## 4 Results
### Optimization of the Damping Effect Control Parameters  
The time history of the rotor angular speed during the phase of its acceleration is defined by a sine function (Figure 7). It follows from the technical parameters that, if no damping is produced by the dampers (e.g., the dampers are not filled with the lubricating oil), the critical speed of the researched rotor is 169.4 rad/s. For the case when the dampers work in the overdamped regime, the critical speed rises to 228.0 rad/s. It implies that the rotor must pass through the area of critical speed during its running up. Figures 8 and 9 show the time histories of the radial displacements of the disc center during the rotor acceleration for two magnitudes of the control current, 0.0 A and 1.0 A. The phase shift of the excitation effect is 4/3 \(\times\)\(\pi\) rad (240\({}^{\circ}\)). The maximum amplitudes are high, 864 and 453 \(\upmu\)m, respectively. As the frequency response curves depicted in Figure 1 show, the initial high damping in the support elements must be changed to a minimum at just the right moment to minimize the oscillation amplitude. The reduction is achieved by switching off the applied current feeding the coils in the magnetorheological squeeze film dampers. At the beginning, the rotor rotates at constant angular velocity of 150 rad/s, the coils of the dampers are fed by the current of 1.0 A, and the phase shift of the excitation effect is 4/3 \(\times\)\(\pi\) rad (240\({}^{\circ}\)). At the point of time of 10.2 s, the rotor starts to accelerate. When the rotor reaches the speed of 195 rad/s, which corresponds to the moment of time of 10.67 s, the applied current is suddenly switched off. Figures 10 and 11 depict the rotor response,  
Figure 8: The time history of the disc center radial displacement (current 0.0 A). Figure 7: The time history of the rotor angular speed. the time history of the disc center displacement in the radial direction, and its trajectory. The results show that, after switching, the current off excessive irregular oscillation of the rotor is induced. The results of the computational simulations supported by the theoretical pieces of knowledge and previous investigations show that, to reduce amplitude of the rotor oscillation during its running up, the current decrease must not be sudden, but it must be distributed in a certain time interval. Therefore, a linear drop in the current from 1.0 to 0.0 A was tested (Figure 12). \(T\) and \(\Delta T\) denote the moment of time of starting and the time of duration of the current decrease, respectively. Figure 11: Trajectory of the disc center after sudden switching the current off. Figure 9: The time history of the disc center radial displacement (current 1.0 A). The optimum values of parameters of the current reduction (\(T\), \(\Delta T\)) are determined by solving an unconstrained optimization problem. The design variables are two--the time of the beginning of the current decrease (\(T\)) and the time of duration of this manipulation (\(\Delta T\)). The objective function is implicit. It is introduced as a maximum lateral displacement of the disc center during rotor acceleration, and this value is determined by means of computational simulations. To solve the optimization problem, the iterative evolution optimization method was applied. This method requires us to determine the initial estimation of the optimum values of the design variables and specification of their increments. Consequently, the initial hypercuboid (a rectangular for the case of two design variables) is constructed, with the center point corresponding to the initial estimation of the optimum solution. The length of the hypercuboid edges is equal to the increments of the design variables. Then, the values of the objective function are determined at the center and each corner point of the hypercuboid. If the minimum value is referred to some corner, a new hypercuboid with the same length as the edges is constructed, with the center at this corner.