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In this article, using finite element method the effects of the preload on the nonlinear dynamic behavior of the noncircular two lobe aerodynamic journal bearing have been investigated. Assuming that the rotor is solid, the governing Rynolds equations for both the gas lubricant and rotor equation of motion in static and dynamic conditions have been derived and performance of the noncircular aerodynamic journal bearing in different conditions has been evaluated. Rung Kutta method has been used to solve the time dependent equations of motions of noncircular aerodynamic journal bearing and its gas lubricant. Using the numerical results, to investigate the motion of the center of the rotor in dynamic conditions, the graphs of frequency response, power spectrum, dynamic trajectory, Poincare map and bifurcation diagram have been plotted. The results show periodic, quasi periodic and chaotic rotor behavior for different bearing preload. It is concluded that appropriate selection of rotor parameters like its preload and suitable design and fabrication of rotor and its bearing can prevent any undesirable perturbed motions of the shaft and both the collision and wear of the rotor and bearing.

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Noncircular lobed journal bearing performance, in comparison with circular types, depends on various design parameters such as tilt and mount angles. Mounting orientation of this kind of bearings with respect to machine frame (mount angle) and also the way of setting their lobes with respect to each other (tilt angle), can change the bearings configuration and as the result their performances. In present study the thermo-hydrodynamic performance of noncircular two, three and four lobed journal bearings for different values of tilt and mount angles, using generalized differential quadrature (GDQ) method, are investigated. The results show that the thermal effects on these bearings performance are considerable and that the thermal consideration makes the results closer to real performance situations. The results of bearings performances due to rise in temperature in rotor, lubricant fluid and bearing shell, when compared to their isothermal conditions, show that viscosity of lubricant as well as load carrying capacity of bearings are decreased, depending on tilt and mount angles especially in case of two lobed bearings. The results also show that the effects of tilt and mount angles on bearing performance are periodic and so it is possible to select these angles suitably for bearings to be optimum.