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OCR for page 342
Control of the Turbulent Wake of an
Appended Streamlined Body
S. Cordier, L. De Lotte (Bassrn d'Essais des Carenes, France)
ABSTRACT
Propulsor te ts at model scale behind a ship
model are faded with the problem Hat Rey olds
m mber similarity camot be met, even if large test
facilities a used
In She work presented we are specifically
concerned with the flow similarity over the --or part
of He hull with ad without apendages This issue
concerns the mea flow into the propulsor (w ke
fraction) ad He pow ring characteristics mch as
sh It speed, thrust ad pow r When cavitation or
hydkoacoustic studies are concerned, it also becomes
impo tat to simulate He th ee dimensional
di tribution of velocities, ad perhaps the turbulent
flow prope ties in the propeller plan Several means
of altering He flow over the hull in order to simulate
R y olds n mber similarity have been st died ad
tested in different laboratories Bo mdary layer
blowing has been selected ad implemented on a
model tested in the GTH
The design of f is set up is briefly described
R sults of LDV measurements a presented which
show how the blowing system modifies He
dishibution of velocities in a ve y effective rmnmer
The characteristics of She wakes generated are
analyzed (wake fraction, harmonic content) m
pa icular with respect to the effect of apendages A
medhod for analyzing LDV measurements in order to
estimate the turbulence in the flow is outlined ad
aplied to She measurements pe formed on She fter-
body Fmally, She effect of She ch tinges m wake on She
steady ad un teady pe forma es of a propeller a
presented
INTRODUCTION
Tests conducted at model scale m naval
hydkody comics are co fronted with She problem of
R y olds n mber similarity which camot be met,
even ff large test facilities are used This similarity
problem requi es the use of extrapolation methods
adapted to She different flows: ship resista e ad
propulsion, flows on lifting surfaces ad propellers,
separated flows, sheet or bubble cavitation, vortex
cavitation, etc The variety of difficulties which
arise f om She differences m he is very challengmg to
She experimental hydkody amici t These issues a
not easily solved by CFD either because of the large
R involved (107 to 105) which create mmerical
problems ad She i fluence of transition which is not
modeled by RANSE codes Finally, aithonfh one can
imagine Hat f 11 scale measurements are the a wer
to these issues, the economical cost ad tech iccl
complexity of performing scientific quality full scale
velocity measurements on a ship have so far reduced
These instep es to a very limited m mber with varying
deg es of success (extent, n mber of components,
ace tray)
We focus our attention in f is pager on She
flow aro Ed She hull ad more precisely in the
propeller disk headed, the velocity field m this plan
detemmines the vol medic flow rate in thepropulsor
disk (wake fraction) ad the pow ring chaateristics
such as rate of tom of the propulsor, f ust, ad
pow r When cavitation or noise is She purpose of She
tests, Hen it is important to simulate the 3D
dimensional mea flow field ad in some instep es
She turbulence levels in She wake
OCR for page 343
The impo tan e of Rey olds m mber effects on
ship wakes in the rage of R from 107 to 7 107 has
been show m the GTH by 3D LDV measurements
behind a single screw merchant ship form ad a twin
screw naval ship model using, in each install e, a
single model with flow velocities as widely spread as
possible; t pically, between I ad 2 m/s at the low
speed end ad 12 m/s, which is She highe t speed
possible in She large test section of the GTH L
Briagon-Marjollet & al, 1995; S. Cordier & al,
1997) These measurements show wide differences m
She w ke fraction ad She th ee dimensional features
of the wake ma. which a due to She changes R
These have proven useful to correct model scale
towing rank w ke ma. to f 11 scale wake
di trrbutions More recent work Evolving She effect
of the propeller has shown how he effects m She
nom mat wake are m odffied by the propeller action
More recently similar investigations have been
performed on a submarine model derived from the
SUBOFF shape ad on She twm screw vessel operated
by SACLANT (the Allia e) This worked sponsored
Through a WEO EUCLID prog am is focused on
evaluating the Hi it of CFD to capture flow details
such as he ad propeller effects
The motivation m this work was to amply this
methodology to submarine work m order to evaluate
He he effects on bare ad Upended submarine hull
forms
As far as She wake f action is concerned,
different empirical formula have been developed for
different types of ships which enable She
exhaolation of the model scale effective wake
f action to full scale These are based on statistical
analysis of families of ships for which reasonably
good full scale data is available For submarines such
data bases are limited in She mmmber of ships ad
when dealing with novel geometries, empirical
methods ape of no help it is fhffefore essential to
oh am a realistic wake at m odel scale to determine the
full scale design point
The goal of this work was to develop ad
demonstrate She effectiveness of a bo mdary layer
conhol method, to study She characteristics of the
wake of She body at different he ad with different
levels of boundary layer control for different
co figurations (with or without apendages) Finally,
f is st dy focused on the effect of the wake
parameters on the perform e of a propeller
operatingbehind this body
The workwasbroken down inn ee parts:
selection ad development of ah mdary layer
control method using CFD
design ad fabrication of He experimental set
up
tests m She GTH ad analysis of measurements
(mea ad turbulent flow, steady ad msteady
forces on the propeller)
BOUNDARY LAYER AND WAKE
CONTROL
Several methods have been imagmed ad
investigated by different authors to reduce the
differences in flow field due to R non-similarity at
model scale Lachle & al, 1983):
use of a low viscosity fluid such as liquid
hell m,
extension of the laminar flow over the body
using heating of He body to reduce He width
of the wake,
reduction of the length of He fore part of the
model,
changes in the test section shape in order to
modify She pressure dishibution ad boundary
layer development on the model,
use of polymers (large molecules) to reduce
the thickening of the boundary layer ad wake,
suction of She bo mdary layer on the body,
blowing of the bo mdary layer
Further examination of these different
solutions quickly points to technical or practical
dffficulties associated with all these schemes For
example, She use of low viscosity fluid is limiting
h case it is not suited to the simulation of cavitation
behavior Furthermore, the size of She models
associated with this technique is so small as to be not
practical h case of the relative sin: of existing flow
velocity measuring devices The use of heating for
laminar flow is not practical in terms of model
fabrication ad its effectiveness is very dependent of
She shape of She body R d i non of the fore pa of
She model implies a modification of geometry which
is not always compatible with the requi ements of She
st dy How ver, when possible, this method is quite
practical Changes in te t sections are not practical
ad not very effective The use of polymer requires
cleamg of the tunnel after use which is not
economically possible Suction of the ho mdary layer
requi es large flow rates which are difficult to
mcorporate in a submarine model
Finally, She possibility of using a blowing
sy rem was considered She most practical This
method had previously been tested wifih Access in a
t m I for the elimination of fihe wall ho mdary layer
OCR for page 344
upstream of a wall mounted fin test set up. Blowing is
relatively simple to implement, does not require a
change in the shape of the body. The effectiveness of
boundary layer blowing is the injection of small
quantities of high momentum flow along the wall of
the body, deep in the boundary layer of the flow. This
momentum is transferred through the action of
turbulent shear forces across the boundary layer.
After a certain distance, the jet corresponding to the
high momentum flow is diffused so that the velocity
profile assumes that of a natural boundary layer. The
use of blowing allows the injection of momentum
with a relatively small flow rate which is convenient
for integration in a model.
Reynolds Average Navier Stokes (RANS)
calculations were performed in 2D axisymetric flow
for different slot positions and slot heights with
varying jet velocities. These parameters (figure 1)
were adjusted so that the velocity profiles in the
propeller plane were monotonous and corresponded
to high Reynolds number velocity profiles. These
calculations allowed the definition of the slot
dimensions, and flow rate required to generate the
appropriate wake.
Systematic calculations were performed using
different sets of parameters (h, Xs, Vj/Vo) and the
resulting velocity profiles were analyzed to identify
the best compromise between jet velocity, slot width
and position. One constraint in this optimization was
to reduce the flow rate as much as possible so as to
reduce the mechanical problems associated with the
integration of the ducting in the model and the
support strut. Hence for a given jet momentum
required to alter the boundary layer profile, the jet
velocity is increased and the jet slot width is
decreased. Then the axial position of the slot is
chosen so that the jet has sufficient time to diffuse
and adopt a realistic boundary layer profile. Reducing
the slot width is very effective in accelerating the
diffusion of the jet. The compromise was found to be
most effective with jet velocity ratios on the order of
2 and a f ow rate coefficient (Cq) of about 1 to 2
percent (Cq = Q/Vo/~ R24.
EXPERIMENTAL SET UP
The tests were carried out in the large test
section of the GTH (Grand Tunnel Hydrodynamique)
located at the Bassin d'essais des carenes in Vat de
Reuil. This test section is rectangular (2m*1.35m),
and 10m in length. The maximum f ow velocity in the
test section is 12m/s. The strut which supports the
model is attached to the roof of the test section.
Figure 2 shows a schematic of the
experimental set up. The model is about 4 m long and
0.45 m in diameter. It is built in 3 main sections:
nose, central section with the blowing system and the
tail which houses the propeller drive motor and the
dynamometer. The blowing section forms a 1 mm
thick slot around the circular periphery of the body
and is set at a 10° angle with respect to the body
surface. The slot is located at about 65% downstream
of the nose of the model. The jet is formed by
accelerating the f ow from a large settling plenum.
This one in turns is supplied with water pumped from
a connection in the tunnel through a f ow meter into
the strut. The pump was installed as low as possible
in the tunnel to reduce the risks of cavitation. The
f ow meter is of the venturi type and was specially
designed and built for this purpose.
vo
as,
xs
Slot angle
!~` . ~ ~ ~
Plenum.F Jet velocity Vj ~ propeller
Ro . Qi . Model
Figure 1: Blowing slot parameters
with h : slot thickness,
Xs : axial position of slot
~ : angle of the slot axis
Vj Vo: non-dimensional jet velocity
Ro : radius of body
~51515151515151~
. Vo ~ ~
blow~ng plenum
motor ~ns~ume~ntation
-
Figure 2: Schematic of the blowing section in the
experimental set up
The tunnel velocity used throughout these
experiments was about 5 m/s and the f ow rate on the
order of 20 1/s. This range of parameters allowed the
blowing coefficient to be varied from 0 to 2.5%
(CqO=0, Cql=0.016, Cq2=0.021, Cq3=0.0254. The
model can be equipped with different appendages
OCR for page 345
which are typically found on submarines (sail, deck,
dive planes, rudders. The results presented here
correspond to two configurations: bare hull and
appended hull (stern dive plane and rudder).
Several types of measurements were
performed during several test campaigns:
· 3D laser doppler velocimetry,
· mean thrust and torque on the propeller,
· fluctuating propeller thrust,
· radiated noise,
pressure fluctuations on the hull.
The measurement of unsteady thrust is useful
in evaluating the low frequency excitation of the
propeller due to wake non-uniformity. The results
presented here are mostly concerned with the LDV
measurements and powering performance.
BLOWING SYSTEM EVALUATION
The blowing system and the behavior of the jet
for different flow rates were evaluated during
dedicated tests. Some of the tests were performed to
calibrate the pressure drop in the pump loop as well
as the flow meter. The behavior of the jet for different
positions downstream of the slot was investigated
through measurement of the velocity profile. These
measurements were performed for different jet
velocity ratios in the bare hull configuration. The
radial distributions of the measured axial velocity for
different flow rates and for different axial positions
are presented in figure 3. The data corresponds to
three axial positions: at the jet nozzle exit, in the
middle of the after-body and in the propeller plane. z+
represents the distance from the model in mm.
Figure 3 shows how the velocity profiles
evolve downstream of the slot from the high velocity
jet at the slot exit which develops a thin boundary
layer along the body and a shear layer with the
incoming boundary layer (figure 3.a). The shear
forces acting on the jet tend to diffuse the jet, reduce
the value of the maximum velocity and increase the
width of the shear layer. Further downstream, the
velocity profiles tend to lose the inflection point
which corresponds to the mixing layer between the jet
and the upstream boundary layer (figure 3.b). Finally,
in the propeller plane, the radial velocity profiles are
monotonous and are similar to the natural wake
(Cq=O) (figure 3.c).
-0,20
CqO
~ Cq1
-0,1 5 Cq2
=+ -0,1 0
N
-O,05
0,00
0 0,25 0,5 0,75 1 1,2;
u/Vo
Figure 3.a: Velocity at the slot exit
-0,20
-0,15
-0,1 0
N
-0,05
CqO
Cq1
Cq2
~ Cq3
0 0,25 0,5 0,75 1 1,2
u/Vo
Figure 3.b: Velocity on the afterbody
-0,8
-0,6
m+
-0,4
-0,2
O
CqO
Cq1
Cq2
Cq3
CqO ( 1 2m/s)
0 0,25 0,5 0,75 1 1,25
u/Vo
Figure 3.c: Velocity in the propeller plane
Figure 3: Measured radial velocity profiles for 4
blowing flow rate (Vo=5 m/s)
OCR for page 346
These figures show the effectiveness of this
boundary layer control system whereby the radius of
the viscous wake can be changed by a factor of
roughly 2 by simply changing the blowing pump
velocity. The final wake velocity profiles with the
blowing system active appear to be similar to the
velocity profiles obtained naturally.
The profiles on figure 3 show velocity data
without blowing (Cq=O) for a tunnel velocity of 5 m/s
(Re=3 107) which corresponds to the natural
evolution of the boundary layer and wake.
Measurements at 12 m/s (Re=7 107) are also shown in
the propeller disk to illustrate the difference in
velocity profiles due to the difference in Reynolds
number. Clearly the range of velocity profiles which
can be achieved cover this difference in Re and most
likely much larger differences in Re, i.e. to full scale
(Re= 109~. The advantage of this system is its
capability to vary the wake rapidly during a test so
that the sensitivity of the propeller performance to
this parameter can be investigated.
Wake fraction
0~5
oo'3
~0,2
0,1
CqO
Cql
Cq2
Cq3
' \\ )K CqO,12m/s
~ ~ -
l l l
0 0,2 0,4 0,6 0,8
wn
Figure 4: Wake fraction as a function of the disk
diameter (without tail planes)
0,5
0,4
In order to quantify the effect of blowing on
the wake, it is convenient to examine the variation of To 093
wake fraction as a function of disk radius. The mean +
wake fraction is calculated as follows: ~ 0,2
Wn=l-s i~('°)dS
prop Sprop 0
0,1
al ~ f lo; prop Fir 03 d dd O
Wake fraction without tad! planes
Figure 4 summarizes the wake fraction data
calculated for 5 values of the radius (20mm, 40mm,
60mm, 80mm et lOOmm) based on the measured
radial velocity distributions on the propeller plane for
4 blowing conditions at 5 m/s and without blowing at
12 m/s. The distribution of the wake fraction as a
function of the disk radius is similar for the different
conditions which confirms the realistic nature of the
resulting wakes. It can be noted that the wake fraction
reduces rapidly as the disk radius is reduced.
Figure 4 shows that significant gains in wake
fractions can be obtained using blowing. Hence, at 5
m/s the nominal wake can be modified by a factor of
two and can very easily compensate for the Re effects
which could be simulated on the model by changing
the tunnel velocity from 5 to 12 m/s.
CqO
Cql
~0 ~Cq2
'` ~ C3. ~ Cq3
· ¢1
·
11 , , , 1
0 0,2 0,4 0,6 0,8
wn
Figure 5: Wake fraction as a function ofthe disk
diameter (with tail planes)
0,5
0,4
0 0,3
~ 0,2
0~
~CqO
C] Cql
Cq2
Cq3
, ~
0 0,05 0,1
wn w/ tail planes - wn w/o tail planes
Figure 6: Effect of tail planes on the radial
distribution of wake fraction
OCR for page 347
The velocity profile at a blowing coefficient
value of Cql is sufficient to match the higher Re
profile. A slight difference in profile slope can be
identified.
Wake fraction with fad! planes
assimilated as a Re effect, higher Re will not only
reduce the wake fraction but also modify the
azimuthal distribution of velocities which will
strengthen the higher harmonics of the wake.
.; ,.
As expected, the presence of tail planes
increases the wake fraction giving a relatively linear
radial distribution of wake fraction (figure 54. The
shape of the distribution in wake is very similar for all
blowing coefficients which shows that the blown
boundary layers have a realistic global effect.
In order to bring to evidence the modification
of the wake fraction due to the tail plane, the
difference between the wake fraction with and
without tail planes is presented on figure 6. The
presence of tail planes results in an increase in wake
fraction for the larger radius. The influence of tail
planes on wake fractions is rather constant for all
blowing coefficients with a maximum value of 0.075
at about 0.3 z+/Ro.
EFFECT OF TAIL PLANES ON THE
WAKE DISTRIBUTION
In order to analyze in further detail the relative
effects of blowing and tail planes, wake maps were
performed for the different configurations. Hence, the
effect of tail planes can be identified by subtracting
the bare hull velocity profiles from the wake maps
measured with tail planes. This subtraction is
performed for the different values of blowing
coefficients. The resulting velocity profiles are shown
on figure 7 for CqO and Cq2. This figure shows how
the tail planes strongly affect the spatial velocity
distribution in the plane of the propeller. The cause of
the disturbance, more than the velocity deficit of the
wake of the tail planes, is the presence of the so-
called horseshoe vortices which are due to the radial
gradient of upstream velocity which creates a radial
gradient of pressure on the thick fins and hence a
vortex pair. The presence of this vortex has for effect
to pump high velocity flow in the area behind the fin
and to concentrate low velocity flow between the tail
planes.
Figure 8 shows the difference in axial velocity
wake maps measured with appendages for two values
of blowing (Cq2 - CqO). The effect of blowing on the
wake upstream of the appendages generates a
modification of the vortex structure which develops
along the root of the tail planes. The axial vortices are
closer to the after-body with stronger velocity
gradients. This difference is hence more important for
the inner radii of the wake. If the blowing effect is
it.
A....
.
. . .
~ . ~ .~
Figure 7.a: Difference in wakes for CqO
(wake with tail planes - wake without tail planes)
.; ..~..k, ......
. (. ..
..
k ::::
:, ,~ if
: ., . A.. :. :,,
Figure 7.b: Difference in wakes for Cq2
(wake with tail planes - wake without tail planes)
Figure 7: Effect of tail planes - (axial component)
:,.
it~,~j~..:
A.
.~...~.-~
~ ,:-.
.~ ..........
.......... ..
~ .. - I.
~ i .~ . ~
.. :~..:.~...
.. .~
.. ... ~~.S
Aft' '.r' ~~,:
<;,' ~
:~. .. ';
. ... :.:.,:.
Figure 8: Effect of blowing on the wake with tail
planes (axial component, Cq2 - CqO)
OCR for page 348
EFFECT OF TAIL PLANES ON THE
WAKE HARMONIC DISTRIBUTION
The most effective way of describing the
azimuthal variations in wake velocity is to perform a
harmonic analysis of the velocities measured at a
given radius. This harmonic decomposition is written
as follows:
00
Wn (r, q) = 2,Vxk (r )cos (kq +i k (r ))
k=0
Relative amplitude of harmonic components
2
1.5
1
with:
Vxk amplitude of the kth harmonic of Vx
Ok phase of the kth harmonic of Vx O
Figure 9 presents the result of this type of
analysis for the axial velocity at the radius r/Ro =
19%. The data on figure 9 is in fact the relative
amplitude of the first 4 harmonics of the wake
obtained by dividing the amplitudes measured with
blowing (Cql, Cq2, Cq3) to that measured without
blowing (CqO). The ordinate scale is hence the
amplitude of nth harmonic with blowing divided by
the amplitude of nth harmonic without blowing.
Although the wake map data showed some
effect of blowing, the harmonic analysis in the form
presented here shows considerable influence of the
harmonic content even for minor blowing flow rates
(Cql). The effect on the 3r~ harmonic is considerable
since it nearly disappears. The importance of the
effect of blowing on the harmonic content is
illustrated on figure 10 where the wake map
corresponding to the fourth harmonic is pictured for
two values of blowing coefficient ~ CqO and Cq24.
The levels indicated are in m/s for a reference
velocity of 5 m/s. For certain values of the harmonic
components, both amplitude and phases of the
harmonic are changed. This type of data illustrates
how a flow modification similar to a Re effect can
have large influences on the unsteady excitation of a
rotor.
TURBULENCE
If the mean velocity distribution (wake) has an
effect on the unsteady performance of apropulsor, the
turbulence of this flow can have some importance on
the behavior of the hydroacoustic response of the
propulsor. It is therefore of interest to look into the
turbulence in the wake and into the way it is affected
by parameters such as blowing and propeller
operation. For this purpose, the velocity
measurements in the turbulent wake upstream of the
propeller disk were performed using a LDV system in
different operating conditions.
r/Ro= 19%
o
111 Cq l
1111 Cq 2
I~Cq3
~-
Harmonic
3 4
Figure 9: Effect of blowing on the harmonic
. .
decomposltlon
........
... (~ ~ .. ~ . ~
·t, .~E . ~<
..............................................
.>i ? ¢
).
3 .,,~
:~
:: :-';
. ~ ~ ~,
:
.:.::,.; ~
... ;< ,.::(
,,., i~
Figure lO.a: Representation of the wake due to the ¢h
harmonic for CqO
~.~
~. .
...:
...i ~
...~t
. .~.,
(..:i;
<~.
~ ;..- t~
~:
~ 1?
.. . ....-..:~.~.
Figure lO.b: Representation of the wake due to the 4th
harmonic for Cq2
Figure 10: Reconstruction of the fourth harmonic of
the wake for blowing coefficients CqO and Cq2
OCR for page 349
However, turbulence measurements using
Laser Doppler Velocimeby are limited in terms of
data rates in He GTH ad using micro bubbles of
water valor ad air, the average data rate is in He
order of I to 2 kH This f equency rage represents
a average value since the data sampling is a ran dom
process which is triggered by the passage of a tracer
m the measurement volume in the GTH, the time
between two consecutive a ival times of tracers
follows ve y closely a exponential distribution
which parameter ~ is equal to the mea samplmg rate
This diary: em on is truncated at high frequency by the
limits of He LDV system Figme 11 is a example of
measured histog am for which the smallest time
1000
1
ferry. al hrne interval (ms)
Figme 11: Histog am of time betw en scamp le arrival
times
This type of random sampling precludes He
direct use of classical algorithm to estimate the
spectral pow r densities Two methods have been
considered:
intemolationmethods
direct atocovaria e calculation
The first method is not relict le a d the second
one was implemented because it only uses
atocovaria e calculations ad does not require the
estimation of data pomts betw en measured pomts
The method used relies on He truncation of the actual
arrival time to the resolution of the clock of the data
Requisition system Hence He data is m the form of a
regularly sampled signal with missing data points
The pectral pow r density of He velocity
fluctuations is obtained by the Fourier transform of
He atocovaria e of this signal This method is
robust a d effective but it is limited in f equency to a
value move which the noise in He pseudo signal is
larger than the signal
Figme 12 presents f ee examples of
t rbulence spectra measured in He wake with ad
without blowing ad with ad without propeller The
hori ontal line in He plot represents He detection
limit of He medhod
The results presented here show Hat the
t rbulence levels are slightly low r when He blowing
>! tem is active ad when the wake is smaller in size
However, when the propeller is operating, He
turbulence levels are much low r due to the
aceleration of the flow This data clearly shows Hat
when turbulence quantities ape requi ed, the effect of
He propeller camot be ignored
Figmre 12 a: Turbulence spectr m
(without propeller, CqO)
Figure 12b: Turbulence spectr m
(without propeller, Cq3)
OCR for page 350
-45
-50
dB
-55
-60 .
Log frequence
Figure 1 2.c: Turbulence spectrum
(with propeller, CqO)
Figure 12: Examples of measured turbulence spectra
in the wake.
POWERING PERFORMANCE
The thrust generated by the propeller with
different flow rates of the blowing system was
recorded. The configuration tested was the one with
the appendages. The trust coefficient, Kt, is plotted
against the advance ratio Jan on figure 11.
T
Kt= r 2D4
~ Vo
J tan = nD
The advance ratio is based on the tunnel speed
and hence, does not take into account the differences
in wake fractions due to the viscous effects and
blowing. Hence, the curves of Kt exhibit a gradual
shift to lower values of Kt as the blowing is increased
which is expected.
In order to asses the effect of wake fraction on
propeller performance, the measured nominal wake
fractions based on the LDV measurements were used
to correct the advance ratios from a behind condition
to an estimated open water performance
JO = JO ~ (1- An) . Figure 12 shows the same values
of Kt plotted against a corrected advance ratio. The
collapse of the curves to a single curve is remarkable.
Hence, when the measured nominal wake is used to
correct the advance ratio, the resulting Kt values are
identical. This shows that although the change in
wake due to blowing induces both complicated
changes in the wake structure (axial vortices, etc..)
and changes in the radial loading of the propeller, the
nominal wake based on the integration of axial
velocities is sufficient to correct the rpm. Inversely, if
the propeller operating point is known at full scale
(ship speed and rpm), it is very easy using the
blowing device to adjust the wake so that this
operating point can be reproduced at model scale with
a realistic wake containing the non-uniformity and the
turbulence content closest to full scale as possible at
model scale. The advantage of performing this
adjustment is that the scaling of the frequencies
associated with shaft speed will match the full scale
values. Furthermore, for cavitation inception studies,
the local cavitation number and the radial distribution
of propeller loading will match closely, leading to
. . ~ . . . .
more accurate predictions of cavitation Inception.
Figure 11: Influence of blowing on propeller thrust
Kt
1 1
CqO
Cq1
. - ~ - Cq2
. - A- - - Cq3
Figure 12: Propeller thrust as a function of corrected
advance ratio
OCR for page 351
CONCLUSIONS
A boundary layer control device has been
st died ad dev loped based on blowing along the
body so fade This device has hen implemented on
a axisymehic sheamlip d body for the pa pose of
simplatmg wakes in the propeller disk with differ nt
values of mea wake fraction This y em has hen
v rived to work as expected ad to provide realistic
v locity profiles on a ban body These v locity
profiles cap compensate for disco pacies in the
R y olds m mber of the flow compared to f 11-scale
ad it is Therefore possible to achiev full scale wake
fraction by adjusting the blowing system flow rate
Wish 6 is . sytel, it is therefop possible to in stigate
She sensitivity of the proppisor performance to
changes in She upstream wake related to R m mber
discrepancies
The effect of She blowing device was st died
on She ban hull wake ad on a Upended wake by
st dying the v locity profiles, She radial wake fraction
di tr~bution, She wake maws, ad the tmbplepoe m the
flow For the la er a special a alysis method has been
dev loped m order to obtain pow r specha of the
t rbplence
The data shows a strong i tip pee of the
ppsheam v locity lield on She perturbation cased by
tail planes which Spears m the v locity field ad She
harmonic decomposition On the other had, the
tmbplepoe lev is do not seam to be Front is affected
by She blowing lev is Howev r, it is clear chat She
propeller action is responsible for a large decrease in
She t rbuIent lev is
After this study oriented towards the
qualffication of His system, it has been found to
provide a effectiv m._ r to She p ed for a variable
wake test set up it has then fop been used for the
design of submarme proppisors where She advance
ratio ad the distribution of circulation plays a key
role in the precise estimation of She noise of the
propplsor
REFERENCES
Briapeop Marjollet L., Cor`fler S., Laureps J-M.,
Rardo J., "Effect of wake scaling on the prediction of
propeller cavitation", CAVES, Deaville, France,
May 1995
Cordfler S., Legrapd F., Pipard J-C., 'Hull ad
shaft wake Iteration", Propeller ad Shafting 1997,
September 1997
Pipard J.C., "Et de experimentale et numeriqp do
sillage en amont d'pp helice" These ECN, 1997
High R Molds Number Flows Using Liapid ad
Gaseous H m, Sprmgff-Verlag, J. RpssellEditepr
Viscous D ag R duction in Bopmdarv Layers
Prog ess in Ashonatics ad Aeronautics, Vol 123,
A AA, D Bushnell et J. Hefp r, edited s
Lapchle G., Gurney G., "Laminar Boundary Layer
St i i it on a Heated Underwater Body", Techmical
Memorandum, Applied R search Laoratory,
PSU/ARL-TM-83-157, Javier 1983
Jessup S., Remmers K., et Berberich W.,
"Comparativ cavitation p rt arm once evaluation of a
naval m face ship prop tier, ISLE 1993, Cavitation
h ception, pp 51-62
Nobach H., Mfdler E., Tropea C., "Efficient
estimation of pow r pecrral density from laser
Doppler a mometer data", E periments in Fluids 24
(1998 ) 499 509
Ramopd A., hhllao P., "Mesa es copplees LDA -
Fils chards, et haitement des sigma LDA", Sime
Comrres FracoPhon de Velocimehie Laser, Ron al
OCR for page 352
DISCUSSION
M L Billet
ARL Pffmsyl mid State University
USA
The mthors have performed m interesting
experiment where they used z walljet on z
streamlmed body to control the me m wake Ingested
by z do..- sheam propeller This has provided some
insite mto mod Ihng higher Rey old mmmber flows
in referee e to pow rmg perfommance How ver, the
in ommg wake into the propeller for this treamlme
body is also controlled by She afterbody adverse
pressme gradient Ed interactions with the
zpp ndr es
The resultmt mean velocity profiles It
different Rey olds mmmber See appropriately similar
However, the IO..'ff Rey olds mmmber wall jet case is
still z wall jet, although complicated by the zfterbody
geometry Ed propeller The distribution of the
turbulent en rgy th oughout the boundary layer
carmot be appropriately similar (see Wyg mski,
Katz, Ed Hover (1) The turbulep e data is not
complete m this paper Ed She measurement method
p eds some clarffication Have the mthors made
measurements of She RMS, me ms Ed length scales
for the cases tested?
As discussed by the mthors, the spatially
nommnifomm flow Ed the temporal variations are of
primary cop rn for the propeller i flow For this
reason, ARL Pemm State has utili ed shorter models
Ed m some cases sheens are added to the nose of
the body to gee rate She predicted me m wake
However, the issue of turbulent en rgy distribution
Ed flowheld harmonic content as z fun non of
Rey olds mmmber remains z critical issue for
cavitation mdhyd oacoustic pe formance
Hz d oacoustic performance is very sensitive
to flow features Ed m my st dies have shown f is
relationship Two mterestmg experiments that relate
appendage wake feat es to noise have l en
conducted by ARL Pemm State Ed Brooktield Ed
Waltz (2)
Two experiments to investigate blowing
from the trailing edge of m appendage have been
conducted at ARL Pemm State The wind tum I
experiment of z conhol-su face-like appendage
show d the ability of hailing-edge blowing to
gee rate z moment mless wake, for z nonlifting,
thee-dimensiopal al foil At m mgle-of.rttack of
ten degrees, z completely moment mless wake using
trailing-edge blowing could not be generated,
becmse of the asymmetry of She original wake A
dual-slot co figuration for zdaptmg hailing-edge
blowing to z lifting foil was then evaluated in z
second experiment, trailing~dge blowing on five
stationary struts located up Ream of z li - e-t lad d f m
was UK 0 porffed With z total flow rate f ough ail
the blowing holes equal to 0 7% of the flow rate
th ough She Ian itself, significant reduction in the
radiated noise at the blade-passing frequep y Ed its
integer multiples was demonstrated The reductions
ringed fi m 15-24 dB for the first four harmonics
Ed from 3-9 dB for the p xt th ee harmonics
Brookfield Ed Waltz (2) have had similar success
using tpuilmg-edge blowing on rotor bodes upstream
of stator bodes m z highbypass~ztio Ian stage
They approximately li fled in th moment m den cit in
the rotor wakes, reducing the zmplit des of the first
two wake harmonics by 70-35% The resulting stator
unsteady loading was reduced up to 10 dB at the
t lad -palm Ire~p~en y
Referee es:
l
2
Wygp mski, toast, Ed Ho Of, "Applicability of
Scaling Laws to She Turbulent Wall Jet," Jourpai
of PluidMffhmics,234,pp 669-690,1992
Brookfield, J. M Ed Waltz, I A, "Trailing-
Edge Blowing for Reduction of Turbomachmery
P m Noise," Journal of Propulsion Ed Power,
Vol. 16, No 1, pp 57-64, Jznuary-Pebruary
2000
AUTHOR'S REPLY
In our paper, figure 3 shows the evolution of
the me m axial velocity profiles from the jet nozzle
exit to the propeller plume m She bare hull
co figuration Pour blowing flow rate w re te ted
Vo=5 m/s) Purth rmore, messuremffpts at 12 m/s
w re also shown in the propeller disk to illushate the
differed e in velocity profiles due to She differed e in
Rey olds mmmber
Fit me 3 show dthat:
the velocity profiles with the blowing
system active appear to be similar to the
velocity profiles obtain d pat rally,
th blowing covers the differed e m Re
betw en 5 m/s Re=3 107) Ed 12 m/s
Re=7 10 ) The r mge of velocity prod les
which c m be achieved seems to
conespond to full scale Rey olds Number
Re=109)
OCR for page 353
Measurements of RMS were also performed
for all the cases but were not shown in the paper. The
radial distributions of the measured RMS axial
velocity are presented below in the propeller plane.
z+ represents the distance from the model in mm and
R the radius of the body.
RMS curves show some differences in the
blowing effect area.
-0.9
-0.8
-0.7
-0.6
-0.1
O.
. . ~ ~ ~
CqO
Cq1
:~: Cq2
Cq3
0 0.02 0.04 0.06 0.08 0.1
urms/Vo
Radial distributions of the measured RMS axial
velocity in the propeller plane for 4 blowing flow
rates (Vo=5 m/s)
DISCUSSION
Michael B. Wilson
David Taylor Model Basin, Carderock Division,
NSWC
This is an interesting investigation and
implementation of a slot blowing boundary layer
control concept aimed at manipulating the velocity
distribution of a model scale turbulent wake at the
plane of the propeller. The criterion of success
should be the degree of accuracy of the simulation of
the main features of a full scale turbulent wake. The
approach taken here hinges on the range of control of
the nominal wake fraction.
Checks on the workability of this approach
are based on test results over a very limited range of
model Reynolds numbers: 3x107 to 7x107, not even a
full decade. As mentioned in the paper, the target
full scale Reynolds number is O(109~. It is also noted
that the testing must have been carried out in
relatively high temperature water, since the kinematic
viscosity value inferred from the stated Rn-values
and the model dimensions indicates that the water
tunnel water must have been at around 39 deg C.
One wonders whether this Rn range is perhaps too
small for good accuracy control in light of the
inherent uncertainties of the experimental process.
For example, in Figure 4 of the paper, the curve of
measured wake fraction wn for the tunnel velocity 5
m/s and CqO is essentially the same as the curve for
tunnel velocity 12 m/s (at Cq = 0), and only slightly
different from the curve for light blowing at Cql.
The relative trends for the CqO and Cql curves
actually change character with respect to the non-
dimensional distance (z+~/ lo. Has there been an
attempt at an uncertainty analysis for the
determination of the wake fraction with the present
scheme?
There is no question that at the higher Cq
values, the variation of the measured u/V0 has the
general appearance of a higher Reynolds number
boundary layer and wake velocity pattern. It is likely
that at a higher blowing rate the distance between the
slot and the propeller should be somewhat larger than
at lower blowing rates so that the flow mixing and
diffusion mechanisms have enough time for the
velocity distribution to settle into the desired
configuration. Are there test results that deal with
this issue?
A real practical question is what is the actual
appropriate target wake velocity distribution at full
scale. A1SO7 can the full scale wake be simulated at
the model scale with a jet blowing flow rate that does
not produce a distorted velocity distribution within
the region of interest. It is suggested in the paper that
this could be the case, but it is not demonstrated here.
Some comments and questions in the nature
of practical testing issues come to mind:
The slot blowing around the periphery of the
body will affect the test section ambient pressure
and its spatial distribution in the vicinity of the
stern and propeller. This pressure would be used
in the calculation of the cavitation number. Was
the magnitude of this influence measured?
The blowing jet will also influence the
distribution of the local surface pressures over
some length of the body. Altered pressure
distributions will affect the measurement of net
hull forces in a water tunnel experiment. Has
this effect been explored?
The cross section area blockage of the model
body in the 2 x 1.35 m test section is 5.9 %,
which is pretty low. Nevertheless it would be
useful to know if there are present any notable
interaction effects or unwanted secondary flows
involving the narrow circular jet in the presence
of the test section wall boundaries.
OCR for page 354
How would f is boundary layer control concept
work with the body et m inclination Ogle to the
oncoming sheam?
AUTHOR'S REPLY
The Rey olds mmmber is based on the model
length Typically Rey olds mmmber effects on ship
wakes c m be show in She GTH in the r mge of Re
from 107 to 7107, which corresponds with flow
velocities betw en I Ed 2 mls Ed 12 m/s highest
speed possible m the large te t section of She GTH)
In our case She model is 4 4 m long The water
mm I em p revere was et around 24° during the tests
Then She r mge of model Rey olds mmmbers was:
2 4~107 to 5 8*l07
All tests w re performed with the same model
Ed the same blowing system The blowing section
fomms c I mm thick slot around the cacular periphery
of the body Ed is set et c 10° Ogle with respect to
the body surface The slot is located et cutout 65%
down tream of She nose of the model As explained
in She paper, the characteri tics of the slot w re found
to be the be t compromise betw en jet velocity, slot
width Ed position m order to minimi e the flow mte
es much es possible Ed to obtain monotonous
velocity profiles in She propeller plane
This optimization was perfommed with 2D
Reynolds Average Navier Stokes RANS)
cclcubtions
In the paper, w concluded that She r mge of
velocity profiles which c m be achieved with blowing
covers dffferff~ces in Re betw en model Ed full
scale Re=109) for She bme h 11 co figuration it is
more difhcult to conclude with the presence of tail
planes The full scale wake distribution in not w 11
k ow inthiscase
The maximal blowing case (Cq3) represents
only 0 15% of the test section flow rate Then we
assumed Nat She blowing system have no i fluency
on the tum I parameters pressure, velocity)
It's true that the blowing jet ch m e. the local
surface pressures Ed then the local friction
do..- stream of the slot How ver we did not measure
the hull resistance since this set up is dedicated to
hyd oacoustic experiments
The boundary Icyer cone ol concept c m work
with c 10° Ogle inclined body with respect to the
upsheam flow However this experimental
co figuration ht. not been investigated yet
Representative terms from entire chapter:
tail planes
