW-H)methodology to compute the far-field acoustic pressure from bubble collapse.Further analysis revealed distinct sound pressure behaviors across different volume fractions:For 25%–40%,time-domain analysis shows that the peak acoustic pressure pulses from the two innermost layers of bubbles are significantly higher than those from the outer layers.In the frequency domain,the octave decay rate of the acoustic pressure levels is relatively low,around−3dB/octave.For 0.5%–25%,four acoustic pressure pulses with similar widths and peak values were observed in the time domain.In the frequency domain,there are three distinct peaks in sound pressure levels(SPL),directly linked to the difference in collapse times of bubbles within the cluster,and the octave decay rate accelerates as the volume fraction decreases,stabilizing at−6dB/octave when the volume fraction is reduced to 17.5%.For 0.005%–0.5%,as the volume fraction decreases from 0.5%to 0.1%,the number of acoustic pressure pulses significantly reduces.Below 0.1%volume fraction,only a single wider pulse is observed.In the frequency domain,the octave decay rate gradually increases with decreasing volume fraction,significantly exceeding−10dB/octave when it drops below 0.1%,reaching up to−11.7dB/octave.

Cavitation bubbles collapse characteristics behind a convex body

Cavitation bubbles behind a convex body were experimentally studied by a high speed camera and a hydrophone synchronously. The experiments were conducted in a circulating water tunnel with five various contraction ratios： β = 0.497, β= 0.6, β= 0.697, β= 0.751, and β= 0.799. The distributions of the cavitation bubble collapse positions behind the five different convex bodies were obtained by combining the images taken by the high speed camera. According to the collapse positions, it was found that no cavitation bubble was collapsed in the region near the wall until the ratio of the water head loss over the convex body height was larger than 20, which can be used to predict if the cavitation damage would occur in the tunnel with orifice energy dissipaters.

ON NECESSITY OF PLACING AN AERATOR IN THE BOTTOM DISCHARGE TUNNEL AT THE LONGTAN HYDROPOWER STATION

The air entrainment for avoiding cavitation damage has been widely used in long free flow tunnels. It is crucial to determine whether an aerator is needed for shorter tunnels. In this article, the bottom discharge tunnel at the Longtan Hydropower Station was involved, for which the free flow tunnel section was only 50.00 m long. The cavitation in the tunnel with and without the aerator was investigated using the physical models of the scale 1/30, through the measurements of cavitation noise. The experimental results show that it is necessary to place the aerator at the inlet of the free flow section for higher reservoir level to protect this tunnel from cavitation damage.