Sine sweep load vs. impact excitations and their influence on the damping coefficients of a bubbly oil squeeze film damper

Squeeze Film Dampers (SFD) suppress excessive vibrations and rotordynamic instabilities in turbomachinery. However, air ingestion into the oil film is a pervasive phenomenon that affects their performance, complicating their analysis and design, and demanding careful experimentation. The type of force excitation affects the damping coefficients since the ensuing dynamic journal motions may lead to a rapid expulsion or to coalescence and entrapment of the air within the lubricant matrix. Experimental force coefficients from a small rotor-SFD apparatus operating with controlled mixtures of air and oil, i.e. emulating degrees of air entrainment, are obtatined from the dynamic response to sine sweep forces and impact loads. The parameter identification procedure renders damping coefficients that are sensitive to the type of force excitation. For impact tests, damping coefficients steadily increase for lubricant mixtures up to 50 % in air volume content. For unidirectional sine sweep load excitations, the damping coefficients are nearly constant even for mixtures with larger air volume fractions. The larger and sustained amplitudes of periodic journal motion induced in the sweep sine tests expel rapidly the air from the mixture, thus leaving a lubricant film that generates invariant dumping coefficients. Conversely, in the impact tests, the journal motions are of small amplitude and short duration thus providing larger damping values; the mixture behaves as a nearly incompressible fluid of larger viscosity.