With a CFD simulation many flow related topics of the hull design can be investigated. Even more than would be possible with a towing tank test, such as the pressure on the bow and the exact flow pattern using streamlines. The slider below contains a number of examples.
The wave height provides useful information to the naval architect about the performance of the hull.
The pressure along the bow indicates exactly where the hull encounters the most resistance.
The streamlines show whether the flow is properly guided along the ship and whether there are no rapid direction changes, or swirls.
The efficiency of the propeller is partly determined by the direction at which the flow of water enters it.
The hull resistance can be optimized using the the CFD results. It can be seen on the right that flow separation and the generation of vortices are reduced after optimization. As a result, the ship's resistance has decreased.
Validation
In addition of the support during the design, the CFD simulation has been validated with the aid of a towing tank test. The CFD simulation has been executed at the same scale as the towing tank test (ship model approximately 4m long). The resistance calculated by the CFD simulation (prior to the test) corresponded very well with the test results. The motion of the ship, vertically and rotation about the latitudinal axis (pitch and heave), seemed to deviate slightly. However, after discussing this with the test facility it turned out to be within the precision of the measurements. A very successful validation! When viewing the images, pay attention to how well the wave along the bow as calculated by the CFD matches the results from the towing tank test.
Comparison between 3D CFD model and towing tank model
With both a CFD simulation and a towing tank test a 3D model is used. The CFD simulation can be performed in full scale and the calculated total resistance can be split directly into frictional resistance and wave resistance. For the towing tank test, a scale model must be used. This makes it impossible to measure the correct frictional resistance and wave resistance at the same time. In practice, the total resistance is measured and the frictional resistance is calculated by formulas. The calculated frictional resistance is subtracted from the total resistance to get the wave making resistance. Since a CFD simulation can be carried out in full scale, this is not necessary when using a simulation.
Forces and position
Due to its sailing, the position of the ship in the water in relation to the position you would expect based on volume (displacement) and weight can change. This affects the calculated resistance, making it important to account for this displacement when executing CFD simulations. In the simulation the ship can pitch and heave.
The graph with the resistance shows that the resistance as calculated by the simulation corresponds very well with the resistance measured in the towing tank.
The graph with the position of the ship shows that the movement as calculated with the simulation also corresponds very well.
Wave pattern
The wave pattern along the bow of the ship changes with the speed of the ship. The faster the ship moves, the higher the waves. The wave making resistance increases by the cube of the speed.
Therefore, it is important that the wave pattern is properly calculated. In the figures, the wave pattern as calculated with the simulation for each speed is compared against the towing tank. The results are very similar. Note that in addition to the waves, the position of the ship also changes in both the CFD and the towing tank test.