FSI: Ball in Water Flow

This project simulates a spherical ball immersed in water flow using ANSYS Fluent coupled with structural analysis through the Fluid–Solid Interaction (FSI) method. FSI is the central theme of the study: rather than treating the ball as a rigid, unresponsive obstacle, the simulation couples the fluid solver with a structural solver so that the flow loads acting on the ball and the ball's structural response are computed together, each influencing the other.

The model is three-dimensional and was created in Design Modeler. It consists of a horizontal tube 0.02 m long and 0.001 m in diameter, with a spherical solid of 0.00009 m diameter placed inside it. Meshing was carried out in ANSYS Meshing with 20,192 elements, and because the coupled response evolves in time, a transient solver is used.

The heart of the methodology is the two-way coupling between ANSYS Fluent and Transient Structural via System Coupling. Because the solid boundary responds to the flow, the mesh adjacent to it must change instantaneously and in step with that response, so dynamic mesh techniques are employed. Smoothing keeps the number of nodes fixed and simply adjusts the mesh by moving or deforming the boundaries, while remeshing is invoked when boundary displacement becomes large relative to the local cell size, regenerating cells that have degraded beyond the acceptable quality limit. The pipe region is defined as stationary, and the wall of the ball is governed by system coupling with the structural solver.

The flow enters the tube at 0.001 m/s and exits at atmospheric pressure. In the structural analysis, the spherical body's wall is designated as a fluid–solid interaction boundary, meaning it can respond to the behaviour of the water flow. The data exchange between the two solvers is defined in the System Coupling settings, where a boundary acting as a source in one solver is mapped to the same boundary as a target in the other. Two data transfers are specified: force is passed from the fluid side to the structural side, and the resulting displacement is passed from the structural side back to the fluid region. In this way the water flow imposes loads on the spherical body, and the body's response feeds back into the flow field. The standard k-ε model is used to close the turbulent flow equations.

After solving, the simulation yields two-dimensional contours of pressure and shear stress over the surface of the spherical body, along with contours of velocity and pressure around the ball on the mid-plane of the tube, all corresponding to the final second of the simulation. On the structural side, contours of deformation and elastic strain are also obtained. As a study in FSI modelling, the project demonstrates how coupling a fluid solver with a structural solver — exchanging force and displacement across a shared interface and using dynamic mesh to track the moving boundary — captures the mutual interaction between a flowing fluid and a deformable solid body.