Ball in Water Flow

FSI Analysis for a Ball in Water Flow — ANSYS Fluent CFD Simulation

Description

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: 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. As the opening project of the FSI: Beginner CFD Training Package, it introduces the core idea of two-way coupling on the simplest possible geometry — a single body deforming under fluid load — establishing the workflow the airfoil, vessel, and turbine cases build on.

Methodology

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 in step with that response, so dynamic mesh techniques are employed: smoothing keeps the number of nodes fixed and 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, so it can respond to 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: 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. The standard k-ε model closes the turbulent flow equations.

Analysis

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. Together these show how the water flow imposes loads on the ball, how the ball deforms in response, and how that deformation feeds back into the flow field. By the end of this project, you'll be able to set up a two-way FSI simulation coupling ANSYS Fluent with Transient Structural through System Coupling, configure the force-and-displacement data transfer across the interface, apply dynamic mesh to track the deforming boundary, and interpret the coupled fluid and structural results that capture the mutual interaction between a flowing fluid and a deformable body.