Two-Phase MRF Model for Wet Clutch Drag Simulation

Wet clutch packs are widely used in today's automatic transmission systems for gear-ratio shifting. The frictional interfaces between the clutch plates are continuously lubricated with transmission fluid for both thermal and friction management. The open clutch packs shear transmission fluid across the rotating plates, contributing to measurable energy losses. A typical multi-speed transmission includes as many as 5 clutch packs. Of those, two to three clutches are open at any time during a typical drive cycle, presenting an opportunity for fuel economy gain. However, reducing open clutch drag is very challenging, while meeting cooling requirements and shift quality targets. In practice, clutch design adjustment is performed through trial-and-error evaluation of hardware on a test bench. The use of analytical methodologies is limited for optimizing clutch design features due to the complexity of fluid-structure interactions under rotating conditions. This article presents a two-phase Multiple Reference Frame (MRF) CFD model for simulating wet clutch behaviors, accounting for detailed design geometry. The model employs the Volume of Fluid (VOF) method to determine the air-fluid interface inside a computational domain. Model setup and simulation parameters, including initial conditions, boundary conditions, and relaxation factors are evaluated in terms of convergence behaviors. The model capabilities are validated against experimental data. Convergence of the CFD solver is demonstrated, capturing peak drag location as a function of rotating speed, until the phase fraction drops to a small value. The CFD model provides analytical insight into complex fluid interactions for grooved rotating plates, complementing hardware-based clutch design processes.