Case - EEXI from Full-scale validated CFD

This case study shows how Aerotak™ can determine the Energy Efficiency Existing Ship Index (EEXI) using a validated CFD model for significantly lower costs than conducting additional towing tank test or sea trials.

Objective

Validate the CFD model using existing sea trial and towing tank results. For future projects, the CFD model can then be used to simulate a self-propelled ship in EEXI condition to attain the EEXI of the vessel.

Validation procedure for EEXI calculation

Extensive validation and verification of; resistance, propeller open-water, and self-propulsion simulations are conducted in both model and full-scale. Doing the validation and verification in multiple steps, ensure a high quality and accuracy of the CFD simulations, which is also appraised by classification societies during their approval process.

Resistance Open-water Self-propulsion

Simulation setup for a full-scale EEXI calculation

For the propeller open-water simulations in model scale, the flow on the blade goes from a laminar regime to transitionally turbulent and finally to fully turbulent. These simulations utilized the k-omega SST turbulence model, but this was derived for fully turbulent flows. Thus, to model the influence of transition, the Gamma ReTheta approach was used, which added the two transport equations for intermittency and transition momentum thickness Reynolds number. The model scale propeller open-water simulations used the k-omega SST turbulence model since the transition model Gamma ReTheta only is compatible with the k-omega SST. The calm free surface in the resistance and self-propulsion simulations were resolved using the volume of fluid (VOF) method in STAR-CCM+. Furthermore, the self-propulsion simulations included modelling of the free surface and rotation of the 3D propeller.

Validation results

All CFD results were sent blind to the shipyard. By sending the results blind without knowing the towing tank results and sea trial results, the shipyard could trust that the CFD results have not been influenced by towing tank results and sea trial results. The full-scale resistance and propeller open-water showed very good agreement with towing tank measurements and predictions as seen in Fig. 2 and 3. The discrepancies between CFD and towing tank test were lower than the towing tank uncertainties. This demonstrated that the CFD model could accurately predict the full-scale resistance and propeller open-water performance.

Similarly, there was an excellent agreement between model scale self-propulsion simulations and towing tank measurements and predictions as illustrated in Fig. 4. However, the full-scale self-propulsion simulations using the traditional approach of including the roughness as a point force estimated by an empirical formula, noticeably underestimated the power from the speed trial measurements.

When predicting the full-scale performance of a vessel, the resistance from surface roughness is very important. Even for new ships with little or no biofouling, there are several factors that can lead to increased hull resistance, e.g., roughness of the paint, welding seams, variation in plate thickness etc. In the present study, the effects of full-scale hull and propeller roughness were implemented and compared using two different approaches. The traditional approach is using empirical formulas to predict the roughness and apply it as a concentrated point force. The alternative method is to account for the surface roughness by modifying the wall functions in the turbulence model. By including the effect of hull and propeller roughness directly into the CFD model, the discrepancies between CFD and speed trial measurements decreased substantially. The delivered power was predicted within 1-4 % of the sea trial results, when the effect of hull and propeller roughness was implemented directly into the CFD model.

Conclusion

With the validated CFD model, Aerotak™ can perform the CFD simulations of vessels and attain the EEXI. Aerotak™ assists the shipowner when the attained EEXI is sent to the classification society for approval. Finally, the shipowner can decide whether Engine Power Limitation (EPL), energy saving devices (ESDs) or other method for improving the attained Energy Efficiency Existing Ship Index are required.

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