Ventilation or cavitation: An experimental study to determine loads on controllable pitch propellers

September 2006

Teus van Beek and Tom IC. van Terwisga

Deift University of TechnolOgy

Ship Hydromechanjcs Laboratory Mekeiweg 2, 26282 CD Deift

Ventilation or cavitation: An experimental

study to determine loads on controllable

pitch propellers

by

Teus van Beek and Tom J.C. van Terwisga

Report No. 1544-p

2006

Presented atthe gth International Symposium on

Cavitation, CAV2006, Wageningen, September 2006

TU'Deift

Deift University of Technology

Page lof 1/1

ABSTRACT

The load variation on a propeller depends mainly on the change of the inflow velocities on the propeller.

And it is essentially the non-uniform velocity distribution in the wúke of theship that causes the thrust and torque to significantly vary throughout one revolution. This load variation but especially the pressure distribution on the blade in the tip area and consequently the blade spindle torque is affected by the occurrence of cavitation and sometimes ventilation These variations in tip loading are important because the life expectancy of the propeller system is a direct consequence of the dynamic loadings. Ventilation may occur at reduced draught conditions but may also occur When the ship operates in heavy waves.

Not much is known about the conditions for limited

ventilation and its effect on propeller loading. For that reason, a series of exploratory tests have been carried out to study the occurrence of cavitation and ventilation and to determine its effects on loading, especially on blade spindle torque.

The experiments were carried out in a depressurized towing tank which is a suitable facility to study cavitation and ventilation. A high speed video camera was used for visualization while force transducers were mounted in the propeller to measure instantaneous propeller blade forces (propeller thrust and torque and individual blade spindle torque). The visualization comprised the observation of ventilationas well asof its interference with cavitation.. For ventilation, a small formation of air entrainment in the tip región of the propeller was observed, just beyond

the propeller's top position

. The appearance of the

ventilation was very similar to cavitation on the blade. The experiments showed that ventilation may readily occur without the blade tip protruding the water surface The blade loading, especially the blade spindle torque was heavily affected and was found to depend on the amount of ventilation. The spindle torque variation was increased while the blade was already over the top, increasing the cyclic loading on the propeller and its components. Therefore, the experiments show that the ventilation changes the dynamic loads considerably. As such, this phemenon should be considered in the lifetime prediction for the controllable pitch propeller.

Sixth International Symposium on Cavitation

CAV2006, Wageningen, The Netherlands, September2006

Ventilation or cavitation: An, experimental study to

determine dynamic loads on controllable pitch propellers

By: Teusvan Beek Wrtsilä Propulsion Netherlands BV andTom van.Terwisga MARIN.

INTRODUCTION

Ship propellers are operating in a non homogeneous inflow. This inflow flow field, normally referred to as the wake field, is

the cause of many propeller design

problems. Especially for single screw vessels such as a fast container vessel, the load variation on a fixed or controllable pitchpropeller depends mainly on the change oftheiñtlow velocity tothe propeller inthe wake field. This 'inflow field variation causes the blade's loading to vary during one revolution. It is therefore not surprising that this inflow variation is predicted and calculated in many design procedures for scantling of propellers The resulting wake dependent fatigue loads are carefully

considered in the strength analysis

of the

main

components such as the blades.

Even when it comes down to the operation of ship

propellers in waves, studies have been undertaken to show that the wake dependent loading variation is still' dominant compared to loadings which normally occur in Waves. In a paper by Hageman and Helle (1979) the analysis' of the loadings during one voyage has ben addressed, taking into consideration the inflow variation

caused by the waves encountered by the ship. The

additional loadings thus predicted led to asmall increase in blade' loadings For the cases studied, only a narrow

stress distribution resulted from ship motions. The

propeller loading. For that reason, a unique series 'of tests

have been carried out to study the occurrence of

cavitation and ventilation and to determine its effects on loading, especially on blade spindle torque.

The experiments Were carried out in a depressurized towing tank which is a suitable facility to study cavitation and ventilation. A high speed video camera was used for visualization while force transducers were mounted in the propeller to measure instantaneous propeller blade forces The visualization comprised theobservation of cavitation as well as of ventilation.

The main interest of the.studies is to explore reasons and backgrounds of the dynamic loadings on controllable pitch propellers. These loadings determine for instance

the life expectancy of the main components of the

propeller system and in extreme cases causeoverload and thus failure ofthesystems or its components. The interest

of ship operators today to have systems with ever

increased reliability and safety should therefore start with a proper investigation of the dynamic loads inoffdesign

conditions. This paper will show the importance of

ventilation in these consideration.

'ON THE BACKGROUND OFVENTILATION For a comprehensive study of ventilation and its effect is referred to the work reported by Minsaas in (l983) Here the ventilated propelleris defined to operate in 3 typical conditions:

-

fully ventilated condition at small advance

rations Where the suction side of the propeller

is dry also when the blade is

in the lowest condition.

- Partly ventilated propeller at high advance ratio where mainly the emerged propeller is

dry.

Unstable condition between these tWo

conditionswith hysteresis.

When a propeller section is

fully cavitating the

lift

coefficient is determined by:

CL=a+"°

(I)

4 ,V2pV,

The pressure difference

p0 - p

is the pressure

difference between the static pressure and the pressure in the cavity on the suction side. The pressure in the cavity is p the vapour pressure. When the propeller is fully ventilated however the suction Side

is open to the

surrounding atmosphere. The pressure difference

therefore will be :

= pgh with h the immersion.

From implémentation of this

pressure difference in

equation (I) a new relation for the lift coefficient under ventilatingcondition isfound:

ir

pgh

CL=a+

2 (2)

4

2pV

2

The main conclusion from this formulation is that the lift is maximized given the submergence of the local blade

section.

Typical and a well know characteristic in the occurrence of ventilation is that the beyond the point of ventilation a

significant drop in thrust and torque of the propeller

occurs. This is however related also to the occurrence and shâpe of the ventilation on the profile. For instance when the blade ventilation starts from the leading edge (very similar as to the occurrence of cavitation on the leading edge) the breakdown of lift or thrust is connected with a condition Where a small iñcrease in angle of attack or loading of the profile will cause a sliong increase in the suction peak behindthe leading edge. The pressure in this peak generates a vortex at the leading edge with separation and ventilation through the vortex. When the propeller loading is small the minimum pressure is at the middle of the profile rather then at the leading edge. In this case only the last part of the section will be covered by air in the caseofventilation Such a type of ventilation will not cause a dramatic decrease in lift and thus thrust as whena leading edgesuction1peak occurs.

lt is however worth to consider that separation is a

condition which supports ventilation 'because the cavity can best be maintained in areas where the velocity is small or-closeto zero.

Whena propeller isoperating close to afree surfaceit has also to be considered that the free surface itself will locally accelerate the water flow the closer the propeller gets to the water -surfáce The acceleration of the water flow is a results of the mirror effect. One-can argue-that in order to have the boundary condition at the water surface satisfied asecond propeller with similar flow field should be positioned. This resUlts iñ an increase in velocity the closerthepropeller gets to the water surface. This implies that the behaviour of the propeller changes when it gets closer-to the water surface. This effect should be borne-in mind when reviewing the details of the experimental studies which are presented in more-detail.

Ventilation has been quite well

known from

its

occurrence With surface piercing propellers were the propellers as partly out of the water. The submergence plays an -iiîìportant role in the loading and- design of such propellers see also Young- and Kinnas(2003).

A more recent study on ventilation has -been presented by Koushan(2006) for steerable thruster units. The main aspects studied- here is overloading during operation in seaways and the effective control of such units in the same conditions. Koushan has neatly outlined the

conclusions are that ventilation is an important parameter in the dynamic loading on steerable thruster units.

In this paper the focus is more on controllable pitch

propellers rather then on thrusters. Instead

of

investigating the bollard condition a freerunning velocity

has been investigated also behind a ship's hull in the

usual manner. The main motive for the investigation was

the occurrence of large blade spindle variation which

were measured on full scale of a controllable pitch

propeller of a fast single screw vessel containership.

These measurements included strain gauge measurements

on the actuating

pin. The full

scale measurements

indicated that in normal free sailing ahead condition

rather sudden sometimes induced by the rudder or

movement of the ship the blade spindle torque variation

increased beyond a factor of 3. This factor could not be

explained by normal variation of the propeller inflow as

occurring in the wake. Investigations and calculations

indicated that the most logical conclusion was that these

increased loadings were a result of ventilation. As initial

calculations did not reveal these phenomena experimental studies were undertaken to investigate these phenomena.

EXPERIMENTAL STUDIES

The incentive to conduct the reported experimental

study was to investigate whether a sudden change in

blade spindle torque Ms could be explained which was

measured frequently in practice on a fast container vessel.

The experimental programme was therefore conducted

with the following objectives:

-Explore the effect of ventilation on the propeller

blade spindle torque

-Explore the sensitivity of ventilation and blade

spindle torque for variations in propeller immersion,

propeller loading and rudder angle and motion.

Conditions were tested were the propeller was clearly

not ventilated and condition where ventilation could be

observed both visually and from the measured signals(Ms and Q). The most pronounced effect on Ms occurs for the non cavitating condition in heavy overload condition. The phenomenon can be described as follows:

Under non-ventilated conditions, the blade spindle

torque increases sharply when the leading edge of the

blade enters the wake peak and the suction pressure in the

leading edge region (that is stronger negative pressure)

increases (Fig. 1). When the leading edge leaves the wake peak again, the suction pressure diminishes and the blades spindle torque gets smaller. The peak in Ms is close to the

I 0 degrees position of the blade reference line. Fig. I

show the graphics of the instantaneous measured blade

spindle torque of an individual blade in a condition

representative for the

self propulsions

point. Both

cavitating and non cavitating conditions are plotted in

Fig I. The results appear to be quite the same. The change

in blade spindle torque is dominated by the change in

inflow velocity at the propeller which is normally called

the wake distribution.

In the ventilated cases, ventilation through the tip

vortex core occurs shortly after the leading edge has

3

passed the wake peak. lt could be observed that when

ventilation occurred, the downstream part of the tip

region close to the trailing edge became fully ventilated.

This implies that after this occurrence, the pressure over

this part of the blade is bound by the atmospheric

pressure, and cannot become lower than that, which it

would do in otherwise non-ventilated conditions. The

relatively high pressure at the after end of the propeller

blade magnifies the blade spindle torque (leading edge

suction pressure and trailing edge overpressure both

increase the blade spindle torque). When the ventity

(note the word ventity is introduced similar as cavitation

and cavity) leaves the aft part of the blade, the blade

spindle torque recovers, but remains significantly higher

until approximately the 90 degrees blade angle position. The following parametric variations were systematically studied:

- the effect of the ambient pressure propeller immersion

- propeller loading

- rudder angle These effects are reviewed here. Effect of ambient pressure

)

Fig. I Effect of occurrence of cavitation on the actuating

forces (in this case no ventilation occurs).

There is an important effect of the ambient pressure on

the character of the blade spindle torque for severe

overload conditions of the propeller. Fig. 2 shows that

cavitation has a strong damping effect on the blade

spindle torque. This can be understood from the limiting

effect cavitation has on the minimum pressure on the

blade. Once cavitation has occurred, the local pressure

cannot be much lower than vapour pressure. lt is also

observed that In the same condition, were ventilation

could not be observed in

the cavitating condition,

condition. This finding is confirmed by the character of

the blade spindle torque, which does not show the

characteristic high amplitude that it showed in the non cavitating condition where ventilation was observed. There are two possible reasons for this attenuating effect:

One reason is that the effect of hydrostatic pressure

on air bubbles that are captured,

is much more important on the volume decrease for the scaled pressure condition than it is for the atmospheric pressure condition. This follows from the ideal gas law, where the decrease in bubble volume of the gas bubbles is proportional to:

Pgo Pga

+pgh

Because the gas pressure at the surface Pgo is much smaller for the scaled pressure condition (some 5% of the atmospheric pressure), the effect of immersion on the decrease of the bubble's volume is noticeably greater. In other words, the bubble size will decrease more rapidly with the increased immersion for the scaled pressure condition.

Another reason is presumable the moderating effect

that sheet cavitation has on the suction peak pressure and its consequent effect on blade tip loading and tip vortex strength.

Flg.2 Effect of ambient pressure on blade spindle torque; The highest torque peaks occur for the highest ambient pressure (left photo).

For the lower loading, which is considered representative for steady operation in practical service conditions, the blade spindle torque is effectively independent of the

4

ambient pressure(Fig. I and Fig.2). Ventilation could in both conditions not be observed and cavitation is limited

to a narrow sheet cavity on the leading edge and a

cavitating tip vortex.

Effect of propeller immersion

Fig. 3 Effect of propeller immersion on blade spindle torque (atmospheric pressure)

An important effect of propeller immersion on blade spindle torque for both (Fig.3) non-cavitating and the cavitating (Fig.4) could be observed. It is thereby noted that the relative increase in the amplitude of the dominant frequency (shaft rate) due to ventilation is essentially the

same (approximately a factor of two), but that the

absolute value for the cavitating case is much smaller (approximately a factor two).

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âfro,SOm.fos. Sñka Cmi be seer e, Opoloi ca.SoSO msest

PO.49nt&

4Aibthe

gIi.I

Ah hóM The ro UeIIIdIe,

1o4 lUs

,1phIth

mhfac 0 DnØec calo (*34Ode5rees

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SAis*h*oe, lAi 15 lolOs. liñO lolo

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Fig.4 Effect of propeller immersion on blade spindle torque (scaled pressure)

Effect of propeller loading

The effect of propeller loading is possibly more important

for the non-cavitating condition (Fig.5) than it is for the

cavitating condition(Fig.6). Where ventilation did occur

for the highest blade loading in the non-cavitating case, it

did not occur for the cavitating case. If anything,

it

showed on the contrary an even smaller contribution for

the blade spindle torque. This is again ascribed to the

moderating effect

of

cavitation on the pressure distribution.

5

significantly affected (approximately 4 degrees amplitude

for a IO degrees of rudder amplitude). This roll angle is

likely to change the propeller immersion and might thus

initiate ventilation. This could however not be confinned by the current tests.

41 M bÓTM, fr NO1S1 Dy ,,eI,ndI,cit,, eI,st 10 d.B,, TE SB 1SBW, eSB

at

O1E 1d1B

Fig? Effect of rudder angle on blade spindle torque

(atmospheric pressure)

bIb4 0Ml thEbd0 0 SB BIT,OIBEBIC cale

FIg.6 Effect of propeller loading on blade spindle torque

(scaled pressure)

7I

4I4

fr A

Fig.5 Effect of propeller loading on the blade spindle torque (atmospheric pressure); the left condition ¡s at the same ship speed but increased model RPM.

Effect of rudder angle

An effect of rudder angle on ventilation or on blade

torque could not be found, neither in the non-cavitating

(Fig. 7), nor in the cavitating condition (Fig.8). Few tests

where conducted with a rudder oscillation

and an

amplitude of 10 degrees. These dynamic tests also show

no effect, which is not surprising if the rudder frequency

is compared with the blade passing frequency, the latter

being much higher (blade tip makes approximately 12

revolutions for one full cycle of the rudder, where rudder

cycle frequency o used from full scale measurements).

The dynamic effect of rudder angle is consequently

negligible.

A possible effect of rudder angle on the phenomenon of

suddenly increased blade spindle torque is consequently

JE sa

ITh,,frnffiewfsoe.

Fig.8 Effect of rudder angle on blade spindle torque (scaled pressure)

CONCLUSIONS

The effect of ventilation, with and without interaction

with cavitation about the propeller blade, has been

experimentally investigated. This was done on a CPP

propeller, operating in the wake of a fast container vessel.

The most relevant findings of this study are summarized

in the following:

Under non ventilated

condition,

the blade

spindle torque

increases sharply when the

leading edge enters the wake peak and the

suction pressure in the leading edge region

increases (that is a stronger negative pressure

occurs). When the leading edge leaves the

wake peak, the suction peak pressure diminishes and the blade spindle torque

diminishes. The peak in Ms is close to the 180

degrees (top position) of the blade reference

line.

In the ventilated case, ventilation through the

tip vortex core occurs shortly after the leading

edge has passed the wake peak. It could be

observed that when ventilation occurred, the

downstream part of the tip region close to the

trailing edge became fully

ventilated. This

implies that after this occurrence, the pressure

over this part of the blade is bound by the

atmospheric pressure, and

cannot become

lower than that, which it would do in otherwise

non-ventilated conditions. This relatively high

pressure near the Trailing Edge of the propeller

blade magnifies the blade spindle torque

(leading edge suction pressure and trailing

6

edge overpressure both increase blade spindle

torque). When the ventity leaves the aft part of

the blade, the blade spindle torque recovers,

but remains significantly higher for a long

time, until

approximately the

90 degree

position is reached.

The main conclusion from the studies conducted for this

paper are that ventilation in certain conditions

will

change the character and loading of a propeller. It is well known that heavy ventilation will have a significant effect

on the powering characteristics such as thrust and torque

(both will be reduced when heavy ventilation occurs).

The paper however shows that ventilating can occur not

only in conditions were the propeller is out of the water

but also when the propeller is fully submerged.

The

ventity may change or affect the balance in the blade

spindle torque when the propeller blade is closest to the

water surface operating in a wake behind a ship at full

speed. The shape of the ventity appears the be similar to

cavitation structures. Vortex type ventities appeared to be entrained in the flow in the leading edge area.

The observations shown here are new in a way that

ventilation has never been observed in behind ship

conditions in both non-cavitating, cavitating and non

ventilating conditions. The blade spindle torque appears

to be a sensitive parameter which is affected strongly by

ventilation. For the case presented a change in rudder

which was set during the observation did not make any

change in the character of the blade spindle torque.

The presented data give more insight in the ventilation

characteristics and in hindsight also

indicate why

preliminary calculation did not capture the character and

mechanism presented here. The observations presented

here provide a basis for improved calculations and also

generate a basis for understanding the phenomena of

ships operating in waves with the propeller operating

close to the water surface. The use of high speed video in

combination with

the propeller operating behind a

complete ship model has been a very powerful tool in

these observations.

REFERENCES

Helle, H.P.E. and Hageman, LA.S.,'Fatigue damage

accumulation in marine propellers',fourth Lips Propeller

Symposium, 1979.

Minsaas Knut J., 'Ship design for fuel economy',

West European Graduate Education in Marine Technology, Eight School in Gothenburg, August 1983.

Young, Y.L. and Kinnas, S.A.,'Numerical analysis of

surface piercing propellers',

Propellers/Shafting 2003

Symposium , Society of Naval Architects and Marine

Engineers.