Squeeze film dampers (SFDs) provide viscous damping to attenuate excessive vibrations and enhance system stability in turbomachinery. SFDs are of special importance in aircraft engines which use rolling element support bearings that, by themselves, do not provide enough damping to ensure safe operation. A modular test rig capable of simulating actual operating conditions in aircraft jet engines is used to test two centrally grooved, end sealed, SFDs. Both SFDs have diameter D and nominal radial clearance c and consist of two parallel squeeze film lands separated by a deep circumferential groove of length LG and depth dG. A short length damper with film land lengths L and a long damper with land lengths 2L are tested. Piston rings seal the damper lands. An ISO VG2 lubricant is supplied to the SFD via three radial holes that discharge lubricant into the central groove. The lubricant passes through the damper lands and across the piston ring seals to finally exit the damper at ambient pressure. Circular orbit tests of amplitude ~0.5c and for static eccentricities varying from 0 to ~0.36c are conducted on the two sealed dampers. The instrumental variable filter method (IVFM) serves to identify the SFD dynamic force coefficients. The parameter identification range is 50Hz to 210Hz for the short damper and 110Hz to 250Hz for the long damper. Large amplitude dynamic pressures measured in the central groove demonstrate that the central groove does not divide the damper in two separate film lands, but the lubricant in the groove interacts with the squeeze film lands, hence contributing significantly to the SFD forced response. Dynamic pressures in the film lands and in the central groove reveal that both dampers operate free of air ingestion or cavitation for the tested static eccentricities and amplitudes of motion. Comparisons to test results for the same SFD configurations but with open ends demonstrate the effectiveness of the end seals on increasing the direct damping coefficients. For the sealed ends short length damper, the added mass coefficients are ~2 times larger and the damping coefficients are ~3.8 times larger than the respective coefficients of the open ends long damper. For the sealed ends long damper, the damping coefficients are ~2.8 times, and the added mass coefficients are ~3.1 times larger than coefficients from the open ends configuration. The identified SFD direct stiffness coefficients are nearly zero except at the maximum static eccentricity for the long damper. Predictions from a novel computational model that include the effects of the central groove, the lubricant feed holes and the end seals are in excellent agreement with results from the short length damper. For the long damper, the predicted damping coefficients are in good agreement with the test results, while the added mass coefficients are under predicted by ~25 percent. Experimental results from the two sealed SFD configurations lead to a better understanding of the effects of end seals as well as central feed groves on the SFD forced performance. The results presented in this thesis will help improve the effectiveness of SFDs aircraft jet engines.

Squeeze film dampers are used in the high speed turbo machinery industry and aerospace industries as a means to reduce vibration amplitude, to provide damping, to improve dynamic stability of the rotor bearing system and to isolate structural components. The effects of cavitation included in previous studies were not effective. The effect of different design parameters were not studied thoroughly as experimental investigation of squeeze film dampers is very expensive. Few of them used numerical investigation but the methods they used are either time consuming or complicated. The present study investigated the feasibility of applying a steady state solver, which is computationally less expensive, for analyzing flow field inside the squeeze film dampers. The behavior of dynamic pressure profiles at different operating conditions, and the effect of a central groove on dynamic pressure profiles were also studied. Simulation results of a 3D case which is similar to the one experimentally studied by Delgado were used to establish if the moving reference frame (MRF) model in Fluent 12.1 can be used. A steady state solver in an absolute frame of reference was used to produce whirling motion of the rotor in this study. The inlet pressure of 31kpa and the whirling speed of 50 and 100Hz were used as boundary conditions. The mixture model with three percent dissolved air in lubricant is used to model multiphase flow. Singhal cavitation model is used to model cavitation. The simulations (50,000 iterations) were run until steady state solutions were reached. The results closely agreed with those obtained experimentally by San Andrés and Delgado. Numerical simulations of three-dimensional cases with an additional central groove on the squeeze film land were also performed to predict the effect of central groove on dynamic pressure profiles. Addition central groove reduces the pressures and forces generated by squeeze film damper.

The accuracy of rotordynamic analysis of aeroengine structures is typically limited by a trade-off between the capabilities and the computational cost of the squeeze-film damper (SFD) bearing model used. Identification techniques provide a means of efficiently implementing complex nonlinear bearing models in practical rotordynamic analysis; thus facilitating design optimisation of the SFD and the engine structure. This thesis considers both identification from advanced numerical models and identification from experimental tests. Identification from numerical models is essential at the design stage, where rapid simulation of the dynamic performance of a variety of designs is required. Experimental identification is useful to capture effects that are difficult to model (e.g. geometric imperfections). The main contributions of this thesis are: • The development of an identification technique using Chebyshev polynomial fits to identify the numerical solution of the incompressible Reynolds equation. The proposed method manipulates the Reynolds equation to allow efficient and accurate identification in the presence of cavitation, the feed-groove, feed-ports, end-plate seals and supply pressure. • The first-ever nonlinear dynamic analysis on a realistically sized twin-spool aeroengine model that fulfills the aim of taking into account the complexities of both structure and bearing model while allowing the analysis to be performed, in reasonable time frames, on a standard desktop computer. • The introduction and validation of a nonlinear SFD identification technique that uses neural networks trained from experimental data to reproduce the input-output function governing a real SFD. Numerical solution of the Reynolds equation, using a finite difference (FD) formulation with appropriate boundary conditions, is presented. This provides the base data for the identification of the SFD via Chebyshev interpolation. The identified 'FD-Chebyshev' model is initially validated against the base (FD) model by application to a simple rotor-bearing system. The superiority of vibration prediction using the FD-Chebyshev model over simplified analytical SFD models is demonstrated by comparison with published experimental results. An enhanced FD-Chebyshev scheme is then implemented within the whole-engine analysis of a realistically sized representative twin-spool aeroengine model provided by a leading manufacturer. Use of the novel Chebyshev polynomial technique is repeatedly demonstrated to reduce computation times by a factor of 10 or more when compared to the basis (FD) model, with virtually no effect on the accuracy. Focus is then shifted to an empirical identification technique. Details of the commissioning of an identification test rig and its associated data acquisition system are presented. Finally, the empirical neural networks identification process for the force function of an SFD is presented and thoroughly validated. When used within the rotordynamic analysis of the test rig, the trained neural networks is shown to be capable of predicting complex nonlinear phenomena with remarkable accuracy. The results show that the neural networks are able to capture the effects of features that are difficult to model or peculiar to a given SFD.

Squeeze Film Dampers (SFDs) are employed in high-speed Turbomachinery, particularly aircraft jet engines, to provide external damping. Despite numerous successful applications, it is widely acknowledged that the theoretical models used for SFD design are either overly simplified or incapable of taking into account all the features such as cavitation, air entrainment etc., affecting the performance of a SFD. On the other hand, experimental investigation of flow field and dynamic performance of SFDs can be expensive and time consuming. The current work simulates the flow field inside the dynamically deforming annular gap of a SFD using the commercial computational fluid dynamics (CFD) code Fluent and compares the results to the experimental data of San Andrés and Delgado. The dynamic mesh capability of Fluent and a User Defined Function (UDF) was used to replicate the deforming gap and motion of the rotor respectively. Two dimensional simulations were first performed with different combinations of rotor whirl speed, operating pressures and with and without incorporating the cavitation model. The fluid used in the simulations was ISO VG 2 Mobil Velocite no. 3. After the successful use of the cavitation model in the 2D case, a 3D model with the same dimensions as the experimental setup was built and meshed. The simulations were run for a whirl speed of 50 Hz and an orbit amplitude of 74 μm with no through flow and an inlet pressure of 31kPa (gauge). The resulting pressures at the mid-span of the SFD land were obtained. They closely agreed with those obtained experimentally by San Andrés and Delgado.

Typical aero-engine assemblies have at least two nested rotors mounted within a flexible casing via squeeze-film damper (SFD) bearings. As a result, the flexible casing structures become highly sensitive to the vibration excitation arising from the High and Low pressure rotors. Lowering vibrations at the aircraft engine casing can reduce harmful effects on the aircraft engine. Inverse problem techniques provide a means toward solving the unbalance identification problem for a rotordynamic system supported by nonlinear SFD bearings, requiring prior knowledge of the structure and measurements of vibrations at the casing. This thesis presents two inverse solution techniques for the nonlinear rotordynamic inverse problem, which are focused on applications where the rotor is inaccessible under operating conditions, e.g. high pressure rotors. Numerical and experimental validations under hitherto unconsidered conditions have been conducted to test the robustness of each technique. The main contributions of this thesis are:• The development of a non-invasive inverse procedure for unbalance identification and balancing of a nonlinear SFD rotordynamic system. This method requires at least a linear connection to ensure a well-conditioned explicit relationship between the casing vibration and the rotor unbalance via frequency response functions. The method makes no simplifying assumptions made in previous research e.g. neglect of gyroscopic effects; assumption of structural isotropy; restriction to one SFD; circular centred orbits (CCOs) of the SFD. • The identification and validation of the inverse dynamic model of the nonlinear SFD element, based on recurrent neural networks (RNNs) that are trained to reproduce the Cartesian displacements of the journal relative to the bearing housing, when presented with given input time histories of the Cartesian SFD bearing forces.• The empirical validation of an entirely novel approach towards the solution of a nonlinear inverse rotor-bearing problem, one involving an identified empirical inverse SFD bearing model. This method is suitable for applications where there is no adequate linear connection between rotor and casing. Both inverse solutions are formulated using the Receptance Harmonic Balance Method (RHBM) as the underpinning theory. The first inverse solution uses the RHBM to generate the backwards operator, where a linear connection is required to guarantee an explicit inverse solution. A least-squares solution yields the equivalent unbalance distribution in prescribed planes of the rotor, which is consequently used to balance it. This method is successfully validated on distinct rotordynamic systems, using simulated data considering different practical scenarios of error sources, such as noisy data, model uncertainty and balancing errors. Focus is then shifted to the second inverse solution, which is experimentally-based. In contrast to the explicit inverse solution, the second alternative uses the inverse SFD model as an implicit inverse solution. Details of the SFD test rig and its set up for empirical identification are presented. The empirical RNN training process for the inverse function of an SFD is presented and validated as a part of a nonlinear inverse problem. Finally, it is proved that the RNN could thus serve as reliable virtual instrumentation for use within an inverse rotor-bearing problem.

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Aero-engine assemblies are complex structures typically involving two or three nested rotors mounted within a flexible casing via squeeze-film damper (SFD) bearings. The deployment of SFDs into such structures is highly cost-effective but requires careful calculation since they can be highly nonlinear in their performance, particularly if they are unsupported (i.e. without a retainer spring). The direct study of whole-engine models with nonlinear bearings has been severely limited by the fact that current nonlinear computational techniques are not well-suited for complex large-order systems. The main contributions of this thesis are: • A procedure for unbalance response computation, suitable for generic whole-engine models with nonlinear bearings, which significantly extends the capability of current finite element packages. This comprises two novel nonlinear computational techniques: an implicit time domain integator referred to as the Impulsive Receptance Method (IRM) that enables rapid computation in the time domain; a whole-engine Receptance Harmonic Balance Method (RHBM) for rapid calculation of the periodic response in the frequency domain. Both methods use modal data calculated from a one-off analysis of the linear part of the engine at zero speed.• First-ever analyses on real twin-spool and three-spool engines. These studies illustrate the practical use of these solvers, provide an insight into the nonlinear dynamics of whole-engines and correlate with a limited amount of industrial experimental data. Both IRM and RHBM are directly formulated in terms of the relative response at the terminals of the nonlinear bearings. This makes them practically immune to the number of modes that need to be included, which runs into several hundreds for a typical engine. The two solvers are extensively tested on two/three-shaft engine models (with 5-6 SFDs) provided by a leading engine manufacturer using an SFD model that is used in industry. The tests show the IRM to be many times faster than an established robust conventional implicit integrator while achieving a similar level of accuracy. It is also shown to be more reliable than another popular implicit algorithm. The RHBM enables, for the first time, the frequency domain computation of the nonlinear response of whole-engine models. Its use is illustrated for both Single-Frequency Unbalance (SFU) excitation (unbalance confined to only one shaft) and Multi-Frequency Unbalance (MFU) excitation (unbalance located on two or more shafts, rotating at different speeds). Excellent correlation is demonstrated between RHBM and IRM.The parametric studies compare and contrast the frequency spectra for SFU and MFU cases. They also reveal the varying degree of lift at the unsupported SFDs. The sensitivity of the response to end-sealing and bearing housing alignment is also illustrated. It is demonstrated that the use of suitably preloaded vertically oriented “bump-springs” at the SFDs of heavy rotors produces a significant improvement in journal lift. It is also shown that the consideration of a slight amount of distributed damping in the structure significantly affects the predicted casing vibration levels, bringing them closer to measured levels, while having little effect on the SFD orbits.