# Can CFD Replace Roll Decay Tank Testing for Ships

The shipping industry is dedicated to ensuring safe navigation of the world's oceans. One of the most critical aspects of maintaining sea safety is to ensure ships' stability, especially in different weather conditions. To address dynamic stability failure modes, the International Maritime Organization (IMO) has been developing the second-generation intact stability criteria (SGISC). The SGISC emphasizes the importance of estimating a ship's roll damping, which is a key factor in assessing a ship's vulnerability.

##### At the BOAT International Design & Innovation Awards 2024, Best Naval Architecture - Displacement Motor Yachts: Milele. Naval Architects: Van Oossanen.

While there is an existing empirical method for estimating roll damping, it has its limitations regarding accuracy and applicability. As a result, computational fluid dynamics (CFD) is being explored. CFD is a powerful tool that simulates the fluid flow around a ship and provides more accurate roll-damping estimates. However, for regulatory purposes, it is crucial to validate the accuracy and reliability of CFD codes. In this blog, we'll learn the significance of roll damping estimation, explore the empirical method and CFD simulation, and compare the results of both methods with tank testing.

**What is a Roll Decay Test?**

The roll decay test helps evaluate a ship's ability to resist rolling motion. In the context of investigating the roll motion of ships, the roll decay experiments are conducted at a reduced scale in a hydrodynamic basin. In these experiments, the vessel is subjected to an off-balance state by applying an external moment, and subsequently, the motion gradually returns to the equilibrium state.

##### Ship's Stability to Resist Rolling Motion.

The test provides valuable information on the extent and duration of a ship's rolling motion following the impact of a wave. Higher damping indicates that the ship has a better ability to reduce its residual roll motion, and thus provides greater stability. It is a crucial test used to assess ships' safety and stability in rough seas.

**Empirical Method**

The empirical method, i.e., IKEDA, used for studying the roll decay of a ship can be broken down into linear, and non-linear components. The linear and non-linear components are explained below.

**Linear Components**

**1. Wave-making Components**

In naval architecture, the wave-making component of a vessel refers to the difference between the damping coefficient at zero knots and the coefficient when the vessel is at speed. To accurately determine the damping coefficient in each section, it is essential to establish a zero-knot baseline. When a vessel is in motion, it generates waves by pushing water to the side. However, this process requires energy not conserved by the vessel's motion.

If the vessel has a forward speed, it is necessary to compute the zero-knot baseline first. The damping coefficient due to wave-making is typically expressed as a fraction of the zero-knot damping, and its value generally ranges from 5 to 30%. Additionally, the size of the wave-making component can be influenced by the vessel's shape and size. For instance, a shallow and wide vessel will generate larger waves, which can affect the overall size of the wave-making component.

**2. Lift Components**

The hull of a vessel can function as a lifting body during forward motion and as a wing during lateral rolling, creating sway and yaw motion. This motion serves as a means of energy dissipation from the system. The lift component is directly proportional to the angular velocity, and its effectiveness depends on the velocity. Moreover, it should be noted that the vessel's length, draft, midship coefficient, and velocity influence the damping coefficient to a great extent.

**Non-Linear Components**

**1. Frictional Component**

The frictional component of a vessel's resistance is affected by speed and Reynolds number, which is determined based on roll amplitude rather than speed. At model scale, this component can be quite significant, ranging from 8-10%, while at full scale, it is modest, only accounting for 1-3%. A similar process for bare hull resistance scaling is followed to determine the frictional component.

First, the relative area of the vessel is computed, followed by the friction coefficient, and then scaling is applied. However, instead of calculating friction on a flat plate, it is determined for a cylinder, which is then scaled to match the size of the vessel. Formulas for equivalent radius and surface area can be used to match the cylinder to the vessel.

**2. Eddy-making Component**

##### Graph for Determining Eddy-making Component.

To determine the eddy-making component, it is essential to compute the half breadth-draft ratio and the area coefficient. Based on these values, the eddy-making component can be determined from the graph. Unlike the lift component, the Eddie-making component decreases with forward speed.

**3. Appendage Component**

The Bilge keel component significantly affects the pressure distribution on a ship's hull. As water flows past the bilge keel, it creates high pressure on one side and low pressure on the other. This pressure distribution can be assumed and used to obtain a coefficient. Similarly, the length of the skeg can be used to make assumptions and obtain a force and pressure coefficient for it.

**CFD Simulation Using Fidelity Fine Marine**

The methodology for CFD simulation of the roll decay of a ship is based on the results obtained by our valued customer, Van Oosanen Naval Architects, using Fidelity Fine Marine for simulation and Hexpress for meshing. The simulation employs overset meshing to consider significant motion. However, it is crucial to note that a large overset domain can lead to wave reflection on the overset boundary. Therefore, caution must be exercised, and the size of the overset domain should be reduced if wave reflection occurs. A wave-damping layer is utilized to coarsen the mesh at the edge of the background domain, and during simulation, adaptive grid refinement refines the free surface.

##### Background Domain Cut at WL (left) and Overset Domain Mesh at Boundary (right).

It is imperative to set a reference velocity for a vessel, even stationary. Without forward speed, the initial conditions determine the heel angle and the initial sink. However, if the vessel has a forward speed, it requires two simulations. The first simulation accelerates and heels the vessel, while the second simulation aims to release the roll motion while maintaining the same speed. This approach ensures a comprehensive analysis of the vessel's behavior in different scenarios, thereby enabling the identification of potential risks and the development of effective mitigation strategies.

**Comparison**

##### Comparison Between Tank Test, Empirical Results and CFD Solutions.

The analysis reveals that the tank system exhibits low initial damping and a natural roll period similar to that obtained from the added mass values of the CFD simulation. However, the added mass value calculated through the empirical method is overestimated compared to the tank system and CFD. The study suggests that the empirical method and CFD effectively determine the natural roll period of a tank test with a high degree of accuracy. Moreover, the results indicate that CFD can potentially replace free decay towing tank tests.

**Conclusion**

The stability of a ship in turbulent waters is a critical aspect that requires careful examination and thorough analysis. One such examination is the roll decay test, which assesses the ship's ability to restore its stability after being rolled to one side. Historically, this test was conducted using a towing tank, which could be costly and time-consuming. However, with CFD, it is now possible to simulate the roll damping using software such as Fidelity Fine Marine. By leveraging this software, engineers can simulate a ship's roll damping, reducing costs and resources while providing reliable results.

Watch the on-demand webinar on Simulation of Ship Seakeeping and Roll Decay Using CFD to learn how Fidelity Fine Marine CFD simulation can help predict a ship's roll decay.