The effect of flow field on drag torque in a wet clutch was examined through a combined numerical and experimental study. Three-dimensional hydrodynamic numerical simulations were carried out, and the drag torque was measured experimentally for a single wet clutch pack. Two-phase flow induced by aeration was visualized in the experiment. In the present drag torque test, the main section was consisted of two parallel circular plates. The plate with the frictional material was rotated. The frictional material was divided into some sections, and radial or circumferential grooves were made on the rotating disk. Automatic Transmission Fluid (ATF) was supplied from the axial center, and ejected into the surrounding open boundary. At low rotation speeds, it was found that the oil flow is of single-phase, and the drag torque is linearly proportional to the rotation speed since the shear stress on the clutch plate increased monotonically. In the single-flow regime, the slope of drag torque curve was controlled with the clearance between the clutch plates. The drag torque reached a peak value at a certain rotation speed, and it decreased gradually after the peak. These observed phenomena were due to the aeration from the inner gap on the disk, and the bubble volume fraction was directly related to the drag torque. The peak of drag torque was controlled by both the flow rate of supplied ATF and the arrangement of grooves on the frictional material. It also was found that the smooth ejection of ATF and the enhancement of aeration led to a reduction in the drag torque.

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