Comparing model tests of a patrol ship’s hull with computational fluid dynamics.
In this user case, Marintek uses Fidelity Fine/Marine and Hexpress for resistance curve prediction of a planning hull and its validation against the model test cases.
Team involved
End User: Eloïse Croonenborghs, Research Scientist at MARINTEK, Maritime division, Trondheim, Norway
Team Expert: Sverre Anders Alterskjær, Research Scientist at MARINTEK, Maritime division, Trondheim, Norway
End Client Expert: Canan TİRYAKİ, STM, Ankara, Turkey
Software Provider: NUMECA International S.A.
The total ship resistance of a new patrol vessel in calm water conditions was evaluated by means of CFD simulations and model tests. The hull form was developed by STM, and its dimensions are tabulated below:
The hull’s total resistance is computed using CFD simulations at different speeds. The simulations were performed with Fidelity FINE Marine at full scale in deep water conditions.
The vessel’s geometry is meshed using Fidelity Hexpress. A boundary layer grid normal to the hull surface is specified to reach y+ values between 30 and 80. Given the diversity of the Froude regime covered in this study, new meshes were generated for each computed speed. Adaptive grid refinement was used with the free surface criterion in the proximity of the hull in the final stage of each simulation to increase the accuracy of the results. The final meshes were composed of about 5 to 7.5 million cells.
For the simulations, propulsion was modeled as a force applied at the center of the action on the water jets. The air drag was modeled as a force applied at the center of the frontal projected area.
Wave pattern at 55 knots (left), bow wave profile at various hull stations, and hydrodynamic pressure on the hull at 55 knots (right).
The hull model is made of foam and wood coated with paint. It has a hydrodynamically smooth surface finish to the linear scale of 1:16. In turbulent stimulation, fine sand grains were glued to the hull along the keel from the bow to station 17.
The resistance tests were performed with the model towed by MARINTEK’s high-speed rig with resistance, trim, and sinkage measurements. In the test setup, the model is free to heave, roll and trim but fixed in all other degrees of freedom.
The effect of air drag on the projected area above the water line is included in the prediction based on the projected area of the vessel.
Bottom and perspective views of the hull and wave pattern.
The conversion from a hull model (numerical or experimental) into a full-scale ship is made using the form factor method. This method assumes that the total resistance can be divided into the viscous resistance and the residuary (due to vorticity, wave making, and wave breaking) resistance CR. The viscous resistance is determined by multiplying the frictional resistance CF with a constant form factor k0, which is identical for the models and the ship. Further, the residuary resistance CR is assumed to be identical for models and the ship.
When numerical or experimental results are converted to total ship resistance RTs, the hull surface roughness effect is considered by means of the empirical formula. The results are presented in terms of non-dimensional total ship resistance CTs.
The following table compares the predicted total ship resistance obtained from the model test approach and the CFD approach. For all speeds, the results agree within 0.7%. The hydrodynamic trim angle agrees within 0.5 deg. This is a satisfying result, given that the trim measurements are not corrected for scale effects and that the CFD mesh could be further refined around the hull, although it was not required for this study.
Comparison of non-dimensional total ship resistance CTs and hydrodynamic trim angle from model test results and CFD results.
The agreement of the CFD prediction to the model test results for total ship resistance in calm water conditions increases Marintek’s confidence in Fidelity CFD solutions for their marine applications.
Company profiles
The Norwegian Marine Technology Research Institute (MARINTEK) performs R&D in ocean technology for a global market, primarily in the maritime, oil and gas sectors, and ocean energy. MARINTEK’s main offices and laboratories are located in Trondheim, Norway. Their commercial services and research activities strategy has always been a rational combination of physical and numerical modeling approaches. Prediction of vessel and propeller performances, design optimization process, wake analyses, and studies on propulsor-hull interaction are just a few examples of CFD applied to ship hydrodynamics.
STM was established in 1991 by the decree of the Defence Industry Executive Committee to provide system engineering, technical support, project management, technology transfer, and logistics support services for Turkish Armed Forces (TAF) and Undersecretariat for Defence Industries (SSM). Also, to develop software technologies for defense systems and establish/operate national software centers for software development, maintenance, or support.
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