# Study on the Effect Mechanism of MRD to Rotor System
## 1 Introduction  
Rotating machinery, especially the aero-engine, is developing towards high flexibility and high reliability, which puts forward new requirements for the vibration of the rotor system. When a rotor system passes through the critical speed, a large deflection of it is produced, which brings safety hazard to the normal operation of the machine. Therefore, the effective measurements to control the vibration are of great significance to the stability and safety of the rotating machine. The supporting system is the only way to transmit vibration. So setting a damper between the bearing and the supporting structure is an effective way to reduce the load and vibration amplitude transmitted by the bearing. Squeeze film damper (SFD) is widely used in rotating machinery because of its simple structure and obvious effect. In 1963, [PERSON] [1] studied the damping mechanism of SFD to the rotor system with unbalanced force and verified that SFD could effectively reduce the vibration by experiment, which was helpful for rotor operation. [PERSON] [2] studied the effect of SFD on the rotor system with misalignment rubbing coupling faults and found that SFD with appropriate film clearance could significantly suppress the nonsynchronous response and reduce the vibrationamplitude. However, if the design parameters are inappropriate, SFD will deteriorate the vibration of the rotor system. [PERSON] [3] found that there was a limit to the unbalance of the rotor system. When the unbalance exceeded the limit, the dynamic characteristics of the rotor system supported on SFD were worse than those supported on the rigid supporting. Also, [PERSON] [4] found that the supporting damping and stiffness had optimum ranges, in which SFD could improve the rotor performance. When they were not in these optimum ranges, the rotor performance supported on SFD was significantly worse than that supported on the rigid supporting. So, SFD, a passive damper, has limitations in that it cannot adapt to system uncertainty and external interference. Active control (e.g., controllable squeeze film damper [5; 6], electromagnetic bearings [7; 8], shape memory alloy actuator [9], piezoelectric actuator [10; 11; 12]) can overcome the limitations of the passive control through adjusting its parameters to adapt to the change of the external conditions. As a kind of intelligent material, magnetorheological fluid (MRF) can rapidly change from liquid state to quasi-solid state under the action of an external magnetic field, and the change process is continuous, controllable and reversible. Magnetorheological damper (MRD) with MRF as lubricating oil is a kind of controllable SFD. It can actively change stiffness and damping under the action of an external magnetic field. Due to these features, MRD has attracted more and more attention in the field of vibration control of rotating machinery. Based on the Reynolds equation, [PERSON] [13; 14] established the model of film pressure of MRD, in which MRF was modelled as Bingham constitutive law. The mechanical characteristics of MRD and the unbalanced response characteristics of the rotor system with MRD were analyzed by numerical simulation and verified by experiment. [PERSON] [15; 16] proposed a disc-type MRD operating in shear mode and studied the effectiveness of MRD for suppressing the vibration of the rotor system under AC sinusoidal magnetic fields by experiment. [PERSON] [17; 18; 19; 20] presented a feasibility study of MRD and compared the two solutions. After that, an experimental table of MRD-rotor was developed and a series of experimental and theoretical researches were carried out. Considering the thermal effect, [PERSON] [21] established a dynamic model of MRD based on Bingham constitutive law and analyzed the influence of temperature and current on the steady-state response of the rotor system. [PERSON] [22] presented an enhanced mathematical model based on a bilinear constitutive law and studied the effect of MRD on the force transmission and vibration attenuation of a rotor system.