AN EXPLORATORY STUDY ON THE WORKING PRINCIPLES OF ENERGY SAVING DEVICES (ESDS) PIV, CFD INVESTIGATIONS AND ESD DESIGN GUIDELINES

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1 Proceedings of the 3 st International Conference on Ocean, Offshore and Arctic Engineering OMAE22 July -6, 22, Rio de Janeiro, Brazil OMAE AN EXPLORATORY STUDY ON THE WORKING PRINCIPLES OF ENERGY SAVING DEVICES (ESDS) PIV, CFD INVESTIGATIONS AND ESD DESIGN GUIDELINES Jie Dang Maritime Research Institute Netherlands (MARIN) 2, Haagsteeg, 678PM Wageningen The Netherlands Guoxiang Dong Shanghai Ship & Shipping Research Institute 6, Minsheng Road, Shanghai 235, China Hao Chen Guangzhou Shipyard International Company LTD 4, S. Fangcun Road, Guangzhou 5382, China ABSTRACT The Maritime Research Institute Netherlands (MARIN) has recently started a Joint Industry Project (JIP) called ESD- JILI, investigating the working principles of energy saving devices (ESDs). Within the framework of this JIP, three ESDs have been selected and thoroughly investigated. They are a Pre- Duct with an inner Stator (PDS), a Pre-Swirl Stator (PSS) with asymmetric blade design and Hub Fins (HFs). The investigations have been carried out by using dedicated force and moment sensors to measure all the components of the ESDs independently during the propulsion tests, by using Particle Image Velocimetry (PIV) to measure the flow before, in-plane and behind the propeller and the ESDs, and by using Smart Ship Model technology (Wijngaarden 2) to simulate the full-scale wake field during the model tests to gain insights on scale effects of the ESDs. At the same time, computational fluid dynamics (CFD) calculations are also carried out in order to further deepen the understanding of the working principles of the selected ESDs, and to assist the ESDs designs under certain guidelines. Some of the results of the study have been published to the Greenship 2 Conference (Dang et al 2). The flow details around the propulsion system with the PDS, which were obtained by both PIV measurements and CFD calculations, and the energy balance of the ship-propulsion system with a PDS are further investigated and reported here. Based on the results of the studies, the principle guidelines for ESD designs for single screw merchant ships have been proposed. KEYWORDS Energy Saving Devices, Propulsion, Efficiency, PIV, CFD INTRODUCTION With the oil price skyrocketing and the implementation of EEDI and EEOI approaching, the issue of green ships has drawn wide attentions again among operators, owners, builders, classifications, shipping regulators, harbour regulators, and governments. As one of the major driving forces on reducing bunker cost and pollution, energy saving issues are becoming more and more important in recent years. 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 (Schneekluth 986, Stierman 987). In the last decades, many new ideas and patents have been proposed, which are tested in towing tanks around the world (Hansen 2, Lee et al 992, Mewis 2, etc.). Several of the ideas are fitted to real ships and tried at full scale. Significant improvements on total propulsive efficiency of the vessel have been claimed. At the same time, doubts exist about the real efficiency gains achieved by fitting those devices, due to the fact that some sea trial measurements do not show any improvement directly, although large efficiency gains have been measured in model scale. It is always difficult to judge sea trial results because the efficiency gains by the ESDs are often in the same order of magnitude as the uncertainties of the sea trial measurements. In addition to this, the scale effects of the ESDs can be rather large, because most of the ESDs are fitted to a place in the boundary layer of the vessel which differs from that in model scale. This is what is often called Reynolds scale effects. Making the situation even worse, no standard extrapolation method, like the ITTC 978 method, is able to take the details of a specific ESD into consideration yet. Copyright 22 by ASME

2 In order to help the shipbuilding industry to understand the working principles of the ESDs and to develop a proper extrapolation method for the model test results, MARIN initiated a Joint Industry Project (JIP) called ESD-JILI which focuses on the working mechanism of a selection of ESDs. Three types of ESDs have been investigated (Dang et al 2) which are considered to be the most promising ESDs for full block single screw merchant ships. They are, - a Pre-Duct with an inner Stator (PDS); - a Pre-Swirl Stator, asymmetric blade design (PSS); - Hub Fins (HFs). Investigations using four different techniques have been carried out, which include, - force and moment measurements of all components of the ESDs and the propeller (Figure ); - Particle Image Velocimetry (PIV) flow measurements around the ESDs (Figure 2 shows the positions); - Smart Ship Model technology to simulate the fullscale wake in model scale tests; - CFD calculations looking into the details of the flow around the ESDs. The results of the study have been partly published to the Greenship 2 Conference (Dang et al 2). It is found from the propulsion tests that the HFs, the PSS and the PDS can improve the total propulsive efficiency of the subject oil tanker by about 2%, 4% and 6%, respectively, both tested with the original model as well as with the Smart Ship Model (meaning in the simulated full scale wake). It is also found that the PDS suffers strong scale effects while the PSS not. The PDS thrust is reduced significantly in the simulated full-scale wake by the Smart Ship Model, compared to the thrust measured behind the original model. However, the power saving by the PDS remains at the same level of about 6%. Figure The sensors used to measure forces and moments With respect to the working principles of the PDS, further studies have been carried out by investigating the flow details and the energy balance of the ship-propulsion system fitted with a PDS, both by PIV measurements and also by CFD calculations. Figure 2 The location of the PIV measurement planes Computational Fluid Dynamics (CFD) The newest state-of-the-art CFD code developed at MARIN is ReFRESCO. ReFRESCO is an acronym for Reliable and Fast RANS Equations (solver for) Ships, Cavitation and Offshore. As the name indicates, the intention of ReFRESCO is to be able to solve all possible hydrodynamic problems encountered in naval designs. It is possible to model complex geometry, like complete shaft lines or energy saving devices. ReFRESCO has already been applied to and validated for propeller flows, flow around complex ships and submarine maneuvering. ReFRESCO solves the Reynolds Averaged Navier-Stokes (RANS) equations for steady or unsteady single or multiphase flows. It employs cell-centered finite-volume discretization and implicit time integration. It can handle structured and unstructured grids with arbitrary cell shapes and hanging nodes. The code is parallelized using MPI (Message Passing Interface) combined with sub-domain decomposition and runs on the private cluster of MARIN. At the inflow of the domain a constant velocity that equals the ship speed is imposed. At the left, right, bottom and outflow of the domain a zero pressure gradient is imposed. All simulations are double body, therefore at the symmetry plane a symmetrical boundary condition is imposed. An advanced method for determining the effective wake field is a coupled approach between ReFRESCO and PROCAL. PROCAL, which stands for PROpeller CALculation is a Boundary Element Method (BEM) code used for propeller analysis and cavitation prediction, developed within the framework of Cooperative Research Ships (CRS). The following is an example of the surface grid of the shippropulsion system with a PDS used for the calculations for the present project. Due to the limitation of the length of the paper, only limited CFD results have been presented in this paper in order to illustrate the ability of the CFD on calculating the details of the flow and its good correlation to the PIV results. No results regarding the coupling of ReFRESCO and PROCAL have been shown, which will be published in the near future. 2 Copyright 22 by ASME

3 Table Original model - shaft power reduction and rotational rate changes, w.r. t. the case without ESDs with PDS with PSS Vs PB Ns PB Ns [kn] [kw] [RPM] [-] [-] % -2.83% -.85% -4.% % -3.5% -3.5% -4.78% 4-5.9% -3.% -4.7% -5.% % -3.2% -4.38% -5.3% 6-8.4% -4.% -6.38% -5.76% Figure 3 An example of the grid mesh for the PDS RESULTS AND DISCUSSIONS Energy Saving, ESD Thrust and the Thrust Deduction In order to get reliable test results, all propulsion test runs were carried out on one day with a well-controlled waiting time in between. The tests were done by fitting ESDs one by one to the ship model by divers without disconnecting the ship model from the carriage (Dang et al 2). The reference propulsion test without any ESD is used as the basis for the comparisons and discussions. The power savings of the PDS and the PSS, with respect to the case without any ESD, are given in Table and 2 for the original ship model and the Smart Ship Model, respectively. Also listed in the tables are the changes of the shaft rotational rates. It is seen that the PDS has somewhat higher power saving than that of the PSS, while both of them are not strongly affected by the changes of the wake fields from the model scale wake of the original ship model to the simulated full-scale wake by the Smart Ship Model. Meanwhile, both the PDS and the PSS reduce the rotational rates of the propeller, resulting in a heavy running propeller behind the ESDs, where the PSS reduces the propeller rotational rates more than that of the PDS, making the PSS less favorable for retro-fit. Since the reduction of the rotational rate of the propeller will reduce the friction losses on the propeller blades, small parts of the power reduction in Table and 2 are from the propeller itself, instead of the ESDs. If the propeller shaft rotational rate is corrected by reducing the propeller design pitch, the power saving will be reduced too. However, the effects will be rather small for both the PSS and the PDS. The details of the measured ESD thrusts and the influence of the wake scaling on the thrusts have been discussed in detail by Dang et al 2. It has been found that the duct and the stator of the PDS generate about 8% and 2% extra thrust in the original ship model wake, respectively, in addition to that of the propeller. These thrusts are reduced to about only 2% and %, respectively, in the simulated full-scale wake by the Smart Ship Model. However, the propulsion tests show 6% power saving on the propeller shaft for both test cases. Table 2 Smart Ship Model - shaft power reduction and rotational rate changes, w. r. t. the case without ESDs with PDS with PSS Vs PB Ns PB Ns [kn] [kw] [RPM] [-] [-] % -2.4% -3.86% -4.69% % -2.32% -4.6% -4.74% % -2.64% -4.46% -4.67% 5-6.3% -2.7% -4.3% -4.47% % -2.37% -4.35% -4.4% When studying the details of the propulsion components such as the thrust deduction and the increased ship resistance with fitted ESDs, the following results have been obtained, as given in Table 3 and Table 4 for the original ship model and the Smart Ship Model, respectively. Please note that the ESDs are treated as part of the propulsor here, but not the appendages of the ship. The thrust deduction analyzed here is based on the total thrust of the propeller plus the ESDs. Due to the thrust deduction, the resistance of the vessel increases when a propulsor is operating. When a vessel is propelled at a constant speed forward, the total propulsor thrust equals the resistance of the vessel plus the suction effect of the propulsor. By fitting the ESDs, the suction effect of the propulsor (including the ESDs) is increased and therefore the resistance of the vessel is increased too. The resistance increase R due to fitting the ESDs can be defined as, () where T p is the total thrust of the propulsor including the ESDs. In addition to the resistance increases by the ESDs, the thrusts of the PDS and the PSS, T_PDS and T_PSS in the tables respectively, are also listed in the tables. It is seen from the results that the thrust deduction fraction t is increased significantly due to fitting either the PDS or the PSS. The positive thrust (T_PDS in the table) generated by the PDS is not able to compensate the increase of the total resistance R. For the case of the PSS, the PSS even generates negative thrust (T_PSS in the table), meaning additional drag. This means that the thrusts generated by the ESDs are partly or completely lost to the thrust deduction. No net thrust gains are found either for the PDS or for the PSS cases. These results provide important information on the shippropulsor interactions, i.e. that the thrust or the drag generated by the ESDs do not provide direct information on whether or not a specific ESD can result in energy saving. This may also 3 Copyright 22 by ASME

4 indicate that during the design of the ESDs, trying to optimize the geometry of the ESDs by using CFD tools and aiming to achieve the highest positive ESD thrust may not automatically guarantee that the designed ESDs will save energy. On the other hand, if no positive thrust can be found by CFD calculations, such as for the PSS and also often found on the wake equalization ducts (WED), it may also not be concluded that these ESDs do not result in power saving. Table 3 Original ship model thrust T, thrust deduction t and the increase of resistance R by fitting ESDs without ESD with PDS with PSS Vs t t T_PDS R t T_PSS R [kn] [-] [-] [kn] [kn] [-] [kn] [kn] Table 4 Smart Ship Model thrust T, thrust deduction t and the increase of resistance R by fitting ESDs without ESD with PDS with PSS Vs t t T_PDS R t T_PSS R [kn] [-] [-] [kn] [kn] [-] [kn] [kn] In order to judge the efficiency gains or losses by fitting an ESD, the energy balance of the flow before and behind the propeller and ESDs must be investigated thoroughly, in addition to understanding the forces and moments on the ESDs. This will be discussed in detail for the PDS in the following sections, by using the PIV and CFD results. Kinetic Energy Losses in the Wake As early as in 92 s, people realized that the thrust deduction could be actually a kind of internal force of the shippropulsion system and might not generate additional losses in the flow (Fresenius 92). This could explain the findings in the last section on the positive ESD thrust and the increase of the thrust deduction, which does not result in power losses on the shaft but savings. However, we understand now that only the potential part of the thrust deduction is a purely internal force while the viscous part does generate energy losses by reducing the total pressure head along a streamline and converting it into heat as energy losses (Korvin-Kroukovsky 956, Tsakonas et al 96). This losses is finally shown as flow kinetic energy losses when the static pressure is fully recovered in the far field. It is hence important to investigate the kinetic energy losses in the wake flow in order to understand the energy saving of an ESD. Dyne (995) elaborated in detail on the energy balance of a ship-propulsion system and identified different kinds of energy gains and losses in the far wake of a propelled ship, although it was incomplete and left a lot of questions to be answered in the author s opinion. An optimal propulsion system is a system which can re-gain all the kinetic energy losses in the wake a so-called fully filled wake condition. At this ideal condition, the deficit of the velocity in the ship s wake will be completely filled by the flow acceleration by an ideal propulsor so that after the ship-propulsion system the total kinetic energy losses will be zero. The power delivered into the system equals then the effective power of the vessel and the total ship-propulsion system reaches the highest efficiency. Since the acceleration of the flow by the propulsor (in order to generate thrust), which results in the ideal losses of a propulsor, is used to fill in the velocity deficit of the ship s wake in behind condition, the total propulsive efficiency of a ship-propulsion system d is always higher than the propeller open water efficiency for a single-screw vessel. The hull efficiency h can be actually explained as the regaining of the ideal losses of a propulsor. Bearing in mind that to optimize the ship-propulsion system is to minimize the kinetic energy losses in the far wake of a ship-propulsion system while maintaining the momentum balance (total thrust equals total resistance), the assessment of the energy saving value of ESDs can be done by studying the kinetic energy in the far wake, measured by PIV and/or calculated by CFD. Herewith we will focus our study only on the PDS, partly because the highest powering savings have been measured by the propulsion tests for the present vessel, and partly because rather strong scale effects have been found on the thrust of the PDS (Dang et al 2). In addition, looking into the details of this complicated ship-propulsion system may lead to a better understanding of the working principles of the energy saving devices in general. Let s define the non-dimensionalized axial kinetic energy going through plane S w in the wake per unit time as, and the non-dimensionalized transverse kinetic energy going through the same plane S w in the wake as, where V a, V t and V r are the 3-D wake velocity components with respect to the ship fixed coordinates in the axial, tangential and radial directions, respectively; V s is the ship speed; and is the wake cross plane on which the kinetic energy of the wake flow behind the ship is integrated, for both cases with and without the operating propeller. Based on this definition, when the far wake of the ship is completely filled by the slipstream of the propulsor, not more and not less, there will be no kinetic energy left in the wake and the total kinetic energy, (4) will be.. When the wake is not fully filled (both under- or over-filled), the value will be larger than.. The higher the value is, the more kinetic energy losses in the wake will be. Fully-separated flow to the far wake, resulting in or negative (2) (3) 4 Copyright 22 by ASME

5 Va/Vs z/r z/r V a, is not considered here. Before we study the more complicated situation of an operating propeller with ESDs, the kinetic energy in the nominal wake fields on the propeller disc area of the vessel, with and without the PDS, is studied first Va: Vs y/r Figure 4 The PIV measurement results on Plane PROP at 5 knots, without propeller, without ESDs, original ship hull.5 Va: include the entire ship wake. In addition, due to the shadow of the propeller dummy hub, part of the wake field on the starboard side has not been measured. On each of the colored areas in Figure 4 and Figure 5, the 3-D velocity fields at,55 individual points have been sampled and used for the calculation of the kinetic energy on the colored area in the wake. The results are listed in Table 5 for both the wake without ESD and the wake with a PDS. Table 5 Comparison of kinetic energy levels on Plane PROP (see Figure 2) in the ship s wake, with and without ESDs (original hull model, without working propeller, at 5 knots) without ESDs with PDS relative difference [%] K ax % K tr % K total % Table 5 shows that the major kinetic energy losses in the wake of the ship are the axial kinetic energy (.46), which is about 2 times larger than the transverse (rotational) kinetic energy (.246). By fitting a PDS to the vessel, both the axial kinetic energy and the transverse kinetic energy are increased, where the increase of the transverse kinetic energy is higher than that of the axial kinetic energy. This could be due to the combined effects of the duct and the stator where the duct accelerates flow in the wake peak and the stator changes the swirl of the flow, including also the friction of the duct and stator which results in additional total pressure head losses in the flow along streamlines. In total, the kinetic energy in the wake is increased by about 2.34%. Seeing that the measured area is much smaller than the total wake area of the ship, the amount of kinetic energy increase by the PDS may result in less than % of the vessel resistance increase, which is in line with the resistance tests (Dang et al 2) r/r=.8 r/r=.7 r/r=.6 nominal wake without ESD nominal wake with PDS r/r=.8 r/r=.7 r/r= Vs y/r Figure 5 The PIV measurement results on Plane PROP at 5 knots, without propeller, with PDS, original ship hull The wake field at the propeller disc plane without an ESD has been measured by PIV and is shown in Figure 4, while the nominal wake with a PDS in the upstream is shown in Figure 5. In both measurements, the same measuring area has been used, which is slightly larger than the propeller disc but does not.4.3 r/r=.8.2 r/r= Angular position [ o ] Figure 6 Axial nominal wake distributions at.6r,.7r and.8r, with and without a PDS. Also shown in Figure 4 and Figure 5 are the propeller disc (the circle with the solid line) and the inner diameter of the PDS duct (the circle with the dashed line). The acceleration of the flow by the duct is clearly seen, such that the axial velocity 5 Copyright 22 by ASME

6 in the wake becomes more uniform, especially in the inner radii. This can also be seen in Figure 6 where the axial components of the flow on 3 key radii of the propeller are plotted, where 8o angular position corresponds to the 2 o clock position. When a propeller is operating in the downstream of a PDS, a more significant accelerating effect of the duct can be expected. The uniformization of the wake by the PDS may result in some axial kinetic energy reduction, meaning energy saving, as proposed by van Lammeren (949) and Schneekluth (986) and well-known as wake equalizing effects..5 Vs Va: In the same way as for the nominal wake field, the kinetic energy levels behind the operating propeller, including the slipstream of the propeller are also evaluated by using the PIV measurements, as shown in Figure 7, Figure 8 and Figure 9 for the cases without an ESD, with the PDS and with PDS plus the HFs, respectively. The wake fields have been measured by triggering the propeller angular position, but only the time averaged values are shown and plotted here for the present paper. For each of the measurements, the 3-D velocity fields have been measured at 49,532 individual points in the colored area in the figures, which has the same size as for the nominal wake field shown in Figure 4 and Figure Vs Va: z/r z/r y/r - Figure 7 PIV measurement results on Plane BH at 5 knots, orignal ship model, with operating propeller, without ESD. Va: z/r y/r Figure 9 PIV measurement results on plane BH at 5 knots, orignal ship model, with operating propeller, with PDS + HFs..5 Vs -.5 y/r Figure 8 PIV measurement results on plane BH at 5 knots, orignal ship model, with operating propeller, with PDS. By carrying out the integration over the colored area in the figures, the kinetic energy levels in the wake of the vessel, including the slipstream of the propeller, are obtained and compared in Table 6. When comparing the kinetic energy in the wake with an operating propeller but without an ESD to that in the nominal wake (Table 5), we found that the axial kinetic energy is reduced slightly by the propulsor (from.46 to.424). This is because of the action of the propeller which increases the axial velocity in the wake ( fills the wake) behind the ship so that the total kinetic energy losses in the wake of the ship are changed (reduced). Looking at the velocity field behind the propeller (Figure 7), it is seen also that the wake is somewhat over-filled, leaving some new energy losses by the propeller into the wake. A more important observation is that the operating propeller results in significant increase of the transverse (rotational) kinetic energy in the wake (from.246 in Table 5 to.822 in Table 6). This swirl of the propeller 6 Copyright 22 by ASME

7 slipstream is clearly seen in Figure 7. The total kinetic energy in the wake is increased by the operating propeller from.76 in Table 5 to.246 in Table 6. These results show that we are still far away from the optimum (the fully-filled wake) and there is still significant room for improvement. Table 6 Comparison of kinetic energy levels on Plane BH in the wake of the ship, with and without ESDs (original ship model, with working propeller, at 5 knots). without ESDs with PDS with PDS + HFs [%] [%] K ax % % K tr % % K total % % On the other hand, the small reduction of the axial kinetic energy in the wake by the propeller reflects the so-called hull efficiency where the total system efficiency benefits from the wake of a single screw vessel. Should the propulsors be placed completely outside of the ship s wake, such as for a twin-screw vessel, the kinetic energy losses of the hull will not be reduced by the propulsor but at the same time the propulsors introduce more kinetic energy losses (the ideal losses ) in their own slipstreams, resulting in an increase of total kinetic energy losses behind the whole ship-propulsor system. When a PDS is fitted to the upstream of a propeller, significant reductions of both the axial and transverse (rotational) kinetic energies in the wake have been found (see Table 6). The total kinetic energy is reduced from.246 to. by more than 9%, where the axial kinetic energy is reduced by about 7.78% and the transverse kinetic energy is reduced by 25.79%. This is in line with the positive shaft power saving measured during the propulsion tests (Table ). This reveals also the working principles of the PDS where the PDS interacts with the ship stern and the propeller in such a favorable way so that the total kinetic energy losses in the wake are significantly reduced, mainly the transverse (rotational) kinetic energy. Furthermore, adding the HFs to the PDS-propeller system does not change the total kinetic energy level of the wake significantly (see last column in Table 6), but it does reduce it further by more than 6%, with the majority coming from the reduction of the transverse (rotational) kinetic energy. It is also clearly seen that in the nominal wake of the ship the transverse (rotational) kinetic energy level is only half of the axial kinetic energy level (Table 5). By operating a propeller in the wake of a vessel, the axial kinetic energy level is reduced slightly, but the transverse (rotational) kinetic energy level is increased significantly (Table 6). In the wake of a single screw ship with an operating propeller, the transverse (rotational) kinetic energy level becomes about 2 times larger than the axial kinetic energy level. This is different from our common knowledge based on the open water propeller operation where the transverse (rotational) kinetic energy losses is only a small portion of the total kinetic energy losses in its slipstream. Due to the wake filling effect, a large part of the axial kinetic energy losses in the wake of the ship are regained by the slipstream of the propeller (regaining the ideal losses ). In such a situation, the transverse (rotational) kinetic energy losses become relatively larger or even dominant in the wake left behind a single screw vessel. This may suggest that reducing the transverse (rotational) kinetic energy losses in the wake of the ship by using ESDs is the key task that can lead to significant power saving of the ship-propulsion system. Limitations The discussions above are based on the measured area, which is only a part of the whole wake of the ship (even with shadows in the nominal wake fields). The axial kinetic energy level outside the measured area is believed to be high and should not be ignored, which is unfortunately not covered during the PIV measurements. This part of the axial kinetic energy losses can only be regained by applying a larger propeller diameter or distributed propulsors, or by changing the hull lines so that more ship s wake is coming into the propeller disc, etc. The values in Table 5 and Table 6 are used to elaborate the working principles of the ESDs in a relative sense, and should not be used in general as absolute values. It should also be pointed out that the discussions above are based on the measurements either on the propeller disc Plane PROP or on the plane in the near downstream behind the propeller Plane BH, which differ from the far wake of the ship, where the static pressure is completely recovered. These discussions are only valid in a relative sense if the static pressure recovery, along the streamlines from the place where the PIV measurements were carried out to the far wake, are the same for all cases, with or without the ESDs. On the other hand, due to (numerical) dissipations, it may not be a bad idea to compare the kinetic energy levels in the near downstream of the ship-propulsion system rather than in the numerical far wake, as long as the static pressure is well calculated so that a fair comparison of the kinetic energy levels at that place can be carried out. Computational Fluid Dynamics (CFD) Computational Fluid Dynamic analysis has been proven to be a very powerful tool for naval hydrodynamics including complex flows around ESDs, not only for the analysis of the flow details including the pressure field (which cannot be obtained by model tests) in order to understand the working principles of the ESDs, but also in assisting ESD designs so that the ESDs can be designed to maximize the total propulsive efficiency of the ship. CFD simulations by using ReFRESCO are also used in the flow analysis for the present project, especially for the flow around the PDS. An example of the un-structured grids used in the calculations are plotted in Figure 3 in the previous sections. Figure and Figure show the calculated axial and transverse velocity fields, respectively, on the propeller disc in the nominal wake of the ship in model scale when there is no 7 Copyright 22 by ASME

8 ESD fitted. By comparing this calculated wake field to the PIV measurements shown in Figure 4, it is seen that the calculated wake field is almost identical to the measured ones where both are in the same model scale. the PDS as found in the PIV measurement. The only difference is the mean axial velocity on the propeller disc where the CFD shows a reduction while the PIV shows a small increase. The merit of using CFD, with respect to PIV, is that it reveals the flow details including the flow separation. Such as for the present PDS fitted to the stern and towed at 5 knots without a working propeller, flow separations have been calculated on the stator fins, as shown in Figure 4 where the reverse flow zones are plotted in dark blue colors. It is seen that the flow separations occur mainly at the connections between the inner ring of the duct and the tips of the stator fins. The largest separation is found on the portside where the angle of attack of the fin to the local flow is the highest, where the fins are trying to re-direct the transverse flow downward. Figure Axial nominal wake field on the propeller disc calculated by ReFRESCO, without ESDs. Figure 2 Axial nominal wake field calculated on the propeller disc by ReFRESCO, with PDS. Figure Transverse nominal wake field on the propeller disc calculated by ReFRESCO, without ESDs. When there is a PDS installed in the upstream of the propeller disc, the wake field is changed significantly, as shown in Figure 2 and Figure 3 for the axial and transverse fields, respectively. When comparing these calculated nominal wake field to the PIV measured ones as shown in Figure 5, it is seen that the major features of the wake field are well captured by the CFD. The vortex shedding on the port side of the propeller disc, at both 8 and o clock positions, and the re-directed transverse flows by the stator fins resemble the measured ones to a great extent. In the meantime, the variations of the wake field in the inner radii of the propeller are clearly reduced by Figure 3 Transverse nominal wake field on the propeller disc calculated by ReFRESCO, with PDS. 8 Copyright 22 by ASME

9 which penetrate the propeller and seen in the slipstream. The kinetic energy in the vortices, is not completely recovered by the propeller, leaving strong, concentrated vorticity in the slipstream of the propeller. This reminds us that while applying an ESD to get favorable interactions with the stern flow and the propeller so that the global kinetic energy losses in the wake behind the propeller are reduced, attention should be paid to possible unfavorable flow separations on the ESDs so that none or minimal amount of additional kinetic energy losses will be generated, which are not able to be recovered by the propeller. Proposed Principle Guidelines for ESD Design Figure 4 flow separation calculated on the stator fins. Besides that fitting the PDS upstream the propeller will generate additional kinetic energy losses (viscous part) due to friction, it is expected to generate also pre-swirl (potential part) that can be regained by the propeller. However, the potential part of the kinetic energy may also not be completely regained due to scale differences. This is seen in Figure 5. Figure 5 Vorticity based on PIV measurements at Plane BH (averaged), 5 knots, original hull form with PDS, calculated from the velocity field of Figure 8. Figure 5 shows obviously that the vortices shed from the tips of the stator fins at the 8 and o clock positions on the portside have much smaller sizes than the propeller blades, Based on the studies presented to the Greenship 2 (Dang et al 2) and from the present paper with further elaborations on the forces and moments measurements on the ESDs and the analysis of the energy balance in the ship s wake by the PIV measurements and the CFD calculations, the following principle guidelines for ESD designs for single screw merchant ships are proposed, where contemporary CFD tools should play an important role. - Under the condition of the momentum balance (total thrust equals the total resistance), and under the condition that the additional energy losses generated by the ESDs is minimized, the judgment on the efficiency gains of an ESD should ideally be based on the kinetic energy savings in the far wake of the ship where the static pressure recovery is complete. Comparing the kinetic energy levels in the near field may be a practical way to prevent numerical dissipation, but care has to be paid to the pressure recovery to make sure the comparison is relative and fair. The thrust or drag on an ESD, in principle, is mainly an internal force in the ship-propulsion system and should not be used as the objective function in the ESD optimization procedure. - Contrary to the normal perception based on typical open water propeller operation where ideal losses is much higher than the rotational losses, the transverse (rotational) kinetic energy left in the wake of the propeller behind a single screw merchant ship (where the wake of the ship is pretty well filled by the propeller) is higher than the axial kinetic energy level. Without making changes to the main dimensions of the hull and the propeller, the aim of the ESD design should first be the reduction of the transverse (rotational) kinetic energy in the wake of the ship. - The axial kinetic energy losses in the wake can be further reduced by applying e.g. a larger propeller diameter, distributed propulsors or changing the lines of the ship so that more wake of the ship will be filled by the operation of the propeller. Further reduction of the energy losses can be achieved by making the wake of the ship more uniform the wellknown wake equalizing effects. 9 Copyright 22 by ASME

10 - Flow separation and concentrated vortex shedding on the ESDs should be minimized. Besides the fact that the kinetic energy losses due to friction on the ESDs cannot be regained, the potential part of the kinetic energy losses generated by the ESD, e.g. in form of vortices, are often in smaller sizes than the propeller and can also not be easily regained. CONCLUSIONS The thrust or drag generated on the ESDs has no direct link to the power savings of the ESDs. It is often found that the more additional thrust generated by the ESD, the higher the thrust deduction will be. Those two forces cancel each other largely, leaving no net thrust gains. The additional thrust generated by the ESD is mainly an internal force of the shippropulsion system which does not change the energy balance of the system. Aiming to maximize the ESDs thrust, by using CFD tools in the design stage, is not likely to lead to the highest possible power saving to the system. On the other hand, an ESD which generates drag, such as a PSS, might well result in positive power saving. For the merchant vessel studied here, the transverse (rotational) kinetic energy loss in the nominal wake is only half of the axial kinetic energy loss. The axial kinetic energy loss in the wake is reduced by an operating propeller. But at the same time, the transverse (rotational) kinetic energy loss is increased significantly by the swirl of the slipstream of the propeller. In the wake of a self-propelled ship, the transverse (rotational) kinetic energy becomes 2 times larger than the axial kinetic energy losses. This differs from the normal perception of the open water operation of a propeller. Reducing the transverse (rotational) kinetic energy in the wake seems to be a good strategy which can lead to significant power savings. Further reduction of the axial kinetic energy losses can be achieved by a propeller with large diameter, by distributed propulsors in the wake, or by redesigning the lines of the vessel so that more kinetic energy losses from the ship come into the propeller disc. Equalizing the wake does also result in energy saving. By fitting the PDS to the upstream of the propeller, the total kinetic energy in the wake is reduced by more than 9% on the area studied, which is mainly from the reduction of the transverse (rotational) kinetic energy rather than the axial kinetic energy. This reveals the working principles of the PDS. Attempting to further improve the PDS-propeller system by adding HFs behind the propeller can reduce the total kinetic energy level by an additional 6%, although additional rotational losses will be produced by the vortex system shed by the fins. CFD tools have shown their powerful abilities to catch the flow field details and the possible flow separations. The findings from CFD agree with the flow phenomena found by the PIV measurements. CFD tools are encouraged to be used for the ESD designs with principle guidelines provided in this paper. ACKNOWLEDGMENTS The authors are grateful for the valuable support from Guangzhou Shipbuilding International (GSI), Shanghai Ship and Shipping Research Institute (SSSRI) and the matching fund of the Maritime Research Institute Netherlands (MARIN) through the Joint Industry Project (JIP) ESD-JILI. The criticism and encouragement of Prof. Dr. Ir. Tom van Terwisga is invaluable. Thanks are also extended to Ir. Bart Schuiling for carrying out the CFD analysis of the PDS with promising results. REFERENCES Dang J., Chen H., Dong G., van der Ploeg A., Hallmann R. and Mauro F. (2). An Exploratory Study on the Working Principles of Energy Saving Devices (ESDs), Proceedings of International Symposium on Green Ship Technology, Greenship 2, Wuxi, China. (available on Dyne G. (995). The Principles of Propulsion Optimization, Transactions RINA, 37, pp Hansen H. R., Dinham-Peren T. & Nojiri T. (2). Model and Full Scale Evaluation of a Propeller Boss Cap Fins Device Fitted to an Aframax Tanker, Proceedings of 2 nd International Symposium on Marine Propulsors, SMP. Hamburg, Germany. Fresenius R. (92). Das Grundsätzliche Wesen der Wechselwirkung zwischen Schiffskörper und Propeller, Schiffsbau, Vol. 23, No., pp and pp3-34. Korvin-Kroukovsky B. V. (956). Stern Propeller Interaction with a Streamline Body of Revolution, International Shipbuilding Progress, Vol. 3, No. 7, pp3-24. van Lammeren W. P. A. (949). Enkele Constructies ter Verbetering van Rendement van de Voortstuwing, Schip en Werf, No. 7, pp -8, April. Lee, J. T., Kim, M. C., Suh, J. C., Kim, S. H. & Choi, J. K. (992). Development of a Preswirl Stator-Propeller System for Improvement of Propulsion Efficiency: a Symmetric Stator Propulsion System, Transaction of SNAK. No. 29(4), Busan, Korea. Mewis F. & Guiard Th. (2). Mewis Duct - New Developments, Solutions and Conclusions, Proceedings of 2nd International Symposium on Marine Propulsors, SMP. Hamburg, Germany. Schneekluth H. (986). Wake Equalization Duct, The Naval Architect 986. Stierman E. J. (987). The Design of an Energy Saving, Wake Adapted Duct, Proceedings of International Symposium on Practical Design of Ships and other Floating Structures PRADS 987, Trondheim, Norway. Tsakonas S. and Jacobs W. R. (96). Potential and Viscous Parts of the Thrust Deduction and Wake Fraction for an Ellipsoid of Revolution, Journal of Ship Research, November, pp-6. van Wijngaarden E. (2). Prediction of Propellerinduced Hull-pressure Fluctuations, Dissertation of the Technical University Delft, ISBN , The Netherlands. Copyright 22 by ASME