A proposed design methodology for successfully developing ESDs
SSPA has long experience with testing most Energy Saving Devices (ESDs) available on the market and has also been involved in many joint research projects developing energy saving solutions. SSPA, and also other parties within the community, has recognised that there can be a risk if ESDs are developed and applied in a standard routine process only based on model scale, not taking into account the full scale flow effect. This article describes additional steps needed in the development process of ESDs and propose a design methodology for successfully developing ESDs. In order to test and evaluate the proposed design methodology two generic research devices have been created: Generic Device GD-OK and GD-GK.
Most feasible ESDs to be selected
Most ESDs are used to enhance the flow into propulsion devices, and aimed at increasing propulsive efficiency as well as reducing energy loss. The ESD should be designed based on optimum trade-off between power reduction (favourable effect) and maximum allowable resistance increase/cavitation risk (unfavourable effect). SSPA’s experience has shown that the design of ESDs is specific for a given ship and that the best gain can be reached if the hull/propeller/ESD is optimised together for each specific ship.
It is a well-known phenomenon that the flow characteristics in model scale differ from the full scale flow field in the wake. This has been taken into consideration for long time in an experience-based manner by e.g. propeller designers, working both for design, evaluation and extrapolation.
Today it is common practice that the performance of ESDs is measured in the towing tank and evaluated with standard extrapolation methods developed for hulls and propellers. Even though there is a lack of validation studies of Computational Fluid Dynamics (CFD) RANS simulations for full scale ship flow and the results are not yet reliable enough to fully predict global quantities, CFD codes can still be used to understand the full scale flow and thereby provide means for better ESD optimisation.
As CFD simulations can be conducted at full scale, the scaling problem inherent in model tests can be avoided and the prediction of full scale performance of ESDs can be improved by a combination of model tests and CFD prediction.
Proposed design methodology
In order to achieve the best possible result, SSPA proposes a design methodology that makes use of all available technical resources in the most effective way to its full extent, but within the limits of its capabilities. The methodology is presented in three steps below.
Step 1: Optimisation of ESD in full scale
The optimum configuration obtained from model tests/model scale CFD simulation might not be the optimum in full scale. Therefore, the optimisation of the ESD should be carried out for full scale performance from the beginning. A wide range of design parameter variation studies are performed using full scale CFD simulation. Typical design adjustments can be dimensioning, positioning and shaping parameters of the ESD.
Step 2: Confirmation by model testing
Based on the evaluation of power gain and detailed analysis of flow characteristics, the most promising ESDs will be selected and tested for confirmation.
As the ESD has been designed in full scale, it cannot be expected that it will perform too well in model scale. Normal towing tank testing is necessary though, both for baseline performance without ESD, but also for validation of the CFD in model scale (to be compared to model scale CFD simulations of the proposed ESD).
Step 3: Full scale wake dummy hull
As the flow characteristics in model scale differ from actual flow fields around the ship in full scale in the wake region, the efficiency gain prediction from model tests is not sufficient for reliable correlations of power saving between model and full scale. On the other hand, the absolute accuracy by CFD computation is still limited, particularly for predicting global quantities such as speed power performance.
To address these issues, and to bridge the Reynolds number range, an additional step introduced is the design of a full scale wake dummy hull, which can create a wake which resembles the full scale wake (non-dimensionally). This idea allows for model testing aiming at higher confidence in the design, both for full scale performance prediction and possible cavitation/vibration risks. Substantial work has been conducted for the first two steps described above. In order to investigate full potential of the proposed design methodology, a research initiative is ongoing in which the method will be applied to the two test examples. This will investigate the complete design methodology and its potential for the future.
Photos and illustrations
In order to test and evaluate the proposed design methodology two generic research devices have been created: Generic Device GD-OK and GD-GK.
Model and full scale wake in plane of an ESD.
The figure above indicates that an approximate 5~6% power reduction can be achieved by different types of ESDs in model scale as compared to the baseline design. SHIPFLOW computations were able to correctly predict the relative ranking for the ESDs tested. Test cases investigated so far indicates that full scale CFD predicts lower power gains than model test full scale predictions.