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Influence of Hydrodynamic Interaction Between Ducted Propellers and Struts on Performance of Azimuth Thrusters

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Influence of Hydrodynamic Interaction Between Ducted Propellers and Struts on Performance of Azimuth Thrusters Fifth International Symposium on Marine Propulsors smp’17, Espoo, Finland, June 2017 Influence of Hydrodynamic Interaction between Ducted Propellers and Struts on Performance of Azimuth Thrusters Isao Funeno1 1Kawasaki Heavy Industries, Kobe, Japan. ABSTRACT Since strong hydrodynamic interaction between ducted propellers and struts of azimuth thrusters is very complicate, it is not so easy to design optimally the ducted azimuth thrusters in spite of the importance. Both the shape and the location of struts affect performance of the ducted propellers such as inflow into propellers, increase of strut drag due to suction of propellers, steering moment of the units and others, which bear a strong relationship of performance tradeoff. In this paper, first in order to improve bollard thrust of the ducted azimuth thrusters, the interaction was discussed by using the Lagally’s theorem theoretically and the CFD (Computational Fluid Dynamics) technique numerically. Particularly effects of the strut cross-sections, relative position between propellers and struts were discussed. Next, in free running condition the strut shape should be Figure 1 Ducted azimuth thrusters. considered to avoid large-scale separation flow, which causes harmful propeller cavitation. The issues were former SMP the author presented the difficulty to obtain studied for changing the both shapes of propellers and optimal design of the ducted azimuth thrusters, in which struts using the CFD computations and the cavitation discussed mainly hydrodynamic interaction between the observations. Finally, the concerns about increase of the duct and the gear housing (Funeno, 2009). unit steering moments and the unit installation onto On the other hand, it is also very important to consider vessels due to larger distance between ducted propellers hydrodynamic interaction between the ducted propeller and struts were discussed in order to design the optimal and the strut of the azimuth thruster. Then, we have to units. study hydrodynamically optimal shape of the struts within mechanical constraints. Of course the thrusters should provide excellent propulsive performance in bollard Keywords conditions as well as free-running conditions. However, Ducted azimuth thruster, Propeller, CFD, Cavitation, there are little references about the hydrodynamic Steering moment. interaction. In this paper the author presents basic concept and 1 INTRODUCTION investigation using CFD in detail in order to obtain larger Ducted azimuth thrusters are very useful propulsion units bollard thrust and higher propulsive efficiency of the on providing those advanced maneuverability and higher ducted azimuth thrusters. Further cavitation erosion risk bollard thrust to such support vessels – tugboats, platform in free-running conditions due to low water speed region, supply vessels, cable layers, drill ships and others. wake behind the strut and the countermeasures are However, it is very difficult to design optimally ducted discussed. Additionally, optimal location between the azimuth thrusters from both hydrodynamic performance ducted propeller and the strut to avoid excessive steering and mechanical viewpoints. As one of the reasons, the moment of the ducted azimuth thrusters and those ducted azimuth thrusters are very complicated and installations onto vessels are discussed and finally consisted of many components such as propeller, duct, conclusions are described. gear housing, strut and others as shown in Figure 1. In the 2 INFLUENCE OF STRUT IN BOLLARD PULL CONDITIONS f/fo m/mo a/ao 2.1 Basic concept in bollard pull condition 1.5 Considering hydrodynamic interaction between a strut incorporating L-drive or Z-drive of power transmission 1.0 system of an azimuth thruster and a rotating propeller behind the strut, the interaction can be almost regarded as an analogy with that between a ship hull and a rotating Fx/Fxo 0.5 propeller behind the stern, in brief an issue of thrust deduction fraction in propulsion tests. The basic 0.0 mechanism concept of thrust deduction can be explained 0.0 0.5 1.0 1.5 2.0 simply using Lagally’s theorem (Milne-Thomson, 2011). f/fo, m/mo, a/ao As with the concept, on the assumption of bollard pull condition of the azimuth thruster, consider simply the Figure 3 Lagally’s force with each nondimensionalized force exerted on a circular cylinder by a sink. Taking factors. Figure 2 with the sink of the strength of ‘m’ at A and the can obtain simply the pressure distribution using the circular cylinder with the radius of ‘a’ at O on the x-axis, velocity potential derived from the complex potential, then the complex potential w(z) is described in the Equation (1) and Bernoulli’s theorem. The pressure is following Equation (1) with the complex variable z, nondimensionalized by the following equation, 푤(푧) = 푚log(푧) − 푚log(푧 − 푓) − 푚log(푧 − 푓′) (1) ∆푝 퐶푝 = 푚 2 (3) 2 1휌( ) Where f’ = a / f =OB and f=OA. 2 푎 Therefore the cylinder is attracted towards the sink with Where Δp: differential pressure. Further the drag the following horizontal force Fx, component of pressure, Cpx is defined as, 2휋휌푚2푎2 퐶푝푥 = 퐶푝 cos훼 (4) 퐹푥 = 2 2 (2) 푓(푓 −푎 ) Figure 4 and 5 show distribution of the pressure and the In that case, conceptually m, a and f can correspond with drag component with the circumferential position, α on the propeller thrust loading, the curvature radius of a strut the circular cylinder by parametrically changing the rear end and the distance between the propeller and the distance f/a respectively. From these figures, it is found strut respectively. Next consider how these factors that the peaks of negative pressure and drag component contribute to Fx. Figure 3 shows the change of non- are shifting from about 40 degree in circumferential dimensional force Fx/Fxo with regards to these non- toward 0 degree, the rear end of circular cylinder with dimensional factors for easy to understand the effect of decreasing the distance f/a. Hence it is very important to the factors on the force. Where, Fxo is Fx at m=a=1 and consider the optimal cross-sectional shape in the vicinity f=2, both a/ao and m/mo are ratio to each unit quantity and of the rear end of strut intensively so that its curvature f/fo is ratio to twofold quantity. From Figure 3, it is radius should be as small as possible to take account of inferred by analogy that the force exerted on the strut by both the diameter and the horizontal location of the the rotating propeller, the induced drag on the strut can be vertical drive shaft inside the strut. increased steeply by the increased propeller thrust In addition, approaching the propeller toward the strut loading, the increased curvature radius of the strut rear rear end makes more efficiently the propeller thrust end and the decreased distance between the propeller and increased due to low induced water speed behind the strut, the strut. the wake gain. However, the strut drag could be increased In more details, consider the pressure distribution on the considerably due to the increased propeller thrust loading surface of the circular cylinder exerted by the sink. We y f/a=1.4 f/a=1.6 f/a=2.0 f/a=2.4 f/a=2.8 0.0 r -2.0 α -4.0 a O B A Cp x m f’ -m -m -6.0 f -8.0 -10.0 0 30 60 90 120 150 180 alpha (deg) Figure 2 Arrangement of circular cylinder and sinks. Figure 4 Pressure distribution on circular cylinder with f/a. f/a=1.4 f/a=1.6 f/a=2.0 the ducted propellers and the strut rear ends were changed f/a=2.4 f/a=2.8 from the original ones. Figure 6 shows the CFD results of 10.0 pressure distribution around the thrusters on the 8.0 horizontal cutting plane at 0.7R above the propeller shaft centers. In these cases the propeller wash direction is from 6.0 left to right. The pressure is nondimensionalized by the 4.0 following pressure coefficient, Cpn, Cpx (푝−푝 ) 2.0 퐶푝푛 = 표 (5) 1휌(푛퐷)2 0.0 2 -2.0 Where, p: static pressure, po: standard pressure, ρ: water 0 30 60 90 120 150 180 density, n: propeller revolution, D: propeller diameter, R: alpha (deg) propeller radius. The both strut sections are called “Type A” and “Type B”, which have relatively larger and Figure 5 Distribution of drag component of pressure with smaller curvature radius of the strut rear ends f/a. respectively. as mentioned in Equation (2) and then the unit thrust Figure 6-(a) shows the pressure distribution in case of the could be decreased totally. section “Type A” close to the ducted propeller. The intensive negative pressures, which cause the induced 2.2 Discussion with CFD Results drag of struts, occur in the both side regions at about one- More specifically, the above mentioned discussion should quarter strut length far from the strut rear end. Figure 6- be verified from flow simulation using the advanced (b) shows that in case of the same section far from the viscous CFD (Computational Fluid Dynamics) technique ducted propeller. The negative pressures around the strut based on the unstructured grid method so as to deal with rear end are considerably weakened by distancing the complicate geometries such as the ducted azimuth ducted propeller from the strut rear end. Next Figure 6-(c) thrusters. The computational method was almost the same shows that in case of the section “Type B” close to the with that described in (Funeno, 2009). But the versatile ducted propeller. The negative pressure region on the both commercial CFD software, STAR-CCM+ by CD-adapco side around the strut rear end are relatively weakened to was used for these computations.
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