Transient Response Of Rotor Research Articles

Dynamic Analysis of a Rotor System Supported on Squeeze Film Damper with Air Entrainment

Squeeze film dampers (SFDs) are widely used in compressors and turbines to suppress the vibration while traversing critical speeds. In practical applications, air ingestion from the outside environment and cavitation may lead to a foamy lubricant that weakens oil film damping and dynamic performance of rotor system. In this paper, a rigid rotor model is established considering both lateral and pitching vibration under different imbalance excitations to evaluate the effect of air entrainment on rotor system. Tests with three different imbalances are carried out on a rotor-SFD apparatus. Volume controlled air in mixture ranging from pure oil to all air are supplied to the SFD. The transient response of rotor is measured in the experiments. The results show that two-phase flow produces significant influence on the system stability and dynamical response. The damping properties are weakened by entrained air, such as the damping on high frequency components of rolling ball bearing. Super-harmonic resonance and bifurcation are observed, as well as the low frequency components due to air entrainment.

Real-Time Aero-elasticity Simulation of Open Rotors With Slender Blades for the Multidisciplinary Design of Rotorcraft

This paper elaborates on the theoretical development of a mathematical approach, targeting the real-time simulation of aero-elasticity for open rotors with slender blades, as employed in the majority of rotorcraft. A Lagrangian approach is formulated for the rapid estimation of natural vibration characteristics of rotor blades with nonuniform structural properties. Modal characteristics obtained from classical vibration analysis methods are utilized as assumed deformation functions. Closed form integral expressions are incorporated, describing the generalized centrifugal forces and moments acting on the blade. The treatment of three-dimensional elastic blade kinematics in the time-domain is thoroughly discussed. In order to ensure robustness and establish applicability in real time, a novel, second-order accurate, finite-difference scheme is utilized for the temporal discretization of elastic blade motion. The developed mathematical approach is coupled with a finite-state induced flow model, an unsteady blade element aerodynamics model, and a dynamic wake distortion model. The combined formulation is implemented in an existing helicopter flight mechanics code. The aero-elastic behavior of a full-scale hingeless helicopter rotor has been investigated. Results are presented in terms of rotor blade resonant frequencies, rotor trim performance, oscillatory structural blade loads, and transient rotor response to control inputs. Extensive comparisons are carried out with wind tunnel (WT) and flight test (FT) measurements found in the open literature as well as with nonreal-time comprehensive analysis methods. It is shown that the proposed approach exhibits good agreement with measured data regarding trim performance and transient rotor response characteristics. Accurate estimation of structural blade loads is demonstrated, in terms of both amplitude and phase, up to the third harmonic component of oscillatory loading. It is shown that the developed model can be utilized for real-time simulation on a modern personal computer. The proposed methodology essentially constitutes an enabling technology for the multidisciplinary design of rotorcraft, when a compromise between simulation fidelity and computational efficiency has to be sought for in the process of model development.

Dynamic Response of Squeeze Film Dampers Operating With Bubbly Mixtures

Squeeze film dampers (SFDs) aid to attenuate vibrations in compressors and turbines while traversing critical speeds. In actual applications, gas ingestion from the environment may lead to the formation of a foamy lubricant that degrades the rotor/bearing system dynamic performance. Impact and imbalance response tests conducted on a rigid rotor supported on SFDs, and aimed to emulate the pervasive effect of air ingestion into the damper film lands, are reported. Two types of squeeze film damper support the test rotor, one is a conventional cylindrical design with a squirrel cage-type elastic support, and the other is a compact four-pad damper with integral wire EDM elastic supports. Both dampers have identical diameter and radial clearance. Controlled (air in oil) mixtures ranging from pure oil to all air conditions are supplied to the SFDs, and measurements of the transient rotor response to calibrated impact loads are conducted. System damping coefficients, identified from acceleration/load transfer functions, decrease steadily as the air content in the mixture increases. However, measurements of the rotor synchronous imbalance response conducted with a lubricant bubbly mixture (50% air volume) show little difference with test results obtained with pure lubricant supplied to the dampers. The experimental results show that air entrainment is process and device-dependent, and that small amounts of lubricant enable the effective action of SFDs when the rotor traverses a critical speed.

Active Control of Gimballed Rotors Using Swashplate Actuation During Shipboard Engagement Operations

An analysis has been developed to investigate the feedback control of gimballed rotor systems using swashplate actuation during transient shipboard engagement operations. The equations of motion for a rigid, three-bladed gimballed rotor system were derived using a Newtonian force summation method. Aerodynamic forces were simulated with a blade-element linear attached flow model. The resulting equations of motion were time integrated along a specified rotor-speed profile. A time domain linear-quadratic-regulator optimal control technique was applied to the equations of motion to minimize the transient rotor response. The physical limits of collective and cyclic pitch inputs in both magnitude and rate were enforced. The maximum transient gimbal tilt angle was reduced by as much as 56% with full knowledge of the ship air-wake environment and 42% with only partial knowledge of the ship airwake environment. Increasing the physical limits of collective and cyclic pitch inputs achieved up to a 70% reduction in the maximum transient gimbal tilt angle.

Vibration analysis of a misaligned rotor—coupling—bearing system passing through the critical speed

The transient response of a misaligned rotor—coupling—bearing system passing through the critical speed has been analysed by using the finite element method (FEM) for flexural vibrations. The coupling has been modelled in two ways: a frictionless joint and a joint with stiffness and damping. From the vibration analysis, the subcritical speeds at one-half, one-third and one-fourth the critical speed have been found when the misaligned rotor—coupling—bearing system passes through its critical speed. The continuous wavelet transform (CWT) has been used as a tool to extract the silent features from the time response of the rotor system. A parametric study has been carried out to investigate the transient response of this rotor system for different angular accelerations in different types of misalignment.

Stability Analysis of Flexibly Supported Rough Submerged Oil Journal Bearings

A theoretical analysis is presented to study the effect of support stiffness and damping on the transient response of flexibly supported rotor-bearing systems, considering surface roughness. The transient response of rotor is plotted in the form of trajectories. The average Reynolds equation, which uses the Patir and Cheng flow simulation model of rough surfaces, is solved by the finite difference method with a successive over-relaxation scheme (Gauss-Seidel), while the equations of motion of both the rotor and bearing are solved by the fourth-order Runge-Kutta method. It has been found that the surface roughness considerably affects the stability of a rotor-bearing system and the stability may be improved greatly by a proper design of bearing supports.