Date Author Address
Maart 2006
Bart van Oers (T(JDelft) and Serge Toxopeus (MARIN) Deift University of Technology
Ship Hydromechanics Laboratory
Mekelweg 2, 26282 CD Delft
TU Deift
DeIft Ufliversity of Technology
On the relation between flow behaviour
and
the lateral force distribution acting
on a ship
in oblique motion
by
Bart van Oers and Serge Toxopeus
Report No. 1473-P
2006
Published: 2d World Maritime Technology Conference, WMTC 2006, 6-10 March, London1 United Kingdom
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Parallel Session F: Ship Motion
Ventilation effects on cavitating wedges and struts
S Gowing, Y Shen, NAVSEA Warfare Center, USAOn the relation between flow behaviour and the lateral force distribution acting on a ship in oblique
motion
Ir B van Oers, DeIft University of Technology, The Netherlands; Ir S Toxopeus, MARIN, The Netherlands
Numerical optimisation of propeller-hull configurations at full scale
Prof L Larsson, Dr B Regnström, FLOWTECH International AB, Sweden; K Han, Prof G Bark, N Bathfield, Chalmers University of Technology, Sweden
A particle-based Lagrangian' CFD tool for free-surface simulation
D Muox, V Gonzalez, M Blain, Next Limit Technologies, Spain;J Valle, CEHIPAR, Spain;
J C Diaz-Cuadra, NAVANTIA, Spain
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Conference Proceedings of
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Part D
http://1 27.0.0.1:1 03/cgi-binlsomsid.exe?page=indexes/stream i parallelsessionfshipmotion
2006-03-13
On the relation between flow behaviour and
the lateral force distribution acting on
a
ship in oblique motion
Bart van Oers,, MSc(Hon), GMRINA
Delfi University of Technology - Ship Hydromechanics Laboratory
Serge ToxopeuS MSc, MSNAME
Maritime Research Institute Netherlands (MARIN:)
Delfi University of Technology - Ship Hydromechanics Laboratory
Author!s Biographies
Bart van Oers graduated in 2005 from Delfi University of Technology, Faculty of'Mechanical Maritime and Materials
Engineering with a specialisation in Ship Hydrodynamics. Currently, he is researching the design optimisation of naval surface vessels m'the context of his PhD project.
Serge Toxopeus graduated in Ship Hydrodynamics from DeIft University of Technology at the Faculty of Mechanical
Engineeringand Naval ArchitectUre in 1996. Since then, he has been Working in the Manoeuvring Department of MARIN
Main fields of competence are ship manoeuvring simulations and practical application of viscoUs-flow calculations for
manoeuvring ships.
SYNOPSIS
This paper presents the results of a research project focussed on the simulation of the viscous flow fields around five vessels.saiiing atanon-zero drift angle. The calculatedflowfields wereusedto investigateflow
features relevant to the lateral-force distribution, thus offering insight in the physics involved in the
manoeuvnng behaviour of ships This insight can be used to improve the manoeuvnng characteristics in the early stages of the design process.
INTRODUCTION
Proper manoeuvrability is. essential for a' ship to perform its task, necessary for both navigation and the avoidance of traffic and natural hazards. Designing such a ship, with suitable manoeuvring characteristics, is,
however not straightforward To be able to do so
it is necessary to predict with sufficient accuracy themanoeuvring behaviour in the early design stage In addition an understanding of the physics involved i e
what causes the forces actingon the vessel, is necessary to determine relevant design changes. To extend the predictive capability and improve the understanding of the flow field, the use of mathematical models with a
better prediction of the flow behaviour is necessary and therefore the viscous flow-solver Parnassos(developed
by MARIN) was extended to simulate the flow around ships sailing at non zero drift angle Promising results withirespect to quality of bothfieldand integrated quantitieswere obtainedl(reported in Li'], [2], [3J and [4:1). The.present paper addresses the relation between flow field and lateral force distribution of vessels in stationary oblique flow. The flow field around the Esso»Osaka for 100 is investigated, highlighting the significant flow features 'introduced 'by the oblique motion. Using the pressure distribution on the hull, the flow aspects relevant
to the lateral force distrtbution are established As a closure a comparison is made between the pressure
distribution of five vessels'toestablish the relationbetween hull shape and manoeuvring characteristics of ships.APPROACH: CoordiflateSystern
The origin of the ship-fixed, righthanded system of axes used in this study is located at the 'intersection of the
water-plane, midship and centre-plane, with x directed aft, y to starboard and z vertically upward. All
coordinates given in this paper are made non dimensional with All velocities are made non dimensional
Computational backgrouDd
The calculations were performed with the MARIN inhoUse flow solverParnassos, ([5]and [6]). This solver is
based on a finite-difference:discretisation of the Reynolds-averaged continuity and momentum equations, tising fl.illy-collocated variables and discretisation. The equations are sOlved with a coupled procedûre, retaining the continuity equation in its original form.
For the calculations, the one-equation turbulence model, proposed by Menter [7J, was used, while avoiding the use of wall-functions. The Spalàrt correction (see [8]) ofthestream-wise vorticity is included.
The results presented in this paper wereall obtained' on structured grids with H-Otopology, with grid clustering near the bow and propeller plane More details regarding the computational domain, the implementation of a drift angle in the calculations, and the applied boundary conditions are found in [3] and [4]. Appendages, free surface:deformation and dynamic trim and sinkage of the vessels were not modelled.
Vessels
Calculations were made of the model scale flows around five ships sailing at a drift angle. Table I shows some main particulars:ofthese.ships togetherwith the Reynolds'numbers used in the calculations.
Table l:Main palliculars
sailing in ballas draught
INFLUENCE OF DRIFT ANGLEON FLOW AROUND THE ESSO OSAKA
To establish the influence of a drift angle, a comparison is made of the flow fields around the Esso Osaka at
3Ø0 and lO°. Viscous effects have the largest influence around the aft body, which therefore becomes the
area of focus of the discussion. Figure 1 through Figure 4 respectively show theaxial flOw velocity at the bow,at
25% and 45% aft of midship and at the propeller plane. Where applicable, letters are used to indicate
relevant parts of the figures.
Three dominant effects govern the flow around a ship sailing at a drift angle. The first, thedisplacement effect, introduces a pressure field around the hull as pressure gradients displace the flow away from (at the bow) and towards the hull (at the forward:andaft shoulders)to follow the local hull curvature. These pressure gradients are accompanied by changes in flow velocity. The second effect, flow separation, can occur when the streamlines curve towards the hull by the displacement effect (a pressUre reduction at the hull surface) but are unable to
follow the curvature of the hull due to an adverse pressure gradient Flow separation can lead to vortex
development. The third effect, the convective property of a vortex, can change the flow velocity by convecting high-velocity fluid towards a place of lower flow velocity and thereby changing the local pressure. Obviously, prediction of the latter two effects requires the useofa viscous flow solver.The main change to the flow field at thebow when sailing inoblique flow is the windward shift of thestagnation point, see Figure I. The pressure changes extend downstream towards the forward shoulders, increasing the pressure atthe windward side, while reducing it at the leeward shoulder.
Osaka [4] MARiNFer,y [9] Series:6O[1] KVLCC2AtI[3] Hopper[IO]
202 .002 .01 N 005 =
2
.002 -0.04 N .004 -y .0.1.-...
Figure 1: Axial velocity distribution at the!bow
Further downstream at the aft shoulders, 25% aft of midship, vortices develop at both bilges, as shown in
Figure 2. The vortex shed at the leeward bilge starts further towards the bow, due to the difference in the flow direction relative to the hull curvature. Both vortices remain close to the hull and grow in strength downstream. The boundary layer shows a large difference in thickness between the windward and leeward side. This difference indicates a delayedseparation of the flow on a concave, inward-curved surface (referto A in Figure 2) thus allowing thedisplacement effect to furtherreduce the pressure at thewindward aft shoulder
Figure 3: Axial velocity distribution at 45% L, aft of midship Figure 2: Axial velocity distribution at the aft shoúlders, 25% L1, aft of midship
At 45% L aft of midship, shown in Figure 3, large changes occur in the flow field as the viscous effects
become more pronounced The zero drift case reveals the rapid increase in boundary layer thickness For 100 drift, the vortex at the leeward side, which developed upstream at the leeward bilge, has detached from the boundary layer near the hull by a combination of an increase in vortex intensity and the receding shape of the
hull (B). The leeward side also shows a large area of flow separation close to the keel, resulting in the
development of an intense vortex (C). This vortex remains close to the hull by the transverse flow direction introduced' by the receding frame shape, which maintains a constant draft. Due to its convective property, this vortex increases the flow velocity near the hull, thus compressing the boundary layer and reducing the local pressure At the windward side the vortex from the windward bilge ends up alongside the hull despite the oblique flow direction'(D).
002
The predicted flow field for 0° drift at the propeller plane, see Figure 4, shows two counter-rotating vortices, which increase the axial flow velocity in the upper part of the propeller disk by convection (E in Figure 4). A more complex flow field is visible for 13=10°. The leeward vortex, reinforced by the flow separation near the keel, merges with a new vortex shed from underneath the propeller hub, explaining its stretched shape (F). Due to the drift angle, the windward vortex is co-rotating with the leeward vortex, which results in an upward movement of the leeward vortex and a downward movement of the windward vortex (F and G).
003
A
D= 00BI
Figure 4: Axial velocity distribution at the propeller plane
RELATION BETWEEN FLOW AROUND THE SHIP AND PRESSURE DISTRIBUTION In this section the influence of the flow features on the pressure distribution (the contribution of the friction to the lateral force is negligible) on the hull of the Esso Osaka is discussed, see Figure 5 through Figure 7. Again, letters are used to refer to relevant parts of the figures. The pressure is made dimensionless using equation (1).
c
'½pV,2
Windward side
A
C
FigureS: C distribution at the bow and forward shoulders, = 0° and J 10°
From the pressure distribution at the bow, shown in Figure 5, three things stand out. First is the windward shift of the stagnation point caused by the oblique inflow (A in Figure 5). Secondly, this shift also affects the pressure distribution near the forward shoulders, increasing the pressure at the windward side while reducing it at the leeward side (B). Third, near the bottom the pressure changes are opposite to those occurring at the forward shoulders. At the windward side, a reduction in pressure is visible whereas at the leeward side the pressure increases relative to the zero-drift case (C). These three changes result from displacement effects, i.e. a change in flow direction relative to the local hull curvature.
Figure6: C,distribution attheaft shoulders and stem, windward side, fl 0°and3 = 100
Figure 6 and Figure 7 show the pressure distribution at the windward and leeward sides of the stern respectively. A comparison between 13=0° and f3°I0° shows a pressure reduction at the windward aft shoulder, introduced by the flow remaining attached to the hull over a longer distance compared to the zero-drift case (D in Figure 6). At the bottom of the stem, the vortex developing at the windward bilge has little influence on the local pressure distribution (E).
E
ILeeward sid
The pressure distributions for the KVLCC2M in this paper are given for a drift angle of 9°
D
= 10°
el
11D10°
G
IGl
FI
FIgure7: C. distribution at the aft shoulders and stem, leeward side, D= 00and = 10°
The area of flow separation responsible for the vortex discussed in the previous section shows up as a pressure reduction visible near the bilge (F in Figure 7). The convective property of the vortex reduces the local pressure and this, together with the flow separation at the stern contour, prevents the shift of the aft stagnation point (G) towards the leeward side. It also limits changes in pressure at the aft shoulder (H). The pressure distribution near the propeller hub (I) is dependent on the position and intensity of vortices on both the windward and leeward side of the hull, as explained in the previous section. In [4] it was shown that the pressure distribution in this area can valy substantially with small changes in drift angle.
RELATION BETWEEN HULLFORM AND PRESSURE DISTRIBUTION
To establish the relation between hull form and lateral force distribution, the pressure distributions on the five hull forms under consideration are compared for 100 drift anglet. The discussion will focus on the bow and stem areas, as these have the largest contribution to the lateral force. Due to space limitations, the five flow fields could not be included but these have been used to further understand the pressure distributions.
Starting at the bow, see Figure 9 and Figure 10, V-shaped frames near the bow are only found on the MARIN Ferry, the other four vessels have U-shaped frames. These differences in frame shape show up as pressure changes around the forward shoulders and near the bottom of the bow, which are more pronounced for the U-framed ships. The MARIN Ferry and the Series 60 have very slender bows, reducing flow displacement and thus the influence of the stagnation pressure at the bow. The bow contour of the Ferry is more rounded and this, together with the slight concave shape of the hull aft of the bulb, considerably reduces the pressure difference between the leeward and windward side of the hull. The flow separates underneath the bow of both the Ferry and Series 60, resulting in a vortex. The very slender frames, the sharp bow contour and the concave waterlines of the Series 60 result in a large high pressure area at the windward side of the bow and a large low pressure area at
leeward. Compared to the Ferry, the blunt bows of the Osaka, the KVLCC2M and the hopper dredger introduce a much higher stagnation pressure and stagnation area on the windward side, while simultaneously allowing the flow to remain attached at the leeward side, thrther reducing the pressure.
At the stern, see Figure 11 and Figure 12, the flow features are more complex. A pressure reduction develops at the windward aft shoulder, as was discussed in the previous section. Its longitudinal extent and magnitude are
dependent on the frame shape and the contour line of the stern. The Ferry and Hopper both with pram-type aft bodiés, show a larger pressureredùctionover a longer distance compared to the U or V shaped frames dûe to an increase in hull curvature relative'to flow direction. A more full aft body obviously increases hull curvature and hence the pressure reduction at the windward aft shoulder (compare for instance the results of the full-bodied Hopperwith those of the slender'Ferry).
The influence of stern shape also influences viscous effects. Flow separation occurs at two locations near the
stern, at the stem: contour and near the bottom of the leeward aft shoulder. At the second: location, the
accompanying vortex reduces the local pressure However due to the (upward) cross flow generated by the pram-typeaft-body of the Hopper, the vortex is located further away' from the hull surface, resulting 'in a smaller redûction ofthepressure. Together with the relatively blunt waterlines in the aft ship, this results ma shiftof the aft stagnation point towards the leeward side. None of the other vesséls shows this shift.
The larger'skegof the Ferry preventsthe leeward shift ofthe stagnatiónpoint, despite havinga similar hull: shape as the Hopper. In addition to the effect discussed above, the skegs of the Hopper and' the Ferry introduce: other effécts. At the windwardside, they displace the flow, increasingthe local pressure. At the leewardside, a vortex develops at the start of the skeg increasing rn strength downstream This vortex reduces the pressure at the leeward side of the skeg, contributing to a higher negative lateral force at the stem. Due to the strong reduction in sectional area in the aft ship of the Hopper, the flow around the windward side of'the stem is accelerated and therefore the pressure òn the' skeg on windward is relatively small compared to the pressure distribution on the skeg:of the Ferry.
In addition to the influences stated above, the longitudinal positions of the forward and aft shoulders and'skeg also 'influence the lateral force distribution by changing the hull curvature and the longitudinal position of pressure changes.
RELATION BETWEEN PRESSURE DISTRIBUTION AND THE LATERAL FORCE DISTBUTION To establish the consequences for the lateral force distribution along, the length o'f the ship, the ship 'is divided
into 10 segments, with the segment boundaries located at even stations. The lateral forces Y,, acting on these segments are made non-dimensional using the' prdjected 'lateral area S, of each segment according to equation (2). Figure 8 'shows' the calculated longitudinal distributions for the five vessels (the results for the KVLCC2M were'obtained,by interpolation bet*een 9° and 12° drift angle).
010 0.00
T
-0.10 -0.20 --0.30 -0.40 Y Y,n .LpV2S I 2 3 4 5 6 7 '8 9 IOstern n- segment -n bow
FigureS: Longitudinal distribution of lateral forceationdritt angle
MARIN Feny KVLCC2M D Osaka Hopper U Series60 (2)
Figure 8 shows that thelargestmagnitude of the lateral' force is generated at the bow and forward shoulders, i.e. in segments 9 and 10 Furthermore it shows that the five foremost segments for all ships contribute to the
lateral force, while the aft' segments experience either negative lateral forces as well as positive forces,
Starting with the influence of the bow shape, it was shown in the previous section that the blunt bow vessels have significantly larger pressure gradients at the bow, explaining the large transverse drag (i.e. large negative lateral force) at the bow and forward shoulders The flòw separation at the slender bows of the Ferry and Series 60 reduces the transverse drag in segment 9 relative to the vessels with the blunt bows The relatively small transverse drag of the.MARIN Ferry in segment IO is caused by a combination of therounded bow contoUr, the concave hull shape aft of the bulb andthe relatively shallow draught (low aspect ratio).
Moving aft, the reduction of the pressure at the windward aft. shoulder is responsible for the positive lateral forces seen for segments I to 4. The higher block vessels all show a positive lateral force iñ segments 2 to 4, introduced, by displacement effects resulting from the increasedhull curvature at the aft shoUlders. The pram-type aft body of the I-lopper has an even larger pressure drop extending over a longer distance explaining the large positive lateral force even in segment I. All vessels show the development of flow separation at the leeward sideof the stem, with varying influence on the lateral force distribution. The ships with a U-shaped aft body, the Osaka, the KVLCC2M and the Series 60, all develop an intense vortex at the leeward side, which reducesthe local pressure and thus results insmallor negative lateral forces in segments I to 2.
At the extreme end of the stern1 different aspects determine the pressure distribution. For the KVLCC2M, and theOsaka therelative vortex intensity determines the pressure distribution in segment I. The .positivetransverse drags (i.e. negative lateral forces) in segments I and 2 of the Ferry and the Series 60 can be attributed to the effect of the skeg and the very narrow and sharp aft body of the Series 60. The influence of the skeg for the Hopper diminishes by the far larger influence of the pram-type aft body and aft shoulder location, resulting ina net positive lateral fôrce.
INFLUENCE OF HULLFORM ONMANOEUVRABILITY
T.'he manoeuvrability of ships cambe dividedrinto two separatequalities: the turning ability andthe directional (or related to that the course) stability For agood turning ability the resistance against turning should be small the yaw moment as a function of drift should be large and the ability to generate a turning moment (by using a steering appendage such as the rudder) should be large. For a good directional stability, the resistance against turning should be large and the yaw moment as a functiOn of drift should .be smalL Therefore, a compromise between the turning ability and the directional stability must always be made, unless a very powerful steering appendage is applied.
For most ships except very slender vessels such as frigates, the required turning ability is easily met but an
acceptable directional stability is hard to achieve. For these ships, the directional stability can be improved best by reducing the yaw moment as a function of drift and increasing the resistance against turning These effects can be achieved simultaneously by increasing the drag against cross flow in the aft ship Based on experience from manoeuvring tests and clarified by the study presented in this paper, this can be realised by moving the aft shoulder forward (e.g. by moving the centre of buoyancy forward) such that the aft ship becomes more slender or by increasing the pressure at the windward side by applying a sharp skeg. Alternatively, this can be achieved by using V-shaped frames instead of U-shaped frames in the aft ship. Stimulating flow separation and vortex development by reducing bilge radius near the stem will reduce the destabilisitig yawing moment while increasing the transversedrag for U-shaped and V-shaped frames.
CONCLUSIONS
Usingthe'results ofcalculationsfor severaldifferent ships, the influence of the hull form on.theflów around five different ships was studied, relating the hull form and the manoeuvring performance of the ship. The following qualitative conclusions are drawn.
Influence of the bow shape:
Sharp, vertical frames generate moretransversedrag thanslender,shallow draught, V-shaped frames. Concave waterlines reduce the transverse drag compared to convex waterlines
Influenceof the aft body shape:
A pram type aft body reduces the transverse drag and increases the destabilising yawing moment as a function ofdrift relativeto U and V-shaped frames.
Series 60 4 o
403
KVLCC2M
4Hopper
ç -- o -e. - - '- - -.Skegs increase the overall lateral drag while reducing the'destabilising yawing moment
Vortex development resulting from flow separation at the leeward side ofthestem increases the lateral drag while reducing the destabilising yawing moment for U and V-shape aft-bodies.
The results above show that themanoeuvring,performance of ships is strongly determined by the shape of the aft body. Therefore, careful attention should be paid to the design of the aft hull form in the early design stage and guidelines are given. The insigjit offered by viscous flow simulations contributes to the assessment of manoeuvring characteristics in the early stages of design Research to include rotational motion in Parnassos further improving the predictive capability, is currently under way.
ACKNOWLEDGEMENTS
Part of the work conducted for this paper has been funded by theCommissionof the European Communities for the Integrated Project VIRTUE. This project is part of the Sixth Research and Technological Development FrameWork Programme (Surface Transport Call). Another part of the project was conducted as an MSc thesis project carried out by Bart van Oers at MARIN under supervision of Serge Toxopeus;
REFERENCES
[I]
Toxopeus, S.L. ; "Simulation and validation of theviscous flow around the Series 60 hull form at loo drift angle". 7" NuTIS Numerical Towing Tank Symposium, October 2004.Toxopeus, S.L.; "Validation Of Calculations Of The Viscous Flow Around A Ship In Oblique Motion". The First MARJN-NMRJ Workshop, pp. 91-99, Tokyo, Japan, October 2004.
Toxopeus, S.L.; "Verification And: Validation Of Calculations Of The Viscous Flow Around KVLCC2M In Oblique MotiOn". 5th Osaka Colloquium, March 2005
Van Oers, B.J. An investigation of the viscous flow around ô ship in oblique motion. MSc thesis, Delft University of Technology, Faculty of Mechanical, Maritime and Materials Engineering, March 2005. Hoekstra M. and Eça, L. "PARNASSOS: An Efficient Method for Ship Stem Flow Calculation", Third Osaka Colloquium on Advanced CFD Applications to Ship Flow and Hull Form Design, pp. 331-357, Osaka, Japan, May 1998;
Hoekstra, M. Numerical Simulation of Ship Stern Flows with a Space-Marching Navier-Stokes Method. PhD thesis, Delfi Uñiversity of Technology, Faculty of Mechanical Engineering and Marine Technology, October 1999.
[7]: Menter, F.R. "Eddy Viscosity Transport Equations and Their Relation to the k-c Model", Journal of Fluids Engineering, Vol. 119, pp. 876-884, December 1997.
[8] Dacles-Mariani, J., Zilliac, 0.0., Chow, J.S. and BradshaW, P. "Numerical/experimental Study of a Wing Tip Vortex in the Near Field", A/AA )ournal, Vol. 33, pp. 1561-1568, September 1995.
[91 Toxopeus SL. and Loeff, G.B. "Model Tests with Segmented Ferry", MARIN Report l5295-l-BT,
October 1999 (Restricted);
MARIN Ferty
MARIN Feny
Señes 60
KVLCC2M
Esso Osaka
Hopper
- -; I,
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