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8/4/2019 Computational Hydrodynamic Analysis of AZIPOD System Propeller-Rudder
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Ocean Engineering 35 (2008) 117130
Computational hydrodynamic analysis of the propellerrudder and the
AZIPOD systems
Hassan Ghassemi, Parviz Ghadimi
Department of Marine Technology, Amirkabir University of Technology, Hafez Ave, No 424, P.O. Box 15875-4413, Tehran, Iran
Received 20 November 2006; accepted 13 July 2007
Available online 27 July 2007
Abstract
A computational method has been developed to predict the hydrodynamic performance of the propellerrudder systems (PRS) and
azimuthing podded drive (AZIPOD) systems. The method employs a vortex-based lifting theory for the propeller and the potential
surface panel method for the steering system. Three propeller models along with three steering systems (rudder and strut, flap and pod
(SFP)) are implemented in the present calculations for the cases of uniform and non-uniform conditions. Computed velocity components
show good agreement with the experimental measurements behind a propeller with or without the rudder. Calculated thrust, torque and
lift also agree well with the experimental results. Computations are also performed for an AZIPOD system in order to obtain the pressure
distributions on the SFP, and the hydrodynamic performance (thrust, torque and lift coefficients). The present method is useful for
examining the performance of the PRS and AZIPOD systems in the hope of estimating the propulsion and the maneuverability
characteristics of the marine vehicles more accurately.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Hydrodynamic analysis; Propellerrudder; AZIPOD
1. Introduction
Propellerrudder systems (PRS) are located behind a
ships stern where they encounter large wake flow. Due to
the hulls presence, the flow distribution into the propeller
is non-uniform and unsteady. Recent improvements made
in the electrical propulsion by the engine manufacturers
provide the azimuthing podded drive (AZIPOD) systems
which are compact propulsion systems, and give excellent
maneuverability. The most significant hydrodynamic ad-
vantage of the AZIPOD system is that the propeller is setin a more regular flow. It utilizes a smaller rudder, i.e. the
flap, located at the trailing edge of a vertical hydrofoil, and
the strut, situated in a similar arrangement as a flap on an
airplane wing, The flap and strut mechanism provides
greater propulsion efficiency and excellent maneuverabi-
lity. The flap is generally used for high-speed crafts when
the pod needs to be locked in neutral position, but could
also function as a controlling element for the course
keeping task.
Until now, hydrodynamics investigations of the propeller
rudder systems, by virtue of different methodologies, have
mainly concentrated on the PRS propulsors. A theoretical
treatment of the propeller and rudder was initially
conducted by Yamazaki (1968) and later numerically by
Yamazaki et al. (1985). Tamashima et al. (1993) and
Matsui et al. (1994) evaluated the PRS propulsion systems
under uniform flow conditions. Matsui et al. (1994) applied
this method in the context of the ship maneuverabilitytaking into account free surface effects. Molland and
Turnock (1993, 1996) investigated experimentally the
propellerrudder performance in the wind tunnel and
developed the propeller theory and rudder lifting line
theory to predict forces on a rudder in a propeller
slipstream. Li (1996) also investigated the propellerrudder
interactions by applying the lifting line theory for the
propeller and the vortex lattice method for the rudder.
For the AZIPOD systems, efficient numerical schemes
are scarce. Rains et al. (1981) applied empirical formulas to
ARTICLE IN PRESS
www.elsevier.com/locate/oceaneng
0029-8018/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.oceaneng.2007.07.008
Corresponding author. Tel.: +982166419615; fax: +982166413028.
E-mail address: gasemi@aut.ac.ir (H. Ghassemi).
http://www.elsevier.com/locate/oceanenghttp://dx.doi.org/10.1016/j.oceaneng.2007.07.008mailto:gasemi@aut.ac.irmailto:gasemi@aut.ac.irhttp://dx.doi.org/10.1016/j.oceaneng.2007.07.008http://www.elsevier.com/locate/oceaneng8/4/2019 Computational Hydrodynamic Analysis of AZIPOD System Propeller-Rudder
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estimate the drag coefficients of different parts of the strut,
flap, and pod (SFP). Wang et al. (2004) investigated
experimentally and numerically (panel code) the push and
pull configurations of the podded propulsion system andthe effect of the pod geometry on the hydrodynamic
performance of the whole system.
More recently, due to the market needs and in order to
gain more efficiency by the AZIPOD systems, marine
researchers have rigorously pursued this topic and much
effort has been devoted to explore it numerically and
experimentally. During the past two years, the 1st and 2nd
T-Pod conferences have been held at the University of
Newcastle (UK) and Universite de Bretagne Occidentale
Brest (France) in 2004 and 2006, respectively, and many
researchers (like Ma et al., 2004; Mohammed Islam et al.,
2004; Sakir Bal et al., 2006; Zhang Lijun and Wang
Yanyin, 2006) presented their latest findings.
Hydrodynamic design of the propellerpod-strut system
has not thus far been adequately explored. It is indeed
imperative that we use more reliable procedures in the
design of such a propulsion system in order to increase the
propulsion efficiency. In an attempt to meet these needs,
the present paper introduces a numerical procedure which
analyzes the hydrodynamic performance of the PRS
and the AZIPOD systems. The method applies Yamazaki
et al.s (1985) method for the analysis of the propeller and
the potential-based boundary element method (so-called
panel method) for the steering system. This combined
method could not only emulate the hydrodynamic behavior
of the simple PRS systems, but also could quite satisfacto-
rily predict the hydrodynamic performance of the more
complex and involved AZIPOD systems. In this work, the
effect of the steering system is also examined on both typesof the propulsors as well as the interactions occurring at
different operating conditions.
2. Prediction method
2.1. Formulation of the problem
2.1.1. Coordinate system
In order to study the flow fields around a rotating
propeller and steering system in a steady flow, a
rectangular coordinate system O-XYZ and a cylindrical
coordinate system O-Xry are defined in space. The origin is
located at the center of the propeller and the X-axis
coincides with the propeller shaft axis as shown Fig. 1. The
steering post is placed behind the propeller and is parallel
to the Z-axis. The distance between the post and the
propeller is X XRud.
2.1.2. Inflow velocity onto the propeller
The propeller is assumed to rotate with a constant
angular velocity o around the X-axis in the negative
direction ofy. Denoting the components of the steady non-
uniform velocity field towards the propeller in the
Cartesian coordinates by (uPX, uPY, uPZ) and in the
cylindrical coordinates by (vPX, vPr, vPy), they are expressed
ARTICLE IN PRESS
Nomenclature
a0(r) geometric pitch of the propeller
a(r) effective pitch of the propeller
c(r) propeller chord length at r
cM(r) distance from leading edge to maximum thick-ness
CP pressure distribution coefficient
D propeller diameter
FRY rudder lift
h(r) pitch of the free vortex
kN(r) Prandtls tip correction factor
MP radial number of intervals
n revolutions per second
N number of propeller blades
nq normal vector
NP circumferential number of intervals
Pi pressure at center of each panel
P0 atmospheric pressureP propeller pitch
PU upper-side pressure
PL lower-side pressure
Q propeller torque
rB hub radius
R propeller radius
SB body surface (steering system)
SW wake surface
SN
outer control surface
T propeller thrust
tL(r) distance from LE to base line at r
tT(r) distance from TE to base line at rtmax(r) maximum thickness at r
tan ag zero lift angle from the base line~V0 upstream inflow velocityV measured axial mean velocity~Vr induced radial velocity~VP inflow velocity to propeller~VR inflow velocity to rudder~VX induced axial velocity~Vt induced tangential velocityXRud propellerrudder stock distance
dF flap angle
dR rudder angle
fP velocity potential on propellerfR velocity potential on rudder
fR right-side potential
fL left-side potential
r water density
G(r,y) strength of bound vortex
H. Ghassemi, P. Ghadimi / Ocean Engineering 35 (2008) 117130118
8/4/2019 Computational Hydrodynamic Analysis of AZIPOD System Propeller-Rudder
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as components of the vector:
~vP vPX; vPr; vPy uPX; uPY sin y
uPZ cos y; 2prn uPY cos y uPZ sin y. 1
In the case of steady and uniform flow, the inflow
velocity simplifies to
~vP vPX; vPr; vPy V0; 0; 2prn, (2)
where V0 is the speed at which water is moving onto the
propeller and n is the propeller rotational speed.
2.2. Vortex-based lifting theory for the propeller
2.2.1. Vortex and potential calculations
The propeller is represented by the vortex system. This is
composed of the bound vortex arranged in the radial
direction on the propeller (it is assumed that the propeller
is replaced by the infinitely many blade-like actuator discs)
and the free vortex shedding from the bound vortex. The
free vortex is distributed on the helical surface with
pitch 2ph(r) without contraction. The strength of the
bound vortex G(r,y) and the velocity potential fP are
determined by the equations of the propeller theory and the
kinematic boundary condition (KBC). An iterative proce-
dure is used to obtain converged values of h(r) on the bladeradius.
The velocity potential fP due to the bound vortex on the
propeller may be expressed (Yamazaki, 1968) as
fP
ZRrB
r0 dr0Z2p
0
Gr0; y0GPx; r; y; r0; y0 dy0, (3)
where
GPx; r; y; r0; y0
r0
hr0
1
RP
r cos y sin y0 r sin y cos y0
R2P X
21
X
RP 4
and
RP
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiX2 Y r0 sin y02 Z r0 cos y02
q.
Other parameters of the Eqs. (3) and (4) are identified in
Fig. 2.
2.2.2. Kinematic boundary condition
The propeller is considered to have a finite number of
blades and a finite chord length. Under these assumptions,
the KBC on the blade is expressed as
2
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 ar2
qNk1cr
r2 hr2
2rhrkNr
0@
1AGr; y
qfPqX
P
hr
r
qfPrqy
P
ar
rvPy vPX, 5
where a(r) is the effective pitch angle and is defined by
ar ka0r r tan ag
1 a0r=r tan ag;
tan ag 2k0tr=cr
1:5 cMr=cr
tLr tTr
cr,
tr tmaxr
2 1
cMr
cr
tLr
cMr
crtTr, 6
kNr 2
pcos1 exp N 1
r
R
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR2 hr2q
2hr
8
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