Rotordynamic Modal Testing via Journal Bearing Lubricant Film - An Experimental Validation

Experimental modal testing is performed in a rotor-bearing system using the journal-bearing lubricant film as an excitation source (shaker). The aerodynamic pressure field between the rotor and the bushing surface is influenced by the pressurized air injection through four orifices orthogonally machined on the bushing surface. The air injections are independently controlled by four piezoelectric stack actuators. Fluid film excitation forces resulting from the aerodynamic pressure variations are dependent on: i) the air supply pressures; ii) the DC amplitudes of the input signals to the piezoelectric actuators, i.e., opening-closing the flow through the orifices; iii) the AC amplitudes of the input signal – a Gaussian-distributed signal – supplied to the piezoelectric actuators; iv) different configuration of active injection orifice; and v) the journal angular velocity. The estimation of damping ratios and natural frequencies is done using a combination of four operational modal analysis (OMA) algorithms. Additionally, classical experimental modal analysis (EMA) is performed using an instrumented impact hammer and the results are used as a benchmark for the validation of the results coming from the novel technique combining the excitation via fluid film with output-only modal analysis algorithms. Good agreements between the two experimental approaches are found. The influence of the amplitudes of the input signal to the piezoelectric actuators, the air supply pressure, the orifice activation layouts, and the rotor angular velocities on the natural frequencies and damping ratios of the rotor-bearing system are investigated using both experimental dynamic testing techniques.