ON THE THEORETICAL PREDICTION OF CAVITATION INCEPTION

After a brief review of the traditional theoretical prediction methods,i.e.bubbledynamics method and pressure parameters method,this paper presents a new prediction method,called“fuzzy prediction method”.The new method consists of two predicting channels(analysischannel and empirical channel)and one scale-effect gate.It can not only give a reliable predic-tion result,but also,better than other method,predict cavitation type on the body.     

CAVITATION INCEPTION OF GATE SLOTS

This paper,starting with field investigation,gives a comprehensive review ofthe hydraulic design and performance of gate slots in 85 existing projects covering 158 gates ofvarious dimension and operating head.Systematic model tests have been conducted in a watertunnel to optimize the configuration of gate slots.Main parameters affecting cavitation inceptionhave been studied,such as the ratio of width to depth,the relative offset,the slope of side wall.as well as the radius of curvature of slot's corner.The scale effects of cavitation modelling arepreliminarily verified.The conformity between model studies and prototype inspections is quiteagreeable.Conclusion and suggestion are made to meet the demand of hydraulic design of gatesolts.     

ZERO-TENSILE STRENGTH FOR RESULT COMPARISON IN CAVITATION INCEPTION TESTS

ＮＯＭＥＮＣＬＡＴＵＲＥＡｉ 　　ｓｅｃｔｉｏｎａｒｅａｏｆｔｈｅｅｎｔｒａｎｃｅｏｆＣａｖｉｔａｔｉｏｎＳｕｓ ｃｅｐｔｉｂｉｌｉｔｙＭｅｔｅｒＣＳＭ　ＣａｖｉｔａｔｉｏｎＳｕｓｃｅｐｔｉｂｉｌｉｔｙＭｅｔｅｒＣｃｏｅｆｆｉｃｉｅｎｔｆｏｒｃａｌｃｕｌａｔｉｎｇｔｈｅｔｅｎｓｉｌｅｓｔｒｅｎｇｔｈｂｙｕｓｅｏｆＣＳＭＰｉ ｌｏｃａｌｐｒｅｓｓｕｒｅａｔｔｈｅｅｎｔｒａｎｃｅｏｆＣＳＭＰ0 ｒｅｆｅｒｅｎｃｅｐｒｅｓｓｕｒｅｏｆｏｎ ｃｏｍｉｎｇｆｌｏｗｉｎｗｏｒｋｉｎｇｓｅｃｔｉｏｎＰｖ ｖａｐｏｒ…     

BLADE SECTION DESIGN OF MARINE PROPELLERS WITH MAXIMUM CAVITATION INCEPTION SPEED

Kuiper and Jessup(1993)developed a design method for propellers in a wake based on the Eppler foil design method.The optimized section is transformed into the three-dimensional propeller flow using the approach of the effective blade sections.Effective blade sections are two-dimensional sections in two-dimensional flow which have the same chordwise loading distribution as the three-dimensional blade sections of a propeller.However,the design procedure is laborious in two aspects:finding an optimum blade section using the Eppler program requires much skill of the designer,and transforming the two-dimensional blade section into a propeller blade section in three-dimensional flow is complex.In this work,these two problems were coped with.A blade section design procedure was presented using an optimization technique and an alternative procedure for the effective blade section is developed using a lifting surface design method.To validate the method a benchmark model of a naval ship was used.This benchmark model was extended by new appendices and a reference propeller,and designed using conventional design methods.This reference propeller was optimized using the new design procedure and model tests were carried out.Special attention was given to the data of the model and the reference propeller,to make the configuration suitable for the Reynolds-Averaged Navier-Stokes(RANS)calculations.     

COMPARISON OF RESULTS ON CAVITATION INCEPTION FOR CHECKINGTHE SCALE EFFECTS

1 .　SCALEEFFECTSOFCAVITATIONINCEP TIONSincethediscoveryofscaleeffectsforthecavita tioninceptiononmodeltestsbyAcheret[1] ,therelatedresearchhasbecomeahotsubject,becauseitcouldleadtoresultstobeincomparableandthusitmightbedifficulttofindoutagenerally acceptedsimilaritylaw .However,thesimilaritylawisnecessary .Eitherforthetransferofmodeltestresultstotheprototypeorfortheresultcomparisonofidenticalorsimilarmodelbod iestestedunderdifferentconditions .ThecavitationcommitteeoftheITTCini…     

CAVITATION INCEPTION WITNESSED BY SOUND PRESSURE LEVEL IN MODEL TEST AND PROTOTYPE OBSERVATION

The paper concerns with the relationship between cavitation inception and Sound Pressure Level (SPL). A case study for the Xiaolangdi Hydro-proiect was taken as a typical example, which demonstrated the standard criterion of SPL indicating cavitation inception. Cavitation inception of an orifice plate used for energy dissipation in a tunnel spillway was witnessed and verified both in model test and prototype observation. A design criterion was proposed for a cavitationfree hydraulic structure which can be served as a reference for designers as well as researchers. Consequently, scale effects were discussed with an aim at suggesting an appropriate factor of safety in design.     

ON THE RELATION BETWEEN CAVITATION INCEPTION AND FLOW PATTERN ON PROPELLER BLADES

Oil flow visuallization on the model propellers with and without tripping devices atthe leading edge is revealed in this paper. Roughing leading edge with sand blasting was to simu-late the flow at full-scale condition. The observations of cavitation inception and the detection ofcavitation erosion were made in uniform and non-uniform flow respectively so as to consider theeffects of flow pattern on cavitation inception and to investigate the causes of difference in thelocations of cavitation erosion between model propeller and prototype. In addition, calculationsof the pressure distributions and the behaviour of boundary layer on the propeller blade were alsodone by using theoretical method. It is necessary to gain a better understanding of cavitation in-ception of the propeller.     

PREDICTION OF UNSTEADY CAVITATION OF PROPELLER USING SURFACE-PANEL METHOD

This paper has predicted the range and volume of unsteady sheet cavitation of a propeller by using the surface panel method. The linearization in cavity thickness is adopted to reduce the computing time and storage space. The iteration scheme between chordwise strips has been used because the range and volume of cavitation are both unknown. The propeller cavitation range determined by the calculation method presented in this paper agrees with the observation results of cavity image at cavitation tunnel very well, and this proves the practicability of the method.     

EXPERIMENTAL ESTIMATION OF A SCALING EXPONENT FOR TIP VORTEX CAVITATION VIA ITS INCEPTION TEST IN FULL- ANDMODEL-SHIP

Tip vortex cavitation noise of marine propeller became primary concerns to reduce hazardous environmental impacts from commercial ship or to keep the underwater surveillance of naval ships. The investigations of the tip vortex and its induced noise are normally conducted through the model test in a water cavitation tunnel. However the Reynolds number of model-test is much smaller than that of the full-scale, which subsequently results in the difference of tip vortex cavitation inception. Hence, the scaling law between model-and full-scales needs to be identified prior to the prediction and assessment of propeller noise in full scale. From previous researches, it is generally known that the incipient caivtation number of tip vortex can be represented as a power of the Reynolds number. However, the power exponent for scaling, which is the main focus of this research, has not been clearly studied yet. This paper deals with the estimation of scaling exponent based on tip vortex cavitation inception test in both full-and model-scale ships. Acoustical measurements as well as several kind of signal processing technique for an inception criterion suggest the scaling exponent as 0.30. The scaling value proposed in this study shows slight difference to the one of most recent research. Besides, extrapolation of model-ship noise measurement using the proposed one predicts the full-scale noise measurement with an acceptable discrepancy.     

A STUDY ON CAVITATION AND CAVITATION DAMAGE IN A CAVITATION TUNNEL

The test results on cavitation and cavitation damage in a cavitation tunnel arepresented in this paper.The cavitator in the shape of 60 deg.symmetric wedge was mounted in-to the inlet of the working section of 20×40×170mm~3 in size.The velocities ranged from8.5m/s to 13.5m/s.At first,the flow structure around the wedge was revealed by a LaserDoppler Velocimetry.Then,the fluctuating pressure signals induced by vortex cavitation werepicked up by a micro-transducer.Cavitation erosion tests were just done at the velocity of13.5m/s.The aim of this work is to investigate the relation between cavitation inception andflow structure and to explore the dependence of cavitation damage on cavitation intensity.Thecavitation intensity which is defined as the power density per unit time can be regarded as the in-put of the test system.The intensity of cavitation damage is supposed to be the consumed powerdensity per unit time and considered as the output of the system of test,The ratio of the output tothe input is as the efficiency of the test system.The test results also show that the output is verysmall in quantity as compared with the input.     

STUDIES ON GREY MODEL DYNAMIC PREDICTION TO CAVITATION DAMAGE OF TURBINES

1 .　INTRODUCTION[1,2 ]Withthedevelopmentofhydroelectricpowerindustry ,therehaveappearedatendencyofusinghighheadsaswellaslargedischarge ,whichmayleadtoseriouscavitationdamageofturbines .Cav itationdamageandfailuresofturbineshaveoc curedfrequentlyinhydroelectric power projectsbothathomeandabroad .Itsmechanismiscom plexanditsinfluencingfactorsarenumerous ,suchasflowvelocity ,flow pressure ,fluctuating pres sure ,qualityofwater ,manufacturingtechnologyofturbines ,structuralcharacteristicsando…     

NUMERICAL SIMULATION OF THE TURBULENT FLUID FLOW AND PREDICTION OF INCIPIENT CAVITATION NUMBER ON ISOLATED SURFACE TRIANGULAR PROTRUSION

ＮＵＭＥＲＩＣＡＬＳＩＭＵＬＡＴＩＯＮＯＦＴＨＥＴＵＲＢＵＬＥＮＴＦＬＵＩＤＦＬＯＷＡＮＤＰＲＥＤＩＣＴＩＯＮＯＦＩＮＣＩＰＩＥＮＴＣＡＶＩＴＡＴＩＯＮＮＵＭＢＥＲＯＮＩＳＯＬＡＴＥＤＳＵＲＦＡＣＥＴＲＩＡＮＧＵＬＡＲＰＲＯＴＲＵＳＩＯＮ￥ＲｅｎＢｉ...     

EFFECTS OF LIQUID QUALITY ON CAVITATION EROSION

The paper concerns with the effects of liquid quality on cavitation erosion, Vari-ous homogeneous liquids such as distilled water, glycerine-water mixtures of different concentra-tion, Kerosene and silicone oil were used as liquid media. Liquids containing solid particles suchas fly-ash ane bakerite powders of different concentration were also studied. Experimental find-ings witnessed the effect of viscosity in attenuating cavitation erosion. Liquids containing solidparticles play dual role of facilitating and alleviating cavitation erosion, in which critical concen-tration and equivalent concentration are in existance. The behaviour of cavitation clouds and theappearance of pittings were also carefully observed to help explain the different effects of liquidquality on cavitation erosion.     

ON CAVITATION TESTS IN VACUUM TANK

In this paper some discussions shown what should be the considerable reasons for difficulty in comparison between results of cavitation tests in vacuum tank and some suggestions are given how to improve the cavitation tests under a condition of vacuum pressure.     

EXPERIMENTAL INVESTIGATION OF CAVITATION IN A SUDDEN EXPANSION PIPE

For sudden expansion pipes,experiments were carried out to study the cavitation inception for various enlargement ratios in high speed flows.The flow velocity of the prototype reaches 50 m/s in laboratory.The relationship between the expansion ratio and the incipient cavitation number is obtained.The scale and velocity effects are revealed.It is shown that Keller’s revised formula should be modified to calculate the incipient cavitation number when the forecasted velocity of the flows in the prototype exceeds the experimental velocity.     

NUMERICAL STUDY OF CAVITATION ON THE SURFACE OF THE GUIDE VANE IN THREE GORGES HYDROPOWER UNIT

Large-area erosions such as rust and obvious cavitation were found on the surface of the guide vane in Three Gorges hydropower units. A numerical explanation of the cavitation is given in this article. At first, based on the characteristic performance curves of the prototype hydro-turbine supplied by ALSTOM together with the actual operating conditions, an operating point is chosen for numerical analysis using the Reynolds-Averaged Navier-Stokes (RANS) equations. The flow passages from the inlet of the spiral case to the outlet of the draft tube are included in the computational domain. The results show that the static pressure on the guide vane surface is much higher than the critical pressure of cavitation. Secondly, a tiny protrusion on the guide vane surface is considered and the problem is simplified to a 2-D problem to study the local detailed flow near the guide vane surface. The protrusion is 0.5 mm in height and is 5.0 mm in width. On the basis of the results of RANS simulations, the 2-D problem is studied using the Large Eddy Simulation (LES). It is shown that there exists a region in which the static pressure reaches a level below the vapor pressure of the water. Thirdly, a cavitation model is included for the 0.5 mm protrusion case and another tiny pit case, with a tiny pit 0.3 mm in depth and 1.0 mm in width. The results show that vapor bubble exists at the protrusion entrance and the pit exit as the low pressure regions.     

A NEW LARGE CAVITATION CHANNEL IN CHINA SHIP SCIENTIFIC RESEARCH CENTER

1 .　INTRODUCTIONIntherecentdecades ,itisrealizedthatitisquitenecessarytocarryouttestsathighReynoldsnumberwithalowbackgroundnoiselevelinordertomeetmoreandmoreurgentneedsofmodernnavyandmaritimeindustry .Anewlargecavita tionchannelisdevelopedinCSSRCfortestingcompletehull/ propulsor/appendagesarrange ments[1] .Itrepresentsoneofthelatestintechno logicallyadvancedlargecavitationtestfacilities.Inthefollowingsections ,thefeaturesandthefirsttestresultsofthechannelwillbedescribed .2 .　DICRIP…     

THE WALL EFFECT ON VENTILATED CAVITATING FLOWS IN CLOSED CAVITATION TUNNELS

For ventilated cavitating flows in a closed water tunnel, the wall effect may exert an important influence on cavity shape and hydrodynamics, An isotropic mixture multiphase model was established to study the wall effect based on the RANS equations, coupled with a natural cavitation model and the RNG k-ε turbulent model. The governing equations were discretized using the finite volume method and solved by the Gauss-Seidel linear equation solver on the basis of a segregation algorithm. The algebraic multigrid approach was carried through to accelerate the convergence of solution. The steady ventilated cavitating flows in water tunnels of different diameter were simulated for a conceptual underwater vehicle model which had a disk cavitator. It is found that the choked cavitation number derived is close to the approximate solution of natural cavitating flow for a 3-D disk. The critical ventilation rate falls with decreasing diameter of the water tunnel. However, the cavity size and drag coeflicient are rising with the decrease in tunnel diameter for the same ventilation rate, and the cavity size will be much different in water tunnels of different diameter even for the same ventilated cavitation number.     

AN ATTRIBUTION OF CAVITATION RESONANCE： VOLUMETRIC OSCILLATIONS OF CLOUD

In order to further verify the proposed theory of cavitation resonance, as well as to proceed the investigations into microscopic level, a series of studies are being carried out on the Warwick venturi. The analysis of the oscillation characteristics of the cavitation resonance has conclusively verified the macro-mechanism proposed through previous studies on other cavitating flows by the authors. The initial observations using high-speed photographic approach have revealed a new attribution of cavitation resonance. That is, the volumetric oscillation of cavitation cloud is associated with the cavitation resonance, which is a collective behaviour of the bubbles in the cloud.     

PROPERTIES OF NATURAL CAVITATION FLOWS AROUND A 2-D WEDGE IN SHALLOW WATER

When a body navigates with cavity in shallow water, both flexible free surface and rigid bottom wall will produce great influences on the cavity shape and hydrodynamic performances, and further affect the motion attitude and stability of the body. In the present work, characteristics of the natural cavitating flow around a 2-D symmetrical wedge in shallow water were investigated and the influences of two type boundaries on the flow pattern were analyzed. The Volume Of Fluid (VOF) multiphase flow method whic...