Browsing by Subject "rotordynamics"
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Item Discussion of Induction Motor Effect on Rotordynamics(2015-05-13) Han, Xu 1986-In this dissertation, the influence of using induction motor in machinery train on rotordynamics is discussed. Two areas are considered ? the use of variable frequency drives (VFDs), which control and drive the induction motor; and the unstable forces due motor eccentricity, which is resulted from motor rotor lateral motion. VFDs ? The dissertation documents fatigue related mechanical failures in VFD motor machinery due to mechanical vibrations excited by drive torque harmonics which are created by PWM switching. Present effort models the coupled system with full electrical system including DC bus, inverter, motor, and an industrial mechanical system including flexible couplings, gearboxes and multiple inertias. The approach extends failure prediction beyond simple occurrence of resonance, to fatigue life evaluation based on Rain-flow algorithm, which is suitable for both steady state and transient startup mechanical response. The use of multilevel inverters is demonstrated having the possibility actually exacerbate resonance and fatigue failure. The model is also compared to an industrial test case, which provides good agreement. Motor eccentricity ? In this dissertation, a MEC modeling method is proposed to calculate both the radial and tangential motor eccentric force. The proposed model is also coupled with the motor electric circuit model to provide capability of transient simulation. FEM (Ansys Maxwell) is used to verify the proposed model. Parametric study is performed on the motor radial and tangential eccentric forces. Also a Jeffcott rotor model is used to study the influence of the motor eccentric force on mechanical stability. A stability criteria of the bearing damping is calculated. The motor radial and tangential eccentric forces are all curved fitted to catch their nonlinearity, which are used in time domain simulation. Nonlinear motions are observed, including limit cycle and jumping phenomena. The results of this dissertation show that both the use of VFDs and the motor lateral motion (motor eccentricity) can cause severe mechanical vibration problem in a rotating machinery train. Both of these two problems need to be carefully concerned in design stage.Item Experimental and theoretical rotordynamic coefficients and leakage of straight smooth annular gas seals(Texas A&M University, 2005-02-17) Kerr, Bradley GrayResults are presented for experimental and theoretical rotordynamic coefficients and leakage of straight smooth annular gas seals. Experimental rotordynamic coefficients were measured and trends in changes of rotordynamic coefficients with operating variables such as rotor speed, back-pressure, fluid preswirl, and seal clearance are analyzed. Experimental results show that cross-coupled stiffness coefficients are highly influenced by fluid preswirl and only moderately influenced by other operating parameters, whereas direct damping is nearly unaffected by changes in operating parameters. Effective damping, a good indicator of stability, is highly affected by fluid preswirl. Although rotordynamic coefficients of straight smooth annular gas seals are assumed to be frequency independent, experimental results suggest a frequency dependent nature at high back-pressures and high excitation frequencies. Experimental results for rotordynamic coefficients and leakage are compared with theoretical predictions of ISOTSEAL, an isothermal-flow, two-control-volume, bulk-flow rotordynamic analysis program. All rotordynamic coefficients are underpredicted. Direct stiffness is poorly predicted while cross-coupled stiffness and direct damping are predicted reasonably well. Leakage is also consistently under-predicted. Theory predicts a slight frequency dependent nature for a limited number of test configurations.Item High Fidelity Simulation of Rotordynamic Morton Effect by Nonlinear Transient Approach(2014-08-06) Suh, JunhoThe present study is focused on accurate prediction of Morton effect problem including journal asymmetric heating and the corresponding long period amplitude oscillations using a nonlinear time transient rotordynamic simulation. For the analysis of the Morton effect problem, variable viscosity Reynolds equation and three-dimensional energy equation are coupled via temperature and viscosity, and solved simultaneously. Three-dimensional heat transfer equations of bearing and shaft are modeled by a finite element method, and thermally coupled with the fluid film via heat flux boundary condition. Asymmetric heat flux into the synchronously whirling rotor is solved by the orbit time averaged heat flux from fluid film to the spinning journal surface. The journal orbit is calculated by the nonlinear transient dynamic analysis of rotor-bearing system with variable time step numerical integration scheme. For the computation time reduction, modal coordinate transformation is adopted for dynamic and thermal transient analysis. This research explains how the thermal bow induced imbalance force develops in spinning journal with time, and how the vibration level is affected by the thermal bow vector. This dissertation is also focused on a new modeling method of three-dimensional thermo-elasto-hydro-dynamic cylindrical pivot tilting pad journal bearing (TPJB). For the computational efficiency, modal coordinate transformation is utilized in the flexible pad dynamic model, and pad dynamic behavior is represented only by means of modal coordinate. Fluid film thickness is calculated by a newly developed node based method, where pad arbitrary thermal and elastic deformation, and journal thermal expansion are taken into account simultaneously. This paper presents a new analysis method for a thermo-elasto-hydro-dynamic tilting-pad journal bearing system to reach a static equilibrium condition adopting nonlinear transient dynamic solver. In the nonlinear transient dynamic solver, physical and modal coordinates co-exist for computational efficiency, and transformation between modal and physical coordinate is performed at each numerical integration time step. Nonlinear time transient dynamic analysis and steady thermal analysis are combined to find the static equilibrium condition of the TPJB system, where the singular matrix issue of flexible pad finite element (FE) model is resolved.Item Identification of rotordynamic forces in a flexible rotor system using magnetic bearings(2009-06-02) Zutavern, Zachary ScottMethods are presented for parameter identification of an annular gas seal on a flexiblerotor test rig. Dynamic loads are applied by magnetic bearings (MBs) that support the rotor. MB forces are measured using fiber-optic strain gauges that are bonded to the poles of the MBs. In addition to force and position measurements, a finite element (FE) rotor model is required for the identification algorithms. The FE rotor model matches free-free characteristics of the test rotor. The addition of smooth air seals to the system introduces stiffness and damping terms for identification that are representative of reaction forces in turbomachines. Tests are performed to experimentally determine seal stiffness and damping coefficients for different running speeds and preswirl conditions. Stiffness and damping coefficients are determined using a frequency domain identification method. This method uses an iterative approach to minimize error between theoretical and experimental transfer functions. Several time domain approaches are also considered; however, these approaches do not produce valid identification results. Stiffness coefficients are measured using static test results and an MB current and position based model. Test results produce seal coefficients with low uncertainties for the frequency domain identification method. Static test uncertainties are an order of magnitude larger, and time domain attempts fail to produce sealIn addition to the primary identification research, an investigation of the relationships between MB force, strain, and magnetic field is conducted. The magnetic field of an MB is modeled using commercial FE software. The magnetic field model is used to predict strain measurements for quasi-static test conditions. The strain predictions are compared with experimental strain measurements. Strain predictions agree with experimental measurements, although strain is typically over-predicted. coefficient measurements.Item Measured and Predicted Rotor-Pad Transfer Functions for a Rocker-Pivot Tilting-Pad Journal Bearing(2012-02-14) Wilkes, Jason ChristopherMany researchers have compared predicted stiffness and damping coefficients for tilting-pad journal bearings (TPJBs) to measurements. Most have found that direct damping is consistently overpredicted. The thrust of this research is to explain the difference between measured and predicted stiffness and damping coefficients for TPJBs, and to provide some confidence to designers that TPJB dynamic coefficients can be accurately predicted. Most analytical models for TPJBs are based on the assumption that explicit dependence on pad motion can be eliminated by assuming harmonic rotor motion such that the amplitude and phase of pad motions resulting from radial and transverse rotor motions are predicted by rotor-pad transfer functions. In short, these transfer functions specify the amplitude and phase of pad motion (angular, radial, translational, etc.) in response to an input rotor motion. A new pad perturbation model is developed including the effects of angular, radial, and circumferential pad motion and changes in pad clearance due to pad bending compliance. Though all of these pad variables have previously been included in different analyses, there are no publications containing perturbations of all four variables. In addition, previous researchers have only perturbed the journal, while both the bearing and journal motions are perturbed in the present analysis, and the applicability of comparing rotor-perturbed bearing impedance predictions to impedances measured on a bearing-perturbed test rig is discussed. This perturbation model was implemented in a Reynolds-based TPJB code to predict the frequency-dependent bearing impedances and rotor-pad transfer functions. Direct measurements of pad motion during test excitation were recorded to produce measured transfer functions between rotor and pad motion, and a comparison between these measurements and predictions is given. Motion probes were added to the loaded pad (having the static load vector directed through its pivot) of a 5-pad TPJB to obtain accurate measurement of pad radial and tangential motion, as well as tilt, yaw, and pitch. Strain gages were attached to the side of the loaded pad to measure static and dynamic bending strains, which were then used to determine static and dynamic changes in pad curvature (pad clearance). Good agreement was found between the amplitude of the measured and predicted transfer functions concerning radial and transverse pad motions throughout the range of speeds and loads tested, while pad tilt was moderately underpredicted. For the bearing investigated, radial pad motions resulting from pivot compliance were as large as 60% of the radial component of shaft motion when operating at 4400 rpm under heavily loaded conditions. Hence, if a dynamic load applied to the shaft resulted in a shaft displacement of 25 microns (1 mil), the pad would displace radially 15 microns (0.6 mils), and the fluid film height would only decrease by 10 microns (0.4 mils). The consequence of this pad motion is that fluid film stiffness and damping forces produced by relative rotor-pad motions are significantly reduced, resulting in a bearing having significantly less direct stiffness and damping than predicted. A similar effect occurs when shaft motions produce significant changes in pad clearance due to pad compliance. For the pad tested here, the measurements show that predicting TPJB stiffness and damping coefficients without accounting for pad and pivot compliance will produce large errors, and is not advised. Transverse pad motion was predicted and observed. Based on phase measurements, this motion is lightly damped, and appears to be caused by pivot deflection instead of slipping. Despite observing a lightly damped phase change, an increase in magnitude at this natural frequency was not observed. Predicted direct stiffness and damping for unit loads from 0-3200 kPa (0-450 psi) fit through 1.5? running speed are within 18% of measurements at 4400 rpm, while predictions at 10200 rpm are within 10% of measurements. This is a significant improvement on the accuracy of predictions cited in literature. Comparisons between predictions from the developed bearing model neglecting pad, pivot, and pad and pivot flexibility show that predicted direct stiffness and damping coefficients for a model having a rigid pad and pivot are overestimated, respectively, by 202% and 811% at low speeds and large loads, by 176% and 513% at high speeds and high loads, and by 51% and 182% at high speeds and light loads. While the reader is likely questioning the degree to which these predictions are overestimated in regard to previous comparisons, these predictions are based on measured operating bearing clearances, which are 20-30% smaller than the cold bearing clearances that previous comparisons were based on. The effect of employing a full bearing model (retaining all of the pad degrees of freedom) versus a reduced bearing model (where only journal degrees of freedom are retained) in a stability calculation for a realistic rotor-bearing system is assessed. For the bearing tested, the bearing coefficients reduced at the frequency of the unstable eigenvalue (subsynchronously reduced) predicted a destabilizing cross-coupled stiffness coefficient at the onset of instability within 1% of the full model, while synchronously reduced coefficients for the lightly loaded bearing required 25% more destabilizing cross-coupled stiffness than the full model to cause system instability. This overestimation of stability is due to an increase in predicted direct damping at the synchronous frequency over the subsynchronously reduced value. This increase in direct damping with excitation frequency was also seen in highly loaded test data at frequencies below approximately 2?running speed, after which direct damping decreased with increasing excitation frequency. This effect was more pronounced in predictions, occurring at all load and speed combinations. The same stability calculation was performed using measured stiffness and damping coefficients at synchronous and subsynchronous frequencies at 10200 rpm. It was found that both the synchronously measured stiffness and damping and predictions using the full bearing model were more conservative than the model using subsynchronously measured stiffness and damping. This outcome contrasts with the comparison between models using synchronously and subsynchronously reduced impedance predictions, which showed the subsynchronously reduced model to be the most conservative. This contrast results from a predicted increase in damping with increasing excitation frequency at all speeds and loads, while this increase in damping with increasing excitation frequency was only measured at the most heavily loaded conditions.Item Measurement of Drag Torque, Lift Off Speed, and Identification of Frequency Dependent Stiffness and Damping Coefficients of a Shimmed Bump-Type Foil Bearing(2014-11-26) Norsworthy, Joshua DThis thesis presents measurements characterizing the static and dynamic performance of a BFB configured with shims of two thicknesses (30 ?m and 50 ?m). Parameters of interest include drag torque, rotor lift off speed, and the estimation of force coefficients of a BFB with shims (30 ?m and 50 ?m thick). The thesis also compares those results to those of the original BFB (without shims). Drag torque measurements during shaft acceleration tests up to 50 krpm show that the lift off speed of both the original bearing and the shimmed bearing increases linearly with applied unit load (W/(LD)). The bearing startup friction factor f=T/(RW) during dry sliding condition for the original bearing (f~0.3) is constant with applied load (W/(LD)), while the bearing in shimmed configurations show a larger friction factor that decreases with load (W/(LD)). Once airborne, a bearing in all three configurations (shimmed and not shimmed) show a similar low (f~0.05 at W/(LD)~ 20 kPa) friction factor that decreases with increasing load. Bearing dynamic force coefficients are estimated over a frequency range of 200-450 Hz, under a specific load ~14.3 kPa. The shims have an unremarkable effect on the foil bearing direct stiffness coefficients. The shimmed BFB shows increased direct damping coefficients (in particular along the static load direction and at high frequencies) while operating at 50 krpm. The energy dissipated is best characterized with a structural loss factor ?, a function of the bearing elastic (K) properties, and the sliding friction characteristics. Over the narrow arbitrary frequency range from 300-400 Hz and for dynamic motions of amplitude of 20 ?m, the bearing without shims offers a ?? 25% larger than the original bearing, demonstrating that the shimmed bearing dissipates more mechanical energy, albeit the standard deviation in the average loss factor of the shimmed bearings is much larger. Measurements of the turbocharger (TC) shaft vibration conducted as the shaft accelerates toward 50 krpm (833 Hz) show that a shimmed BFB reduced subsynchronous whirl motions of the TC shaft apparent with the original BFB (WFR~0.30). When supported on a BFB with 50 ?m shims, the TC shaft operates free of subsynchronous whirl motions. Shimming, therefore is a simple, economical way of increasing energy dissipation in BFBs thereby improving their rotordynamic performance. Alas shimming also offers some undesirable characteristics such as higher startup torque requirements. Note however, that once airborne, the drag friction factor of a shimmed BFB is similar to that of the original bearing.Item Modifications to a two-control-volume, frequency dependent, transfer-function analysis of hole-pattern gas annular seals(Texas A&M University, 2007-04-25) Shin, Yoon ShikA rotordynamic analysis of hole-pattern gas annular seals using a two-control-volume model, Ha and Childs and frequency dependent transfer-function model, Kleynhans and Childs is modified with four features. The energy equation is added, and real gas properties are used instead of the ideal gas equation of state. The depth of the hole-pattern is made variable with the axial distance along the seal. And last, the addition of deep grooves to hole-pattern seals is analyzed, and the code??????s predictions for the influence of a groove are compared with test data.Item On the characteristics of fault-induced rotor-dynamic bifurcations and nonlinear responses(Texas A&M University, 2004-11-15) Yang, BaozhongRotor-dynamic stability is a very important subject impacting the design, control, maintenance, and operating safety and reliability of rotary mechanical systems. As rotor-dynamic nonlinearities are significantly more prominent at higher rotary speeds, the demand for better and improved performance achievable through higher speeds has rendered the use of a linear approach for rotor-dynamic analysis both inadequate and ineffective. To establish the fundamental knowledge base necessary for addressing the need, it is essential that nonlinear rotor-dynamic responses indicative of the causes of nonlinearity, along with the bifurcated dynamic states of instability, be fully characterized. The objectives of the research are to study the various rotor-dynamic instabilities induced by crack breathing and bearing fluid film forces using a model rotor-bearing system and to investigate the applicability of the fundamental concept of instantaneous frequency for characterizing rotor-dynamic nonlinear responses. A comprehensive finite element model incorporating translational and rotational inertia, bending stiffness and gyroscopic moment is developed. The intrinsic modes extracted using the Empirical Mode Decomposition along with their instantaneous frequencies resolved using the Hilbert transform are applied to characterize the inception and progression of bifurcations suggestive of the changing rotor-dynamic state and impending instability. The dissertation presents and demonstrates an effective approach that integrates nonlinear rotor-dynamics, instantaneous time-frequency analysis, advanced notions of dynamic system diagnostics and numerical modeling applied to the detection and identification of sensitive variations indicative of a bifurcated dynamic state. All presented studies on rotor response subjected to various system configurations and ranges of parameters show good agreements with published results. Under the influence of crack opening, the rotor-bearing model system displays transitional behaviors typical of a nonlinear dynamic system, going from periodic to period-doubling, chaotic to eventual failure. When film forces are also considered, the model system demonstrates very different behaviors and failures from different settings and ranges of control parameters. As a result, a dynamic failure curve differentiating zones of stability and bifurcated instability from zones of dynamic failure is constructed and proposed as an alternative to the traditional stability chart. Observations and results such as these have important practical implications on the design and safe operation of high performance rotary machinery.Item Rotordynamics/discharge water-hammer coupling via seals in pump rotordynamics(Texas A&M University, 2004-09-30) Zhang, KaikaiA new closed-loop frequency-domain model is developed to incorporate the water hammer effect with pump rotordynamics, in order to investigate the sub-synchronous instability problem observed in a field pump. Seal flow-rate perturbations due to eccentricity are calculated from Soulas and San Andres's seal code. A complete transfer function matrix between rotor motion and reaction force due to pressure perturbation is developed in detail. Stability analysis with transfer-function'add-in' modules is conducted in XLTRC2. Seal clearances and the reaction force angle are found to be important in shifting natural frequencies and damping. The sub-synchronous instability observed in field is duplicated successfully with double-clearance seals.Item The influence of internal friction on rotordynamic instability(Texas A&M University, 2004-09-30) Srinivasan, AnandInternal friction has been known to be a cause of whirl instability in built-up rotors since the early 1900's. This internal damping tends to make the rotor whirl at shaft speeds greater than a critical speed, the whirl speed usually being equal to the critical speed. Over the years of research, though models have been developed to explain instabilities due to internal friction, its complex and unpredictable nature has made it extremely difficult to come up with a set of equations or rules that can be used to predict instabilities accurate enough for design. This thesis deals with suggesting improved methods for predicting the effects of shrink fits on threshold speeds of instability. A supporting objective is to quantify the internal friction in the system by measurements. Experimental methods of determining the internal damping with non-rotating tests are investigated, and the results are correlated with appropriate mathematical models for the system. Rotating experiments were carried out and suggest that subsynchronous vibration in rotating machinery can have numerous sources or causes. Also, subsynchronous whirl due to internal friction is not a highly repeatable phenomenon.Item Validation of computer-generated results with experimental data obtained for torsional vibration of synchronous motor-driven turbomachinery(Texas A&M University, 2004-09-30) Ganatra, Nirmal KirtikumarTorsional vibration is an oscillatory angular twisting motion in the rotating members of a system. It can be deemed quite dangerous in that it cannot be detected as easily as other forms of vibration, and hence, subsequent failures that it leads to are often abrupt and may cause direct breakage of the shafts of the drive train. The need for sufficient analysis during the design stage of a rotating machine is, thus, well justified in order to avoid expensive modifications during later stages of the manufacturing process. In 1998, a project was initiated by the Turbomachinery Research Consortium (TRC) at Texas A&M University, College Station, TX, to develop a suite of computer codes to model torsional vibration of large drive trains. The author had the privilege of developing some modules in Visual Basic for Applications (VBA-Excel) for this suite of torsional vibration analysis codes, now collectively called XLTRC-Torsion. This treatise parleys the theory behind torsional vibration analysis using both the Transfer Matrix approach and the Finite Element approach, and in particular, validates the results generated by XLTRC-Torsion based on those approaches using experimental data available from tests on a 66,000 HP Air Compressor.