# Effect of Rotational Speed Variation on the Flow Characteristics in the Rotor-Stator System Cavity
## 3 Results and Discussion
### Flow Field Distribution Characteristics
plays a leading role in the flow direction, and the circumferential velocity of the fluid in the reflux area under the action of viscous force gradually decreases. A similar situation is not observed in Figure 10c,d. The inertial force of the superimposed flow formed by the main flow and the reflux gradually decreases along the radial direction, and the dynamic and static interference between the rotor and stator dominates the flow in the cavity. Therefore, starting from \(\mathrm{L_{A3}}\), the fluid in the rotor boundary layer has a smooth transition between the circumferential velocity and the recirculation zone, and there is no large negative velocity gradient, as in \(\mathrm{L_{A1}}\). The axial distribution of \(V_{\mathrm{u}}\)\({}^{*}\) in the rotor-stator system shows a monotonous increasing trend. When the fluid is in motion, owing to the adhesion between the fluid and the solid surface and the interaction between the molecules in the fluid, the fluid undergoes shear deformation. The shear strain rate represents the gradient of fluid velocity along the thickness direction of the fluid. The distribution law of the shear strain rate along \(\mathrm{L_{RB}}\) and \(\mathrm{L_{RC}}\) of the rotor-stator system cavity under different rotating Reynolds numbers is shown in Figure 11.  
Figure 11a reveals that, at the position of \(r^{*}<0.48\), the shear strain rate in the rotor boundary layer basically does not change with an increase in the rotating Reynolds number. In the boundary layer of the rotor, the maximum peak appears at \(r^{*}=0.35\). Because the axial flow is squeezed by the main vortex close to the stator, the flow path gradually becomes smaller, and the velocity of the fluid with inertial force reaches a maximum near this point. At \(r^{*}>0.48\), as the fluid moves in the radial direction, the \(V_{\mathrm{tan}}\) generated by the inertial force in the rotor boundary layer gradually decreases. However, the circumferential velocity of the fluid induced by the adhesion of the rotor surface has a gradually increasing influence on the flow in the boundary layer of the rotor, and the shear strain rate under different working conditions is inversely proportional to the rotating Reynolds number. As the figure shows, the flow within \(Re_{\Phi}=2.12\times 10^{6}\) to \(4.23\times 10^{6}\) produces a trough at \(r^{*}=0.5\)-\(0.7\). After this point, the rotation in the rotor boundary layer plays a dominant role in the change in the shear strain rate. The radial variation law of the shear strain rate at the center of the cavity is shown in Figure 11b. The variation trend under different working conditions is almost the same, and wave peaks appear near \(r^{*}=0.12\). The reason for this peak is the velocity gradient between the fluid in the recirculation zone and the incoming flow from the inlet. In the dashed box near the outlet of the cavity, the shear strain rate of different rotating Reynolds numbers changes regularly, and this position roughly corresponds to the area where the secondary vortex is separated, as shown in Figure 9. The appearance of a vortex in the rotor-stator system cavity shows a trend of first increasing and then decreasing within a certain range on \(\mathrm{L_{RC}}\). For example, the position of the main vortex in \(Re_{\Phi}=2.12\times 10^{6}\) is \(r^{*}>0.25\). The box shows that, when \(Re_{\Phi}=3.53\times 10^{6}\), the gradient of the shear strain rate  
Figure 11: Distribution of shear strain rate (**a**) monitoring line of \(\mathrm{L_{RB}}\); (**b**) monitoring line of \(\mathrm{L_{RC}}\). approaches zero, so there is a critical rotating Reynolds number that induces the separation of the secondary vortex.