An Exploratory Study on the Working Principles of Energy

Symposium on Green Ship Technology (Greenship'2011)
Wuxi, China, October 2011
An Exploratory Study on the Working Principles of
Energy Saving Devices (ESDs)
Jie Dang1, Hao Chen2, Guoxiang Dong3,
Auke van der Ploeg1, Rink Hallmann1 and Francesco Mauro1
1
Maritime Research Institute Netherlands (MARIN), Wageningen, The Netherlands
2
Guangzhou Shipbuilding International (GSI), Guangzhou, China
3
Shanghai Ship and Shipping Research Institute (SSSRI), Shanghai, China
ABSTRACT
A new Joint Industry Project (JIP) has been initiated
recently by MARIN, called ESD-JILI (机理)
机理), looking
into the working principles and scale effects on Energy
Saving Devices (ESDs). Three ESDs have been chosen
for the investigations in the first phase. They were a preduct with a supporting stator in the duct, a pre-swirl stator
with asymmetric blade design and Propeller Boss Cap
Fins (PBCF). Measurements of forces and moments on all
components of the ESDs have been carried out in selfpropulsion model tests with dedicated sensors. Particle
Image Velocimetry (PIV) technique has been used in the
investigation of the detailed flow around the ESDs. In
order to investigate the scale effects in model tests, a fullscale wake field was approximated by a ‘smart ship
model’. Computational Fluid Dynamics (CFD) calculations were carried out both for designing the smart ship
model and also for the detailed flow around the ESDs.
Some findings and fundamental issues on scale-effects of
the ESDs are addressed in this paper.
Keywords
ESDs, Propulsion, Efficiency, Energy Saving, PIV
1 INTRODUCTION
With the oil price skyrocketing and the implementation of
EEDI and EEOI approaching, the issue of green ships is
almost daily brought into discussions with operators,
owners, builders, classifications, shipping and harbour
regulators and governments. As one of the major driving
forces on reducing bunker cost and pollution, energy
saving issues have become important again recently.
Both for new buildings and also for retro-fits, Energy
Saving Devices (ESDs) are widely accepted as important
measures to improve the ship’s total propulsive
efficiency. An ESD can be a pre-duct, a pre-swirl stator
and fins located upstream of a propeller, or a post stator, a
rudder bulb, rudder fins, a PBCF or a twisted rudder in
the downstream. There are many types of ESDs. A review
of the ESDs and the MARIN research activities in this
field is given recently by Hooijmans et al (2010).
Many new ideas and patents have been proposed in the
last decades, which have been tested in the towing tanks
around the world (Hansen 2011, Lee et al 1992, Mewis
2011, etc.). Several of the ideas have been fitted to real
ships and tried at full scale. But very limited trial data are
available in the public domain. According to a limited
number of publications, the best achievement in
improving efficiency by adding an ESD to an existing
vessel is more than 10% according to the model tests.
Feedbacks from full-scale sea trials have shown typically
up to 5% improvement on total propulsive efficiency.
These achievements are already very promising, although
discrepancies between predictions and sea-trials are often
seen.
But until now, there is very limited information on why
and how a specific ESD can improve the total propulsive
efficiency of a vessel, except for the working principles
described by the inventor himself. There is only a limited
number of studies which provide an understanding of the
working mechanism of the ESDs. Therefore in the past,
many ESDs were designed by trial-and-error method and
the designers had to hope that their designs would be
proven in reality. In addition to these uncertainties, an
ESD suffers often from strong scale effects. The
achievement in full scale is often different from what is
found in model scale. This makes the application of the
ESDs at full scale sometimes questionable.
Thanks to the rapid development of CFD, the working
principle and the scale effects on ESDs are becoming
clearer than before (Heinke 2011 and Çelik 2007). In
addition, CFD provides guidance on ESD designs so that
the performance of ESDs can be optimized even for fullscale conditions and flow separation, often found on misaligned ESDs, can be prevented (Zondervan et al 2011).
In order to help the shipbuilding industry to understand
the working principle of the ESDs and to develop a proper
extrapolation method for the model test results, parallel to
other projects on ESDs, MARIN recently initiated a new
Joint Industry Project (JIP) called ESD-JILI with focus on
the working mechanism of a selection of ESDs. For the
first phase of this project, only three types of ESDs have
been investigated which are considered to be the most
promising ESDs for full block merchant ships. They are,
- a pre-duct with a supporting stator in the duct;
- a pre-swirl stator with asymmetric blade design;
- a PBCF.
Investigations using four different techniques have been
carried out, which include,
- the force and moment measurements of all the
components of the ESDs and the propeller;
- the Particle Image Velocimetry (PIV) flow
measurements around the ESDs;
- the ‘Smart Ship Model’ technique to simulate the
full-scale wake;
- CFD calculations (This will be published soon
but will not be discussed in the present paper).
We have chosen two oil tankers for the study in the first
phase of the JIP. One of the ships is a 80,000DWT tanker.
Only the studies of the ESDs, selected and fitted to this
tanker, are presented and discussed in the present paper.
2.1 ESD Models and sensors
Three ESDs have been designed for this vessel
empirically by the participants of this JIP. Figure 2 shows
some examples of the assembled ESD models.
Figure 2 The pre-duct with a supporting stator (L) and the
pre-swirl stator (R), painted black for PIV measurements.
The photos in Figure 3 show part of the ESD models
made of Nylon by rapid prototyping. The PBCF has a
lower pitch angle than that of the propeller at the root.
2 TEST SETUPS AND CFD SIMULATIONS
The subject vessel for the study is a 80,000DWT oil
tanker developed by GSI and SSSRI by using CFD and
model tests. The vessel had been optimized already for its
overall powering performance without ESDs. The main
dimensions of the vessel are listed in Table 1.
Table 1 Main dimensions of the 80,000DWT oil tanker
Length
Breadth
Draught
Lpp
B
T design
224.90
32.26
12.20
m
m
m
A large ship model has been made of wood with a scale
ratio of 23.94. During the model tests, a bronze stock
propeller No. 5880R has been used. The main parameters
of the stock propeller in full scale are given in Table 2.
Table 2 Main parameters of the stock propeller
Diameter
Pitch ratio
Blade area ratio
Number of blades
D
P0.7R/D
AE/A0
Z
6.120
0.894
0.430
4
m
Figure 3 The pre-duct (left) with a supporting stator (centre)
and the PBCF (right).
In order to measure the forces and moments during the
resistance and propulsion tests on all components of the
ESDs and also on the propeller, the pre-duct was
equipped with two 1-component sensors to measure the
longitudinal force (thrust or drag) on the duct. Also the
stator longitudinal force (thrust or drag) and the stator
torque were measured by a hollow shaft sensor at the
stator hub which is coaxial with the propeller shaft. At the
end of the propeller shaft and close to the propeller hub,
the thrust and the torque of the propeller were measured
simultaneously. Figure 4 shows the details of the sensors
and their locations.
The calculated wake fields for model- and full-scales are
shown in Figure 1. No flow separation has been found.
Figure 4 The sensors used to measure forces and moments.
Figure 1 Comparison of calculated nominal wake field for
model scale and full scale. (geosim – geometrically similar)
The PBCF was fitted to the end of the propeller shaft.
There was no force sensor in between. The force and
moment generated by the PBCF were measured together
with the propeller by the propeller shaft sensor.
2.2 Particle Image Velocimetry (PIV)
To investigate the flow details around the ESDs (in front
of the ESDs, in the ESD and propeller planes, and behind
the propeller and the PBCF, with or without working
propeller), 3-D PIV has been used. The following figure
shows the PIV system installed on the carriage of the
Deep Water Towing Tank at MARIN.
made only to the aftbody only. The first basis form makes
the upper part of the gondola more slender, the second
one makes the lower part of the gondola more slender,
and the third one makes the buttock higher than the
original hull form. The computed wake fields are
compared to the full-scale wake field of the original hull.
By making systematic variations with interpolation in the
design space with the three basis hull forms, hundreds of
CFD calculations have been carried out. Finally, the
candidate shown in Figure 7 has been chosen as the smart
ship model for the present study.
Figure 5 The PIV system used in the towing tank of MARIN.
The PIV system is a DANTEC system built specifically
for MARIN where three windows are opened on a
streamlined body, one for the laser sheet and the other
two are for the cameras. The system is installed at the post
side of the ship model. At a certain angle, the view of the
cameras may be obstructed by the propeller or the ESDs.
In order to measure the flow on the starboard side, reverse
towing of the ship model has been used.
Figure 7 Comparison of the smart ship model (red thin lines)
to the original hull form (black thick lines), not to scale.
Propeller plane
ESD plane
In order to prevent reflection on the models, the stern of
the ship model and all the ESDs, including the propeller,
were painted black, as shown in the Figure 2. The water
was seeded with polyamide particles of a mean diameter
approximately 60 µm. The system was calibrated before
the tests with a multi-level calibration target. Figure 6
shows the locations of the measured cross-section planes.
Figure 8 Comparison of computed full-scale wake of original
hull form with that of the smart ship model, top: axial
velocity, bottom: transverse velocity.
propeller plane
Figure 6 The PIV measurement plane locations.
2.3 Smart ship model
A so-called ‘smart ship model’ technique (Schuiling et al
2011) is used in the present studies. The smart ship model
is in principle only a Wake Field Generator (WFG) so that
a wake field similar to the full scale wake can be
generated during model tests. The propulsion test results
with a smart ship model is aimed at the force and moment
measurements on the ESDs and the relative powering
effect of the ESDs, but is not interpreted in an absolute
sense. The details of the CFD used for the smart ship
model design can be found in van der Ploeg et al (2000).
In order to find the geometry of the smart ship model,
three basis hull forms have been designed that are
modifications of the original form. Changes have been
duct plane
Figure 9 Axial wakes, top: 0.7R and 0.5R on the propeller
plane, bottom: 1.0Rduct and 0.9Rduct on the pre-duct plane.
The wake fields at both the propeller plane and the ESD
plane are plotted in Figure 8 and Figure 9, with
3.1 Resistance tests with ESDs
It sometimes occurs that flow separation takes place at the
stern during the model tests due to too low Reynolds
number combined with a full block and poorly designed
aftbody of a ship with small model size. When an ESD is
fitted to those ship models, such as for the wake
equalizing ducts (WED), it may change the local flow
situation and reduce flow separation, resulting in a
fictitious improvement of the powering performance of
the vessel. But the CFD calculations, as shown in Figure
1, indicate that this will not take place for the present ship
with the large model used here. In addition to the bare
hull, resistance tests have been carried out for the model
fitted with the ESDs. Table 3 shows the resistance
changes by fitting the ESDs during the resistance tests.
Table 3 Resistance with and without ESDs (tank values)
no ESD
Vs
Rs
knots
kN
12
797
13
922
14
1080
15
1274
16
1522
pre-duct with stator
T_ duct
Rs
∆Rs
kN
%
kN
800
0.41%
-0.98
931
0.99%
-0.97
1077
-0.25%
-0.47
1279
0.44%
-0.24
1514
-0.48%
-0.22
pre-swirl stator
Rs
∆Rs
kN
%
−
−
−
−
−
−
1290
1.30%
−
−
It is shown clearly in this table that the pre-swirl stator
generates drag which acounts for about 1.3% of the total
vessel resistance at 15 knots. This is expected since
generating a swirl in the flow costs energy. The pre-duct
generates drag too (see T_duct, which are all negative),
although the drag on the duct is rather small (less than
0.1% of the total resistance Rs). It is believed to be
mainly due to the friction drag on the wetted surface of
the duct. But when comparing the total resistance Rs for
the pre-duct with a supporting stator to that of the
resistance without ESDs, some small increase or decrease
on resistance (less than ±1%) is found. This suggests that
in some conditions, the stator inside of the pre-duct may
generate thrust. This suggests also that, instead of the
assumed pre-swirl function, the stator in the duct may
convert some rotational energy of the ship wake flow into
net thrust (acting as a post-stator). The setting angles of
the stator blades, the tranverse velocity of the ship’s wake
and the interaction with the duct can have strong effects
on it.
During the propulsion tests, both vessel speed variation
tests (from 12 knots to 16 knots) and propeller load variation tests (at the design speed of 15 knots for different
shaft RPMs) have been carried out. When analyzing the
test data, it has been found that the forces on the ESDs are
strongly dependent on the propeller thrust. After nondimensionalizing the measured thrust and torque on the
ESDs by using the propeller shaft rotational rate and the
propeller diameter in the same way as for the propeller
thrust and torque coefficients, thrust and torque
coefficients of the ESDs are obtained. Linear relations can
be seen between the ESD thrust and the propeller thrust as
given in Figure 10 for the original hull form.
0.05
0.04
0.03
wake
scaling
propeller thrust at model scale
In order to get reliable test results, all test runs have been
carried out on one day with a well-controlled waiting time
in between the measuring runs. The tests have been
carried out by fitting ESDs one by one to the ship model,
with some combined cases with the PBCF. The reference
propulsion test wihtout ESD is used as the basis for the
comparisons.
As can be seen in Figure 3, the stator blades are fixed to a
stator ‘hub’ (a bossing) which includes a small part of the
gondola. The stator force and moment measured during
the propulsion tests include also the forces acting on the
hub. In order to obtain the blade forces, the forces on the
hub have been subtracted from the measured forces found
from the resistance tests by a dummy stator hub without
blades. It should be pointed out that the interaction
between the propeller and the stator hub is also included
in the stator blade forces during the measurements.
propeller thrust at full scale
3 FORCE AND MOMENT MEASUREMENTS
3.2 Pre-duct with a supporting stator in the duct
ESD Thrust Coefficient Kt
comparison to the full scale wake field calculated, where
the circle with solid lines show the propeller disc and the
dashed lines show the outer diameter of the pre-duct. It is
seen that the full scale wake field is well approached by
the smart ship model, except for the inner radii at 12
o’clock position where the flow deficit is still higher than
in full scale. The deviations are also slightly bigger in the
lower part of the disk compared to the upper part,
especially for the propeller.
0.02
0.01
Kt-Stator (ESD Pre-Duct with stator)
Kt-Duct (ESD Pre-Duct with stator)
Kt-Total (ESD Pre-Duct with stator)
0.00
-0.01
0.16
0.18
0.20
0.22
0.24
Propeller Thrust Coefficient Kt
0.26
0.28
Figure 10 Measured ESD thrust coefficients – original hull.
All the points shown on Figure 10 are the mean values of
the raw data sampled during the propulsion tests. The
linearity of the relation is evident between the ESD thrust
and the propeller thrust. There is some scatter for the
stator thrust. It can be due to the subtraction of the
dummy hub drag from the resistance test results where the
effect of the propeller on the stator hub and the possible
gap effects, between the stator hub and the hull and
between the stator and propeller hubs, are not included.
When matched with the open water characteristics of the
propeller, the ESD thrust coefficients can also be
expressed as a function of the propeller advance ratio as
shown in Figure 11.
As indications, also plotted in both Figure 10 and Figure
11 are the operational point of the propeller at 15 knots in
model and full scales. For single screw vessels, the
operational point of the propeller can be rather different
for model scale and for full scale, depending on the wake
The test results for the original hull form leads to the
following conclusions:
- The pre-duct generates a thrust which is as high as
about 8% of the propeller thrust at 15 knots.
- Surprisingly, the supporting stator in the duct
generates also a positive thrust (approximately 2% of
the propeller thrust), implying that the stator does not
generate pre-swirl to the propeller. Instead, it converts
rotational energy from the wake of the hull into net
thrust (including also interaction with the duct).
- The pre-duct thrust decreases with the increase of the
propeller advance ratio, similar to a conventional
ducted propeller, implying a worse performance in full
scale.
- In contrast, the stator thrust increases with the increase
of the propeller advance ratio, implying a better
performance in full scale.
3.3 Pre-swirl stator
With the same trend as that of the supporting stator in the
pre-duct, the thrust of the pre-swirl stator increases also
with increasing propeller advance ratio. But all the thrust
measured on the pre-swirl stator for all the propulsion test
runs with the original hull form showed negative values.
This means that the pre-swirl stator generated drag, as
shown in Figure 13.
As one would expect, the energy cost to generate the preswirl flow to the propeller should be recovered by the
propeller so that there are ideally no rotational losses in
the slipstream of the propulsor and the rudder.
0.020
ESD Thrust Coefficient Kt
scaling factor (VA/V)s/(VA/V)m. For the present vessel, this
factor has been chosen as 1.113 according to statistics.
Kt-Stator (ESD Pre-Swirl stator)
0.010
0.000
0.05
-0.010
0.16
Kt-Stator (ESD Pre-Duct with stator)
Kt-Duct (ESD Pre-Duct with stator)
0.03
wake
scaling
At full scale J
At model scale J
ESD Thrust Coefficient Kt
0.04
0.18
0.20
0.22
0.24
0.26
Propeller Thrust Coefficient Kt
0.28
0.30
Figure 13 Thrust on the pre-swirl stator – original hull.
Kt-total (ESD Pre-Duct with stator)
When the pre-swirl stator was tested behind the smart
ship model, the same trend with the same level of drag
force on the stator has been measured as that behind the
original hull form, which is plotted in Figure 14. This
suggests that the present pre-swirl stator may be less
sensitive to scale effects from model scale to full scale.
0.02
0.01
0.00
-0.01
0.40
0.45
0.50
0.55
0.60
Propeller Advance Coefficient J
0.65
0.70
Figure 11 ESD thrust coeff. versus propeller J –original hull.
0.03
ESD Thrust Coefficient Kt
0.020
Kt-Stator (ESD Pre-Swirl stator)
0.010
0.000
-0.010
0.16
ESD Thrust Coefficient Kt
Kt-Stator (ESD Pre-Duct with stator)
Kt-Total (ESD Pre-Duct with stator)
0.00
-0.01
0.18
0.20
0.22
0.24
Propeller Thrust Coefficient Kt
0.26
0.22
0.24
0.26
Propeller Thrust Coefficient Kt
0.28
0.30
3.4 PBCF
0.01
0.16
0.20
Figure 14 Thrust on the pre-swirl stator – smart ship model.
Kt-Duct (ESD Pre-Duct with stator)
0.02
0.18
0.28
Figure 12 Measured ESD thrust coefficients - smart ship
model.
When the same pre-duct with the same supporting stator
in the duct is fitted behind the smart ship model during
propulsion tests, the measured duct and stator thrusts were
reduced dramatically, as shown in Figure 12. The duct
generated still a positive thrust which is only about 2.5%
of the propeller thrust at 15 knots, while the stator in the
duct generates negligible positive thrust.
This suggests that the scale effects from model scale to
full scale, both for the present pre-duct and the supporting
stator, are significant, especially for the pre-duct.
In order to understand the working mechanism of the
PBCF, open water tests with reverse test setup have been
carried out. A quasi-steady open water test technique was
used so that a range of open water characteristics from
J=0 to KT=0 can be obtained efficiently from only one test
run.
The test setup is shown in Figure 15. The MARIN open
water test POD has a very slender body. Previous studies
on reverse open water tests have shown only very small
differences between the reverse open water test and the
normal open water test. Due to the relative comparative
nature of the present study, no correction has been made
for the influence of the POD.
The 5 open water tests consist of 1 test for the propeller
with conventional cap and 4 tests with the PBCF at 4
different position angles as shown by the markers on the
propeller hub in Figure 15. The 4 relative PBCF blades
position angles have been 21, 24, 27 and 30 degrees,
defined between the trailing edge of the propeller blade at
the root and the leading edge of the PBCF. All the tests
have been carried out with a constant propeller shaft
rotational rate of 900 RPM.
total thrust and decreases the total torque on the
propeller shaft. They are about ∆KT=0.0017 and
∆KQ=-0.00035 at J around 0.2.
If this PBCF effect at low J values explains the effect of
the PBCF in behind condition, it will have a contribution
to the improvement of the total performance of the vessel
by about 2% with the present PBCF design, which is by
no means carefully designed for the present study.
Optimizing the PBCF design may lead to higher gains.
0.006
PBCF setting angle 21 deg.
PBCF setting angle 24 deg.
PBCF setting angle 27 deg.
PBCF setting angle 30 deg.
0.005
∆ KT
0.004
0.003
Figure 15 Reverse open water test setup and position angles
between the blades of the propeller and the PBCF.
0.002
The results show that the PBCF has hardly any effect on
the total propeller open water efficiency around its mean
operation condition of about J=0.5 for the present oil
tanker. Only +0.47% (max.) relative improvement has
been found at all 4 PBCF position angles.
0.000
0.8
With PBCF (setting angle 27 degrees)
0.8
PBCF setting angle 21 deg.
PBCF setting angle 24 deg.
PBCF setting angle 27 deg.
PBCF setting angle 30 deg.
0.0003
0.0001
-0.0001
-0.0005
0.6
K T, 10KQ , η 0
0.0005
0.2
0.4
0.6
Propeller advance coefficient J
-0.0003
Without PBCF
0.7
0.0
∆ KQ
One representative results for the PBCF position angle of
27 degrees is plotted in Figure 16.
0.001
0.0
0.5
0.2
0.4
0.6
Propeller advance coefficient J
0.8
Figure 17 Influence of PBCF on total thrust and torque.
0.4
4. PIV measurements
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Propeller Advance Coefficient J
0.8
0.9
1
Figure 16 Open water test results with and without PBCF
It is indeed that the PBCF does have a strong influence on
the total KT and KQ at lower J values where KT is
increased and KQ is decreased a lot by fitting a PBCF.
This influence should not be over-looked since the blade
root of a propeller with PBCF is working in a more
heavily loaded condition with lower inflow velocity (see
Figure 1) than the mean velocity on the propeller disc in
behind of a single-screw full-block vessel.
When subtracting the open water results without PBCF
from the 4 open water results with PBCF and plotting the
differences, we obtain Figure 17. This figure shows,
- At the operational condition (around J=0.5), the PBCF
has very small effects on the propeller thrust (about
∆KT 0.0005 to 0.001). It has almost no effect on the
torque.
- The effect of PBCF is not so sensitive to the position
angle in the range from 24 to 30 degrees as tested.
Only when the leading edge of the PBCF is very close
to the trailing edge of the blade root on the pressure
side (at 21 degrees), the effect of PBCF position
becomes distinguishable.
- At low J values, a clear effect of the PBCF on the
thrust and torque can be seen. The PBCF increases the
As described in Figure 6, the flow details on 8 vertical
and transverse planes in the stern area of the vessel have
been measured, with and without the working propeller
and with and without the ESDs. For the plane BH behind
the propeller, the unsteady flow has been measured by
triggering the propeller blade angular positions, with and
without the PBCF. A large amount of valuable data has
thus been obtained, only limited results can be discussed
in the scope of the present short paper.
Left: without ESD, Right: the pre-duct with a supporting stator, Circle: the
inner diameter of the pre-duct
Figure 18 PIV measurements without working propeller, 15
knots, original hull form.
Figure 18 shows the nominal wake field at planes PS2
(without ESD) and SB1 (with pre-duct and a supporting
stator in the duct). For the present vessel, it is seen that
the pre-duct has hardly any influence on the flow at the
upstream of the duct when there is no working propeller.
When there is a working propeller just downstream of the
pre-duct, the suction effect of the propeller is clearly seen
in the flow field just in front of the duct leading edge
(plane SB1, Figure 19), when compared to Figure 18. The
figure on the right-hand side of Figure 18 shows that the
pre-duct accelerates the flow further so that the wake peak
at 12 o’clock position becomes narrower.
-
with working propeller only, which is reduced by the
pre-duct with stator and further reduced by the PBCF.
The effect is also clear in the vorticity plots. The
PBCF reduces the total strength of the vorticity and
makes the flow rotation less concentrated.
The axial and transverse velocities behind the PBCF
are reduced significantly by the PBCF. This may
suggest pressure increase at the end of the PBCF hub.
5 Extrapolation of powering tests with ESDs
Left: without ESD, Right: pre-duct with supporting stator, Circles: duct/prop
Figure 19 PIV measurements with working propeller, 15
knots, original hull form. (plane SB1, upstream of pre-duct)
Extrapolating the powering model test results with ESDs
to full scale remains a very challenge task. In the first
phase of this JIP, we decided to use the MARIN standard
method (Holtrop 2001) but taking into consideration of
the findings discussed in the last sections as much as
possible.
5.1 Extrapolation method
During the extrapolation of the model test results to full
scale, we treat the ESDs as part of the propulsor instead of
appendages. Since the thrust of the ESDs have been
measured during all the tests, the total thrust of the ESDs
and the propeller are seen as the total thrust of a propulsor
to this vessel. The following corrections to the MARIN
standard 3-D extrapolation method has been proposed in
order to take the ESDs into consideration.
propeller
where Tp is the propeller thrust, TESD is the total thrust of
the ESD and ∆TESD is the viscous correction to the ESD.
pre-duct with a supporting stator + propeller
When the thrust forces on the ESD (positive or negative)
is taken into consideration in the extrapolation
programme, the wake scaling effects found and discussed
in Section 3.2 and 3.3 are automatically taken into
account. The Reynolds effects on the ESD thrust (friction
and pressure drags) have been taken into consideration by
the viscous correction which is calculated according to,
where the ITTC 1957 formula of the flat plate is used for
the friction coefficients and the form factor k2 is
empirically calculated as a function of the thickness to
chord ratio as 2t/C.
pre-duct with a supporting stator + propeller + PBCF
Figure 20 PIV measurements at plane BH (averaged), 15
knots, left: velocity, right: vorticity, original hull form.
Behind the working propeller, PIV measurements have
been carried out with and without the ESDs, including the
PBCF. Selected results are shown in Figure 20, both for
the velocity field (on the left) and also for the vorticity
field (on the right). The following observations are made
from the measurements,
- Comparing the vorticity with and without the ESD, it
is clearly seen that there is vortex shedding on the
portside, associated with the two stator blades which
have larger angles of attack.
- Comparing the velocity fields behind the propeller at
the shaft, a strong swirl can be seen for the situation
This extrapolation method has been used both for the
model tests for the ship model with the original form, as
well as for the smart ship model. As we have pointed out
at the beginning of this paper, the smart ship model is
used only for relative comparison. For the smart ship
model test results, we have tuned the form factor 1+k and
the correlation allowance CA so that the same level of the
total resistance of the vessel has been reached. For the
smart ship model, the wake scaling factor is set to 1. The
parameters used in the calculations are listed in Table 4.
Table 4 Extrapolation factors
Original hull form
Smart ship model
1+k
1.200
1.165
CA
0.00030
0.00036
(VA/V)s/(VA/V)m
1.113
1.000
When the correlation factors listed in Table 4 are used in
the speed-power prediction for the basis cases without the
ESDs, the same shaft power levels at the same vessel
speed have been reached precisely, both for the original
hull form and for the smart ship model.
By using these two cases without ESDs as the basis for
the relative comparisons, the following performance
predictions and power savings have been obtained after
fitting the ESDs.
5.2 Performance prediction
The predicted shaft power levels based on the original
hull form and on the smart ship model are shown in
Figure 21 and Figure 22, respectively.
It should be pointed out that the thrust deduction behind
the smart ship model with ESDs can be different from
that behind the original ship model or from that without
the ESDs, even if the comparison is in a relative sense.
But this effect cannot be easily subtracted from the test
results. It was decided to leave this out of the present
paper. Further details are being investigated by using
CFD calculations and will be published soon.
6 CONCLUSIONS
The forces and moments on all the ESD components were
measured during the model tests, the smart ship model
successfully improved the similarity with the full-scale
wake and the PIV measurement results provided insights
into the flows about the ESDs. The present JIP studies on
ESDs shed important light on the working principles of
the ESDs and the scale effects, which will lead to an
improved extrapolation of model tests with ESDs in the
near future. The major findings of the present study are
listed below.
-
Figure 21 Speed power predictions based on original hull.
-
Figure 22 Speed power predictions based on smart ship
model.
It is seen that both model tests predict more than 4%
energy saving when fitting the pre-swirl stator. The PBCF
improves the performance by more than 2%, as expected
in Section 3.4. The most energy saving comes from the
pre-duct with a supporting stator in the duct, about 6%.
Surprisingly, the pre-duct with a supporting stator
performs also very well in the simulated full scale wake
field, although the duct thrust is dramatically reduced.
This can only be explained that the stator in the duct
works also differently in the simulated full-scale wake.
This will be made clear when the PIV measurements are
carried out for the smart ship model.
Some additional propulsion tests have also been carried
out by combing the PBCF with the pre-swirl stator and
the pre-duct with a supporting stator, respectively. In both
cases, the total propulsive efficiency has been further
improved. But the improvement is only approximately
1.0% at 15 knots. This can be understood from Figure 20.
-
-
The resistance test results on the original hull model
fitted with ESDs show that both the pre-swirl stator
and the pre-duct generate drag. But the supporting
stator in the pre-duct converts the flow swirl from the
hull into net positive thrust.
The propulsion test results on the original hull form
fitted with the pre-duct with a supporting stator show
that both the pre-duct and the supporting stator
generate positive thrust. While the test results of the
pre-swirl stator show clearly drag force on the stator.
Both the propulsion test results on the original hull
model and on the smart ship model, fitted with ESDs,
show clearly linear relations between the ESD thrust
(or drag) and the propeller thrust. The thrust of the
pre-duct increases with the increase of the propeller
thrust, while the thrust of both stators decreases. This
implies that the wake scaling from model to full scale
will result in worse performance for the pre-duct and
better performance for the stators.
The scale effects on the pre-duct used in the present
study are significant. The duct thrust reduces
dramatically from model scale to full scale. While the
scale effects on the pre-swirl stator used in the present
study are very limited.
The reverse open water tests with PBCF show that the
PBCF has hardly any effect on the total propeller open
water efficiency around its mean operation condition.
But it has strong effects at low advance ratio J when
the propeller is heavily loaded. This may explain why
a considerable improvement on the total propulsive
efficiency has been found by fitting a PBCF to the
propeller in behind condition.
The results of the PIV measurements on the unsteady
flows about the ESDs, triggered by the propeller
angular position, clearly show the suction effect of the
pre-duct; the vortex shedding from the stator blades;
the propeller tip vortices and the strong hub vortex
when there is no ESD. By fitting the pre-duct with a
supporting stator, the hub vortex is reduced. When a
-
PBCF is fitted, the hub vortex in the slipstream of the
propulsor (including ESDs) is completely eliminated.
The model test results show about 6% improvement
by fitting the pre-duct with a supporting stator, about
4% improvement by fitting a pre-swirl stator and
about 2% improvement by fitting only a PBCF to the
propeller, on the total propulsive efficiency for a
tanker in full scale. For the present study, the ESDs
are by no means optimized. Further improvements are
still possible.
DISCLAIMER
The present study is purely aimed at understanding the
working principles of the selected ESDs in general. The
ESDs used in the study may be, by coincidence, similar to
other patented products. No designs with detailed
geometry from any 3rd party have been used in the present
study.
ACKNOWLEDGEMENT
The authors are grateful for the valuable criticism,
corrections and comments from Prof. Tom v. Terwisga of
TU Delft and Mr. Jan Holtrop of MARIN on the draft of
this paper.
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