Hydrodynamic Optimization of Energy Saving Devices in Full Scale

Kim, K., Leer-Andersen, M., Orych, M.

Proceedings, 30th Symposium on Naval Hydrodynamics, Hobart, Tasmania, Australia, 2-7 November 2014


The hydrodynamic effects of Energy Saving Devices (ESD) are often tested at model scale Reynolds number in a towing tank. However, the flow characteristics in the towing tank differ significantly from full scale, especially in the wake region. Still it is common practice that the performance of ESDs is measured in a towing tank and the fuel savings is estimated by extrapolation methods. The predicted power saving may sometimes turn out to be too high, despite the fact that this appendage cause an additional viscous drag, or too low although the device has solid hydrodynamic effects for power savings. Generally it is difficult to reliably predict full scale performance of this type of devices from standard model tests at equivalent Froude number in the towing tank. One likely cause of such over/under prediction is the scaling problem inherent in model tests using ITTC method, which does not specifically take such devices into account.One may think full-scale estimates of energy savings could be made based on correlations between model tests and full scale trials results. The problem is that such a reliable data is not easily available due to a number of reasons, limited previous experience with the devices, lack of accuracy in full scale speed trial measurements and lack of transparency of the saving in actual operations.CFD solves this problem offering the capability to compare model-scale results with full-scale results by making calculations for both cases. The comparison offers better support for extrapolations from model test predictions to full scale conditions. The absolute accuracy by CFD computation is still limited, particularly for predicting global quantities such as speed-power performance in full scale. However, the effect of Reynolds number on ESDs can be investigated on the basis of fundamental aspects of the hydrodynamics of those ESDs by analyzing the local flow characteristics thus improving the performance.Although there exists several technical problems still unresolved as mentioned above, the design of ESDs should be based on model tests combined with full scale CFD and the energy saving potential should be confirmed by sea-trial tests.This paper describes additional steps needed in the development process of ESDs and presents a design methodology for the successful development of ESDs. The design methodology considers the ESD design by integrating all available technical resources to its full extent but within the limits of their capabilities.