The design and energy saving effect prediction of rudder-bulb-fin device based on CFD and model test
Introduction
In order to reduce the operational costs arising from the ever-increasing cost of fuels and also comply with various regulations for shipping industries, a number of measures have been proposed. One of the most recommended measure is reduction of energy requirements by ships. This can be achieved through: reduction of hull resistance, reduction of required on board equipment power, design of more efficient propulsors, use of hydrodynamic energy saving devices and partial substitution of fuel power by renewable energies. For optimum energy saving effect, a combination of these measures should be considered, say, an optimized hull form appended with a more efficient propeller and hydrodynamic energy saving devices. Nordbank (Nordbank, 2013), conducted a survey on implementation of energy efficiency measures. The study showed that about 42% of the respondents preferred ship retrofitting. It was also noted that optimization or modification of rudder and/or propeller was the most popular retrofit at 33% followed by design optimization of bulbous bow and the hull.
Many energy saving devices (ESDs) have been proposed with significant energy saving effects for ships. Current focus in relation to ESDs is on optimizing their energy saving effect and ESD combination systems. Su et al. (2020) showed that ESDs can effectively enhance the propeller slipstream and have a positive interference effect on a large-scale hull. Mizzi et al. (2017) investigated influence of various geometries on PBCF. The net energy efficiency improvement of 1.3% and a reduction in the hub vortex was recorded. Furcas et al. (2020) analyzed three novel wake equalizing duct (WED) designs. A base reduction of the total ship resistance larger than 2% was achieved by altering angle of attack, while the maximum saving (about 4%) was achieved when non-symmetric configurations were considered. Chang et al. (2019) investigated the effects of a fan-shaped Mewis duct on propeller performance. They noted that there exists an optimum design value for all of the duct parameters. Sakamoto et al. (2019) in their study on pre-swirl and post swirl stators for three hull forms showed that vertical distribution of the twist angle of its rudder and the distance between propeller boss cap and the rudder bulb should be modified to improve saving effect.
One of the widely used ESD in modern ships is a combination of a rudder bulb and rudder thrust fins. The rudder bulb is devised to improve the efficiency of a ship's propeller by minimizing hub drag caused by separation and pressure pulses (Carlton, 2019). Similarly, the rudder thrust fins improves the propulsive efficiency by producing thrust since they operate in the propeller induced rotational flow. As discussed by Liu Ye-bao eta al. (2012), the energy saving of a combination of rudder bulb and rudder thrust fin is more than that of rudder bulb. According to the International towing tank Conference (ITTC) (ITTC 1999), the combination of rudder bulb (COSTA propulsion bulb) and rudder fin (rudder-appended thrust fins) showed energy gains of 4–14% at model scale and 4–7.4% at full scale trials. Jie Dang et al. (2012) noted that the best achievement in improving efficiency by adding an ESD to an existing vessel is more than 10% according to the model tests, although full-scale sea trials have shown up to 5% improvement on total propulsive efficiency. With the Energy Efficiency Design Index (EEDI) regulations in operation, these levels of savings are sufficient to justify investment into the rudder bulb and rudder thrust fin devices. However, there is little or no information on how the various parameters influence the energy saving effect of these devices.
Recent decades have witnessed magnificent developments in computational fluid dynamics (CFD) and most importantly its application during ship design. This is evident from the regular workshops held especially those on numerical methods on model ship flows. Ponkratov (2017) presented a comparison of the results obtained from modern numerical methods with those from sea trial measurements compiled by Lloyd's Register (LR). The aim was to assess and establish the capabilities of numerical tools in ship scale and also to increase confidence in ship scale CFD. Application of CFD in full-scale ship performance prediction has been instrumental in addressing the scale effects manifested during extrapolation of model scale data to full scale. In references (Castro et al., 2011; Tezdogan et al., 2015; Wang et al., 2016), a full scale KRISO Container Ship was effectively used to study hull-rudder-propeller interactions under various conditions. Hai-long et al. (2016) and Tezdogan et al. (2015) in their studies noted that CFD data calculated at full scale was more reliable compared to model scale. Zhao (2015) noted the existence of scale effects attributed to differences in the flow field between normal simulation and complete-similarity theory simulation (CSTS). There has been remarkable advances in application of CFD to investigate ship seakeeping and maneuvering characteristics and their response to head and bi-directional waves (Huang et al., 2021; Jiang et al., 2021; Le et al., 2021; Yao et al., 2021; Zhang et al., 2021).
Computational fluid dynamics has been critical in the research on energy saving devices. Sakamoto et al. (2020) used viscous CFD to study two merchant ships appended with a type of pre-swirl duct energy saving device. The numerical method was able to capture the nominal and total wake distributions in the vicinity of the propeller plane, asymmetric wake due to the induced velocity of the propeller and the axial momentum defect due to the presence of the ESD. Song et al. (2019) showed that an integrated simulation of a ship-propeller-rudder-stern flap was a more accurate means of predicting the energy-saving effects of stern flaps than model resistance. Tacar et al. (2020) studied a rudder gate system through model experiments and complementary CFD analyses. Dong et al. (2018) used RANS to analyze scale effects for open water performance of a tip-rake propeller. From the presented literature, it is clear that CFD can be used to study ship hydrodynamics with adequate certainty. It is common knowledge that model tests and sea trials, especially for ships appended with ESDs, are constrained by cost requirements, high technical skills, time constrains and a number of physical and environmental parameters that can influence measurements. Numerical methods are of great value in complementing model test and could perhaps be used during early design and optimization stages.
The main purpose of the paper is to investigate the effects of rudder bulbs and rudder fins geometrical on ship performance using numerical simulations and model test. CFD code STAR CCM+ is used to predict the energy-saving effect for rudder-bulb-fin devices. The geometry of a 35000DWT bulk carrier was used in this study. In order to validate the CFD method, bare hull resistance, open water, and self-propulsion experiments are carried out at model scale (λ=29.197). Based on the self-propulsion point for ship without ESD, the hydrodynamic performance prediction of eight hull-propeller-rudder-bulb models and eleven hull-propeller-rudder-bulb-fin models are carried out. The optimized rudder-fin and rudder-bulb are then validated by model tests.
Section snippets
Numerical method
The numerical method comprises; geometrical modeling, computation domain and boundary condition setup, grid generation and discretization of governing equation and physics modeling.
Open water performance prediction
Open water simulations are used to determine non-dimensional coefficients for; thrust (KT), torque (KQO) and open water efficiency (ηo). These characteristics are used together with those obtained from self-propulsion to evaluate ship performance. The dimensionless coefficients are calculated as follows:
The propeller motion was simulated using two techniques; Moving reference frame (MRF) and dynamic overlapping mesh. For the MRF method, a boundary-based interface
Model test experiment
Model test is essential in data validation and verification. In the current work, the experiments were performed in accordance with ITTC procedure and recommendations for model tests (ITTC 2002). A wooden ship model (λ=29.127) was used for the 35000DWT single screw bulk carrier with a bulbous bow fore ships. At first, open water, resistance and propulsion tests were performed for the ship without ESD. After which the conventional rudder and hub were replaced with the modified rudder and hub
Grid sensitivity study
Grid sensitivity was carried out using the resistance and open water prediction cases. For resistance, simulations were carried out for ship moving at design speed of 14 kn The time step was controlled by the Courant Number (C) defined as; C = uΔt/Δx, where u is the velocity magnitude, Δt is the time-step of the numerical model, and Δx is the spacing of the grid in the numerical model. Three grids were considered where systematic refinement was carried out by varying the grid size using a ratio
Conclusions
The present work investigates the influence of rudder bulb diameter, thrust fin; span, chord length and angle of attack on propulsive efficiency. The CFD method for ship self-propulsion simulation is validated by the model test data and found to be adequate and reliable for analysis of hull-propeller-rudder interaction.
Among the findings from the current work, the following important conclusions can be drawn;
First, a computation method for calculating energy saving effect for rudder bulb and
Credit author statement
OBWOGI Enock Omweri: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data Curation, Writing - Original Draft. SHEN Hai-long: Conceptualization, Resources, Validation, Writing - Review & Editing, Visualization. SU Yu-min: Supervision, Funding acquisition
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research work was supported in part by Science and Technology on Underwater Vehicle Laboratory, through Harbin Engineering University; and the Chinese Scholarship Council, through People's Republic of China Government (CSC No. 2018DF019500) in collaboration with the Kenyan Government.
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