Numerical simulation of spheres in relative motion using dynamic meshing techniques

The commercial Computational Fluid Dynamics (CFD) code ANSYS CFX was used to simulate the flow around three dissimilar spheres in relative motion over a large range of Reynolds numbers (Re) from 102 to 106. The simulations utilise a six degrees-of-freedom Rigid Body Dynamics (RBD) solver to predict the motion of spheres in response to external forces. The simulations were intended to provide a benchmark of the code in its ability to accurately predict the flow around multiple submerged bodies, such as submarines and unmanned underwater vehicles (UUV) in relative motion. The simulations were found to be in good agreement with both experimental data for the drag predictions and analytical solutions for the simulated motions.

Due to the large relative motions between the spheres, the CFD simulation domain undergoes significant deformation, requiring dynamic meshing techniques to maintain the integrity of the mesh and solution. A number of options including mesh deformation and adaptive re-meshing, immersed solids, turbulence models, and the interface with the RBD solver were evaluated to optimise the time and resource utilisation, while maintaining acceptable accuracy and stability. The study identifies the merits of the different options to simulate multiple bodies in relative motion and provide time dependent hydrodynamic data at sufficient accuracy and speed to enable dynamic coupling with a control system for manoeuvring simulation of underwater vehicles.