Maritime University of Szczecin
Akademia Morska w Szczecinie
2010, 20(92) pp. 5–12 2010, 20(92) s. 5–12
Distribution of flow velocity generated by propellers of twin
propeller vessel
Rozkład prędkości przepływu generowanego przez pędniki
statku dwuśrubowego
Teresa Abramowicz-Gerigk
Gdynia Maritime University, Faculty of Navigation, Department of Ship Operation Akademia Morska w Gdyni, Wydział Nawigacyjny, Katedra Eksploatacji Statku 81-345 Gdynia, al. Jana Pawła II 3, Poland
Key words: propeller jet, flow velocity distribution, model tests, CFD methods Abstract
The paper presents the results of experimental investigations of the flow generated by the controllable pitch propeller. The influence of the propeller pitch, rate of turn and rudder angle on the velocity filed measured behind the rudder for two different water depths were investigated. The model tests were carried out using twin propeller twin rudder car-passenger ferry model in 1:16 scale. The results of the experimental analysis allowed for the quantitative assessment of the velocity components distribution. The predicted flow field generated by the propellers near the quay wall during unberthing has been computed using CFD methods. Słowa kluczowe: strumień zaśrubowy, rozkład prędkości przepływu, badania modelowe, metody CFD Abstrakt
W artykule przedstawiono wyniki badań modelowych przepływu indukowanego przez śrubę nastawną. Badania dotyczyły wpływu skoku i prędkości obrotowej śruby oraz kąta wychylenia steru zaśrubowego na pole prędkości mierzone za sterem, dla dwóch głębokości wody. Wykonano je na modelu promu pasażersko- -samochodowego w skali 1:16. Wyniki badań umożliwiły ocenę ilościową rozkładu poszczególnych składo-wych pola prędkości strumienia zaśrubowego. Przewidywane pole prędkości, indukowane przez pędniki wokół rufy statku, podczas odchodzenia od nabrzeża, obliczono z wykorzystaniem metod CFD.
Introduction
The fast development of intermodal and multi-modal transportation is followed by EU initiatives promoting short sea shipping. In 2004, the adopted revision of the Trans-European Network Transport Guidelines (TEN-T) gave a legal basis to the concept of Motorways of the Sea which main aim is fast development of intermodal transportation chains with efficient trans-shipment centres in ports. This means the growing movement of self-manoeuvring vessels like ferries and container vessels routinely covering the roundtrip routs between two ports, more arrivals and departures of vessels with larger draft, available power and diameters of propellers and thrusters.
Growing requirements with respect to safety impose the design and operational constraints, especially for berthing and unberthing manoeuvres. The main factors considered in the risk analysis are the human factor, available thrust of ship propellers and thrusters, the influence of weather conditions and ship interaction with berthing facility: berthing energy and the effect of propellers and thrusters jet flows on the quay wall and bottom protection.
Investigation of ship’s propeller wash is necessary for civil engineering to determine seabed scouring, sediment deposition, drift of sediments into the protected areas, stability of concrete plates and stones in bottom protection structures.
The proper prediction of the propeller jet velocity field during self-berthing is also necessary for the determination of allowable operational conditions.
The formation of a propeller jet is a complex phenomenon. Velocities can exceed 10 m/s [1]. The strong jet flow can last for the distance of several propeller diameters. The major concerns are the downstream propeller jet up to eight times the propeller diameter and the decay of the maximum axial components of velocity [2].
The prediction of the amount of scour produced by a propeller jet and evaluation of the efficiency of different protection structures can be done using a set of empirically derived equations based on the averaged velocities of propeller jet flow calculated according to the empirical formulas [1, 3, 4, 5, 6]. In the recent studies the numerical models are investigated [2, 7, 8].
The paper presents experimental investigations of propeller jet velocity field behind the rudder, generated by the working propeller of twin propeller twin rudder car-passenger ferry model, measured for different propeller settings. This allowed to study the effect of propeller’s pitch, rate of turn as well as rudder deflection and water depth to draft ratio on the volume of flow rate of propeller jet. The flow field around the vessel during unberthing has been predicted using CFD methods.
Velocities distribution of propeller jet flow
The velocity field induced by a propeller com-prises of axial, tangential and radial components. The axial component is parallel to the axis of pro-peller rotation, the tangential and radial components are perpendicular and coincident to the radius.
The axial components have the highest velocity magnitudes [2]. The observed distribution of axial component of propeller jet velocity is presented in figure 1 [1].
Due to the obstruction effects of the propeller hub the two zones of flow pattern, depending on flow character, can be distinguished: zone of flow establishment and zone of established flow.
The results of model tests carried out and cited by Wilson et al. [1], Chin & Li [9] and Lee et al. [10] confirm that the maximum propeller jet velocity in the zone of flow establishment mainly depends on the efflux velocity and propeller geo-metry. The efflux formation of the jet depends on the propeller geometry.
At the distance of 3 Dp (propeller diameter) the propeller outflow stream is fully established.
In the zone of flow establishment the axial velocities at 0.5 Dp are approximately symmetrical
about the propeller axis, the maximum velocities occur beside the propeller axis at the radial distance of 0.238 Dp [9] from the axis of propeller hub.
The propeller hub reduces the axial velocities in the center of the propeller jet.
In the zone of established flow the maximum axial velocity appears close to the axis of the propeller hub. Rotation speed of propeller is the crucial factor for the formation of jet flow and the axial velocity is dominant component [9].
In the near wake region, the axial turbulence intensity decreases gradually whereas the vertical turbulence intensity remains almost constant as the wake moves downstream due to the rotation of the propeller. For the heavy loading conditions, the axial turbulence intensity is concentrated in the wake region, the energy of the wake flow is diffused as the flow moves downstream [10].
In 2003 Dargahi [6] presented a three-dimensio-nal numerical simulation of ship-induced propeller flows and numerical solution of the sediment continuity equation. The observed phenomena considered the propeller jet influenced by the rudder (Fig. 2).
Fig. 2. Schematic description of the two jet flows behind the rudder ([6] – originally presented by Dargahi in 2003)
Rys. 2. Schemat dwóch strumieni zaśrubowych za sterem ([6] – wcześniej przedstawiony przez Dargahi w 2003 r.)
7 m 0. 5 m Propeller Bottom
Rudder Upper Jet
Lower Jet Circular regions – diameter 0.7 D0 1.988 m Water surface D0 Y X
Fig. 1. Schematic view of a propeller jet showing two zones of flow and efflux velocity distribution ([1] – originally presented by Hamill and Johnston in 1993)
Rys. 1. Schemat strumienia zaśrubowego pokazujący dwie strefy przepływu i rozkład prędkości ([1] – wcześniej przedsta-wione przez Hamilla and Johnstona w 1993 r.)
Zone of established flow Zone of flow establishment R V Rm V0 x = 0.35Dp 2Dp Rm Dp Rh R V Vmax
The rudder situated behind the propeller splits the jet produced by the propeller – into to parts directed upwards and downwards accordingly. The lower jet is similar to a flat wall boundary layer flow with the logarithmic distribution of velocity profiles in the lower jet.
In an unrestricted area the kinetic energy of the jet dissipates proportionally to the distance from the propeller. In another case the slipstream deflects to the nearest boundary surface and observed increase of velocity is dependent on the roughness of this surface.
The distribution of jet velocities is also influ-enced by the shape of ship stern. It is especially important for twin propeller vessels with skegs or with the form of the aft part of ship separating the propellers jets.
Model tests of jet flow generated by twin propeller twin rudder ferry
Assumptions and program of model tests
Model tests of bollard pull were carried out in the experimental test setup constructed in the lake at Ship Design and Research Centre of the Foun-dation for Safety of Navigation and Environment Protection in Ilawa-Kamionka, Poland.
The experimental test setup is presented in figure 1. The sea bed was modeled by the flat floor positioned horizontally at the required depth.
The large manned model of car passenger ferry in 1:16 scale was used in the model tests. The main particulars of the model are presented in table 1.
Table 1. Main particulars of the car-passenger ferry model Tabela 1. Wymiary główne modelu promu pasażersko-samo-chodowego
Displacement [m3] 4.89
Length over all [m] 10.98
Length between perpendiculars [m] 9.64
Breadth [m] 1.78
Draft [m] 0.42
CB 0.687
Dp [m] 0.319
Propeller number of blades 4
Model scale 1:16
The model was equipped with two four blades, controllable pitch propellers of the inward direction of revolution and two rudders.
The experimental investigations of propeller wakes are predominantly carried out using point-wise experimental techniques: hot film, Pitot tube and LDV (Laser Doppler Velocimetry), which
measure flow velocities at discrete points by scan-ning the flow field with an array of measurement probes [10].
Fig. 3. Experimental test setup Rys. 3. Stanowisko pomiarowe
For the presented measurements of the propeller jet a multi-hole pressure probe was used. The difficulties of a distinct probation are mainly due to the complex turbulence in the jet flow.
The range of measurements was limited to velocity vectors slant 60 and uncertainty 3% FS.
The three-dimensional velocity vectors in the propeller jet were measured within the range of the area of a control surface 0.304 0.304 m posi-tioned behind the rudder in the distance of 0.13 m (0.41 Dp) from the rudder trailing edge (equal to the
Fig. 4. Coordinate system used for measurements and visualization of velocity field. Position of control surface for the measurements behind the port side rudder
Rys. 4. Układ współrzędnych przyjęty przy pomiarach i wizu-alizacji pola przepływu. Położenie powierzchni kontrolnej przy pomiarach ze sterem po lewej burcie
z x
x
y
distance of 0.81 Dp form the propeller disk), centered and perpendicular to the propeller axis.
The coordinate system used for measurements and visualization of velocity field, position of control surface for the measurements behind the port side rudder is presented in figure 4.
Influence of water depth to draft ratio on the flow generated by propellers
The jet produced by the propeller corresponds to the forces generated on the hull.
During the maneuvers in restricted waters, due to the interaction effects, the effective forces generated on the hull are dependent on the restrictions of the area and due to very complicated, turbulent flow and scale effects it is very difficult to determine the general model of the full scale interactions [11, 12, 13, 14].
The influence of the wall effect on the sway force and yawing moment is different in the berthing and unberthing cases. During both berthing and unberthing in shallow water condi-tions the strong suction is induced by the propeller jet. It results into an attraction between the stern and wall during the berthing and attraction along the whole length of the hull during unberthing [12].
The results of previous investigations of bollard pull [12] show the significant influence of water depth to draft ratio and the distance to the berth on the effective forces generated on the hull by working propellers.
The measured surge force for the ship in bollard pull conditions is presented in figure 5.
Fig. 5. Surge force induced on the hull due to propellers action as a function of ship-berth distance compared to longitudinal force induced in unrestricted waters in bollard-pull condition Rys. 5. Siła wzdłużna indukowana na kadłubie przez pędniki w zależności od odległości od nabrzeża porównana z siłą na wodzie otwartej zmierzoną na uwięzi
The water depth to draft ratios h/T = 1.2 and
h/T = 3 were assumed as shallow and deep water
conditions accordingly.
The distance to the berth is defined as follows (1):
B
b (1)
where: – distance between the wall and model centreline, B – model breadth.
Nondimensional force is defined by Equation 2:
T gL Fx Fx 2 5 . 0 ' (2)
where: Fx – measured surge force, – water density, g – acceleration of gravity, L – model length between perpendiculars, T – model draft.
The influence of water depth on the propeller jet is presented in figures 6 and 7. The assumed control surface for the calculation of the volume’s velocity of the jet’s flow rate [m3/s] was the circular area of the diameter equal to the maximum range of the pressure probe used for the measurements 0.304 m. Propeller pitch and corresponding rate of turn for the propeller settings are given in table 2.
Table 2. Propeller settings Tabela 2. Nastawy pędników
Propeller setting pitch rpm [o] [1/min]
Dead Slow Ahead 5 818
Slow Ahead 10 953
Half Ahead 15 1025
Full Ahead 19 1025
Volume of flow rate of propeller jet in depen-dence of water depth, propeller pitch and corres-ponding propeller rate of turn given in table 2, measured behind the rudder is presented in figure 6. The significant dependence of water depth to draft ratio on the velocity field measured behind the rudder can be observed for higher propeller settings. The dotted lines approximating the w(H) functions are plotted for 95% uncertainty level.
The observed 18% increase of the volume of flow rate in shallow water corresponds to 40% increase of the forces and moments generated on the rudder.
Rotation speed of the propeller plays the most important role in formation of the propeller jet flow [9]. The observed influence of propeller revolutions on the axial velocity field is stronger in shallow water conditions. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 0,5 1 1,5 2 b F x' x 1 0 3
Slow Ahead shallow water Dead Slow Ahead shallow water Slow Ahead open water Dead Slow Ahead open water Slow Ahead deep water Dead Slow Ahead deep water
0,0 0,2 0,4 0,6 0,8 1,0 1,2 0,5 1 1,5 2 b F x' x 1 0 3
Slow Ahead shallow water Dead Slow Ahead shallow water Slow Ahead open water Dead Slow Ahead open water Slow Ahead deep water Dead Slow Ahead deep water
0,0 0,2 0,4 0,6 0,8 1,0 1,2 0,5 1 1,5 2 b F x' x 1 0 3
Slow Ahead shallow water Dead Slow Ahead shallow water Slow Ahead open water Dead Slow Ahead open water Slow Ahead deep water Dead Slow Ahead deep water
Fig. 6. Volume of flow rate of propeller jet in dependence of water depth, propeller pitch and corresponding propeller rate of turn given in table 2, measured behind the rudder, position of the control surface at the distance 0.41 Dp from the rudder
trailing edge, 0.81 Dp form the propeller disk
Rys. 6. Wydatek strumienia zaśrubowego w zależności od głębokości wody, skoku śruby i obrotów podanych w tabeli 2, zmierzony za sterem. Położenie powierzchni kontrolnej 0,41 Dp od krawędzi steru, 0,81 Dp od płaszczyzny śruby
The influence of rudder deflection on the mean axial velocity within the same circular control surface show the higher decrease of the velocity in this area in shallow water conditions (Fig. 7).
Fig. 7. Volume of flow rate of propeller jet for Full Ahead settings, in dependence of water depth and rudder angle, measured behind the rudder, position of the control surface at the distance 0.41 Dp from the rudder trailing edge, 0.81 Dp
form the propeller disk
Rys. 7. Wydatek strumienia zaśrubowego dla Całej Naprzód w zależności od głębokości wody i kąta wychylenia steru, zmierzony za sterem. Położenie powierzchni kontrolnej 0,41 Dp od krawędzi steru, 0,81 Dp od płaszczyzny śruby
Distribution of propeller jet flow velocity components behind the rudder of twin propeller twin rudder ferry
The 3D measurements of velocity field allowed to study the distribution of velocity components: axial (vx), horizontal (vy), vertical (vz), tangential (vt) and radial (vr) within the assumed control area. The visualization of the measured axial velocity (vx), for deep water conditions and zero rudder angle is presented in figure 8.
The axial component has the velocity magnitude 1.8 m/s which correspond to 7.2 m/s in full scale. This magnitude is higher than magnitudes of other components. The horizontal and vertical
compo-nents magnitudes are 10% and 22% of the axial velocity. The tangential and radial components magnitudes are both 10% of the axial velocity magnitude.
The higher and lower jets observed for a single propeller single rudder ship showed in figure 2 are shifted horizontally from the propeller axis due to the part-tunnel effect of the aft body form which partly separates thrust streams from two propellers (Fig. 9).
This results in additional sway force and yawing moment generation when the propellers do not produce the equal thrust.
Shallow water effect results in the similar shape of the area of higher velocities. However the area is widen in both horizontal and vertical directions and slightly shifted towards the ship centre line and base plane accordingly.
The effect of 35 helm to port is the movement of the propeller jet to the port side and smaller area of higher velocities observed within the control surface.
The horizontal velocities distribution show the area of highest negative velocities (port side direc-tion) with the magnitude 0.2 m/s which corresponds to 0.8 m/s in full scale. The area is placed under the propeller axis, mostly close to the propellers vertical centre plane.
The vertical velocities distribution show two areas of highest velocities of opposite – up and down directions placed on the right and left sides of propeller axis accordingly.
The area of the maximum positive vertical components is moved from the vertical propeller axis in the direction of ship centre line and the
0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0 5 10 15 20 25 30 d [o]35 W [m3/s]
deep water shallow water 0 0,02 0,04 0,06 0,08 0 5 10 15 H [o] 20 W [m3/s]
deep water shallow water
Fig. 8. Axial velocities (vx) distribution measured behind
the rudder trailing edge at the distance of 0.81 Dp form the
propeller disk
Rys. 8. Rozkład prędkości osiowych pomierzony za sterem w odległości 0,81 Dp od płaszczyzny śruby
highest magnitudes are over the horizontal pro-peller axis.
The rotating effect of propeller causes the spiral movement of the flow, which boundary is the boundary of propeller slipstream with zero axial velocities. Ideally the radial velocity is positive above the propeller axis and negative below it. The observed irregularity of the radial velocity distribution is due to the influence of the tangential and axial velocities [9].
The largest rotation speeds induce the negative crest of radial velocities under the propeller axis, whereas, the medium rotation speeds produce the largest radial velocity at the propeller axis. This indicates the effect of strong turbulence within the jet flow [9].
The velocities in tangential direction can be about 40% of axial flow and induce considerable effects under the propeller in shallow water [9]. The tangential components in deep water conditions observed for the ferry model show values of about 10% of the axial velocities magnitude.
Model tests of jet flow generated by twin propeller twin rudder ferry
The flow field during unberthing was calculated using RANSE model, STAR CCM+ code [15]. The non-stationary model met the requirements of the real nature of the flow. The aft part of a hull form is presented in figure 9. The polyhedral mesh of about one million cells was generated. The view of the mesh in the aft part of a hull is presented in figure 10.
Fig. 9. The aft part of a hull form of the model Rys. 9. Część rufowa kadłuba modelu
The CFD computation of flow field allows to predict the time and space distribution of the magnitudes of pressures and velocities over sea
bottom and quay wall for different procedures of berthing manoeuvres.
Fig. 10. View of the mesh in the aft part of a hull form Rys. 10. Siatka obliczeniowa w rufowej części kadłuba
Flow visualisation for unberthing (port and starboard propeller settings Slow Astern and Slow Ahead accordingly) is presented in figure 11.
Fig. 11. CFD prediction of streamlines in the aft part of the car-passenger ferry during unberthing. Distance from the berth 0.1 ship breath, water depth to draft ratios h/T = 1.2
Rys. 11. Prognoza linii prądu w rejonie rufy promu podczas odchodzenia od nabrzeża. Odległość od nabrzeża 0,1 szero-kości statku, bezwymiarowa głębokość h/T = 1,2
The distance from the berth wall and centre line of the ship model is 1.07 m, which corresponds to 0.1 ship breadth from the ship side, distance between the edge of the model transom stern and berth in longitudinal direction is 0.75 m, the depth to draft ratio is 1.2. The assumed propeller thrusts correspond to the values measured at bollard pull conditions.
Velocity distribution of the flow generated on the sea bottom during unberthing is presented in figure 12.
The maximum velocities over the sea bottom calculated for the model correspond to about 3 m/s in real scale. The calculated velocities over the
surface of the quay wall reach 0.57 m/s for the model and 2.3 m/s for the ship.
Fig. 12. Velocity distribution of the flow generated during unberthing on the sea bottom. Distance from the berth 0.1 ship breath, water depth to draft ratios h/T=1.2
Rys. 12. Rozkład prędkości przepływu na dnie akwenu generowany podczas odchodzenia od nabrzeża, bezwymiarowa głębokość wody h/T = 1,2
The flow over the sea bottom calculated for Half Ahead / Half Astern settings of propellers in real scale has the magnitude equal to 5.6 m/s and near the quay wall 4.7 m/s.
Conclusions
The objective of this study was to identify the mean velocities produced by the propeller of twin propeller twin rudder ferry and the variations in the axial, radial and tangential velocity components of the propeller jet including the rudder effect.
The main limitation of the physical measure-ments of propeller wash is the size of the propeller – the diameter of the model of controllable pitch propellers used in tests corresponded to 5.104 m in full scale. Consequently experiments of the propeller jet velocity distribution are carried out using scale models and the prediction of the full scale values is limited by the scale effects.
The tests confirmed the general assumptions of the velocity distribution of propeller jet influenced by a rudder and the aft body form. The practical application of the presented results is the prediction of the influence of thrust streams generated by the self-manoeuvring vessel on the sea bed scouring.
The axial velocities of propeller jet have the highest mean values and magnitudes and they are considered in scouring predictions, however for the big, high powered self-manoeuvring vessels, operating in shallow water conditions the radial and tangential velocities at the exit of propeller jet are not neglectful. The tangential velocities can induce
considerable scouring effects under the propeller in shallow water [9].
The numerical calculations of the complex view of velocity field generated during berthing of the twin propeller twin rudder ferry can be further used in reliability analysis of quay wall and bottom protection structures.
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Acknowledgements
The paper presents a part of results of the research projects sponsored by Polish Ministry of
Science and Higher Education: 4 T12C 01029 “The influence of the berth type and water depth on efficiency of the steering and propulsion devices during ship berthing and unberthing” and N N509 293635 “Safety of berthing of ships in the Motorway of the Sea transportation system”, conducted at Gdynia Maritime University, Poland. The open water model tests were carried out in collaboration with Ship Design and Research Centre in Gdańsk, Poland and Shiphandling Research and Training Centre of Foundation for Safety of Navigation and Environment Protection in Iława-Kamionka, Poland.
