|
Items in cart [0] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 789
24th Symposium on Naval Hydrodinamics
Fukuoka, JAPAN, 8-13 July2002
Propeller Wake Analysis Behind a Ship by Stereo PIV
G. Calcagno, F. Di Felice, M. Felli, F. Cerebra
iNSEAN (Istituto Nazionale di Studi ed Esper~enze di Architettura Navale)
ABSTRACT
An experimental investigation of a five blade MAU
propeller wake behind a Series 60 Cb=0.6 ship model
has been performed using Stereo Particle Image
Velocimetry (Stereo-PIV) in a large free surface
cavitation tunnel. The investigation of the wake at
different longitudinal stations and its evolution in
phase with the propeller has pointed out the
capability of Stereo-PIV in resolving the complexity
of the flow field. The blade viscous wake, which
develops from the blade surface boundary layers, the
trailing vortex sheets that are due to the radial
gradient of the bound circulation, and the velocity
fluctuation distributions are identified and discussed.
The complex interaction between the hull wake and
propeller is described through the evolution of the
mean velocity components and the vorticity fields.
In the near field the effects of turbulent diffusion and
viscous dissipation, which cause a rapid decay of the
velocity gradients in the trailing edge wake, are also
examined.
INTRODUCTION
In the last 20 years the application of advanced
optical measurement techniques like laser Doppler
velocimetry (LDV) has provided a deep insight into
complex flow fields like propeller flows. Most of the
actual knowledge on propeller flow, especially
regarding the turbulence characteristics, derives
from the application of the LDV measurement
technique (Min. 1978; Kobayashi, 1982; Cenedese et
al., 1985; Jessup, 1989, Chesnack et al. 1998, Stella
et al., 2000) and is today routinely used by the main
research organizations in the world and in different
fields. LDV will allow, also in the future, to obtain
valuable information with high accuracy on the
mean and fluctuating velocity fields in complex
flows which is of relevance for their physical
modeling and for the validation of computer codes.
However, as any experimental technique, besides its
undeniable advantages, LDV has some limitations:
it can hardly give an idea of the spatial
characteristics of large coherent structures
which are generally encountered in complex and
separated flows, due to its single point
measurement nature;
it can induce significant errors on the intensity
of unsteady vertical structures, due to its fixed
location and time averaging nature;
it needs long periods of operation of the facility
to get a whole velocity field, increasing the
testing costs and leading to difficulties in
unsteady flows or when the facility working
characteristics have to be kept constant for long
periods of time.
This is the case of the experimental investigation of
the propeller wake in a non-uniform inflow. This
analysis requires a sufficiently dense grid into the
whole measurement plane, for each different
propeller angle, in order to resolve the flow
structures during the propeller revolution (Fell) et
al., 2000; Esposito et al., 2000; Di Felice et al.,
2000~.
From this point of view, Particle Image
Velocimetry (PIV) technique, as it allows the
instantaneous measurement of the velocity at a
plane, offers many advantages over single point
techniques: the experimental analysis could be fast
and easily conducted by acquiring images at each
angular position of the blade, drastically reducing
the testing time. Over the past decade PIV technique
has experienced a considerable progress and is today
considered to be a powerful whole field measurement
technique, continuously broadening its range of
applications. The growth of the PIV technique due to
the improvement of the hardware components is
OCR for page 790
clear: high-energy and nanosecond pulse duration
lasers, high-resolution and low-noise CCD cameras,
fast frame grabbers, as well as faster computers and
large data storage hard disks are among the major
factors that have raised the capabilities of the
measurement approach. Recent literature
demonstrates the applicability of the technique to the
naval field, in particular in towing tank applications
(Guj et al. 2001) and in the case of the propeller flow
(Cotroni et al., 2000; Di Felice et al 2000) even if
these investigations were limited only to the two
velocity components in the measurement plane
(planar PIV or 2C-PIV). However, it is apparent
that this information is not always sufficient to
characterize the flow field especially for propeller
flows where the presence of strong three-dimensional
flow structures with strong velocity gradients
requires the knowledge of all the velocity
components.
Stereoscopic PIV, using two cameras to view the
flow from two perspectives, is the obvious extension
of planar-PIV for measuring all three velocity
components in a plane. Two components of velocity
nominally perpendicular to the camera optical axis
are measured from each camera viewpoint. The pair
of two-dimensional velocity vectors for a point in the
flow are then combined to yield a three-dimensional
velocity vector. By combining the vector fields from
the two cameras, the three-dimensional velocity field
for the plane in the fluid is computed.
In the present work Stereo-PIV is applied to the
analysis of a ship model with propeller in the large
INSEAN Circulating Water Channel. The
experiment is a partial replica of the Toda et al.
(1990) experiments: measurements were performed
By_
,`
~ ' 1 tax
in several cross-planes behind of a Series 60 Cb=0.6
ship model with a 5-blade MAU propeller. The
Series 60 model was selected for the experiments to
complement the many previous studies with this
geometry.
In the following sections, results of the wake
survey are discussed pointing out to the measurement
technique capabilities in resolving the wake
structures and outlining the major problems in
applying stereo PIV in a large facility .
EXPERIMENTAL SET-UP
Measurements were carried out at the INSEAN
Circulating Water Channel, a free surface cavitation
channel with a 10 m length, 3.6 m wide and 2.25 m
depth test section which allows a 5.2 m/s water flow
maximum speed. More information regarding the
facilities capabilities can be found at
http://www. in scan. it
The ship-model was a Series 60 Cb=0.6, 6.096 m
length, conforming to the standard offsets with some
minor modification of the stern geometry to allow
the propeller installation. The propeller was a 5-
blade MAU propeller, with the following features:
diameter D=22 1.9 mm, pitch-diameter ratio
P/Do7=1.03 1, expanded area-disk area ratio
AJAo=0.74. Tests are carried out at the propeller
angular velocity of 6.7 rps with the tunnel water
velocity set to 1.22 m/s, corresponding to a Froude
number Fr = 0.16. In the presentation of the results
and the discussion to follow, a Cartesian coordinate
system is adopted in which the x,y,z axes are in the
direction of the uniform flow, starboard side of the
hull and upward, respectively. Measurements were
Figure 1: Ship, propeller model and location of the
measurement planes. The propeller is shown at ~ = 0° Figure 2: Experimental set-up
OCR for page 791
performed in three cross-planes orthogonal to the
shaft and located downstream the propeller disk,
respectively at x/L.= 0.9997, 1.0000, 1.0187 (figure
1) where L is the model length. In a reference frame
with the propeller center, the above measurement
planes are located at x/D= 0.59, 0.76 and 1.85
respectively. In the first plane, measurements
without the propeller were also performed to have an
estimate of the propeller inflow. The locations of the
last two measurement planes are similar to the Toda
et al. (1990) experiment while the first one is very
close to the propeller and was selected to test the
stereo-PIV capability of analyzing regions with
stronger velocity gradients as expected in that plane.
The experimental set-up is shown in Figure 2.
The light sheet, generated by a two-head Nd-YaG
laser was delivered to the measurement plane by
means of underwater optics. The laser sheet
generation rate was 10 Hz, with an energy output of
200 mJ per pulse. A rotary 3600-pulse/revolution
encoder supplied the actual propeller position as an
electrical trigger signal to the synchronizer. This in
turn provided the trigger signals to the two flash
lamps and two Q-switches of the double-head laser,
as well as to both cameras. Therefore, the image
acquisition was synchronized with the propeller
angle. During the test campaign, 129 acquisitions at
a given propeller angle were performed in order to
obtain the mean velocity field. Propeller angles from
0° to 69° have been considered with a step of 3°.
STEREO PIV SYSTEM SETUP
In the present experiment the angular displacement
method was adopted for the optical configuration of
the stereo-PIV system. The setup consisted of two
cameras with a resolution of 1280x1024 pixels each
and a depth of 12 bits per pixel. The first one (left
camera) was located 2 m downstream the ship model
in an underwater housing, with an orthogonal view
of the measurement plane, and the second one (right
camera) outside the tunnel test section looking at the
measurement plane through the tunnel access
windows. To reduce the strong diffraction and
aberrations due to the thick window and to the
water/glass/air interfaces a prism filled with water
has been placed in front of the right camera.
The adopted configuration is not symmetric and
has never been presented before in previous Stereo-
PIV measurements. The main reasons that led to this
choice are the following:
.
.
a symmetrical configuration, with two cameras
looking at the measurement plane through the
windows on the opposite sides of the test section,
which guarantees the maximum accuracy
(Prasad, 1993; Weesterweel and Van Cord,
1999) because of the large angle between the
cameras, was not practical. (size of the facility,
limited length of the camera cables, need to
control remotely the focus and Scheimpflug
angle of the farther camera located in the
opposite side of the test section with respect to
the control room).
with the adopted configuration the underwater
camera was directly measuring the cross-flow
components V and W. maximizing the accuracy
on these components. Furthermore the
Scheimpflug angle needs not to be remotely
controlled which would increase the complexity
with a larger underwater case to host all the
necessary equipment. In such way, all the errors,
in the stereo reconstruction, are confined to the
estimate of the longitudinal component only.
the adopted configuration, with an angle
between the right and left cameras ranging from
36° to 40°, depending on the measurement
plane, is the most accurate in the evaluation of
the out-of-plane component, among all possible
optical configurations having the cameras on the
same side of the test section. This result has
been assessed on a test bench by measuring a
known displacement of a reference object placed
on a translation stage.
IMAGE ANALYSIS AND STEREO
RECONSTRUCTION
As the first step toward the determination of the
three velocity components at the measurement plane,
the images are processed to obtain the vector fields
viewed by the left camera and the right camera. The
acquired images were analyzed using an algorithm
in which the window offset correlation method has
been implemented (Westerweel 1997). Furthermore
a recursive processing method was used by
implementing a hierarchical approach in which the
sampling grid was continually refined and also the
size of the interrogation windows was reduced
during the iterations. During the iteration the
interrogation window was also weighted by using a
Gaussian function that was stretched in the direction
of the window offset to further improve the signal to
noise ratio of the correlation function (Di Florio et
OCR for page 792
al., 2001~. In the last iteration the windows were also
overlapped to obtain a better reconstruction of the
whole flow field especially in the regions with strong
gradients. This procedure has the added capability of
Figure 3: Image preprocessing: a) original image; b)
mean image over 129 acquisitions; c) final image with
propeller removed in the background obtained subtracting
(b) from (a)
applying interrogation windows with size smaller
than the particle image displacement increasing both
the dynamic range and the spatial resolution. To
eliminate the remaining spurious vectors, each data
set was subjected to a validation procedure to detect
and replace spurious displacement vectors (Keane
and Adrian, 1992~. For the results presented in the
following sections, a final window size of 32x32
pixels, with 75% overlap between two adjacent
windows, has been adopted as the best compromise
in terms of spurious vector reduction and spatial
resolution. This window size was equivalent to 7 x 7
mm2 in real space.
In the stereo reconstruction the procedure
described by Soloff et al. (1997) is used. The camera
views were calibrated using a special target
providing a mesh of 20X20 dots in two planes. This
calibration was required to determine the
transformation function needed to reconstruct the 3
velocity components from the two separate planar
Figure 4: Longitudinal mean velocity component
obtained by stereo reconstruction without (a) and
with (b) image preprocessing
OCR for page 793
PIV measurements. Besides the geometrical
correction of the perspective, this non-linear
transformation took also into account the optical
distortions introduced by the presence of multiple
interfaces (air, glass and water).
In the adopted configuration, the measurement area
was defined by the overlapping region between the
separate views and had a dimension of about 250
mm X 200 mm that was sufficient for investigating
the whole propeller disc.
The analysis of the acquired images presented
some difficulties due to the fact that both left and
right cameras were imaging the rotating propeller in
the background of the measurement plane. The
propeller, even if black painted with care, was
scattering the light diffused by the particles,
especially for the plane x/L=.9997 closer to the
propeller masking the particles and locking the
velocity at the propeller speed especially in the
regions in proximity of the hub and at the blade
edges. This led to the erroneous evaluation of the 3
components of the velocity field in the region
extending along an horizontal radius from the hub.
To overcome this problem the images have been
pre-processed: a mean image (figure 3b) has been
calculated by using all the acquired images at a
given angle and this reference image has been
subtracted from the actual image before the analysis
(figure 3b). This procedure allows the elimination of
the background propeller image (figure 3c) and
drastically improves the final result. A comparison of
the mean field obtained with and without such
procedure at x/L = 0.9997 for ~ = 0° is shown in
figure 4. The preprocessing procedure was used only
in the analysis of the measurement plane closer to
the propeller. For the planes at x/L = 1.000 and
1.018 the preprocessing of the images was not
necessary because of the larger distance between the
propeller and the measurement planes. Moreover the
small depth of field of the camera objectives reduced
the occurrence of the above problem.
MEASUREMENT UNCERTAINTY
A comprehensive discussion on the uncertainty
and the accuracy of the PIV technique, especially for
stereo-PIV, is out of the goal of the present work and
this aspect is a complex topic with many open points.
A detailed analysis can be found in Prasad (20001. In
the following, the main assessments necessary to
qualifier the present results and to outline the major
problems are reported.
1100
1 000
900
800
700
.X 600
>500
400
300
con
1100
1 nnn
900
In
_ 700
·~600
>500
400
300
200
100
U (mars)
1~
I.
do.~
oa
- lo* camera field
5W 10W
X trim - l
5W 10W
X pixel
Istantaneous 3D vector field (3 = 0°
Vd Mag
-;; ' if I; "A 5 44075
.~, , . - .i ,
., . : ,., :: .:+ A ,.,. in,.
,,, I,,>,—TSAR 2~ .S
: 4.3526
ant ~
. 1 ~815
O
Figure 5: Left (a) and right (b) camera instantaneous field
with the 3 D stereo reconstruction (c)
OCR for page 794
The uncertainty on velocity measurements by
each single camera is mainly due to the error on the
particle displacement evaluation which can be
normally considered less than 1/lOth of a pixel for
the present image analysis algorithm (Raffel et al.,
1998), which is equivalent to approximately 4 cm/s
in terms of velocity. This error is essentially present
in the measurement of the instantaneous flow field,
in particular in the evaluation of the cross flow
components which are directly measured by the
underwater camera as explained before. The error in
the measurement of the longitudinal component is
related mainly to the stereo reconstruction. In the
present case test bench results pointed out errors
around 5%, but in the facility with a longer focal
length of the objectives and the window aberration
the previous number could be underestimate. This
error, which seems to be relatively large, is a typical
value for a stereo-PIV measurement and is
depending on many factors such as the adopted
optical configuration (angle between the cameras),
optics aberrations, numbers of dots in the calibration
target, numbers of planes used in the calibration,
etc. The errors due to light reflections from the hub
and from the blade edges were important in flow
field regions mapped in the right or left camera in
proximity of reflection spots. The moving propeller
mean Held ~ = 0°
~ e,~f,~~ 4, ~ ~,~/~~ ~ ~ I.
J (mIs)
1.E
1.24
o.
in the background of the measurement plane is
another source of error. Even if pre-processing the
images mostly removes the propeller image and
reduces this effect, the correlation peak is still locked
at the propeller velocity, in the regions where there is
a lack of particle traces. In the post-processing
phase, the validation procedure is very effective to
detect such spurious vectors due to the large
difference between the flow and the blade velocity
especially at the tip of the blade. Detected erroneous
vectors are eliminated and replaced by interpolation.
Nevertheless, spurious vectors might also be
validated biasing the statistics especially in the
proximity of the hub where the flow and propeller
velocity become closer. This effect is relevant
especially for the second order statistics.
The accuracy of the mean velocity field definitely
depends on the number of acquired samples and on
the shape of the velocity probability distribution
function. The probability density fimction in the tip
vortex core and in the blade wake markedly differs
from the Gaussian. Furthermore, a lack of data and
less samples available for statistics have been
observed in these regions. However by using the t-
Student distribution (for which the confidence
interval at 95%, is +1.96*rms /NI(N-1), with N=129),
it is possible to estimate the uncertainty on a velocity
~1 100
, .' r ~ . ~ ~ }, ~ ~ ~ ~ ~ t ,' ~ ,'
, ,'
`, :,
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 _
-100 -50 0 50 100 150
Y (mm)
Figure 6: Mean field over 129 acquisitions.
O
-50
1
OCR for page 795
component to be about 1/6th of the measured velocity
rms.
RESULTS
Instantaneous Flow Field
An example of an instantaneous flow field obtained
in the first measurement plane for the angular
position O=0° is shown in figures 5a and Sb. For
graphical reasons vectors were skipped by a factor of
four. From the led and the right field, the 3D
velocity field can be evaluated, as shown in figure
5c. The reconstructed flow field shows many missing
vectors due to the fact that the number of spurious
vectors is approximately the union of the right and
left ones. During the mean values calculation,
performed using only validated vectors and not the
interpolated ones, the holes in the reconstructed
mean field were recovered as shown in figure 6
where the 3D field is shown after averaging over 129
. . .
acquisitions.
Hull Wake And Propeller Inflow
Before going into the details of the propeller wake,
and to better understand the behavior of the flow
field during the propeller rotation, the mean field
and the vorticity field at x/L=0.9997 measured
without the propeller are shown in figure 7 and 8.
The mean field has been obtained averaging over
500 acquisitions. Although in the present case
measurements of the propeller inflow were not
exhaustive due to limited optical access with the
Stereo-PIV set-up, this information can give an idea
of the nominal wake and of the propeller inflow,
even if obtained slightly downstream the propeller
plane. The mean velocity field is characterized by a
sharp and strong axial velocity defect due to the
diminishing cross section of the hull at the stern.
The cross flow generally is directed upwards and
towards the hull center-plane where it tends to roll
up generating a diffused circulation well resolved by
the vector field and vorticity field.
For the considered measurement plane, data from
Toda et al., obtained by using Pilot tubes are
available for comparison. Figure 9 shows the axial
wake profiles performed at the same Froude number.
The differences between the velocity profiles is
rather apparent with a larger thickness of the
velocity defect in Toda et al. (1990~. The difference
can be related to the difference in the Reynolds
number, which in the present experiment is almost
double, as well as to the different characteristics of
the facilities.
Near Propeller Wake Evolution
In the following discussion the evolution of the wake
in phase with the propeller angular position at
x/L=.9997 will be presented.
In figure 10 the evolution of the longitudinal
component U is shown for the propeller angles
ranging from 0° to 63° with a step of 9° (this range
corresponds to a blade passage). The main
characteristics of a propeller wake working with non
Figure 7: Mean velocity field at x/L=0.9997. Left side
axial component. Right side cross-flow components
Figure 8: Velocity modulus (right) and vorticity field (left)
OCR for page 796
uniform inflow due to the hull wake can be pointed
out:
the propeller wake loses the axisymmetrical
morphology typical of an isolated propeller and
the largest longitudinal velocity are achieved in
the lower part of the propeller disc and at radial
positions r/R=0.6-0.7 (R propeller radius) .
the thin wake released by each blade can be
recognized in the longitudinal component plots
even if the wake thickness is approximately of
the same order of magnitude of the spatial
resolution of the PIV measurement. This leads
to an underestimation of the velocity defects due
to the smoothing effect of the interrogation
window
the tip vortex are visible in the velocity map and
are pointed out by the strong velocity gradients
at different angular positions at about r/R= 0.9.
-1 oo
-150 _
-20D _
1 1
O.g
N
_ ~
O.8 o ~ 0.6 0.5
~0.9
I;-''
X (mm)
Figure 9: Ship model wake without propeller at
x/L=.9997. Comparison with Toda et al. (1990)
Tip vortices and blade wake are more evident in
the V and W contour plots shown in figure 11 where
the evolution of the cross-flow components is shown
for a blade passage with a step of 18°. The above
figures clearly show the strong circulation generated
by the hub vortex that characterize the propeller
slipstream.
The passage of the blade in the narrow wake of
the hull at 12 o'clock generates strong three-
dimensional effects. In the axial velocity contour
plots, when the blade is in the range of 18°< ~ < 36°,
two parallel stripes of maximum and minimum
velocity near the blade tip can be noticed. This
phenomenon points out to the presence of a strong
vortex structure having the axis parallel to the
measurement plane.
The strong three-dimensional effects due to the
blade passage in the sharp hull wake can also be
noticed in the vorticity evolution shown in figure 12.
From the previous figure the following
considerations can also be done:
- the trailing vorticity, shed from the blade trailing
edge, is well identified and consists of two layers
of opposite sign which overlap at about r/R =0.7,
in the blade section of maximum loading.
- the link of the tip vortex with the blade trailing
vorticity is more apparent with respect to the
mean velocity field where this information is
almost confused or lost;
- the vorticity field provides information of the
radial distribution of the blade loading and points
out to the differences between the five blades due
to the different respective inflow conditions;
- the wake at 6 and 12 o'clock position is strongly
distorted and fragmented as a consequence of the
strong inflow variations in these angular
positions.
An example of the important modifications of the
shape and intensity of the wake released by the blade
during the revolution, is shown in figure 13. The
location of the wake, identified by the vorticity field,
is reported for the five blades for the propeller angle
= 0°. The strong modification of the wake shape
stresses the importance of considering the angular
variation of the propeller inflow, which most of the
time is ignored in some CFD calculations by
integrating the nominal wake along the
circumference at a given radius.
Figure 14 shows the evolution of the total
turbulence intensity distribution
~I(ui)2 +(V)2 +(W)2
where U7' V' and w' are the standard deviations of
the velocity components. Even if the confidence
interval on this second order statistical estimator is
rather limited, due to the fact that only 129 samples
have been evaluated, some important features of the
turbulent wake can be outlined:
- the maximum values of turbulent intensity are
achieved in the tip and hub vortex cores;
OCR for page 797
~-
F: `~.i ~.-iiN .~.~.~ i - ~.~. \~;i \.~;,,;~..~ - , .~..~...~... - :J - ....-}i~ i6.. :~ni ;.~.i. ~ 7.i ~d ., v~r ~ .~..~-i ~ :;
Hi, :'.,~j~A
Figure 10: Longitudinal velocity U (m/s) at different angles (0°,9°,18°,27°,36°,45°,54°,63°) for plane x/L=O.
OCR for page 798
~ - ~=
non Moo -also .0,40 -0.30 o.2o "o.1o O.OO O.lo 0.20 0.30 0.40 0.50 O.60 0.70 t.: jr
Figure 11: Cross-flow V,W (m s) at different angles (0°,18°,36°,54°) for plane X/L=0.999
OCR for page 799
Figure 12: Vorticity ( s-l) at different angles (0°,9°,18°,27°,36°,45°,54°,63°) for plane x/L=0.999
OCR for page 800
60 _
40 _
_ _
~ _
~5
-
{D _
20
o
- ~ 5= = /J
0.4
U.b O.8
rl R
Figure 13: Shape of the trailing vorticity wake for each blade
for propeller angle O= 0°
- the trace of the hull wake, represented by a
vertical stripe of turbulence, can still be noticed
in the measurement plane;
· the passage of the blade in the hull wake
produces a strong turbulence generation due to
the interaction of the hull wake with the blade
tip vortex;
· the intense spikes in the turbulence level
distribution located at the blade trailing edge,
near the hub and at different propeller angles are
due to the erroneous inclusion in the statistical
calculation of some spurious vectors locked at
the blade velocity. This effect is attributed to the
background motion of the propeller which was
not completely removed by the image
preprocessing and by the vector post-processing
validation not able to completely filter spurious
measurements as explained previously.
Longitudinal Wake Evolution
The description of the wake longitudinal
evolution will be limited only to the propeller angle
~ = 0° due to the restricted space. The distribution of
the mean velocity field at the three measurement
planes is shown in figure 15 where the longitudinal
component U and cross-flow are reported
The evolution along the longitudinal axis of the axial
velocity component U shows:
the wake contraction between the first two
planes (they are very close together and also
close to the propeller plane) that causes an axial
acceleration as pointed out by the higher axial
velocity achieved in the second measurement
plane;
the smoothing effect due to turbulent diffusion
and strong dissipation between the first two
planes and the last one which is farther
downstream as also pointed out by the stronger
radial gradients of the axial velocity component
for the first two planes comparing with the last
one;
· the rapid decay of the tip vortex with respect to
the hub vortex which is still characterizing the
flow in the last measurement plane
A better description of the vortices evolution is
provided by the vorticity field shown in figure 16.
The following features on the three different
transversal planes can be noticed
· a very strong distortion of the wake structure
due to the different pitch of the wake shed from
the trailing edge and the pitch of the tip vortices
can be noticed from the first to the second plane;
very strong diffusion and dissipation of the blade
wake, which is spreading very quickly Dom the
first to the second measurement plane. In the
latter, the hub and the tip vortices are still
present, even if very attenuated, while the blade
wake is almost totally dissipated;
The trace of the tip and hub vortices are still
apparent in the last measurement plane pointing
out that the wake is still in phase with the
propeller far downstream.
In figure 17 the turbulence distribution of the V
component is reported as an example of the
turbulence field evolution because the fluctuating
field of the other components shows a similar
behavior. The turbulent wake released by the blade is
quickly dissipated and diffused downstream. The
same process of wake deformation and broadening,
due to the action of the tip vortices and of the hub
vortex, observed in the vorticity plots, is also seen in
the turbulence level distributions. The high level of
turbulence generated by the blade passage at 12
o'clock is convected downstream and dissipated even
if a trace is present in the far downstream plane.
CONCLUSIONS
The analysis of a propeller wake behind a Series 60
ship model, in a large water channel, has been
performed by using stereo-PIV. Both instantaneous
and averaged velocity fields are achieved, the latter
after phase sampling averaging over the same
OCR for page 801
Figure 14: Total turbulence (m/s) at different angles (0°,9°,18°,27°,36°,45°,54°,63°) for plane x/L=O.99
OCR for page 802
angular position of the propeller blade. The
experimental results, in terms of velocity and
vorticity fields, reveal some of the different
contributions to the complex propeller flow field of a
propeller working behind a ship:
1. The viscous part of the wake generated by the
boundary layers on the blade surfaces.
2. The potential part of the wake deriving Dom the
vortex sheet at the blade trailing edge.
The varying loading conditions of the blade
during the revolution which causes a strong wake
deformation.
4. The complex and three-dimensional behavior of
the tip vortex when passing through the narrow
hull wake.
The rapid spreading of the propeller wake in the
downstream flow where the wake is faded and
smoothed by turbulent diffusion and viscous
dissipation.
From the experimental setup point of view and with
regards to the fitture implementation in standard
ship model testing procedures, the stereo-PIV has
shown a number of advantages compared to the well
assessed LDV technique. In particular, and
considering the limited time usually given to these
tests combined with the management and technical
difficulties typical of a large testing facility, the PIV
technique can provide results within a short period.
Instead, the LDV technique requires up to three-four
times more testing time to obtain he same
information, which consequently translates into
additional costs of facility occupancy. The
measurement time is drastically reduced with the
stereo-PIV method, where the plane of measurement
is mapped instantaneously and provides all three
velocity components in one single step, while the
LDV technique requires a long scanning of the
interrogation domain. In this sense, the PIV
approach offers the Deedom of extending the wake
survey to a larger number of areas of interest, with
very limited setup changes.
The major drawbacks of the PIV technique are a
reduced accuracy with respect to the LDV technique
and a huge quantity of information generated both at
the measurement and at the processing time (about
200 Gb in the case of the presented results): one
must address the critical problem of storing,
managing and processing this information without
compromising the tests costs by extended data
processing time.
Acknowledgements
The authors are grateful to the INSEAN Circulating Water
Channel personnel and to Mr. Tiziano Costa who
supported the PIV measurements. This work was
sponsored by the Italian Ministero dei Trasporti e delta
Navigazione in the frame of the [NSEAN 2000-2002
research plan.
REFERENCES
Cenedese, A.., Accardo, L., Milone, R., "Phase sampling
in the analysis of a propeller wake", Experiments in
Fluids, Vol. 6, 1988, pp. 55-60.
Chesnack C., Jessup S.D., "Experimental characterization
of propeller tip flow", Proceedings of the 22th Symposium
on Naval HvdrodYnamics, Washington D.C., 1998.
Cotroni, A., Di Felice, F., Romano, G.P., Elefante, M.,
"Investigation of the near wake of a propeller using
particle image velocimetry', Experiments in Fluids, 2000,
pp. 227 - 236.
Esposito, P., Salvatore, F., Di Felice, F., Ingenito, G.,
Caprino, G., "Experimental and Numerical Investigation
of the un steady Flow around a Propeller", Proceedings of
the 23th Symposium on Naval Hydrodynamics, Val De
Reuil, France, 1998.
Di Felice, F., Romano, G.P., Elefante, M., "Propeller
Wake Analysis by Means of PIV", Proceedings of the 23th
Symposium on Naval Hydrodynamics, Val De Reuil,
France, 1998.
Di Felice, F., Felli, M., Ingenito, G., "Propeller wake
analysis in non uniform inflow by LDV", Proceedings of
the Propeller and Shafting Symposium, Virginia Beach,
2000.
Di Florio, D., Di Felice, F., Romano, G.P., "Windowing
and Deformation of PIV Images for the Investigation of
Flow with Large Velocity Gradients", Proceedings of the
4th International SYmposium on Particle Image
Velocimetrv, Gottingen, Germany, 2001.
Felli M.. Di Felice. F.. Romano. G.P. Installed
, , ~ 7 ~
Proneller wake analysis by LDV: phase sampling
technique", Proceedings of the gth International
Symposium on Flow Visualisation Edimburgh, 2000.
Guj, L., Longo, J., Stern, F., "Towing Tank PIV
Measurement System, Data and Uncertainty Assessment
for DTMB Model 5512", Experiments in Fluids, Vol.31,
2001, pp. 336-346.
Jessup, S.D., "An experimental investigation of viscous
aspects of propeller blade flow". Ph. D. Thesis, The
Catholic University of America, Washington D.C., 1989.
Keane R.D., Adrian R.J., "Theory of cross-correlation
analysis of PIV images", Applied Scientific Research.
Vol.49, 1992,pp.191-215.
Kobayashi, S., "Propeller wake survey by laser Doppler
velocimeter". Proceedings of the International Symposium
on the Application of laser-Doppler Anemometry to Fluid
mechanics, Lisbon, 1982.
Min. K.S, "Numerical and experimental methods for
prediction of field point velocities around propeller
blades". Dep. of Ocean Engineering, Report no. 78-12,
MIT, 1978.
Prasad, A. K., Adrian RJ, "Stereoscopic particle hnage
Velocimetry applied to liquid flows". Exp in Fluids with
OCR for page 803
Figure 15: Mean flow field evolution: longitudinal component (m/s) (left) and cross-flow W. V (m/s) at 0°
propeller angle for planes X/L=0.999, 1.00, 1.018
an orthogonal view of the measurement plane Vol 15,
1993, pp. 49-60
Prasad, A.K., "Stereoscopic Particle Image Velocimetry",
Experiments in Fluids, Vol.29, 2000, pp. 103-116.
Rafael, M., Willert, C., Kompenhans, J., "Particle hnage
Velocimetry", Springer ISBN 3-540-63683-8, 1998.
Soloff, S.M., Adrian, R.J., Liu, Z.C., "Distortion
Compensation for Generalized Stereoscopic Particle Image
Velocimetry", Meas. Sci. Technol., Vol.8, 1997, pp. 1441-
1454.
Stella, A., Guy, G., Di Felice, F., "Propeller flow field
analysis by means of LDV phase sampling techniques",
Experiments in Fluids, Vol.28, 2000, pp. 1-10.
Toda, Y., Stern, F., Tanaka, I., Patel, V.C., "Mean Flow
measurements in the boundary layer and wake of a series
OCR for page 804
- i
Figure 16: Vorticity (s A) for 0° angle at planes
x/L=0.999, 1.00, 1.018
60 Cb=0.6 model ship with and without propeller", Journal
of Ship Research, 34, 4, pp. 225-252, 1990.
Westerweel J., Van Oord, J., "Stereoscopic PIV
measurements in a turbulent boundary layer", Particle
Image Velocimetrv: progress toward industrial application
Kluwer, Dordrecht, 1999.
Figure 17: Turbulence intensity of the V component (m/s)
for O° angle at planes x/L=0.999, 1.00, 1.018
Westerweel, J., "'Fundamentals of Digital Particle Image
Velocimetryll, Meas. Science and Technology Vol.8, 1997,
pp. 1379-1392
OCR for page 805
DISCUSSION
S. Nishio
Kobe University of Mercantile Marine, Japan
1) The Stereo-PIV technique enables us to obtain the
three-component of the velocity field on a plane, but
the three-component of vorticity distribution cannot
be obtained with a single plane velocity distribution.
In mean time, the authors have measured on one pair
of plane at x/L=0.9997, 1.0000, which are very close
and parallel. It may possible to obtain three
component of vorticity using those data, and it would
be interesting to see the change of three-dimensional
vorticity field.
AUTHORS' REPLY
This would be very interesting to compute and we will
do in a future work. At the moment it is possible to
notice that there is a very strong component of the
vorticity with axes in the measurement plane as you
can see in the axial velocity contour plot, fig. 10.
When the blade is in the range of 18°< ~ < 36° two
parallel stripes of maximum and minimum velocity
near the blade tip can be noticed. This phenomena
points out the presence of a strong vortex structure
having the axis parallel to the measurement plane.
DISCUSSION
S. Nishio
Kobe University of Mercantile Marine, Japan
2) The authors have applied a pre-processing on the
original image to extract the background image effects,
and it seems to work appropriately. By the way, the
original image Fig.3 (a) has large gradient of
illumination from left to right. The discusser is afraid
if the gradient of illumination still remains on the final
image Fig.3 (c), and it affects on the uncertainty of
final velocity distribution.
AUTHORS' REPLY
2) Actually subtracting the mean image seems to
eliminate most of the gradients of illumination of the
final image. This is due to the fact that gradients of
illumination are also in the mean image and then they
are subtracted.
In any case adopting this procedure there has been a
tremendous improvement in the quality of analyzed
images and then in the uncertainty of final velocity
distribution.
DISCUSSION
B.-G Paik
Pohang University of Science and Technology, Korea
I think the ship wake is important for the
understanding of propeller wake. Because the ship
wake connects the propeller wake behind a ship with
the propeller wake in P.O.W. I think the measurement
of ship wake at the propeller plane will be helpful for
your better understanding of propeller wake.
AUTHORS' REPLY
Actually we did those measurements in the propeller
plane. You can find the plot in figures 7 and 8.
DISCUSSION
B.-G Paik
Pohang University of Science and Technology, Korea
Another question regarding the perspective error
defined by (IJ2D-U3D)/Uo in longitudinal wake
measurements: In my case for measuring longitudinal
wake using stereoscopic PIV, maximum two percent
error has occurred. So, if you let me know the
maximum perspective error. It will be helpful for my
future research.
Thank you for your exciting research.
AUTHORS' REPLY
We did a systematic analysis on the cameras relative
positions to evaluate the perspective error. With the
adopted configuration the error in the measurement of
the longitudinal component is less than 5%.
Representative terms from entire chapter:
propeller wake
