Displacement Type Ship Resistance Analysis with CFD
The resistance of a ship, also known as drag, is a measure of the force required to propel the vessel through water at a given speed. It is an important factor to consider in the design and operation of a ship, as it affects the fuel consumption and overall performance of the vessel. There are various methods for analyzing and predicting ship resistance, including model testing, empirical formulae, and computational fluid dynamics (CFD). In this article, we will focus on the use of CFD for analyzing the resistance of displacement type ships.
What is CFD?
CFD is a numerical method for solving problems involving fluid flow and heat transfer. It involves the use of computers to simulate the flow of fluids, such as water or air, and to predict the associated phenomena, such as the drag experienced by an object moving through the fluid. CFD can be used to analyze a wide range of problems, including the flow around ships, aircraft, and other objects.
Why Use CFD for Ship Resistance Analysis?
There are several advantages to using CFD for analyzing ship resistance. First, it allows for the prediction of ship resistance with a high degree of accuracy. This is because CFD takes into account the detailed flow patterns around the ship, which can have a significant effect on the resistance. In contrast, empirical formulae and model testing are based on simplified assumptions about the flow around the ship, which can lead to less accurate results.
Second, CFD allows for the analysis of the ship in a range of operating conditions, such as different speeds and water depths. This can be useful for optimizing the ship design for different missions or for assessing the performance of the vessel under different conditions.
Finally, CFD allows for the analysis of the ship in the presence of external factors, such as waves, currents, and wind. This can be important for predicting the performance of the ship in different environments.
Types of Ship Resistance
There are several types of ship resistance that need to be considered in the analysis of a vessel. These include:
Friction resistance: This is the resistance due to the friction between the water and the hull of the ship. It is typically the dominant form of resistance for displacement type ships.
Form resistance: This is the resistance due to the shape of the ship. It is affected by the hull form, appendages (such as rudder and propeller), and the angle of attack of the ship.
Wave-making resistance: This is the resistance due to the generation of waves by the ship. It is particularly important for high-speed vessels, but it can also be significant for displacement type ships at high speeds.
Residuary resistance: This is the resistance due to factors other than the ones mentioned above, such as viscous effects, compressibility, and roughness of the hull.
CFD Modeling of Ship Resistance
The CFD analysis of ship resistance involves the creation of a numerical model of the ship and the surrounding fluid. The model includes the geometry of the ship, the boundary conditions (such as the velocity of the fluid), and the material properties of the fluid.
There are several steps involved in the CFD modeling of ship resistance:
Geometry preparation: The first step is to create a digital model of the ship. This can be done using computer-aided design (CAD) software or by scanning a physical model of the ship. The model should include all the important features of the ship, such as the hull, appendages, and any other structures that affect the flow around the ship.
Mesh generation: The next step is to create a mesh that defines the computational domain. The mesh should be fine enough to capture the details of the flow around the ship, but not so fine that it becomes computationally impractical. There are various techniques for generating meshes for ship resistance analysis, including structured grids, unstructured grids, and hybrid grids.
Solution setup: The next step is to set up the CFD solver and specify the boundary conditions. This includes defining the velocity of the fluid at the inlet and outlet boundaries, as well as the atmospheric pressure at the free surface.
Solution: The CFD solver is then used to solve the governing equations of fluid flow and heat transfer. This involves iteratively updating the values of the flow variables (such as velocity and pressure) until a converged solution is obtained.
Post-processing: The final step is to post-process the results of the CFD simulation. This involves visualizing the flow around the ship and extracting the desired quantities, such as the resistance and propulsion forces.
CFD Validation and Uncertainty
One important aspect of using CFD for ship resistance analysis is the validation of the results. This involves comparing the predicted resistance from the CFD simulation to experimental data or results from other methods. This can be done using metrics such as the coefficient of determination (R2) or the root-mean-square error (RMSE).
It is also important to consider the uncertainty in the CFD results, which can be due to various factors such as the accuracy of the input data, the mesh resolution, and the choice of turbulence model. One way to quantify the uncertainty is through sensitivity analysis, which involves varying the input parameters and observing the effect on the output.
In conclusion, CFD is a powerful tool for analyzing the resistance of displacement type ships. It allows for the prediction of resistance with a high degree of accuracy, the analysis of the ship under a wide range of operating conditions, and the consideration of external factors such as waves and wind. However, it is important to validate the results of the CFD simulation and consider the uncertainty in the predictions.
CFD can be used in conjunction with other methods, such as model testing and empirical formulae, to improve the accuracy and comprehensiveness of the ship resistance analysis. It can also be used in the optimization of the ship design to improve the fuel efficiency and overall performance of the vessel.
In summary, CFD is a valuable tool for understanding and predicting the resistance of displacement type ships, and it has the potential to greatly enhance the design and operation of these vessels.
Eray Ceylan
Naval Architect and Marine Engineer