The flow field of an open grooved two-disc system was studied. The system includes a rotating finite disc and a stationary finite disc. The rotating disc has radial grooves. The numerical results for the air-oil two-phase flow inside the open grooved two-disc system calculated by the CFD code FLUENT were proposed. The results are discussed and compared with the published test results. The results indicate that the groove affects the transitional characteristics from a single-phase flow to an air-oil two-phase flow of the flow field. In the same flow area, the radial oil flow coefficient is enhanced with a larger groove number. The oil mainly discharges through the groove and the air flows into the non-grooved area from the upstream side of the groove. The effects of the angular velocity on the oil volume fraction and the drag torque become greater when the disc is grooved. It is weakened in the high-velocity operations. The drag torque can be reduced by increasing the groove number in the same flow area. The axial force applied on the disc is gradually decreased with the increase of the angular velocity and becomes close to zero finally. The increase of the groove number reduces the maximum axial force. The results can be used for the optimisation of the Tesla pump and wet clutch. (C) 2015 Elsevier Ltd. All rights reserved.