An Immersed Rigid Bodies CFD simulation is a computer model of objects moving through a fluid. This type of Fluid-Structure Interaction (FSI) CFD is very complex but also very important for engineering. We can use it to study things like underwater vehicles or parts moving inside machinery. In this Immersed Rigid Bodies Fluent analysis, we use the Dynamic Mesh feature. This lets the grid in our simulation move and change shape as the objects move. The 6DOF (Six Degrees of Freedom) solver allows the bodies to move and rotate freely, based on the forces from the fluid. Our study uses the methods from the key reference paper by Cruchaga et al. [1] to ensure our model is accurate.

• Reference [1]: Cruchaga, Marcela A., Christian M. Muñoz, and Diego J. Celentano. “Simulation and experimental validation of the motion of immersed rigid bodies in viscous flows.” Computer Methods in Applied Mechanics and Engineering33-40 (2008): 2823-2835.

Figure 1: An image from the reference paper [1] showing the motion of immersed bodies.

Simulation Process: Fluent Setup, Dynamic Mesh and 6DOF

To perform this Immersed Rigid Bodies Dynamic Mesh CFD study, we first created a 2D computational domain containing two circular cylinders. The fluid was not water; it was a highly viscous fluid with a density of 1000 kg/m³ and a high dynamic viscosity of 0.8 kg/m·s. In ANSYS Fluent, we used the transient solver because the motion of the cylinders changes over time. We activated the Dynamic Mesh model, using both smoothing and remeshing techniques. This is very important because it allows the grid to adapt as the cylinders move, keeping the calculation accurate. We also used the powerful 6DOF Solver Fluent capability, which calculates the fluid forces on the cylinders and then moves them realistically in a fully coupled way.

Figure 2: Mesh generated for immersed bodies 6DOF analysis

Post-processing: CFD Analysis, Hydrodynamic Drag and Terminal Velocity in Viscous Flow

The graph of the drag coefficient acts as a diagnostic map of the hydrodynamic forces acting on the cylinders. From an engineering standpoint, this graph tells the story of the fluid resisting the motion of the objects. The drag coefficient smoothly increases from about 1.4 to 2.5 over the one-second simulation. This trend is expected and correct; as the cylinders accelerate, the resistance from the thick, viscous fluid grows stronger. A critical check of our simulation’s accuracy is that the drag curves for both the left and right cylinders are almost identical. This confirms that the simulation is balanced and the forces are being calculated correctly for both bodies.

Figure 3 A graph of the drag coefficient from the Drag Prediction CFD analysis, showing the force on both cylinders over time.

The velocity contour and the velocity graph together explain the result of these forces. The cylinders accelerate until their speed becomes almost constant at 1.05 m/s. This is the terminal velocity. It is the point where the drag force from the fluid has become so large that it almost perfectly balances the force making the cylinders move. The velocity contour shows the flow field that creates this drag. We can see the low-velocity wake region behind each cylinder, which is a key part of the pressure drag. We can also see the fluid speeding up to 1.26 m/s in the narrow gap between the two bodies. The most important achievement of this simulation is its ability to accurately couple the fluid forces with the body’s motion, correctly predicting the terminal velocity by calculating the changing drag force as the bodies accelerate through a highly viscous fluid.

Figure 5: A contour of velocity from the Viscous Flow Simulation, showing the flow field around the two moving cylinders, combined with a graph of their velocity over time.