The project simulates the free movement of a floating boat under the influence of a headon wave via a Multi-phase 6-DOF (degree of freedom) transient analysis. The boat is considered as a stationary floating body in a water tank with an open top. A single wave is generated to create disturbance in the water and simulate the motion of the boat. For simplicity of the analysis a laminar model was used.
The main purpose of this project is to demonstrate how free floating body motion can be predicted via simulation to provide insight and aid in marine vehicle design and development.
The geometry consists of a boat body enclosed in a tank. The geometry is shown in Figure 1 with the boat highlighted in red. (Note: This is an inverted model, thus the fluid area around the boat is a solid volume while the boat is empty space.)
Figure 1: Floating Boat - Geometry
To reduce the size of computational domain, only half of the original domain was meshed using the hex-dominant parametric meshing algorithm on the SimScale platform. The resulting mesh (flow domain only) consists of approximately 0.5 million cells and is shown in Figure 2 below.
Figure 2: Floating Boat - Mesh
The flow domain consists of two fluid phases, water and air, thus a Mutiphase Analysis must be done.
Figure 3: Floating Boat - Mesh
The initial distribution of the water phase is set under Initial Conditions by using a geometry primitive named ‘water-initial-state’.
The wave is then generated using another geometry primitive named ‘Wave’.
Figure 4: Floating Boat - Mesh
The domain boundaries are considered as Walls with the top surface as an Open Inlet-Outlet boundary. A Symmetry condition is applied to the side surface based of geometry and flow requirements.
The 6 DOF (degree of freedom) model can be added under the Advanced Concepts item in the SimScale Project Tree, Solid Body Motions. Moreover, in accordance with the symmetry condition, constraints were applied for the rotations and lateral motion was allowed only in the vertical direction. The boat was taken as a solid body with a mass of 740 kg and appropriate values for the moment of inertia.
The simulation was run for 4 seconds of simulation time with automatic time stepping and a maximum courant number (CFL) of 0.5 ( It is recommended that the CFL should be < 0.5 and preferably around 0.3). The job was run on 8 compute cores and took about 5 hours to complete.
Results and Conclusions
The results below (processed using the local version of ParaView, an open source visualization toolkit) show the transient changes in phase fraction of water (Figure 5) and flow velocities along with the free motion of the boat due to the fluid forces. This simulation shows how the free floating body motion can be predicted via simulation to provide insight and aid in Marine vehicle design and development.
Figure 5: Phase Fraction animation
Figure 6: Velocity (m/s) animation