Cavitation Research and Ship Propeller Design

of Leen van Wüngaarden

Edited by Arie Biesheuvel and GertJan F. van Heijst,

Published by Kiuwer Academic Publishers, P. O.Box ¡Z

3300 AA Dordrecht, ISBN 0-7923-5078-2, Reprinted from

Applied Scientflc Research, Volume 58, Nos. ¡

- 4

TU Deift

Faculty of Mechanical Engineering and Marine Technology Ship Hydromechanics Laboratory

Arie Biesheuvel and GertJan F. van Heijst (Eds.)

In Fascination of

Fluid Dynamics

A Symposium in Honour of

Leen van Wijngaarden

FLUID

MECHANICS

AND

ITS

APPLICATIONS

Volume

45

Series

Editor:

R.

MOREAU

MADYLAM

Eco/e

Nationale

Supérieure

d

'I-Jvdrau/ique de

Grenoble

Boîte

Postale

95

38402

Saint

Martin

d'Hères

Cedex,

France

Aims

and

Scope

of

the

Series

The

purpose

of

this

series

is

to

focus

on

subjects

in

which

Huid

mechanics

plays

a

role.

fundamental

As

well

as

the

more

traditional

applications

of

aeronautics,

hydraulics,

heat

and

mass

transfer

etc..

books

will

be

published

dealing

with

topics

which

are

currently

in a

state

of

rapid

development.

such'

as

turbulence,

suspensions

and

multiphase

fluids,

super

techniques.

modelling

iiumerical

and

flows

hypersonic

and

lt is

a

widely

held

view

that

it is

the

interdisciplinary

subjects

that

w'i!lIl

receive

intense

In Fascination of

Fluid Dynamics

A Symposium in Honour of Leen van Wtjngaarden

Edited by

ARIE BIES:HEUVEL

f.M. Burgers Centre för Fluid Mechanics,

University of Twente, Enschede,

The Netherlands

and

GERTJAN F. VAN HEIJST

J. M. Burgers Centre Jbr Fluid Mechanics,

Eindhoven Un ive rsity of Technology,

The Netherlands

Reprinted from Applied Scientific Research, Vol. 58, Nos. 1-4 (1997/8)

Published by Kiuwer Academic Publishers,

RO. Box 17, 3300 AA Dordrecht, The Netherlands. Sold and distributed in North, Central and South America by Kiuwer Academic Publishers,

101 Philip Drive, Norwell:, MA 02061, U.S.A.

In ali other countries, sold and distributed by Klüwer Academic Publishers

RO. Box 322, 3300 AI-1 Dordrecht, The Netherlands.

Printed on acid-free paper

All Rights Reserved

© 1998 Kluwer Academic Publishers

No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical,

including photocopying, recording or by any information storage and retrieval system, without written permission from the copyrightowner.

Preface, by Arie Biesheuvel and GertiJan F. van Heijst

ixX

P.J. ZANIDBERGEN / images of Leen van Wijngaarden

1-12

A. PROSPERETTI / A Brief Summary of L. van Wijngaarden's Work

Up Till His Retirement

13-32

G. KUIPER I Cavitation Research and Ship Propeller Design

3 3-50

M.P. TULIN / On the Shape and Dimensions of Three-Dimensional

Cavities in Supercavitating Flows

5 l-61

WERNER LAUTERBORN and CLAUS-DIETER OHIL / The Peculiar

Dynamics of Cavitation Bubbles

_3-76

J.R. BLAKE, Y. TOMITA and R.P. TONG / The Art, Craft and Science

of Modelling Jet Impact in a Co1'lapsing Cavitation Bubble

77-90

D.F. DE LANGE and G.J. DE BRUhN / Sheet Cavitation and Cloud

Cavitation, Re-Entrant Jet and Three-Dimensionality

91-114

V.K. KEDRINS Kil / The IordanskyKogarkovan Wijngaarden Model:

Shock and Rarefaction Wave Interactions in Bubbly Media

115-130

RENÉ MOREAU / MHO Turbulence at the Laboratory Scale:

Estab-Fished Ideas and New Challenges

_131-147

GJ.F. VAN H'EIJST and H.J.H. CLERCX / Selforganisation of Quasi-2D

Flows in a Rectangular Container

149-168

N, RILEY / The Fascination of Vortex Rings

169-189

JAMES LIGHTHTLL / Ocean Spray Modelling for Tropical Cyclone

PRAM-lU VALIVETI and DONALD L. KOCH / Instability of

Sedi-menting Bidisperse Particle Gas Suspensions

_275-303

R ZENIT, M.L. HUNT and C.E. BRENNEN

_{/ On the Direct and}

Radi-ated Components of the Collisional Particle Pressure

_in

Liquid-Solid Flows

_305-317

JIM B.W. KOK / The Fokker-Planck Equations for Bubbly Flows and

the Motion of Gas Bubble Pairs

_319-335

PETER D.M. SPELT and ASHOK S. SANGANI./ Properties and

Aver-aged Equations for Flows of Bubbly Liquids

_337-386

KNUD LUNDE and RICHARD J. PERKINS / Shape Oscillations of

Rising Bubbles

_387-408

P.C. DUINEVELD / Bouncing and Coalescence of Bubble Pairs Rising

at High Reynolds Nurnber in Pure Water or Aqueous Surfactant

Solutions

_409-439

JACQUES MAG;NÀUDET and DOMINIQUE LEGENDRE / Sorne

Aspects of the Lift Force

on a Spherical Bubble

_441-461

P.DM. SPELT and A. BIESHEUVEL / Dispersion of Gas Bubbles in

Large-Scale Homogeneous Isotropic Turbulence

_463-482

J.C.R. HUNT / Qualitative Questions in Fluid Mechanics

_483-501

Bies/,euve/ and GJ. F Vail Heifst (eds), 1,1 Fa.ccination of Fluid Dynamics.

33

© l998

Kiuwer Acímdemnic Publishers. Printed in the Netherlands.

Cavitation Research and Ship Propeller Design

G. KUIPER

Marin, P.O. Box 28, 6700 AA Wageningen, The Netherlands

Abstract. The role of cavitation research in the design of ship propellers and the influence of

re-search on propeller design is reviewed. The historical development of rere-search on bubble cavitation is

an example of a lack of communication between research and designi Research on sheet cavitation is

starting now and simplifications such. as two dimensional cavitation are being made. It is argued from

observations on propellers that the use of two-imensionaI cavitaties is not a proper simplificatiön to investigate sheet cavitation An illustration is also given of the gap between the assessment of the risk of erosion on propeller models and research on erosion. Finally, the simplifications of tip vortex inception and the problems of the incçption speed of propellers are discussed.

Key words: propeller, cavitation.

1.

introduction

It is an honor and a p1eas.ire to give a paper on the occasion of the formal retirement

of Leen van Wijngaarden. He started his career at Marin (then Netherlands Ship

Model Basin) and the problems encountered there have always had his attention:.

I have always felt the cooperation with Leen as an illustration of the fact that

technology and science are different and that they need each other.

The difference between science and technology is in the direction of thought:

scientific research wants to undérstand the phenomena and have insight in their

behavior, technology wants to design working equipment in specific applications,

where control: of the phenomena is required. Abstraction from environmental

com-plexity in a laboratory is essential for scientific research, in technology there is the

complexity of the design and the impossibility to isolate various

parameters.

The idea that technology is applied science is a gross simpl:ification of

technol-ogy and leads to conflicts between scientists and designers. Designers will find the

scientific results insufficient for their application and scientists will blame

design-ers for the complexity of their designs. A technical univdesign-ersity has this problem

i:ntemally, leading to a controversy between designers and scientist, especially

since Twente University gives itself the epitheton "Entrepeneurial University", In

practice there is a distinction between scientific results and technology.

J:n the meantime, both science and technológy develop rapidly and the

distinc-tions shift accordingly. Especially the development of computational fluid

Figure 1. Example of a ship propeller (courtesy: Esscher Wyss.

between scientific numerical results and technology has been forgotten and this has

already lead to serious disappointments

on both sides.

In this paper I will try to illustrate the relation between science and

_technology

in the field I have been working on: cavitation on ship propellers. It is not my

intention to review literature, and L will focus on the viewpoint of the designer.

2. The Problems of the Designer

The first question is why cavitation is of interest for the propeller

designer. The

answer is in the detrimental effects cavitation often has. There

_{are three main}

categories of detrimental effects: erosion, noise and vibrations. In case of severe

cavitation it may also cause thrust breakdown, but that problem is

experienced

only with a restricted class of ships.

Why not eliminate cavitation? In uniform flow this might be possible in some

cases, but the propeller is generally mounted in the wake of the ship in order to

re-claim part of the energy which is lost by the resistance of the ship (hull

efficiency).

This wake, however, is strongly non-uniform and the resulting inflow variations of

the blades make cavitation generally unavoidable. To limit the extent of cavitation,

propeller blades are relatively very thin in comparison to airfoils and the blade area

ratio is high, leading to a high aspect ratio of the blades (Figure 1).

The most common effect of cavitation is erosion. It means that the material of

the propeller is eroded due to the impact of cavitation on the blade. An example

of the effect of erosion is given in Figure 2, where the trailing edge of a propeller

Figure 2. Detail of an eroded ship propeller.

Figure 3. Stern of a cruise liner.

blade is eroded due to cavitation generated at the leading edge. When this occurs

near the blade root the strength of the blade is decreased and the propeller will

fail eventually. The eroded surface shows a typical roughness, illustrating that the

erosion is due to many strong impacts

_{on the surface over small areas.}

A very common effect of cavitation is vibration of the hull. This is caused by

rapid fluctuations of the cavity volume. The vibrations of the hull lead to structural

failures, malfunctioning of equipment and problems with personal comfort of the

Figure4. Navy frigate (courtesy Royal Netherlands Navy).

This problem is e.g. sensitive for cruise liners. An important element of a cruise

is the daily meal and the stern offers a nice view. When sailing, the open stern decks

are sheltered and attractive. So the restaurant is located below the open decks

on

the stern, which is at the square windows in Figure 3. Although

at a very attractive

location, this places the designer for a difficult problem because the two propellers,

absorbing each some 20 MW in power, are located some 6 meters below the floor

of the dining room, where a noise level below 60 dB and

a very low vibration

level is required. If only a tiny fraction of the

_{power is diverted into noise and}

pressure fluctuations, the meal will certainly no longer be pleasant. Isolation with

e.g. floating floors is possible, but only in the higher frequency range. The emphasis

in propeller design is therefore nowadays in the frequency range between blade

frequency (some 10 Hz) and 150 Hz.

Noise generation is important also for Navy ships such

as the M-Frigate in

Figure 4. The requirements of the crew are less demanding than on a cruise liner,

but the radiated noise poses problems for the detection equipment and creates a

target for noise-seeking weapons.

The presence of any type of cavitation increases the radiated noise

_drastically

and it becomes important to avoid cavitation altogether. The speed

_{at which}

cav-itation occurs for the first time is called the inception speed. The prediction

and

increase of the inception speed is

a major topic for navy ships. In this case not

only the design condition at a straight course, but also cavitation inception of the

propeller during manoeuvres is important. The strength of the radiated noise of

cavitating propellers is also important, but has a lower priority than the inception

speed.

Fortunately, cavitation is not always dangerous. it is possible to design

a

cavi-tati:ng propeliler with low pressure fluctuations and without erosion, butin that

case

cavitation has to be tightly controlled. This is the main problem of the propeller

designer and it is also the area where cavitation research and design practice

are

still far apart.

3. Bubble Cavitation

The development of cavitation research is an interesting example of the

interac-tion between the designer and scientific knowledge. When steam turbines could

generate enough power to obtain a high ship speed cavitation became a problem

immediately in Parson's test ship, the Turbinia, in 1895-1897

[1]. The problem

was thrust breakdown due to cavitation. This has been investigated in cavitation

tunnels in the first half of the 20th century. Although around i 920 the phenomenon

of erosion became apparent on full scale propellers, the emphasis in cavitation

design remained on the effects of cavitation on thrust. The search was mainly for

blade sections which Were free of cavitation in uniform flow and the developments

in the calculation and design of airfòil sections has been used to improve propeller

design. Cavitation was mainly observed at model scale and the assessment of the

observations was highly empirical.

An impulse for cavitation research came from the work of Knapp and Hollander

in 1948 [2]. Typically for the research approach the problem was simplified by

using a hemispherical headform instead of a prope1ler Contrary to propellers the

calculation of the pressure distribution was possible for this body. The technique of

high speed photography was used to make detailed observations of the phenomena.

These two approaches, simplification and detailed observations of the phenomena,

lead to new developments.

The cavitation observed was bubble cavitation, where small gas nuclei in the

flow expand in a low pressure region and collapse again when the

pressure

in-creases. These cavities move with the flow, which is why this type of cavitation

is also called travelling bubble cavitation. With the knowledge of the

pressure

distribution and with the detailed observations Knapp and Hollander could describe

the behaviour of the observed cavitation using the inviscid equations of motion.

Their approach also formulated the basis for nuclei effects on cavitation

incep-tion, as will be discussed later. Ever since that time cavitation research has focussed

on bubble cavitation (Figure 5).

Thermodynamic effects, viscous effects, effects of non-spherical shape, effects

of the initial size (gas content) on the bubble dynamics, implosion and the

occur-rence of .the microjet, interaction of the nuclei with the boundary làyer, interaction

of bubbles with an acoustic pressure field, all have been invest:igated. The focus

of cavitation research was even further restricted to cavitation inception and scale

Figure 5. Bubble cavitation on a profile.

effects, while on fully developed cavitation the attention went to supercavitation,

which was strongly driven by the Navy.

_{On partially cavitating foils and propeller}

blades much less research work was done.

Bubble cavitation makes

_{a lot of noise, at least at model scale, and bubble}

cavitation got the reputation of being erosive. The propeller designer consequently

avoided this type of cavitation and designed propellers with less camber and higher

angle of attack, which exhibit sheet cavitation. Also at full scale, bubble cavitation

got the reputation of being erosive, although the basis of that reputation is poorly

documented.

For decades, there has been a large gap between cavitation research and

pro-peller design. Although bubble cavitation

_{remained a risk for the propeller}

de-signer, the problems on propellers were in the behaviour of sheet cavitation. Bubble

cavitation was carefully avoided and sheet cavitation was judged on the basis of

model tests.

Does this gap still exist? An important development has been that nowadays

more full scale observations of cavitation on propellers are being made. Although

very few propellers show extensive bubble cavitation, it has

_{become clear that the}

maximum size of the cavitating bubbles at full scale is very small. An example is

given in Figure 6, where extensive sheet cavitation at the outer radii of the propeller

blade is accompanied by streaks of bubble cavitation at inner radii. These streaks

of bubble cavitation look like thin clouds. The end of such

a region of bubble

cavitation is generally not very violent, and this raises the question why bubble

cavitation is so strongly erosive.

Also at model scale this question arises. As a side effect testing with leading

edge roughness, which was used to reduce scale effects on inception of sheet

cav-itation, it was found that leading edge roughness could generate ample nuclei [3].

At model scale with smooth propeller blades relatively large isolated bubbles

_are

observed, as in Figure 7. This type of cavitation is very noisy indeed. However,

when abundant nuclei

_{are present in the incoming flow the number of bubble}

cavi-ties increases and the maximum size decreases. This is illustrated in Figure

8

where

the abundant amount of nuclei on the propeller surface is generated by roughness

elements at the leading edge. Although

_{the trailing edge of the cavity is sharp,}

its appearance is not

_{very violent. Moreover, the amount of noise generated by}

cavitation as in Figure 8 is much less than generated by cavitation as in Figure 7.

,

Figure 6. Full scale cavitation observation with incidental bubble cavitation at inner radii.

Figure 7. Bubble cavitation on a model propeller (low nuclei content).

This supports the suspicion that bubble cavitation is less erosive than generally

thought. The possibility to allow bubble cavitation is

_{even more alluring since}

modern blade sections with delayed inception of cavitation have the characteristics

of laminar flow profiles and tend to exhibit bubble cavitation.

This information has to go back to the cavitation research

_{community. Maybe}

the propeller designer was wrong in avoiding bubble cavitation at all costs. In

cavitation research the single bubble approach has to be modified to reduce the

maximum bubble size and maybe to take the presence of many bubbles into

ac-Figure 8. Bubble cavitation with an abundance of nuclei.

count. On the other hand, cavitation research on sheet cavitation is urgently needed

in order to control induced fluctuation pressures, as will be discussed below.

4.

Cavitation Induced Pressures

Since on propellers bubble cavitation was avoided, the necessary consequence was

that sheet cavitation occurred. Sheet cavitation has

two effects: its growth and

col-lapse causes pressure fluctuations in the flow and the shedding

of cloud cavitation

at the trailing edge causes erosion. Erosion will be discussed later. The problem

of cavitation induced hull excitations was encountered in the seventies when with

increasing engine power the propeller loading was increased or the ship speed was

increased. This problem was attacked at model scale in cavitation

tunnels.

The problem of cavitation induced vibrations is not difficult in principle.

Cav-itation on the propeller blades acts as a monopole, which means that the pressure

fluctuations in the fluid are in phase. The ship hull, which is relatively

_stiff,

in-tegrates these pressure fluctuations and this leads to large excitation forces. The

cavity volume variations act as a distributed source, which can be treated

nu-merically and from which the pressure fluctuations in

_{an unbounded fluid can}

be derived. Effects of the free surface and of the solid boundary of the

hull do

complicate the problem, but at present that is not the main problem.

The main

problem is the determination and control of the cavity shape. This shape,

more

specifically the volume distribution, has to be calculated

very accurately, because

the pressure fluctuations are proportional to the second derivative in time of the

Figure 9. Measurement of the fluctuating pressures on a ship model.

When cavitation is simulated correctly at model scale the induced pressure field

at the ships hull can be measured by an array of pressure transducers (Figure 9).

There are several problems in the experimental determination of the

pressure

fluctuations and the relation with full scale measurements. The main problem is

the hydroelastic behaviour of the ship, and sometimes also of the model, which

makes it difficult to distinguish between excitation pressures and hull response. A

considerable amount of development has to be done in this area.

The problem of the designer is in the control of sheet cavitation. In cavitation

research the problem has generally been simplified by using two-dimensional foils

and two-dimensional cavitation. Initially calculations were linearized and

analyt-ical, e.g. [6], the present development is in potential flow calculations with panel

methods, e.g. [7].

The problem is that steady two-dimensional cavitation cannot exist. At the

clo-sure of the sheet cavity the flow has to be tangential to the profile on the wetted side

and has to be at the vapor pressure at the cavity side of the closure. In potential this

leads flow to a singularity, which can only be overcome by releasing either the

pressure condition or the tangential flow condition or both in the vicinity of the

cavity closure. However, this so-called closure condition has a strong impact

on

the solution, that is on the calculated cavity length and shape.

In real flows the closure of the cavity leads to a stagnation point and a re-entrant

flow into the cavity. This re-entrant flow or jet moves upstream inside the cavity

until it hits the upper boundary of the cavity, which leads to the shedding of the

downstream part of the cavity. An extreme case is shown in Figure 10, where the

re-entrant flow hits the cavity surface close to the leading edge of the cavity and

the whole sheet cavity is shed and collapses in the flow. Note that this collapsing

cloud is highly three-dimensional.

Figure 10. Shedding of cavitation in steady inflow due to the re-entrant jet.

Figure ¡1. Closure of a two-dimensional cavityon a foil.

This mechanism can be simulated using potential flow calculations, as shown

elsewhere in this symposium by de Lange. These calculations are not without

nu-merical difficulties, indicating that in the re-entrant jet viscous effects may also be

important. The trailing edge of a sheet cavity breaks up in highly three-dimensional

structures, as is shown in Figure 11. This problem of shedding

_{becomes even}

more important in the unsteady case. As an example the collapse of a cavity on

a propeller blade passing a sharp wake peak is shown in Figure 12.

When the attention is limited to steady flow

_{a strongly three-dimensional sheet}

is shown in Figure 13.

The trailing edge of the sheet is perfectly smooth,

_{while at most part of the}

trailing edge the re-entrant flow is seen to move towards the tip. A small secondary

cavity is observed at the closure of the sheet. At the inner radii of the cavity some

cloudy shedding is observed at the trailing edge. This is the region where the radial

gradient of the cavity length decreases.

This is also the case in Figure 14, where

_{a re-entrant flow in outward direction}

is observed at inner radii. At outer radii the cavity is more two-dimensional and

shedding occurs, leading to typical cloud cavitation, which is assumed to be

ero-Figure 12. Collapse of sheet cavitation on a propeller blade passing a sharp wake peak.

Figure 13. Stable sheet cavitation on the blade of a propeller model.

sive. This propeller blade has an unloaded tip, which reduces the cavity

_{length in}

the tip region.

These experimental observations show that, in order

to obtain a stable cavity in

steady inflow, the re-entrant jet has to be removed. This can only be the case in

three-dimensional flow, where the region of shedding and cloudy cavitation

can be

restricted to a small part of the sheet. In many cases this is the cavitating tip vortex.

The presence of the re-entrant jet is still important for the calculation of

the cavity

Figure 14. Sheet cavitation on a propeller with unloaded tip.

A simple sink can absorb the flow. Calculations using this approach

are underway

and show very promising results.

The foregoing implies that cavitation calculations and especially the validation

of the calculations by experiments should not be done on two-dimensional foils.

Two-dimensional foils are the most difficult to investigate and their behaviour with

shedding of cavitation is more complex than

_{can be calculated at present.}

Validation of calculations is generally done by comparing the cavity length. This

is not the relevant parameter. Because the main risk of unsteady sheet

cavitation is

vibrations, the cavity volume should be validated.

A strong interaction between the designer and his experiences and the cavitation

researcher is mutually stimulating and can keep both

on the right track.

5.

Cavitation Inception

5.1. NUCLEI

As mentioned above the approach of Knapp and Hollander formulated the basis for

scale effects on cavitation inception, because it related the growth of gas nuclei into

cavities with their initial size. In this field the exchange between cavitation research

and the ship designer has been good, especially because the towing tanks

were

eager to eliminate scale effects on cavitation inception. All efforts were put into

the control of nuclei, up to now with limited results. An illustration is found in the

recommendations of the cavitation committee of the ITTC (International Towing

Tank Conference) which every three years reviews the developments in cavitation

research relevant for ship propellers. A recommendation in 1966 was: "Systematic

nuclei entrained: in the flow", in 1986 a recommendation was: "Efforts should be

devoted to the devekprnent of instrumentation to determine the size and releat'ive

distribution of spherical: gas nuclei in the test water". Research

on cavitation

in-ception has long been focussed on nuclei alone, as in the case of bubble cavitation.

Nuclei still play an important role and control of the nuclei content is important, if

only for the repeatability of tests. The measuring of the nuclei content in cavitation

tunnels is slowly icreasing, the device which is most frequently being used is the

cavitation susceptibility meter using a venturi to create a low

pressure region. The

use of this device is still incidental, however. Only one large cavitation tunnel (the

GTH in Val de Reuil, France) installed extensive equipment to control the nuclei

content.

Sheet cavitation has a different mechanism of inception than bubble cavitation

or tip vortex cavitation, its inception conditions are much less sensitive to the nuclei

content [5]. Therefore, for the majority of propellers (with sheet cavitation) the

nuclei content is less critical. This is why towing tanks have been able to investigate

many propellers without sophisticated nuclei control. Sheet cavitation has its own

specific problems, however.

5.2.

Viscous. EFFECTS ON INCEPTION

Until 1970 all efforts to understand scale effects were directed towards nuclei

con-trol. The effects of the state of the boundary layer became apparent after work of

Arakeri' and Acosta [8] on headforms and Casey [9I on a foil. A laminar boundary

layer was shown not to cavitate when the mean pressure was below the vapor

pres-sure. This work came from researchers without stimuli from any design problem!

Such a laminar flow region is very common on propeller models. An example of the

state of the boundary layer is shown in Figure 15, where the results of a paint tests

on the suction side of a propeller blade are given. The streaks pointing outwards

indicate a laminar region, the streaks in radiai direction at the outer radii indicate

turbulent flow. In this figure the sudden cut-off of the laminar region is caused by

a laminar separation bubble at the leading edge. Cavitation inside the radius of the

radius of the cut-off did nt occur. An increase of the nuclei content of the flow did

not change that situation.

The viscous scale effects on inception have strengthened the desire to

carry out

cavitation tests at maximum Reynolds numbers, so that laminar flow could be

elim-inated. This has lead to cavitation tunnels with very high flow velocities in the test

section (LCC in Memphis, USA; GTH in Val de Reuil, France; Hykat in Hamburg,

Germany). However, the pressure gradient is very important in maintaining laminar

flow, and even at a very high Reynolds number, laminar flow can occur.

The question is if the boundary layer at high Reynolds numbers becomes

tur-bulent due to instability and: natural transition or due to surface imperfections.

This still has to be investigated. Indications are that the blade surface is generally

dominant in generating transition.

Figure 15. Paint streaks on a propeller blade showing regions of laminar and turbulent

boundary layer.

At MARIN the Depressurized Towing Tank is used for

cavitation simulation.

This makes a better modelling of the propeller hull interaction and of the free

surface possible, but it makes it

_{necessasy to maintain the Froude number and thus}

to low Reynolds numbers. This has lead to the application of artificial leading edge

roughness at the leading edge of the propeller to stimulate turbulence and cavitation

inception.

Cavitation research on the effects of surface roughness has been limited. For the

designer many questions remain unanswered. Such questions are the mechanism

of inception and the inception conditions at the roughness elements, the minimum

roughness size to be effective for inception stimulation and the maximum

rough-ness size allowed without changing the mean pressure distribution

_{on the surface.}

As a side effect leading edge roughness was found to generate nuclei, indicating

that some type of micro-cavitation occurs on the roughness elements. This has been

used as an efficient mechanism to generate nuclei locally, which is important

_when

the radiated noise is measured.

6. Erosion

The only tool a designer has to predict erosion qualitatively is observation of the

cavitation. Shedding of clouds ofcavitation behind

_{a sheet is stili the most erosive}

type of cavitation. In a wake field this. can occur in a very small part of the propeller

revolution. The cavitation causing the erosion of Figure 2 is shown in

Figure 16.

The erosive part ofthe cavity

can be seen on the tip of blade 5, where an isolated

cloud cavity collapses just on the edge of the blade. it is

_{not difficult to miss such}

a detail during observations at model scale. The implosion does not

occur at every

revolution, and is visible only in

_{a very small range of blade positions. Many}

pho-tographs have to be taken to find it, which is why video observations are sometimes

preferred. The resolution of video observations is less than photographs,

_however,

and this increases the chance to miss relevant features of the cavitation.

Cavitation research has not given the designer

_{a reliable method to measure the}

erosivity of cavitation. Experiments with the removal of stencjl ink

_{by cavitation}

have been only partially successful, since the calibration and extrapolation of the

speed of erosion is difficult. As a result ink tests can predict the location of erosion,

but not the magnitude of the risk of occurrence or the speed of erosion. This is not

much more than can be derived from visual observations, however.

In cavitation research the question of the mechanism of erosion is not yet

re-solved. The old discussion is if erosion is caused by local high

_{pressures at the}

re-entrant jet of collapsing bubbles or by shock waves transmitted to the surface.

Interesting data were obtained recently by Phillipp and Lauterborn [10], who firmly

concluded 'that only bubbles collapsing on the surface, caused erosión. Erosion was

not found at the locatjon of the re-entrant jet, but at the location of the

_imploding

ring-shaped cavity around the re-entrant jet. This. would confirm that on propellers

Figure 17. Cavitating tip vortex on a heavily loaded tip.

an aid in the assessment of cavitation. This development is interesting and may

lead to new techniques to assess erosion.

The designer problem in erosion is the collapse cloud cavitation. These clouds

are often shed by unsteady sheet cavitation. Steady sheet

_{cavitation, however, is}

also shedding clouds of vapor at its trailing edge. For cavitation research it is

important to investigate the mechanism of erosion in bubble clouds as occur at

the trailing edge of bubble cavitation. The results of Leen van Wijngaarden and his

co-workers as Ompta [11] and Buist [12] should be used to develop scaling rules

both for noise and erosion. This has

_{not yet been done.}

7. Vortex Cavitation

The relation between cavitation research and the propeller designer is not complete

without mentioning vortex cavitation. Tip vortex cavitation is important because it

is often the first type of cavitation which occurs and therefore it determines the

inception speed of Navy Ships. Tip vortex cavitation in a more developed stage has

caused broadband excitation of the ships hull in

some cruise liners.

Inception of tip vortex cavitation is the

_{most difficult topic in cavitation}

Extrapola-Figure ¡8. Developed tip vortex cavitation.

tion from model scale to full scale is done in a crude manner, based on the ratio of

the boundary layer thickness between model and full scale.

There is a gap between the problems of the designer and those in cavitation

research. In cavitation research the problem is generally simplified by making the

problem two-dimensional with the vorticity concentrated in the viscous

core. This

vortex is usually generated by an elliptical foil, e.g. [13]. Such a foil, however, has

a high loading at the tip. On a propeller the cavitating tip vortex is then similar to

that on an elliptical foil, as shown in Figure 17.

However, the propeller designer tries to increase the inception speed of

a Navy

ship by unloading the tip. Tip vortex cavitation occurs in that case on or very close

to the blade surface and not in the developed tip vortex downstream of the tip.

The vorticity is not axisymmetrical in that case and there are no theoretical

mod-els available to simulate or calculate the flow. Application of full NavierStokes

solvers may reveal more of the controlling parameters in wetted flow, but there is

still some time to go before these techniques can be used in propeller design.

Inception can be calculated without cavitation by finding the minimum

pres-sure. In case of developed tip vortex cavitation the situation is more difficult, as is

shown in Figure 18. The complexity of the tip vortex and the modelling of

vortex

researcher to find the relevant parameters and simplifications. As shown above

sometimes cavitation research has been solving problems that were less relevant

for the designer. Sometimes the désigner has been too uncritical and unimaginative

in using the results of cavitation research. This may lead to

unnecessary design

restrictions. This is why the contacts between Marin and Leen

van Wijngaarden

always were very rewarding. I hope that will remain so in the future.

References

BurrilI, L.C., Sir Charles Parsons and cavitation. Transactions of the Institute of Marine

Engineers 63 (1951) 149-167.

Knapp, R.T. and Hollander, A., Laboratory

_{investigations of the mechanism of cavitation.}

Trans. ASME7O(1948) 419-435.

Kuiper, G., Cavitation Inception on ship propeller models. Thesis, Technical University Deift

(1981).

Kuiper, G., Some experiments with specific types of cavitation on ship propellers. ASME J. Fluids Eng. 1(1982) 105-114.

Gindroz, B. and Billet, M.L., Influence of nuclei on the cavitation inception for different types of cavitation on ship propellers. In; Proceedings_{ASME mt. Symp. Symposium on 'Gavitation} Inception, New Orleans, FED-Vol. i77 (i993)pp. _1-14.

Geurst, J.A., Linearized theory for partially cavitated hydrofoils. 'ini'. Shipbuilding Progress 6 (1956) 369-384.

Fine, N. and Kinnas, S.A., A boundary element method for the analysis of the flow around 3D cavitating hydrofoils. J. Ship Research 37 (1993) 213-224.

Arakeri, V.H. and Acosta, A.J., Viscous effects in the inception of cavitation on axisymmetric bodies. ASMEJ Flüids Eng. 95(1979) 519-528.

9.. Casey, M.V., The inception of attached cavitation from laminar separation bubbles

on

hydro-foils. In: Proceedings Int Conference_{on C'avitation, Edinburgh (11974) pp. 9-16.}

110. _{Pillipp, A. and' Lauterborn, W., Cävitation erosion by single laser-produced bubbles. J. Fluids}

Mech. (to be published).

Il.

_{Ompta,. R., Oscillations of a cloud of bubbles of small and not so' small amplitude, J. Acoust.}

Soc. Ame,: 82 (1987) 1018-1033.

Buijst, J., On the origin and acoustical behaviour of_{cloud cavitation. Thesis Twente University}

(1991).

Fruman, D, Duque, C., Pauchet, A., Cerrutti, P. and' Briancon-Marjollet, L, Tip vortex roll-up

© 1998 Kluwer Academic Publishers. Printed in the Netherlands.

Cavitation Research and Ship Propeiller Design

G. KUIPER

Marin, P.O. Box 28, 6700 AA Wageningen, The Netherlands

Abstract. The role of cavitation research in the design of ship propellers and the inflüence of re-searchon propeller design is revieWed. Thehistorical development of research on bubble cavitation is an example of a lack of communication between research and design. Research onsheet cavitation is starting now and simplifications such as two dimensional cavitation arebeing made It is argued from observations on propellers that the use of two-dimensional cavitaties is not a proper simplification to investigate sheet cavitation. .n illustration is alsogiven of the gapbetween the assessment of the risk of erosionon propeller modelsandresearch onerosion. Finally, the simplifications of tip vortex inception and the problems of the Inception speed of propellers are discussed.

Key words: propeller, cavitation.

1. Introduction

It is an honorand a pleasure to give a paper on theoccasion of the formal retirement of Leen van Wijngaarden. H started his career at Marin (then Netherlands Ship Model Basin) and the problems encountered there have always had his attentiow

I have always felt the cooperation with Leen as an illustration of the fact that

technology and science are different and that they neçd each other.

The difference between science and technology is in the direction of thought: scientific research wants to understand the phenomena and have insight in their behavior, technology wants to design working equipment in specific applications, where control of the phenomena is.required. Abstraction from environmental com-plexity in a laboratory is essential for scientific research, in technology there is the complexity of the design and the impossibility to isolate various parameters.

The ideathat technology is applied sdience is a gross simplification of technol-ogy and leads to conflicts between scientists anddesigners. Designers will find the scientific results insufficient for their application and scientists will blame

design-ers for the complexity of their designs A technical univdesign-ersity has this problem

internally, leading to a controversy between designers and scientist, especially since Twente lJniversity gives itself the epitheton "Entrepeneurial University". In practice there is a distinction between scientific results and technology.

In the meantime, both science and technokgy develop rapidly and the distinc-tiöns shift accordingly. Especially the development of computational fluid dynam-ics(CFD)has an impact. Butalso inthis field it can beobserved thatthe distinction

Figure 1. Example of a ship propeller (courtesy: Esscher Wyss.

between scientific numerical results and technOlogy has been forgotten and this has already lead to serious disappointments on both sides.

In this paper I will try to illustrate the relation between science and technology in the field I have been working on: cavitation, on ship. propellers, It is not my intention to review literature and I will focus on the viewpoint of the designer

2. The Problems of the Designer .

The first question is why cavitation is of interestfor the propeller designer. The

answer is in the detrimental effects, cavitation often has.. There are three main

categories of detrimental effects: erosion, ñoise and vibrations. In case of severe

cavitation it may also cause thrust breakdown, but that problem isexperienced

only With a restricted class of ships.

Why not eliminate cavitation? In uniform flow this might;be possible in some cases,. but. the propeller is generally mounted in the wake of the ship in order tore-claim partof theenergy which is löst by the resistance of the 'ship (h011 efficincy). This wake;howeveris strongly non=uniform-and-the resulting-inflow--vanations of the blades make cavitation generally unavoidable. To limit the extent of cavitation, propeller blades are relatively very thin in comparison to airfoils and the blade area ratio is high, leading to a high aspect ratio of the blades (Figure 1).

The most common effect of cavitation is erosion It means that the material of the propeller is. eroded due to the impact of cavitation on the blàde. An example of the effectof erosion is 'given in.Figure 2, where the trailing edge of a propeller

Figure 2. Detail of an eroded ship propeller.

Figure 3. Stern of a cruise liner.

blade is eroded due to cavitation generated at the leading edge. When this occurs

near the blade root the strength of the blade is decreased and the propeller will

fail eventually. The eroded surface shows a typical roughness, illustrating that the erosion is due to many strong impacts on the surface over small areas.

A very common effect of cavitation is vibration of the hull. This is caused by rapid fluctuations of the cavity volume. The vibrations of the hull lead to structural failures, malfunctioning of equipment and problems with personal comfort of the

Figure 4. Navy frigate (courtesy Royal NetherlandsNavy).

The presence of any type of cavitation increases the radiated noise drastically and it becomes ituportant to avoid cavitation altogethet The 'speed at which cav-itation occurs for the first time is called the inception speed. The prediction and

increase of the inception speed is'. a major' topic for navy ships. In this' case not only the design condition at a straight course, but also cavitation inception of the

propeller during manoeuvres is important. The strength of the radiated noise of

This problem is e.g. sensitive for cruise. liners., An important element of a cruise is the daily meal and the stem offers a nice view. When sailing, the open stern decks

are sheltered and attractive So the restaurant is located below the open decks on the stem, which is at the square windows 'in Figure 3. Although at a very attractive location, this places the designer for a difficult problem because thetwo propellers, absorbing each some 20 MW in power, are located sorne 6 neters below the floor of the dining room4 Where a noise level belòw .60 dB and 'a very low vibration

level is required. If only a tiny fraction of 'the power is diverted into noise and

pressure fluctuations, the meal will certainly no longer be pleasant. Isolation with e g floating floors is possible, but only in the higher frequency range The emphasis in propeller design is therefOre nowadays in the frequency. range between blade frequency (some 10 Hz) and 150 Hz.

Noise generation is important also for Navy ships such as the M-Fngate in

Figure 4. The requirements of the crew are less demanding than on a cruise liner, but the radiated noise poses problems for the detection equipmènt and creates a target for noise-seeking weapons.

cavitating propellers is also important, but has a lower priority than the inception

speed.

Fortunately, cavitation is not always dangerous. It is possible to design a cavi-tating propeller with low pressure fluctuations and without erosion, but in that case cavitation has to be tightly controlled This is the main problem of the propeller designer and it is also the area where cavitation research and design practice are still far apart.

3. Bubble Cavitation

The development of cavitation research is an interesting example of the interac-tion between the designer and scientific knowledge. When steam turbines could generate enough power to obtain a high ship speed cavitation became a problem

immediately in Parson's test ship, the Turbinia, in l895-l897 [1] The problem

Was thrust breakdown: due to cavitation. This has been investigated in cavitation tunnels in the first half of the 20th century. Although around f920 the phenomenon

of erosion became apparent on full scale propellers the emphasis in cayitation

design remained on the effects of cavitation on thrust The search was mainly for

blade sections which Were free of cavitation in uniformi flow and the developments

in the calculation and design of airfoil sections has been used to improve propeller design. Cavitation was mainly observed at model scale and the assessment of the observations Was highly empirical.

An impulse for cavitation research came from the work of Knapp and Hollander in 1948 [2]. Typically for the research approach the problem was simplified by using a hemispherical headform instead of a propeller. Contrary to propellers the calculation of the pressure distribution was 'possible for this body. The technique of

high speed' photography was used to make detailed observations of the phenomena.

These two approaches, simplification and detailed observations of the phenomena, lead to new developments.

The cavitation observed was bubble cavitation, where small gas nuclei in the flow expand in a low pressure region and collapse again when the pressure in-creases. These cavities move with the flow, which is why this type of cavitation is also called travelling 'bubble cavitation. With 'the knowledge of the pressure distribution and with the detailed observations Knapp and Hollander could describe the behaviour of the observed cavitation using the inviscid equations of motion.

Their approach also formulated the basis for nuclei effects on cavitation incep-tion, as will bediscussed later. Ever since that timecavitation research has focussed on bubble cavitation (Figure 5).

Thermodynamic effects, viscous effects, effects of non-spherical shape, effects of the initial size (gas content) on the bubble dynamics, implosion and the occur-rence of the microjet interaction of the nuclei with the boundary layer interaction of bubbles With an acoustic pressure field, all have been' investigated. The focus of cavitation research was even further restricted to cavitation inception and scale

effects, while on fully developed cavitation the attention went to supercavitatión, which was strongly driven by the Návy. On partially cavitating foils and propeller

blades much less. research work was done.

Bubble cavitation makes a lot of noise, at least at model scale, and bubble

cavitation got the reputation of being erosive. The propeller designer consequently

avoided this type of cavitation and designed propellers with less camber and .higher

angle of attack which exhibit sheet cavitation Also at full scale bubble cavitation got the reputation of being erosive, although the basis of that reputation is poorly

documented.

For decades, there has been a large gap between cavitation research and

pro-peller design Although bubble cavitation remained a risk for the propro-peller

de-signer, the problems onpropellers. were in the behaviour of sheet cavitation. Bubble cavitation was carefully avoided and sheet cavitation was.judged' on the basis of

model tests.

Does this gap still exist' An important development has been that nowadays more full scale observations of cavitation on propellers are being made. Although very fewpropellers show extensive bubble cavitation, it has become clear that the maximum size of the cavitating bubbles at full scale is very small. An example is given in Figure 6, where extensive sheetcavitation at the outer radii ofthepropeller blade is accompanied by streaks of bubble cavitation at inner radii. These streaks

of bubble cavitation look like thin clouds. The end of such a region of bubble

cavitatiOn is generally not very violent, and this raises the question why bubble

cavitation is so strongly erosive.

Also at model scale this question arises. As:,a side effect testing With leading edge roughness, which was used to redúce scale effects on inception of sheet cay-itation, it was found that leading edge roughness could generate ample nuclei [3]. At model scale with smooth propeller blades relatively large isolated bubbles are observed, as in

when abundant nuclei are present in the incoming flow the number of bubble'

cavi-ties increases and the maximûm Size decreases. This is illûstrated in Figure 8 where

the abundant 'amount of nuclei on the propeller surface iS generated by roughness

elements at the leading edge. Although the trailing edge of the cavity is sharp, its appearance is not very violent. Moreover, the amount of noise generated by

Figure 6. FIlscale cavitation observation withincidehtal bubbIecavitation atinner radii.

Figure 7 Bubblecavitation on a model propeller (low nucleicontent).

This supports the suspicion that bubble cavitation is less erosive than generally

thought The possibility to allow bubble cavitation is even more alluring since

modern blade sections with delayed inception of cavitation have the charactenstics of laminar flow profi1s and tend toexhibit bubble cavitation.

This infonnation has to go back to the cavitation research community Maybe the propeller designer Was Wrong in avoiding bubble cavitation at all costs. In cavitation research the single bubble approach has to be modified to reduce the maximum bubble size and maybe to take the presence of many bubbles into

ac-Figure'8. Bubble cavitation withan abundance of nuclei.

count. On the other hand, cavitation research on sheet cavitation is urgently needed in order to control induced fluctuation pressures, as will be discussed below.

4. Cavitation Induced Pressures

Since on propellers bubble cavitation was avoided, the necessary consequence was that sheet cavitation occurred; Sheet cavitation: has two effects its 'grówth and col-lapse causes pressure fluctuations 'in thç flow and the shedding of cloud cavitatiön at the trailing edge causes erosion. Erosion will be discussed lateE.. The problem of cavitation induced hull excitations was encountered in the seventies, When With increasing engine power the propeller loading was increased or the ship speed was increased This problem was attacked at model scale in cavitation tunnels

The problem of cavitation induced' vibratioñs is not difficult in principle.. Cav itation on the propeller blades acts. as a monopole, which means that the pressure

fluctuations in the fluid are in phase The ship hull which is relatively stiff

in-.tegrates these pressure fluctuations and' this 'leads to large excitation forces. The

cavity volume variations act as a distributed source, which can be treated nu-merically and from which the pressure fluctuations in an unbounded fluid can be denved Effects of the free surface and of the solid boundary of the hull do complicate the problem, but at present that is not the main problem The main problem is the determination and control of the cavity shape. This shape, more

specifically the volume distribution, has to be calcUlated very accurately4 because the pressure fluctuations are proportional to the second denvative in time of the

Figure 9. Measurement of the fluctuating pressures on a ship model.

When cavitation is simulated correctly at model scale the induced pressure field at the ships hull can be measured by an array of pressure transducers (Figure 9).

There are several problems in the experimental determination of the pressure fluctuations and the relation with full scale measurements The main problem is the hydroelastic behavióur of the ship, and sometimes also of the model, which makes it difficult to distinguish between excitation pressures and hull response. A considerable amount of development has to be done in this area.

The problem of the designe! is in the control öf sheet cavitatiOn. In cavitation research the problem has generally been simplified by using two-dimensional foils and two-dimensional cavitation. Initially calculations were linearzed and analyt-ical, e.g. [6], the present development is in potential flow calculations with panel methods, e.g [7].

The problem is that steady two-dimensional cavitation cannot exist. At the do-sureof the sheet cavity the flow has to be tangential to the profile on the wetted side and has to be at the vapor pressure atthe cavity side of the closure In potential this leads flOw to a singularity, which can only be overcome by releasing either the pressure condition or the tangential flow condition or both in the vicinity of the cavity closure. However, this so-called closure condition has a strong impact on the solution that is on the calculated' cavity length and shape

In real flows the closure of the cavity leads to a stagnation point and a re-entrant flow into the cavity. This re-entrant flow or jet moves upstream inside the cavity until it hits the upper boundary of thé cavity, which leads to the shedding of the downstream part of the cavity An extreme case is shown in Figure 10 where the re-entrant flow hits the cavity surface close to the leading edge of the cavity and the whole sheet cavity is shed and collapses in the flow. Note that this collapsing cloudis highly three-dimensional.

Figure 11, Closure of a two-dimensional cavity on a foil.

This mechanism can be simulated using potential flow calculations, as shown elsewhere in this symposium by de Lange. These calculations are not without nu-merical difficulties, indicating that in the re-entrant jet viscous effects may also be

important. The trailing edge of a sheet cavity breaks up in highly three-dimensional

structures, as is shown in Figure 11. This problem of shedding becomes even

more important in the unsteady case. As an example the collapse of a cavity on a propeller blade passing a sharp wake peak is shown in Figure 12.

When the attention is limited to steady flow a strongly three-dimensional sheet

is shown in Figure 13.

The trailing edge of the sheet is perfectly smooth, while at most part of the

trailing edge the re-entrant flow is seen to move towards the tip. A small secondary cavity is observed at the closure of the sheet. At the inner radii of the cavity some cloudy shedding is observed at the trailing edge. This is the region where the radial gradient of the cavity length decreases.

This is also the case in Figure 14, where a re-entrant flow in outward direction is observed at inner radii. At outer radii the cavity is more two-dimensional and shedding occurs, leading to typical cloud cavitation, which is assumed to be

Figure 12. Collapse of sheet cavitation on a propeller blade passing a sharp wake peak.

Figure 13. Stable sheet cavitation on the blade of a propeller model.

sive. This propeller blade has an unloaded tip, which reduces the cavity length in the tip region.

These experimental observations show that, in order to obtain a stable cavity in steady inflow, the re-entrant jet has to be removed. This can only be the case in three-dimensional flow, where the region of shedding and cloudy cavitation can be

restricted to a small part of the sheet. In many cases this is the cavitating tip vortex.

The presence of the re-entrant jet is still important for the calculation of the cavity shape, but the calculation can be cut off as soon as the re-entrant jet is formed.

i

Figure J4 Sheet cavitation ona propeller with uñloaded tip.

A simple sink can absorb the flow. Calculations using this approach are undçrway

and show very promising results.

The foregoing implies that cavitation calculations and especially the validation of the calculations by experiments should not be done on twodimeñsional foils. Two dimensional foils are the most difficult to investigate and theff behaviour with shedding of cavitation is more complex than can be cálculated at present.

Validation of calculations is generally done by comparing the cavity length This is not the relevant parameter. Because the main risk of unsteady sheet cavitation is vibrations, the cavity volume should be vlidated.

A strong interaction between the designer and his experiences and the cavitation researcher is mutually stimulating and can keep both on the right track.

5. Cavitation Inception 51. NUCLEI

As mentioned above the approach of Knapp and Hollander formulated the basis for

sca1e effects on cavitation inception, because it related the growth of gas nuclei into cavities with their'initial size.. In this field: the exchange between cavitation research

and the ship designer has been good, especially because-thetowing-tanks-were

eager to eliminate scale effects on cavitation inceptiÓw All efforts were put into the control of nuclei, up to now with limitedresults. An illustration: is found in the recommendations of the cavitatiòn committee of the ITTC (International Towing Tank Conference) which every three years. reviewsthe developments in cavitation research relevant for ship propellers..A recommendation in 1966 was: "Systematic studies should be undertaken to determine the screening. effect. of bodies on gas

nuclei entrained in the flow". In 1986 a recommendation was:, "Efforts should be devoted to the development of instrumentation to deterniine the size and releative distribution of spherical gas nuclei in the test water". Research on cavitation in-ception has long been focussed on nuclei alone, as in the case of bubble cavitation. Nuclei still play an important role and control of the nuclei content is, important, if only for the repeatability of tests The measunng of the nuclei content in cavitation tunnels is slowly icreasing the device which is most frequently being used is the cavitation susceptibility meter using a venturi to create a low pressure region. The use of this device is still incidental, however. Only one large cavitation tunnel(the GTH in Val de Reuil France) Installed extensive equipment to control the nuclei

content.

Sheet cavitation has a different mechanism of inception than bubble cavitation

or tip vortex cavitation. Its inception conditions are much less sensitive to the nuclei

content [5]. Therefore, for the majority of propellers (with sheet cavitatioñ) the

nuclei content is less cntical This is why towing tanks have been able to investigate

many propellers without sophisticated nuclei control. Sheet cavitation has its own specific problems, however.

5.2. Viscous EFFECTS ON INCEPTION

Until 1970 all efforts to understand scale effects weredirected towards nuclei con-trol. The effects of the state of the boundary layer became apparent after work of Araken and Acosta [81 on headforms and Casey [9] on a foil A laimnar boundary layer was shown not to cavitate when the mean pressure was below the vapçr pres-sure. This work came from researchers without stimuli from any design problem!

Such a laminar flow region is very common on propeller models. An example of the

state of the boundary layer is shown in Figure 15 where the results of a paint tests on the suction side of a propeller blade are given. The streaks pointing outwards indicate a laminar region, the streaks in radial direction at the outer radii indicate turbulent flow. In this figure the sudden cut-off of the laminar region is caused by a laminar separation bubble at the leading edge Cavitation inside the radius of the radius of the cut-off did not occur. An increase of the nuclei çontent of the flow did not change that situation.

The viscous scale effects on inception have strengthened the desire to carry out cavitation tests at maximum Reynolds numbers so that laminar flow could be ehm mated. This has lead to cavitation tunnels with very high' 'flow velocities in the test

section (LCC in Memphis, USA;rGTH in Val de Reuil, France; Hykat 'in Hamburg,

Germany) However the pressure gradient is very important in maintaining laminar flow, and even at a very high Reynolds number, laminar flow can occur.

The question is if the boundary layer at high Reynolds numbers 'becomes tur

bulent due to instability and natural transition or due to surface imperfections.

This still has to be investigated 'Indications are that the blade surface is generally dominant in generating transition.

Figure 15. Paint streaks on a propeller blade showing regions of laminar and turbulent

boundary layer.

At MARIN the Depressurized Towing Tank is used for cavitation simulation.

This makes a better modelling f the propeller hull interaction and of the free

surface possible but it makes it necessary to maintain the Froude number and thus to low Reynolds numbers. This has lead to theapplication of artificial leading edge

roughness at the leading edge of the propeller to stimulate turbulence and cavitation

inception.

Cavitation research on the effects of surface roughness has been limited For the designer many questions remain unanswered. Suçh questions are the mechanism of inception and the inception conditions at the roughness elements, the minimum roughness size to be effective for inception stimulation and the maximum rough-ness size allowed without changing the mean pressure distribution on the surface As a side effect leading edge roughness was found to generate nuclei5 indicating

that some type of micro-cavitation occurs on the roughness elements. This has been

used as an efficient mechanism to generate nuclei locally, which is important when the radiated noise is measured.

6. Erosion

The only tool a designer has to predict erosion qualitatively is observation of the cavitation. Shedding of clouds of cavitation behind a sheet is still the most erosive type of cavitation. In a wake field this can occurin a very small part of the propeller revolution. The cavitation causing the erosion of Figure 2 is shown in Figure 16.

The erosive part of the cavity can be seen on the tip of blade 5, Where an isolated

cloud cavity collapses just on the edge of the blade. It is not difficult to miss such a detail during observations at model scale. The imploson does not occur at every revolution, and is visible only in a yery small range of blade positions. Many pho-tographs have to be taken to find it which is why video observations are sometimes preferred. The resolution of video observations is less than photographs, however,

and this increases the chance to miss relevant features of the cavitation.

Cavitation research has not given the designer a reliablemethod to measure the erosivity of cavitation Expenments with the removal of stencil ink by cavitation

have been only partially successful, since the calibration and extrapölation of the speed of erosion is difficult. As aresult ink tests can predict the location of erosion, but not the magnitude of the risk of occurrence or the speed of erosion. This is not

much more than can be derived from visual observations, however.

In cavitation research the question of the mechanism of erosion is not yet

re-solved. The old discussion is if erosion is caused by local high pressures at the

re-entrant jet of collapsing bubbles or by shock waves transmitted to the surface.

Interesting data were obtained recently by Phillipp andLauterborn [101, who firmly

concluded that only bubbles collapsing on the surface, causederosion. Erosion was not found at the location of the re-entrant jet, but at the location of the imploding ring-shaped cavity around the re-entrant jet. This would confirm that on propellers it is important to see if implosion occurs on the blade High speed video could be

Figure 17. Cavitating tip vortex oña heavily loaded tip.

an aid in the asseSsment o:f cavitation. This development is interesting and may leid to new techniquesto aSsess erosion.

The designer problm inerosion is the collapse cloud cavitation. These cluds aie often shed by unsteàdy shet cavitation. Steady sheet cavitation h.'ever, is also shedding clouds of vapor at its trailing edge For cavitation research it is important to investigate the mechanism of erosion in bubble clouds as occur at

the trailing edge of bubble cavitation. The results of Leenvan Wijngaarden aiid his

èo-workers as Cmpta [11] and Buist 12 shoúld be ued to develop scaling rules

both foi noie aúd erosion. This has not yet been done.

7. Vortex Cavitatiòn

The relation-between cavitation research and the propeller-designer-is-not-complete

without mentioning vortex cavitation Tip vortex cavitation is important because it is often the first type of cávitation which occurs and therefore it determines the inception speed of'Navy Ships. Tipvortex cavitation ina more developed stagehas caused broadbandexcitation of the ships hull in sOme cruise liners,

Inception Of tip vortex 'cavitation is the most difficult topic in cavitation