*5.1. Flow Field Visualization*

The flow field simulation and the visualization test were conducted under the condition of the rotating speed of the grooved disk ω = 18.8 rad/s, the disk spacing *H* = 1.8 mm, and the inlet volumetric flow rate of oil *Q* = 40 mL/min. The comparison of oil volume fraction distribution between the simulation result and the visualization test of the flow field is shown in Figure 3. The 3D contour of the oil phase distribution is presented in Figure 4. In general, a two phase flow was exhibited in the flow field in the simulation. The simulation demonstrated that the oil film was continuous and complete near the inlet of the flow field. The air entrainment and the decrease of oil volume fraction took place radially between the inlet and the outlet, presenting an air–oil two-phase flow. There was a clear interface formed between the continuous oil phase and the two-phase area. Near the outlet surface, the oil volume fraction became close to zero, indicating an air phase flow. Previous studies also showed that a sufficient flow rate could ensure that the oil phase filled the entire flow field at a given disk speed and gap [24]. However, in practice, the flow rate was not enough when the rotating speed increased continuously. Then, the air entered the gap from the periphery of the disk gap to ensure the continuity of the flow, which could be observed from the streamline simulation of air phase in Figure 5. In the visualization test result, the inner and the outer radii of the grooved disk were specified as the inlet and the outlet, as marked in Figure 3. A relatively stable boundary between the continuous oil phase and the aerated part could also be clearly observed. The boundary was considered to be the air–oil interface, as shown in Figure 3. Obviously, the visualization test also showed the same flow. In conclusion, the simulated oil volume fraction distribution correlated well with the visualization test result. The CFD model was therefore validated through this comparison.

Figure 5 illustrates the contour surface of ϕoil = 0.75 and the streamline of fluid flow. From the contour surface of ϕoil = 0.75 and the 3D contour of oil phase distribution in Figure 4, the oil phase occupied more area on the stationary disk surface than the rotating disk surface. The flow of oil gathered around the grooved zone. From the flow streamline in Figure 5, it can be noted that the oil flowed circumferentially along the ungrooved surface within the continuous film. The oil phase flowed into the radial groove and then came out, retaining circumferential flow status. Outside the interface of the continuous oil film, the oil phase was aerated and became a two-phase flow. The two-phase flow mainly discharged through the radial groove. Near the outer edge of disks, the air phase entered through the radial groove and flowed circumferentially in the ungrooved area. From the above observations, it can be deduced that a better cooling effect around the groove could be achieved due to a stronger mixing and heat exchange of oil and air around the grooved area. In practice, the oil flow was mainly used to provide sufficient cooling capability, especially in the high-speed operation. Thus, a sufficient assessment of the flow inside the groove was helpful for the heat dissipation.

**Figure 3.** Comparison of the oil volume distribution between simulation and visualization test.

**Figure 4.** 3D contour of oil phase distribution for a 36◦ section of flow field.

**Figure 5.** Streamline figure of fluid flow with the ϕoil = 0.75 contour surface shown.
