Determination of hydrodynamic loads generated by an operating thruster propeller

ABSTRACT

For a thruster designer in order to carry out accurate strength calculationS and develop an appropriate control system it is necesuary to know the exact values of hydrodynamiC loads observed during the steady-state as well as during the transient modes of the thruster operation (starting-up, reversing, varying the pitch of the thruster controllable pitch pro-peller (CPP), etc.), moreover so, the values of th mentioned dynamic loads may be substan-tially higher compared to thoc of the statio-nary ones.

In the present paper, the theoretical cal-culation procedure for the unst.ady flow in the thruster tunnel is proposed, calculated and experimental data are compared, oscillog-raph-aidod analysis of the thruster operation under conditions of the atmospheric air p-aenOe at propeller disc as well as the results of the calculation of hydrodyflamic loads ge-nerated by the reversing CPP of a full-size thruster arc given.

1flesearch Scientist

2SonJ.or Research Scientist, ih.D.

3Tochnicnl Director

DEThRMINAT1ON O' IIYDRODYNAMIC LOADS GLD4LDATED BY AN OPjRATING 1.ILRUSTSR PROPELLER

1ikhail N. Grjbkov1, Victor I. GruziflOV2, Valentin U'. VailyeV3

Research & 'rodiic t ion AssOciation "VtNT" 11, Tchaikovskodo Str., Moscow, 121099,

USSR

NOM}NCLA'1'URL

A2 - thruster outlet section area - propcili.Or (WOfl ratio

1) - propeller diameter - advance ratio, J5 TECHNISCRE UNjVEflSfl'JT Laboratorjwn voor

&mech

MekeJwegz2aco

Deift

- exit flow area reduction coefficient r

0

K r

KQ - propeller shaft torque coefficient,

a

---5-r ' nD'

- propeller thrust coefficient,

=

--- dynamic damage coefficient

3 - length of the thruster tunnel LID - tunnel length - propeller dianieter

ratio

1 - reduced length of the thruster tunnel

n - propeller speed

p - propeller induced pressure dir-Cu roTi CO

P - thruster thrust

iUpOlieL Iiyd voitytinni.C tC)r([1IC - driving cnginO shaft torque - ehaft line friction torque

2J

c c

_iT

T - propeller thrust

t - current time

tr

- timo required for the C?? blades to change their poitiOfl froiii

"Full Ahead" to "b'ull Astern"

vi ,v2,

- flow velocity at corresponding sections

v - flow velocity at propeller disc

z - number of propeller blades

I - Moment of inertia' shaft line - steady-state kinetic energy

coe-fficient allQWinS for the

flow-field nonuniformity in

corres-ponding sections

- steady-state momentum coeffi-cient allowing for the flow-field nonuniformity in the thruster

tunnel

- steady-state hydraulic resistance coefficient for the thruster tun-mel

ft

- density of water

A 66 - Moment of inertia entrained mass of water

1. PROBLEM STATEMSNT

Thu existing calculation procedure for

the determination of external loads which

effect the' elements of the thruster is based

on hydrodyflaifliC calculation schemes for th

propeller torque and thrust ( for CPP5 the

spindle torque is also to be determined) of

a thruster operating under stationary condi-tions. ThuS, while calculating the thrust

bearing life, the maximum

axial force is

ta-ken as equal to the propeller thrust plus t ho f i-cr, ti, the ru ii u c ti on arbox . l3oth

values are amenable to a sufficiently

accu-rate computation and the thruster tht'iist

magnitude is also confirmed by numerous

mo-del and full-scale tests. The thruster

dy-namic parameters are considered with the

help of dynamic damage coefficient Kä the

value of which for a thruster is not

sub-ject to explicit specifications and

de-pending on a particular firm is taken

with-in a wide range from 1.15 to 3.0. For the

correct choice of

1t''i'necebary to

evaluate the actual dynamic axial forces ge-nerated on the shaft of the thruster during its starting, reversing or during ship's

mo-tiono and In :ea conditions.

2. EQUA'I'IONS FUR TIlE VsRJING TIIRUSThR

EQUIPPED VITH A FIXED PITCH 1ROPELLER

(1')1P) AND WITfl A CONTROLLABLE PITCH

PROPELLER (C??)

Strictly speaking, in order to

calcula-te the dynQ.ic loads impact on the thruster,

one should jointly consider the equations describing both the unsteady-state water flow in the thruster tunnel and the unsteady

state parameters of the thruster propeller.

There exist a number of procedures for the calculat:ion of propeller parameters which to

a dreater or omaller degree take account of

the unsteady mode of Its operation

11, 21.

In the present case, considering the

actual values of time required to change

the

direction of thrust of the thruster equipped

with a CPP or to start and reverse tho

thruster equipped with a PPP one can use a

quasi-stationary approach which provi4es the

parameters of thruster propellers with an

accuracy sufficient for operating purposes. Therefore, in the present paper the propeller

performance curves arc taken as prodetermifle4

and equal to steady-state valueS, while tran-sient vulues arc taken acocunt o1 by

introdu-cing tho iritufltUfleOU2 advance speed on the propeller disc. i.e. by changing its

opera-ting mode compared to the steady-state value

and allowing for the entrained maso of water.

By introduction of a ocries of simplifying

assurrip Lions and tr:n,foriiiatioflO, the unsteady flow in the tunnel of the thruster (See Pig.

1) can be reduce4 to a Bernulli equation for

the unstoady motion of flow in the thruster

Liiiiuu]. er' I tton irli

p,1J)U1-

2H.2

4- 1_i_ (1)

The theoretical and experimental,

deter-minetlon,. of unsteady-state momentum

_ft

kinetic ëi)egY.'th and local

reels-tanoe ' coefficients used in equation (1) involve very complex computations. Therefore, by introduction of a series of simplifying assumptions, described, in particular, in.

3) and bearing it in mind that pressure

differentie.l on the propeller is equal to

P

J)flE)Kr(hJ,

P/D)

(2)

and torque on the propeller shaft

Q=

(3)

one can on the banjo of (1) obtain 'the £01-lowing equations for the reversing thruster with a CPP (n cont.) and with a FPP

(P/D conat.) respectivcly - -

(%2

.DR'r(Z,P/D)

-

.,

).4f

., L)2 Ki-(%1) L15 n,

The value of the thruster thrust is de-termined by applying the unsteady-state flow momentum law to the' reference surface which embraces' the th±üster and the apprqpriate portion of the dhip'o hull end crosseS the flow at a 'sufficientlY large distance from the ship's aide.

PC

- r4

1)

-

_(c(a.

t.17)

,1.1 /(q

1(d, plo)

P10=

/(t)

3. RE.;ULTS 01' TIlE EXPERIMENTS AND THEIR

COM-PARISON NITH THE CALCULATED VALUES

Vlith the purpose of evaluating the propo-sed method of dynamic loads calculation for

practical use, model experiments were carried

out with the thruster propeller diametr equal

to 200 mm. Considering the complexity of manu-facturing a remotely_controlled small size CPP and consequently, owing to imposnibilitY of

tenting the thruster in the pitch chan(e mode, measurements were conducted in the process of

starting the thruster equipped with a FPP.

The testing was carrie4 out at the hydro-dynamic tank owned by the Research and Pro-duction Association "VINT". During model

Lct tunnels with L/D ' 1.4; L/D - 3.2 and lID 5.2 were used. The parameters or the model thruster propeller wero as follows:

7. 4; A0/A 0.36; rID 0.9.

in the pr000ss of the experiment, pro-peller speed n, thrust T and shaft torque Q wore measured with the help of a propeller

dynamometer. Besides that, the magnitude of

the thruster thrust was measured by mans of

yet another dynamomoter. Frequency signels from the dynamometex's were trasmitted to mo-nitoring devices which transformed them into nnnlouC ones and in this form they wcre re-corded on a mirror_glllvanotfleter oscilloi-raph.

Propeller thrust yalues measured against corresponding thruster drive acceleration laws are represented by the full line in Fig.2 and

fig.3. The propeller thrust curve calculated without allowing for dynamics at = const. and at instantaneous speed is shown by the

dotted line. The reSults of the computations carried out according to the proposed proce-dure are represented by dots. In a similar way, the thruster thrust and propeller shaft torque plots are shown in Fig.4 and Fig.5 respectively.

A fairly good agreement of calculated and experimental data characterizing the thruster

acceleration parameters during its starting

ev:t dent from the above graphs confirms the authenticitY of the prop000d procedure for the calculation of the thruster unsteady-state modes of operation.

It is also seen from the graphs that the thruster hydrodynalfliC characteristics

observ-ed during its starting substantially difCer from the steady-state values. In the process of wnt..r acceleration in the thruster tininul, inertia forces cause the propeller operating point shift in the direction of smaller

ud-vance ratios which leads to a

correspond-ing increase in the values of and KQ.

When nominal speed is reached, the

hydrody-namic characteristics of the thruster with

some time delay acquire their steady-state

values.

CALCULATION OF DYNAMIC CHARACTiRISTICS OP

A FULL-SIZE THUSTR dITH A CPP IN RJVERSi

CONDITIONS

The calculations (See Pig.G. . .9) were

performed for a 500 kW thruster with the

CPP die. D 2.0 m end speed n = 4.067

s1.

The pitch change was taken as subject to li-near law while the tunnel length L/D and

pitch change time tr varied.

'roul V.1g. 6. . .9 it tolloWs that the moe I

dallguiuuD, froiii thu poin I ut view uf i'x lur-nál loads, is the pitch change mode in case

of long tunnels and short reversing time. Thus, at tr 6 see and L/D - 7 the value of thrust on the propeller shaft is 2... 2.5 times as large as the nominal value. The

situation with the torque is also dangerous

because boidu3 exceeding its nominal value

it can also change its sign (See J?ig.8)

which leads to impact loads in the thruster right-angled gearbox.

OPLd1AT1ON UN PER CONI)TT1 0N3 01

AIil-INI'LOW

In the course of servIce und during full-scale and model teuts there sometimes occurs cecil 1)hnOmufloli US infloW of atTUo!J-pheric eir to the thruster propeller disc. It is caused by an inadequate submergence of the propeller axis.

One may single out at least three cases of air suction to the thruutr propeller

zo-ne: continued, periodic and accidental uix' suction.

Thruster operation under conditions of

a continued air suction takes place when the thruster propeller axis is evidently too close to water surface and is characteriwod

by an uninterrupted inflow of air to the

propeller disc. In conditions 01' air presence the propeller begins to work in a water/air

m.i.xtulo with a iimall value of upuclfic

donsi-ty which naturally leads to a multiplied fall in the value of propeller thrust,

thruster thrust and consumed power. In this

case, external loads on the structure elements

don't exceed their nominal values and the

efficiency of the thruster is very low. Periodic suction of air to the propeller disc originates at depths close

to critical

and can be explained by the fact that in case of atmospheric air entrainment to the

propel-ler area there takes place a sharp fall in the value of thrust and torque due to

fornia-tion of water/air mixture. The pressure

dif-ference generated by the operating propeller

tends to zero. This leads to decrease in the

value of pressure drop and diminishes the

flow velocity at the thruster inlet. When the pressure drop at the leading edge reach-es a value which is not adequate for the

ge-no Ia ti CII of air Inflow, the propeller begins to upuruto iii wutur uIivi.rOIllIII1 I with a normal

value of and at flow speeds substantially

lower compared to the ateady-tate value. The propeller operating point shift to smaller advance values leads to increased thrust and

Inlet presdure drop and with the pressure

drop reaching its critical value the inflow

of air jhionomenOn reappears.

Accidental air suctiOn happens in case of

change of flow conditions at the inlet due to

seas or ship's motions. The physics of the process is ijimilar to that observod in the case of the periodic air inflow mode.

The accidental and periodic modes of air

in Li ow appear the most danoroUs onos from time point o.t view of accompanying loads which

atI'oCt the elements of the thruster... 'f'h theoretical olUtiOfl of this problem

involve very coiup lox coumpu tat! ons, therefore, an attempt wuc undertaken to deternhine the

loads experimentallY. Thus, nieasuremaflts of thruster thrust performed by means of a transducer on a model with 0 0.15 m showed the possibility of its near two-fold increase compared to the steady-state nominal value.

Similar results nero obtained from the oscil-lograph-aided recordings of thrust and torque on the 5imiift of a model thirustor with D

6. CONCLU3ION3

Opera tioñally aôcurate co].culatioñn

tjon-cribing the unoteady-etate modes or thruster operation Euch am starting or reversing can

be performed with the help of known equa-tions used in hydromOChafliCs. The results

of calôulatiOfls carried out according to the

proposed procedure are in good agreemcit with experinicntal data.

Dynamic ldidc affecting tho thruster elements and units during its unsteady-state operation depend to a substrtiitial degree on the length of the tunnel, starting or.re-versing time, driving engine parameters and

other factors and can exceed the steady-state values in the ordär of 2.. .2.5 times.

These data mUst be taken account of in the

process of thruster design.

Lipis V.]3., Propeller FlydrodyflasliCe during Ship 'sMotiOflS, Sudostroyenie, 1975.

Rusetskii A.A., Hydrodynamics of

Controllable Pitch Propellers,

Sudo-stroyenie, 1968.

Popov D.N., NonstatlOflarY Ilydro-mechanical J'roc0000fl, Mashinootroyonle,

1, N 300 250 200 150 100 50

Fig.1 Design ocheme of the thruster

L/D=3.2 A/A =0.36 Z=4 P/D 0.9 =Const)

n, ci

20 10 0 t, C .0.0

04 0.8

1.2 1.6 2.0 2.4 2.8 3.2

Fig. 2 Comparison of enlct,latCd and experimental values of propeller tIirint. in the proc005 of starting the thrUCtfr

- experimental dala calculaLod data

T, N 250 200 150 100 50 0 L/D=5.2 Ae/Ao=0.3'6 Z=4 F/D=0.9

/

/

/

/ I (Kt=Const fl

r, ci,

20 10 0.0 0.4 0. 1.2 i..6 2.0 2.4 2.8 3.2 C

?i. 3 Co:parisoo of calculated and experimental values of propeller thrust in the process of s.tartin

t:e thruster ezrirnental data calculated data Pe, N 400 350 300 250 00 1 50 1QO 50 L/D=1..4

Ae/Ao=0.38 Z4

P,'D= 0.9 n, C-40 30 20 I0 r r'

Pi.4 Comparison of calculated ad expe1nental values of

thruster thrus.t in the process of startin the

thrs ter

-

eoeriental 'lata caluiated data I /

/

0, -. 0.E r a 3.2

0, Nrn

10 9 8 7 0 A 1 0 L/D=5.2 Ae/Ao0.36 Z=4 P/D=0.9 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

.4

p C 30 20 I0 0

Pig.5 xneri:nental and calculated values of propeller

hydrodynamic torque in the process of starting the thruster eeriTental data calculated data T/T. 2 t2!E 1L....t221

IIAWARUF

WA All

/1

Design cve3 of truste :r::ee

orocesE o C?P !:c c:-:ze "i?ull Astern" / 0 (Kq=Const) /

'I.

_/

/

/

/

-I

I. / -I

IIVAR5.0

_IrA,j

U..

- U..

_-U.

WE_U

Wi_Ui

I UIU

Pig.7 Desi curves of thrüstr thrust in the process

of itch change

I

=

0.6

0.5 1.0

_tltr

Design curves of h'do'namjc propeller torque

in the process of CP pitch change fron

"Full Ahead" to '?].i Astern"

Z 3 /A0 0.52 P/D = 0.9 D = 2 n = 4.067 LID = 7 Z 3 A/A = 0.52 PlO 0.9

D-2m n4.067

L/D=7 t=12s

Pe/?e

t6

0.4 0.6 0.2 0.2 1.0 1 2 t/t,

T/To. 0 I

1

Z 3 A/A 0.52 P/D = 0.9 3. 2'rn n 4.067

_tr

12s AL I

uIT1Ii

RIWA$

rAW4.

AWAIi

LID 7 N m 6 0 , N 253 200 150 103, 50 LID = 5.3 D 0.2 m n = 22 s Z 3 A/A0 = 0.36 BID = 0.9 0 10

t,s

?ig.9 Design curves of thruster propeller thrust

in the process of pitch change

S

50

-0 1 ₂

t,

Pig. 10 0sciiiorerns of thruster propeller torcue and

thrust under condjtjos of atnosherjc air