When objects fall through water or air, complex forces make them spin, flip, and change direction in ways that are hard to predict. Six Degrees of Freedom (6DOF) dynamic mesh simulation gives engineers a powerful way to see exactly how these forces work. In this project, we used ANSYS Fluent to create a computational fluid dynamics (CFD) model of a simple box falling into water. Unlike basic simulations, our 6DOF approach captures all possible movements – forward/backward, up/down, side-to-side, plus rotation around all three axes. The Volume Of Fluid (VOF) multiphase model helps us track the boundary between air and water, while the dynamic mesh constantly updates to follow the box’s changing position. This kind of fluid-structure interaction (FSI) analysis is extremely valuable for designing safer packaging, improving material handling systems, and predicting how structures behave during floods or ocean deployment.

Figure 1- Box falling into the water

Simulation Process

The simulation’s easiest part is the 2D computational domain and performing an initial mesh. A combination of Volume Of Fluid (VOF) multiphase model and Dynamic Mesh is necessary to reach acceptable results. Plus, a user-defined function (UDF) must be written to define box motion by its way of falling. It is a 6 DOF approach that involves all exerted forces by water on the box, resulting in a realistic prediction. Last but not least, the mesh will be continuously refined by Smoothing and Remeshing methods.

Post-processing

Look at what happens when our box hits the water! The top image shows the magic moment of impact where the box has just broken through the water’s surface. The blue area is water and the red area is air, with a rainbow-colored boundary between them showing the splashy water surface. Our square box isn’t just dropping straight down – it’s tilting as it falls! This happens because water pushes differently on each side of the box, making it rotate. The water surface isn’t flat anymore either – it’s making waves that move away from where the box hit. These waves are super important because they show how energy moves from the falling box into the surrounding water. We successfully captured these realistic surface waves and the box’s natural rotation, which many simpler simulations can’t do.

Figure 2: Air volume fraction contour showing the box breaking through the water surface

Now check out the second image showing how water flows around our falling box. The swirly lines reveal something amazing – there are little spinning whirlpools forming above and below the box! These vortices (that’s the fancy word for water spinning in circles) happen when fast-moving water gets forced around the box’s corners. Notice how the fastest water (yellow-red colors) squeezes around the edges of the box, reaching speeds over 2 meters per second! The box creates a wake behind it too, just like boats do. We achieved highly detailed flow visualization that perfectly shows how objects create turbulence underwater. And remember, this is just one frozen moment from our animation video that shows the whole journey of the box falling through water. The full video would show how these patterns constantly change as the box sinks deeper and finds its final position at the bottom.

Figure 3: Velocity streamlines around the falling box displaying vortex formation