On experimental techniques for the determination of the vortex cavitation on ship propellers

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On Experimental Techniques for

the Determination of Tip Vortex

Cavitation on Ship Propellers

G. Kuiper

Report 1204-P

July 1999

Presented on the 3rd ASME-JSME Joint Fluids

Engineering Conf, San Francisco, California,

ISBN O-7918-1961-2, Paper FEDSM 99-7302

TU Deift

Faculty of Mechanical Engineering and Marine Technology

Ship Hydromechanics Laboratory

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ABSTRACT

The accuracy of tip vortex inception measurements on propellers is investigated using visual observations, acoustic

measurements and measurements of the cavitating vortex

diameter. The conclusion is that visual observation is at least as

accurate as the other methods. When the propeller is to be

optimized in

order to delay inception

it is necessary to distinguish the location and type of vortex at inception. A

distinction is made between trailing vortex cavitation, local tip vortex cavitation and leading edge tip vortex cavitation. Each type is expected to be controlled by different parameters. In

further investigations these vortex types are to be used for the suppression of tip vortex cavitation.

INTRODUCTION

The present paper addresses the determination of inception of propeller tip vortex cavitation. It has been demonstrated that vortex cavitation on a foil in a laboratory set-up often shows an

erratic behavior, especially so near inception, e.g. Van

Rijsbergen and Kuiper (1996). This is even more so for vortex cavitation originating from the tip vortex of a propeller blade. The often whimsical behavior is largely caused by a complicated

pattern of vortices, shed near the blade tip. Although vortex

cavitation inception on foils has had considerable attention, e.g. ITTC (1993), these studies almost always address a foil with an elliptic circulation distribution.

As part of a US and Dutch Navy sponsored program on tip vortex cavitation, three procedures were studied at MARIN with

respect to their

ability to accurately determine cavitation

inception on model scale, yet showing sufficient resolution and information for a further analysis of the propeller design. The

traditional technique is through visual observations. First the

accuracy of this method was evaluated. Two other techniques

Proceedings of the 1999 ASMEIJSME

Fluids Engineering Division Summer Meeting

July 18-23, 1999, San Francisco, California

FEDSM99-7302

ON EXPERIMENTAL TECHNIQUES FOR THE DETERMINATION OF TIP VORTEX

CAVITATION ON SHIP PROPELLERS

by

Torn van Terwisga, Gert Kuiper and Martijn X. van Rijsbergen MARIN, Haagsteeg 2, 6708 PM WAGENINGEN

The Netherlands

were evaluated, viz, an inception criterion through acoustic

measurements and a cavitation inception criterion through the relation between vortex cavity diameter and cavitation number.

The technique of visual observations is subsequently further

evaluated with respect to analysis and optimization of cavitation

i ncept ion.

The focus of this investigation is on propellers with a

strongly unloaded tip, where inception of tip vortex cavitation is delayed.

NOMENCLATURE

cavitating core radius (m) propeller diameter (m)

propeller advance coefficient; J

nD

propeller thrust coefficient; K =

pn2D4

rotation rate of propeller (sec5.

pressure at

center of the

test section at

cavitation inception (N/rn2)

P', vapour pressure (N/rn2)

Pa - pressure at center of the test section(N/ii)

T - propeller thrust (N)

Lia tunnel velocity (m/sec)

p density of water (kg/rn3)

T

cavitation inception number;

2

cavitation number at inception pressure p

po_ P.

pnD

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-VISUAL OBSERVATIONS

Test set-up

For visual observations on a propeller tip vortex, a propeller in a uniform inflow, operating in the MARIN Large Cavitation Tunnel will be used. A cross section of this tunnel is presented in Figure 1. For each point where visual observations are made,

the operating condition of the propeller is determined by the

non-dimensional cavitation number a and thrust coefficient Kr or advance ratio J.

£&M

Fig. I. Test set up propeller uniform flow tests in MARIN large Cavitation Tunnel

The advance velocity U is determined from the pressure

over the contraction of the test section. The velocity from these pressure readings is calibrated with LDV measurements at the

propeller position. The thrust is measured behind the shaft

bearing and sealing, necessitating a small pressure dependent

correction on the propeller thrust

reading. The maximum deviation from the niean advance axial velocity in the propeller plane is approx. 2.5% and the turbulence intensity of the flow in the test section is approx. 1 .2 %. The water quality is controlled by the total air content, Inception tests are normally conducted at a total air content of approx. 5.5 No effect of air content could

be discerned for increasing values of the air content. Higher

values of the total air content were not used because this limits

visibility at the lowest cavitation numbers and increases the risk of undesired gaseous cavitation.

The propeller leading edge and tip are covered with a carborundum strip of 40 tm average grain size to ensure a

turbulent boundary layer on the propeller blades. The width of the strip has a maximum of 5 mm at the blade root and tapers toward the blade tip (Fig. 2). A grain coverage ratio of 50% is pursued, generally leading to coverage ratios in between 30 and

70%.

Fig. 2. Distribution of sand roughness along Leading Edge

and Tip

Procedure

Vortex cavitation inception from visual observations is

determined using stroboscopic illumination of the propeller. In

the procedure used at MARIN, first the blade angle position

where cavitation occurs first is determined. For this blade angle

position, inception is subsequently determined. The effect of elevation and of spatial distortions of the flow field is thus

excluded. It is important to realize, that when using this stroboscopic technique, one samples pictures that are

essentially uncorrelated, as every exposure or video fraiiìe

stores the picture pertinent to one blade passage. The observer

consequently applies a statistical criterion to call inception,

based on a sufficiently large number of revolutions (typically of

the order of 2000).

Vortex cavitation inception is defined as the condition at which an incipient vortex cavity is visible for some 30 to 50% of the time. In case of a weak vortex, the vortex cavity may occur as a trace of bubbles rather than a string type cavity (Fig. I lb).

In this case, the above criterion is less defined and vortex

cavitation inception is defined as the condition where some 10

to 20% of the vortex trajectory is covered with bubbles. The

application of these inception criteria is done by the observer.

A short study showed that the repeatability of desinent tip

vortex cavitation was not different froiìi that of incipient

cavitation. In the present tests inception has been used.

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Precision of inception measurements

The aim of cavitation inception measurements on a

propeller is to predict the cavitation inception speed of a vessel

propelled with the tested propeller. The inception speed is determined by the intersection of the line of operational

conditions of the ship and the cavitation inception line of the propeller, as illustrated in Fig. 3.

1(ti*.dPiLlO

K1

Fig. 3. Representative cavitation inception diagram with propeller operational line.

Because the operational conditions generally show little variation in KT over the speed range of interest, we will consider the accuracy of the inception measurement at a constant KT value in terms of the precision of the cavitation index.

The uncertainty of the cavitation index at inception is

caused by the accuracy of the propeller thrust and rotation rate

measurement at inception and by the precision with which

inception is determined. The first cause of uncertainty is

typically of the order of 0.5 %. The latter cause is dominant and determines the total uncertainty.

Cavitation inception is a highly stochastic phenomenon. The precision with which inception can be determined depends on the following aspects:

Blade geometry tolerances

Reynolds number and/or velocity effects Water and flow quality

Quality of iiiterpretation

From a large number of repeat tests on the same propeller, conducted in a time span of approx. 1 .5 year, a rough estimate

could be obtained on the importance of the above aspects.

Their estimated contribution to the precision of vortex

cavitation inception is presented in Figure 4. Reynolds number effects (,sec e.g. Keller [1994] and Rood[1997].)were not

investigated. e ffsn(qdl) Rn sn(Rn) le sn(eg) 4 6 8 10 12 standard e,ro( L%)

Fig. 4. Assessment of standard errors after source.

The precision estimate due to the blade geometry is

assessed from tests on three different five bladed model

propellers, two of which were made by MARIN and were hand finished according to standard naval propeller practice. Because measurements on different blades were conducted at nearly the same time, water and flow quality can be assumed to be equal

for all blades, as is the Reynolds number. The difference

between the distinct blades must therefore be caused completely by geometric tolerances and the quality of

interpretation.

The effect of interpretation of the inception criteria was

assessed from many repeat tests with the same observer, and a set of tests with two different observers. Although the sample of different observers is too small to draw general conclusions, no significant distinction between the two observers could be

not iced.

Incidentally, during a set of repeat measurements, all

measurements within half an hour, the scatter in inception

number indicated that there were two distinct inception numbers

around '=1.0l and 1.08, each showing the expected

repeatability. The inception numbers showing this step are

presented in Table I.

Standard errors after source

Table i Inception number o normalized with o' from test I for five repeat measurements at Rn09=3.2 106, turbulence

tri pp i ng through carborundum strip

lt is believed that this is caused by flow conditions on the propeller tip, similar as observed by e.g. Arndt and

Maimes[1997] and Pauchet et al [1996] on aNACAI6O2O profile. Slight variations in the inflow can cause two alternative

conditions of the the boundary layer flow, affecting both the lift

o'

1.00 2 1.01 3 1.09 4 1.03 1.07

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and cavitation inception. This incidental behaviour could,

however, be distinguished from scatter

From the experimental data collected, a distinction between the effect of nuclei density and the effect of flow quality could not be made. lt is expected however, that the nuclei density is responsible for the scatter in the majority of cases.

lt is concluded that the uncertainty in visual inception is in the first place caused by geometric tolerances in the model

propeller, followed by uncertainties due to water and flow

quality. The uncertainty due to interpretation of inception is marginal for a skilled observer.

ACOUSTIC INCEPTION

Test set-up

At full acoustic inception determines the quality of the propeller. To study the relation between visual cavitation inception and acoustic inception, the noise in the Cavitation Tunnel was measured by a hydrophone mounted in a water filled recess that was mounted against the tunnel wall. The vessel was separated from the tunnel water through a sound transparent window made from perspex. The sound signal from the hydrophone and the ambient tunnel pressure were recorded

on tape for further analysis.

The noise signal was analyzed by counting the number of pulses (referred to as number of events) exceeding a certain threshold in a time interval of0.5 sec. This number was collected and plotted over many time intervals. The threshold level was chosen such that the number of events were approximately zero in the non-cavitating condition. In doing so, the background noise level of the cavitation tunnel was eliminated. Cavitation inception can then be associated with a distinct change in the event rate.

Res ui/s

The event rate as a function of ambient pressure for a five bladed propeller is presented in Figure 5. The inception

conditions obtained from visual observations are indicated with grey bars.

Visual observations show a difference in cavitation

inception between the different blades. The first blade (blade 2) shows visual inception at an ambient pressure of sorne 107 kPa, whereas the last blade (blade 4) shows inception at sorne 88 kPa.

Acoc,ticIflcOptIOfl

O IO 20 00 40 50 50 70 80 00 1W 7W IA IA IC IA

Fig. 5. Time traces of pressure and number of acoustic events.

For the propeller under investigation, the event rate was associated with tip vortex cavitation . Two discontinuities in the number of events can be discerned, one after some 30 sec., the other after some 105 sec. These discontinuities can be associated with cavitation inception of blades 1,3 and blade 5 respectively. Inception of blade 4 is more difficult to discern due to the increasing level of the already cavitating blades. lt is

noted that the discontinuities in the event rate occur earlier than

the visual inception points. This is partly caused by the

definition of visual inception, where the cavity should be visible

for sorne 30-50% of the time. The differences in inception

pressure between the two procedures are however sufficiently small (some 4%), to make both procedures useful. The major problem with the acoustic inception procedure is that distinct cavitation patterns and positions are not discerned. This

problem is illustrated by the greater scatter in number of events with decreasing ambient pressure, caused by the increasing cavitation.

It is concluded that the correlation between visual and acoustic inception is satisfactory, but that the lack of resolution in the acoustic procedure prohibits an extensive analysis of the inception results.

INCEPTION FROM CAVITY DIAMETER MEASUREMENTS

An alternative way of determining vortex cavitation

inception of trailing tip vortices is found from the relation

between mean cavity diameter and the cavitation numberA possible form for this relation can be derived from the spiral vortex model as worked out by Kuiper 198l]:

C

typ _(I)

where a = cavitating vortex core radius.

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Once the coefficients c and p in eq. (1) are determined from a best fit method with experimental data, an inception criterion can be derived from a minimum diameter criterion. The precision with which inception can be determined is thus dependent on the precision with which this relation can be established.

To assess the precision of this relation, the coefficients have been determined for a propelleN at different operating points (J-values). Vortex cavity diameter measurements were typically made at some 5 to 9 cavitation numbers at one advance ratio.

The diameter measurements were made using video

observations which were analyzed by a digital image processor. This niade it possible to make many diameter readings.

Diameter readings were mainly made at locations in the tip vortex at 45 behind the tip. This corresponds to approx. 0.95 of the maximum chord length behind the blade, measured along the pitch line. The vortex was stroboscopically illuminated, so that the vortex of a particular blade remained visually steady in the same position. For each condition some 30 seconds of video tape were recorded. For each of these conditions, some 5 frames were analyzed, and from each frame some 30 diameter readings were made of the vortex cavity at slightly different positions

along the vortex (Fig. 6). The postprocessor uses the differences in color intensity between cavity and background to determine the edge of the cavity. The vortex axis was selected manually to allow proper diameter measurements.

Fig. 6. Digital image processing of the tip vortex diameter. The -y relation is affected by several variabies, such as

position of observation, blade geometry, KT, gas content and increasing or decreasing ambient pressure during the

experiment. This effect is reflected in the parameters p and c. Figure 7 shows the effect of the thrust coefficient Kr on these regression parameters.

Correlation between Kl and c

at thftelent al, 035*0010 (between b,adlets

r. 002 c(365( a cl559( 0(7881 (ColsoElport) e 0(369)

.

--015 020 075 070 035 610 Kl

Fig. 7. Effect of thrust coefficient on regression parameters p and c

There is significant scatter in the value for p, especially for the lower thrust coefficients where the diameter of the vortex

cavity is less defined. The standard deviation of p shows a

corresponding increase with decreasing thrust coefficient. These results suggest that a value of p=2 is representative for all cases, a value which was found earlier I!Kuiper, 1979]. In another case however, a value of 1.5 gave a better fit

[Kuiper,1981]. The coefficient e fits a fourth power function of K. which is consistent with the roll-up models of Moore and Saifman [1973] and Rossow[1973j:

A method to determine cavitation inception from the

cavitating diameter relation is to define cavitation at a fixed minimum diametyer. This was investigated by calculating the cavitating core diameter from the regression curve of the ac-a relations at the visually determine inception index. This has been done for 16 independent inception measurements.

Figure 8 shows for all the measurements in the data set cavitating core radius at the visually determine inception index. An uncertainty in the diameter readings is indicated corresponding to a 95% confidence level.

Fig. 8. Cavity diameters and their 95% confidence levels for several inception numbers.

Although Fig. 9 indicates a constant inception diameter, there is a significant scatter in the inception diameter and the uncertainty is of the calculated dianìeter is still high. Based on the standard errors in the regression coefficients p and c, the standard error in inception number is calculated to vary from 7 to 31% in this data set. lt should be kept in mind that this

scatter contains also the uncertainty of the visual observations. One of the factors affecting the cavitating vortex diameter is the effect of diffusion on the diameter of the cavitating vortex [Briançon-Marjollet et al.,l996]. Gas diffusion into the vortex

core was found to affect the a-c relation significantly, especially at high gas contents. An illustration is given in Fig.9.

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Re5treion fit of tent no I4067

A FURTHER EVALUATION OF VISUAL

OBSERVATIONS.

One of the main advantages of visual observations is the fact that the location and type of incipient cavitation can be

discerned. This is a necessity when vortex inception occurs at

or close to the propeller tip. Together with the fact

that

alternative methods did not produce significantly more accurate results, it was decided to further evaluate the visual observation technique.

Tip vortex cavitation on a propeller with unloaded tip

occurs very often close to the propeller tip or even attached to it. This requires an evaluation of the inception mechanisms and the parameters controlling it.

The lip loading ofpropeller.

When the flow on a propeller blade remains fully attached to the blade surface, vorticity is shed into the flow at the trailing edge of the blade. In that case the strength of the vortex sheet depends on the radial loading distribution ofthe propeller blade. In this respect there is no difference with an airfoil .Tip vortices

on airfoils have been subject of extensive research because the

presence of a tip vortex endangers the landing or take-off of

following aircraft. These investigations were therefore focussed

on understanding and controlling viscous diffusion of the

completely rolled-up tip vortex far downstream of the wing with a heavily loaded tip.

The mean angle of attack in the tip region of a propeller

with unloaded tip will always be close to zero in the design

condition. In this situation the rolled-up vortex is weak and local minimum pressures occurring at or close to the propeller tip may determine cavitation.. A comparison between Fig. 10.

Comparison of loading distribution ofa foil and a propeller with unloaded tip.

The elliptical loading distribution of a planar elliptical foil and the radial loading distribution of a propeller is shown in Fig.l0. The span of the propeller is measured from the radius of maximum loading and tììade non-dimensional with the distance

from the location of maximum loading to the tip.

Because a propeller nearly always operates

in a

non-uniform velocity field the blade tip experiences loading

variations during one blade revolution. The critical conditions for tip vortex cavitation are at the maximum and the minimum

angle of attack.

o.

propeler

Fig. 10. Comparison of loading distribution of a fOil and a navy propeller.

This situation has not been investigated yet in great detail,

although a great number of concepts to delay cavitation

inception have been tried experimentally [Platzer and Saunders,

1979]. In this situation roll-up of the trailing vortex sheet is still

of minor importance and local flow around the tip and leading

edge determines cavitation inception. Very few measurements or

calculations are available close to the tip. An example is the work of Dacles-Mariani and Zilliac [1995] for elliptical and rectangular foils, which has been compared to measurements by Corsiglia and Jacobsen[197l]. Most attention has been given to the vortical flow downstream of the tip, where roll-up of the trailing vortex begins [Jessup, 1989; Fruman et al, 1992]. The flow in those regions is related to only one type of tip vortex

cavitation, as defined below.

Trailinc._' vortexcavitation.

OEm Ls

O OE.00 I.E 2 25 3 35

Fig. 9. Effect of pressure variation on the ac-cs relation at

high gas content.

lt is concluded that further investigations are required to

determine if this approach can reduce the uncertainty of

inception measurements. There was another reason, however,

not to pursue this approach in the context of this project.

Measurements of the cavitating diameter were also carried out

on a propeller with unloaded tip. In that case it was only

possible to determine relations between the cavitating diameter and the cavitation index at high propeller loadings. In the design condition tip vortex inception took place close to the tip or at

the tip itself. This makes that for unloaded propeller tips, on

which this investigation was focussed, the

method of

measuring the cavitation diameter was found to be inappropriate.

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Cavitation inception (the beginning of cavitation) in the

rolled-up tip vortex occurs downstream of the blade, where the

tip vortex has increased in strength sufficiently so that the

minimum pressure in the vortex core becomes lower than the critical pressure. In this case cavitation inception occurs iii the

flow downstream of the tip and at inception short pieces of

tube-like cavitation occur. (Fig.! lb). At lower pressures these

parts coalesce to form one long cylindrical tip vortex cavity,

often not yet attached to the tip. This type of inception will be

called trailing vortex inception in this paper. This type of

cavitation has been investigated extensively and theoretical

models [Rossov, 1973, Staufenbiehl, 1984, Rule and Bliss, 1998 1

are related to this type of vortices. Reduction of the tip loading ofa propeller delays this type of cavitation.

Fig. li Trailing vortex inception Local tip vortex cavitation.

On propellers with an unloaded tip cavitation inception

often occurs at or close to the propeller tip, where the roll-up of the trailing vortex sheet is of minor importance. In this case a

short cylindrical cavity occurs attached or nearly attached to

the propeller tip. (Fig. 12). This type of cavitation inception is

called local tip

vortex inception. When the

pressure is

lowered,local tip vortex cavitation increases in length and

merges into the rolled-up trailing vortex or the rolled-up

cavitating trailing vortex grows upstream to connect with the local tip vortex cavitation.

Inception of local cavitation occurs when there is a three

dimensional flow around the propeller tip, which causes

separation at the tip contour. Local roll-up occurs and the minimum pressure in this tip vortex is so low that cavitation

inception takes place. For the three-dimensional flow a certain amount of tip loading is necessary, and it can be expected that the local geometry of the tip is an important factor in this type of inception.

Fig. 13. Leading edge tip vortex cavitation.

Purpose of and di/ficulties in the determination of the type of

vortex.

The purpose of the distinction between these types of

vortex cavitation is to distinguish between mechanisms that are

causing inception. As mentioned above trailing vortex

cavitation is controlled by tip loading. lt can be expected that local tip vortex cavitation is strongly affected by the local tip

geometry. Leading edge vortex cavitation is expected to be controled by the pressure distribution at the leading edge at inner radii and also by transport of vorticity towards the tip.

Fig.12 Local tip vortex cavitation.

Leading edge vortex cavitation inception.

Vorticity can also he generated at the leading edge of a

blade section due to a sharp low pressure peak

. When

separation occurs a separation bubble s formed at the leading edge. In combination with a radial velocity component, this separation zone develops into a leading edge vortex, where

vorticity is transported in radial direction. This is very similar to

the vortex which develops at the leading edge of a swept wing or delta wing. Since the blade sections of a propeller are thin in comparison to airfoils (typically the thickness to chord ratio in the outer radii is around 5 percent) such a leading edge vortex

develops already at low angles of attack.

Leading edge vortex cavitation occurs predominantly at a

high blade loading. When the propeller tip is unloaded, the

leading edge vortex leaves the blade at lower radii. Cavitation

inception of such a leading edge vortex is shown in Fig. 13. This

leading edge vortex cavitation may may merge with local tip vortex cavitation .When the local tip vortex is connected to the

leading edge vortex, transport of vorticity from the leading edge also determines inception of the local tip vortex.

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This distinction is therefore necessary for the design of

propeller blades with delayed tip vortex inception.

In sorne cases the distinction between the various types of cavitation is difficult. When the inner radii of the propeller are

lightly loaded relative to the tip the leading edge vortex cavitation will coincide with local tip vortex cavitation and

cannot be distinguished. Propellers with a strongly unloaded tip, however, will have a higher loading at inner radii to produce the required thrust and leading edge vortex cavitation will occur.

There is a special difficulty in the determination of the leading edge vortex cavitation. The detection of this type of

cavitation is in many cases hampered by the presence of sheet cavitation.(Fig. 15). In inception conditions at full scale no sheet cavitation is present. However, because of the scale effects on vortex cavitation inception, cavitation in the tip

vortex at model scale occurs at much lower pressures. Sheet

cavitation inception is not subject to such scale effects. At

model scale the sheet may therefor blur or the presence of a leading edge vortex or even affect its strength.

In this project inception of leading edge vortex cavitation has been determined when a vortex is seen protruding out of the sheet (Fig.15). lt still has to be investigated whether and how the cavitating sheet affects a leading edge vortex.

Fig. 14. Leading edge tip vortex cavitation in combination with sheet cavitation.

When the design mean loading at the tip is low, there is the

possibility that a further decrease of the loading causes a

negative local tip vortex, while the leading edge vortex is still positive. This leads to a mutual repulsion of the vortices. The result is a cavitation pattern as shown in Fig. 16.

Fig.15. Wrapping of opposite leading edge and local tip vortex cavities.

CONCLUSIONS

The traditional technique of detecting cavitation inception from visual observations, using a criterion based on a

percentage of time presence of the cavity appeared to be the most practical criterion for analysis purposes. For optimisation

of the propeller the type of vortex needs to be carefully

defined., including the position of occurrence, as well as the way the cavitation was visualized.

An acoustic criterion, based on the exceedence of a threshold level of events appeared to give a satisfactory

correlation with the visual inception criterion used here. It's resolution and its ability to distinguish between different types of cavitation are,however, inferior to visual observations.

A cavitation inception criterion based on the experimentally derived relation between cavity diameter and cavitation number

was still hampered by the lack of accuracy and by being

restricted to higher tip loadings.

In a further analysis of visual observations, three types of vortex cavitation in the tip region are distinguished by their position of appearance.

Trailing vortex cavitation occurs well behind the blade tip (order of one maximum chord length behind the blade tip along the pitch surface. lt's ruling mechanism is the rolling up of the shed trailing vorticity sheet.

Local tip vortex cavitation appears as attached cavitation to the blade tip and has its position of maximum strength close to this tip. It's ruling mechanism is local separation of the flow with a sufficient pressure gradient rectangular to the detachment line and sufficient axial velocity.

Leading edge vortex cavitation typically occurs beyond a radius of O.85R. It's ruling mechanism is similar to that of local tip vortex cavitation.

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w

The leading edge vortex and the local tip vortex may mix up, and

their combined vortex will eventually mix up in the trailing

vortex.

In the experimental evaluation of propeller designs on tip vortex cavitation inception, it is of paramount importance to use an accurate propeller geometry. This requires NC machined

propeller models in practice. Further work is required on the effect of water and flow quality on propeller vortex inception and on the interference between sheet and leading edge vortex cavitation. This should lead to improved scaling rules for an improved prediction of full scale cavitation inception. Recent and future developments in visualization technology and image postprocessing will have to be used to come to these requested improvements.

REFERENCES

Briançon-Marjollet L., Merle, L.; 'Inception, development and

noise of a tip vortex cavitation', 21" Symposium on Naval

Hydrodynamics, 1996

ITIC (1993); 'Report of the Cavitation Committee', 20th International Towing Tank Conference, San Francisco, 1993

Keller, AP. (1994); 'New scaling laws for hydrodynamic cavitation inception', The second International Symposium on cavitation', (Kato, H., ed.), Tokyo, pp. 327-334

Kuiper, G.; Cavitation inception on ship propeller models',

Ph.D. Thesis, 1981

Pauchet, A., Viot, X and Fruman, D.H. ; 'Effect of upstream

turbulence on tip vortex roll-up and cavitation', ASME-FED

Conference, Vol. 1, 1996

Rood, E.P. (1997); 'Critical, pressure scaling of Schiebe headform

traveling bubble cavitation inception, 1997 ASME Fluids

Engineering Division Summer Meeting, FEDSM'97-3265

Van Rijsbergen, M.X. and Kuiper, G.; 1'vlodelling a cavitating vortex', 1997 ASME Fluids Engineering Divsision Summer

Meeting, FEDSM'97, June 22-26 1997

N4cCormick, B.W.;'On cavitation produced by a trailing vortex from a lifting surface', Trans. ASME, J. Basic Eng., pp369-379),

1962.

Corsiglia VR., 1971 'An Experimental Investigation of Trailing Vortices Behind'

Jacobsen R.A. , a Wing With a Vortex Dissipater, aircraft wake turbulence and its detection plenum press, ny. 1971

Dacles-Mariani, 1995, Numerical/Experimental Study of a

Wingtip Vortex in the Near Field, AIAA JOURNAL, VOL.33

NO.9, pp1561-1568

Jessup, S.D., 1989, An Experimental Investigation of Viscous

Aspects of Propeller Blade Flow. THESIS,Catholic Univ. of

America, Washington D.C.

Fruman, D.H., Duque, C., Pauchet, A., 1992, Tip Vortex Roll-zip and Cavitation, 19°' Symp. On Naval Hydrodynamics, Seoul. Kuiper, G., 1979 Modelling of Tip Vortex Cavitation

on Ship Propellers, 411-I LIPS PROPELLER SYMP. DRUNEN

McCormick, J., 1962 On Cavitation Produced by a Vortex

Trailing from a Lifting Suiface ASME J. BASIC

ENG.,PP369-379

Moore, d.w., Saffman,p.g., 1973 "Axial Flow in Lasminar Trailing Vortices", Proc. Royal Soc. London,A333, p49 I-508.

Platzer,G.p.,sou 1980 Tip /ortex Characteristics And Delay

On A

Der, W.G., Three Dimensional Hydrofoil, proc. 19th attc,ann

arbor.

Rossow, V.J., 1973, On The Invisced Rolled-Up Structure Of Lift Generated Vortices, j. Aircraft, vol.10, no.11, nov. 1973

Rule, ja., Bliss, D.B, 1998, Prediction of Viscous Trailing Vortex Structure from Basic Loading Parameters AIAA Jounial Vol.36,

pp2O 8-2 IS

Staufenbiel. R.W., 1984, Structure of Lift Generated Rolled-Up Vortices, J.Aircraft Vol.2lNo.10