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This paper presents numerical and experimental investigations about grooved casing treatment with the help of a high-speed small-scale compressor rotor First, the numerical investigation seeks to offer a contribution of understanding the working mechanism by which circumferential grooves improve stall margin. It is found that stall margin gain due to the presence of circumferential grooves arises from the suction-injection effect and the near-tip unloading effect. Based on that, the philosophy of design of experiment is then set up. Finally, parametric studies are carried out through systematical experiments. It is found that the orthogonal experiment and the factorial analyses are successful in identifying the 'best casing configuration' in terms of stall margin improvement. However, the ineffectiveness of the deduction from simulations suggests that the secondary flow circulations on stall margin gain should not be neglected, and the overall contribution of each groove to stall margin gain depends on its unloading effect and the compound effect of suction-injection. Further numerical investigation will focus on how to set up quantitative criteria to evaluate the compound effect of suction-injection and the unloading effect on stall margin gain respectively in each groove. [DOI: 10.1115/1.4003000]

In the field of rotor balancing, the traditional influence coefficient method is often employed along with weighted least squares in order to reduce amplitude vibrations, typically at selected rotating speeds such as critical or operating ones. Usually, these weights are manually selected by the operator. In this paper, an automatic procedure for the balancing mass estimation based on robust regression methods is introduced. The analysis is focused on high breakdown-point and bounded-influence estimators. The effectiveness and robustness of the proposed balancing procedure are shown by means of simulations of a 180 MW gas turbo generator of a power plant. [DOI: 10.1115/1.4001842] Keywords: rotor balancing, robust estimation, high breakdown-point estimators, bounded-influence estimators

The paper considers three issues in flexible rotor and magnetic bearing systems, namely, the control of rotor vibration, control of transmitted forces, and prevention of rotor contact with auxiliary bearings. An adaptive multiobjective optimization method is developed to tackle these issues simultaneously using a modified recursive adaptive controller. The proposed method involves automatic tuning of the weighting parameters in accordance with performance specifications. A two-stage weighting strategy is implemented, involving base weightings, calculated from a singular value decomposition of the system's receptance matrices, and two adjustable weighting parameters to shift the balance between the three objective functions. The receptance matrices are functions of rotational speed and they are estimated in situ. The whole process does not require prior knowledge of the system parameters. Real-time implementation of the proposed controller is explained and tested by using an experimental flexible rotor magnetic bearing system. The rotor displacements were measured relative to the base frame using four pairs of eddy current displacement transducers. System stability is ensured through local PID controllers. The proposed adaptive controller is implemented in parallel, and the effectiveness of the weighting parameters in changing the balance between the transmitted forces and rotor vibrations is demonstrated experimentally. [DOI: 10.1115/1.4001060]

The problem of determining the worst-case mistuning pattern and robust maximum mistuning forced response of a mistuned bladed rotor is formulated and solved as an optimization problem. This approach is exemplified on a two-degrees-of-freedom per blade disk model, two three-degrees-of-freedom per blade disk models, and a mistuned two-stage bladed rotor The results of the optimum search of the worst-case mistuning patterns for the lumped parameter models are analyzed, which reveals that the maximum blade forced response in a mistuned bladed disk is associated with mistuning jump, which causes strong localization of the vibration response in a particular blade. The mistuning jump-localization phenomenon has been observed for all of the numerical examples, and it is also demonstrated that the highest response was always experienced by a blade of mistuning valve jump. The two- and three-degrees-of-freedom per blade disk models are also for determination of its sensitivity coefficients with respect to mistuning variation. Studies show that there is not a threshold of mistuning beyond which the maximum forced response levels off, or even drops, as the degree of mistuning is increased further The maximum magnification factor is found to increase as the mistuning level is increased and reaches a maximum value at the upper limit of the mistuning level The influence of the multistage coupling is revealed by comparing the results of single-stage analysis with that of the multistage case. The computed results have been compared with the Monte Carlo simulation produced, and it is demonstrated that the accuracy and efficiency of the maximum amplitude magnification factor computed by the presented method can be better than that of Monte Carlo simulations. [DOI: 10.1115/1.4001054] Keywords: mistuned bladed disk, random mistuning, maximum amplification magnification, the worst-case mistuning pattern, mistuning jump-localization phenomenon

This paper studies the effects of shaft rub on a rotating system's vibration response with emphasis on heat generation at the contact point. A 3D heat transfer code, coupled to a 3D vibration code, was developed to predict the dynamic response of a rotor in the time domain. The shaft bow is represented by an equivalent bending moment and the contact forces by rotating external forces. The seal ring is modeled as a linear spring, which exerts a normal force to the rotor. The tangential force is then calculated as the product of the normal force with the friction coefficient. Stable or unstable spiraling and oscillating modes were seen to occur in well defined shaft speed zones. In the main, for the configurations studied, the shaft vibration was found to be unstable for speeds below the first critical speed and stable for speeds above the first critical speed. Limit cycle behavior was observed when the phase angle between the unbalance force and the response was around 90 deg. The vibration behavior with rub during startup and shutdown was studied by considering the effects of acceleration/deceleration rate, friction coefficient, and mass unbalance. It was found that friction coefficient and increasing mass unbalance amplified the rub effects while acceleration/deceleration rate reduced it. [DOI: 10.1115/1.4000904] Keywords: Shaft/seal rub, coupled analysis for heat transfer and vibration, thermal bending moment, rub at startup and shutdown

Prior one-control-volume (1CV) models for rotor-fluid interaction in labyrinth seals produce synchronously reduced (at running speed), frequency-independent stiffness and damping coefficients. The 1CV model, consisting of a leakage equation, a continuity equation, and a circumferential-momentum equation (for each cavity), was stated to be invalid for rotor surface speeds approaching the speed of sound. However, the present results show that while the 1CV fluid-mechanic model continues to be valid, the calculated rotordynamic coefficients become strongly dependent on the rotor's precession frequency. A solution is developed for the reaction-force components for a range of precession frequencies, producing frequency-dependent stiffness and damping coefficients. They can be used to define a Laplace-domain transfer-function model for the reaction-force/ rotor-motion components. Calculated results are presented for a simple Jeffcott rotor model acted on by a labyrinth seal. The model's undamped natural frequency is 7.6 krpm. The fluid properties, seal radius [R.sub.s], and running speed [omega] cause the rotor surface velocity [R.sub.s][omega] to to equal the speed of sound co at to = 58 krpm. Calculated synchronous-response results due to imbalance coincide for the synchronously reduced and the frequency-dependent models. For an inlet preswirl ratio of 0.5, both models predict the same log-dec out to [omega] [approximately equal to] 14.5 krpm. The synchronously reduced model predicts an onset speed of instability (OSI) at 10 krpm, but a return to stability at 48 krpm, with subsequent increases in log-dec out to 70 krpm. The frequency-dependent model predicts an OSI of 10 krpm and no return to stability out to 70 krpm. The frequency-dependent models predict small changes in the rotor's damped natural frequencies. The synchronously reduced model predicts large changes. The stability-analysis results show that a frequency-dependent labyrinth seal model should be used if the rotor surface speed approaches a significant fraction of the speed of sound. For the present example, observable discrepancies arose when [R.sub.s][omega] = 0.26co. [DOI: 10.1115/1.4000895]

A simplified analytical approach for modeling the synchronous instability phenomenon known as the Morton effect is presented. The analysis is straightforward and easily applied to any rotor supported on fluid film bearings. The analysis clarifies the interaction of three distinct machine characteristics, which combine to create a case of the Morton effect. Some example calculations are shown illustrating the possible types of spiral vibration. In addition, an analytical approach is described for estimating the magnitude of the shaft temperature difference in a journal bearing as a direct function of the shaft orbit. It is significant that this method can readily be applied to any type of journal bearing, from plain sleeve bearings to tilting pad bearings. Example calculations using the method are shown. [DOI: 10.1115/1.4001512]

A series of simplified rod-fastened rotors, which have different surface contact roughness are manufactured and their modal parameters under different pretightening force, are measured in free-free state. The concept of surface contact stiffness is introduced to simulate the influence of pretightening force on modal parameters of these simplified rod-fastened rotors using finite element method. The experiment measured results are compared and fitted to the finite element analysis results and the relationship between contact stiffness and contact stress is established in which the contact stress is defined by the pretightening force. The relationship is then applied on the modal analysis of a real gas turbine rotor, and its modal test results and finite element analysis results are consistent with each other, proving that the relationship and the described determination method of contact stiffness based on modal test and finite element analysis are effective. [DOI: 10.1115/1.4000591] Keywords: rod-fastened rotor, contact stiffness, contact stress, natural frequency, modal analysis

Ball bearings are important mechanical components in high-speed turbomachinery that is liable for severe vibration and noise clue to the inherent nonlinearity of ball bearings. Using experiments and the numerical approach, the nonlinear dynamic behavior of a flexible rotor supported by ball bearings is investigated in this paper. An experimental ball bearing-rotor test rig is presented in order to investigate the nonlinear dynamic performance of the rotor systems, as the speed is beyond the first synchroresonance frequency. The finite element method and two-degree-of-freedom dynamic model of a ball bearing are employed for modeling the flexible rotor system. The discrete model of a shaft is built with the aid of the finite element technique, and the ball bearing model includes the nonlinear effects of the Hertzian contact force, bearing internal clearance, and so on. The nonlinear unbalance response is observed by experimental and numerical analysis. All of the predicted results are in good agreement with experimental data, thus validating the proposed model. Numerical and experimental results show that the resonance frequency is provoked when the speed is about twice the synchroresonance frequency, while the subharmonic resonance occurs due to the nonlinearity of ball bearings and causes severe vibration and strong noise. The results show that the effect of a ball bearing on the dynamic behavior is noticeable in optimum design and failure diagnosis of high-speed turbomachinery. [DOI: 10.1115/1.4000586] Keywords: ball bearing, rotor, experiment, nonlinear vibration

The methods employed to perform rotordynamics calculations of industrial machines are rather standard and usually allow forecasting the dynamic behavior of the considered machines. Anyhow, in some cases, in order to obtain high level of accuracy, the model has to be updated to fit experimental results, and standard modeling methods have to be improved. In this paper, the updating of the torsional model of a steam turbogenerator is presented. In order to fit the eigenfrequencies calculated using the standard model and the natural frequencies measured on-field, a modeling improvement is proposed, considering partially the dynamics of the components usually modeled as rigid disks. The proposed method has also the aim to preserve the physical meaning of the model. Finally, the new model is updated, and a very good fitting is obtained between eigenfrequencies and experimental natural frequencies. [DOI: 10.1115/1.4000287] Keywords: rotordynamic, model updating, torsional vibrations, natural frequencies

In this study, a modeling approach has been developed to take multiphysical effects into account in the prediction of the rotordynamic behavior of high speed minirotating machinery with a moderate flow confinement. The temperature increase in the confinement and the flow induced forces resulting from the surrounding fluid have been studied and these models are combined with the structural finite element models for determining the rotordynamic behavior. The structure has been analyzed via finite elements based on Timoshenko beam theory. Flow induced forces are implemented to the structure as added mass-stiffness-damping at each node representing the structure in the fluid confinement. A thermal model based on thermal networks in steady-state has been developed. This model is used to calculate the heat dissipation resulting from air friction and temperature increase in the air gap as a function of rotation speed. At each rotation speed, the temperature in the air gap between the rotor and stationary casing is calculated and air properties, which are used for the calculation of flow induced forces are updated. In this way, thermal and fluid effects in medium gap confinements are coupled with the rotordynamic model and their effects on stability, critical speeds, and vibration response are investigated. The experimental results are reported and compared with the theoretical results in an accompanying paper (Part II). [DOI: 10.1115/1.4000787]

In Part I of this work, a theoretical analysis showed that the surrounding air in the closed confinement between rotor and casing has a significant effect on the dynamic behavior of high speed minirotors. In order to validate the developed theoretical model an experimental setup is designed and the dynamic behavior of the rotor with medium gap confinement is studied. The experimental setup has flexible supports, which consist of beams with adjustable length. The support stiffness is changed by altering the beam length. Modal analysis of the rotor is done in free-free conditions in order to test the capability of the rotordynamic model without the supports and multiphysical effects. The experimental and simulation results agree well with a difference of 1%. Then modal analysis of the whole structure is done at standstill and during operation in the absence of the casing. In this way, multiphysical effects are eliminated and only support effects on the dynamics of the structure are observed. The supports appear to have significant effect on the natural frequencies of the flexural modes of the system. Different support modeling techniques are studied and adequate equivalent models are obtained. These models are then implemented into the structural model of the rotor. Finally, multiphysical effects are tested at different speeds with different support stiffnesses. Experiments are performed with and without the casing for determining the change in the natural frequencies and onset of instability. The surrounding fluid has a significant effect on the stability of the system while the natural frequencies do not change significantly. The experimental and theoretical results are in fair agreement for predicting the natural frequencies and the onset of instability. [DOI: 10.1115/1.4000788]

In the most part of an enclosed rotor-stator system with separated boundary layers, the flow structure is characterized by a central core rotating as a solid body with a constant core-swirl ratio. This behavior is not always observed in an isolated rotor-stator cavity, i.e., without any centripetal or centrifugal throughflow, opened to the atmosphere at the periphery: Recent works have brought to evidence an increasing level of the core-swirl ratio from the periphery to the axis, as in the case of a rotor-stator with superposed centripetal flow. The present work is based on an asymptotical approach in order to provide a better understanding of this process. Assuming that the boundary layers behave as on a single rotating disk in a stationary fluid on the rotor side, and on a stationary disk in a rotating fluid on the stator side, new analytical relations are obtained for the core-swirl ratio, the static pressure on the stator, and also the total pressure at midheight of the cavity. An experimental study is performed: Detailed measurements provide data for several values of the significant dimensionless parameters: 1.14 [less than or equal to] [10.sup.-6] x Re [less than or equal to] 1.96, 0.05 [less than or equal to] G [less than or equal to] 0.10, and 0.07 [less than or equal to][10.sup.4] x Ek [less than or equal to]-- 2.65. The analysis of the results shows a good agreement between the theoretical solution and the experimental results. The analytical model can be used to provide a better understanding of the flow features. In addition, radial distributions of both core-swirl ratio, dimensionless static pressure on the stator, as well as dimensionless total pressure at midheight of the cavity, which are of interest to the designers, can be computed with an acceptable accuracy knowing the levels of the preswirl coefficient [K.sub.p] and the solid body rotation swirl coefficient [K.sub.B]. [DOI: 10.1115/1.4001576] Keywords: rotor-stator, hot-wire anemometry, turbulent flow, analytical model

To prevent rotor/stator contact in a rotor/magnetic bearing system, auxiliary bearings may be located along the shaft and at the magnetic bearings. Rotor responses after a contact event may include periodic trapped modes where repeated contact may lead to highly localized thermal stresses. This paper considers an active auxiliary, bearing system with a control strategy designed to limit the trapped contact modes in a rotor/magnetic bearing system that are induced by rotor unbalance. The controller is evaluated from a system model and its responses to short duration contact events. An active auxiliary bearing model is introduced to the system where the dynamic response of the bearing is dependent on the controller. From a harmonic decomposition of rotor/bearing contact, dynamic controllers are sought, which limit the numbers of possible periodic solutions for a given rotor unbalance and operating speed. A case study is performed considering a simple two degree of freedom system with passive and active auxiliary, bearings. Recovery of a rotor trapped in an asynchronous contact mode is shown with variation of the auxiliary bearing controller parameters. [DOI: 10.1115/1.3204644]

The nature of dry-friction whip and whirl is investigated through experimental and numerical methods. A test rig was designed and constructed to demonstrate and record the character of multimode dry-friction whip and whirl. These tests examined steady-state whip and whirl characteristics for a variety of rub materials and clearances. A simulation model was constructed using tapered Timoshenko beam finite elements to form multiple-degree-of-freedom rotor and stator models. These models were reduced by component mode synthesis to discard high-frequency modes while retaining physical coordinates at the rub location to model rotor-stator interaction using a nonlinear contact model with Coulomb friction. Simulations were performed for specific test cases, and compared against experimental data; these comparisons are favorable. Experimental data analysis showed multiple whirl and whip regions, despite claims of previous investigators that these regions are predicted analytically but not produced in simulations or experiments. Spectral analysis illustrates the presence of harmonic sidebands that accompany the fundamental whirl solution. These sidebands are more evident in whip, and can excite higher-frequency whirl solutions. [DOI: 10.1115/1.3204658]

In this paper the contact mechanism between stator and rotor will be considered in detail, which plays a key role in ultrasonic motors. A planar contact model for the stator-rotor interaction in traveling-wave type ultrasonic motors is derived, including rotor flexibility and differenciating between stick and slip regions in the contact zones. The model analysis shows that depending on the motor's operating conditions, complicated contact behavior may occur with several stick-slip subzones in each contact zone. The typical nonlinear resonance observed in ultrasonic motors can be explained with the present analysis. Both the stiffness of the contact layer and of the rotor may drastically influence the speed-torque characteristics. The results will contribute to a better understanding of the contact mechanics in ultrasonic motors. [DOI: 10.1115/1.4000380] Keywords: ultrasonic motor, piezoelectric motor, contact, traveling-wave, stick, slip, speed-torque characteristic

When both ends of an elastic continuous rotor are supported simply by double-row self-aligning ball bearings, the geometrical nonlinearity appears due to the stiffening effect in the elongation of the rotor if the movement of the bearings in the longitudinal direction is restricted. As the rotor becomes more slender, the geometrical nonlinearity becomes stronger. In this paper, we study on unique nonlinear phenomena caused by both of the nonlinear spring characteristics and an initial axial force in the vicinity of the major critical speed [[omega].sub.c] and twice [[omega].sub.c] in a very slender continuous rotor. When the rotor is supported horizontally, the difference in support stiffness and the asymmetrical nonlinearity appear as a result of shifting from the equilibrium position. By the influences of the internal resonance and the initial axial force, the nonlinear resonance phenomena become very complicated. For example, the peak resonance splits into two peaks, and these two peaks leave each other and then one becomes a hard spring type while the other becomes a soft spring type, respectively. Moreover, almost periodic motions and chaotic vibrations appear. In this paper, we prove the above phenomena theoretically and experimentally. [DOI: 10.1115/1.4000841]

In rotor-bearing systems, squeeze film dampers (SFDs) provide structural isolation, reduce amplitudes of rotor response to imbalance, and in some instances, increase the system threshold speed of instability. SFDs are typically installed at the bearing supports, either in series or in parallel. In multispool engines, SFDs are located in the interface between rotating shafts. These intershafi dampers must ameliorate complex rotor motions of various whirl frequencies arising from the low speed and the high speed rotors. The paper presents experiments to characterize the forced response of an open ends SFD subject to dynamic loads with multiple frequencies, as in a jet engine intershaft damper. The test rig comprises of a stationary journal and a flexibly supported housing that holds the test damper and instrumentation. The open ends SFD is 127 mm in diameter, 25.4 mm film land length, and has a radial clearance of 0.125 mm. The damper is lubricated with ISO VG 2 oil at room temperature (24[degrees]C, feed pressure 31 kPa). In the experiments, two orthogonally positioned shakers deliver forces to the test damper that produce controlled amplitude motions with two whirl frequencies, one fixed and the other one varying over a specified range that includes the test system natural frequency. The test data collected, forces and motions versus time, are converted into the frequency domain for parameter identification. The identified viscous damping coefficients are strong functions of the amplitude of journal motion, lying within predictions from classical formulas for circular centered orbits and small amplitude motions about an eccentric journal position. The damper inertia coefficients agree well with predictions derived from a fluid flow model that includes the effect of the feed groove. [DOI: 10.1115/1.3159374]

In this paper, a new rotor-ball bearings-support-stator coupling system dynamic model with rubbing coupling faults is established for practical aeroengine. In the model, the rubbing fault is modeled, the stator motion is considered, the flexible support and squeeze film damper are established, and the nonlinear factors of ball bearing, such as the clearance of the bearing, the nonlinear Hertzian contact force between balls and races, and the varying compliance vibration because of the periodical variety of the contact position between balls and races, are modeled. The numerical integral method is used to obtain the system responses, the effect of support stiffness on rotor responses is studied using a vibration amplitude-rotating speed plot, and the characteristics of the rubbing fault is analyzed using a 3D cascade plot. An aeroengine tester with a stator is established to carry out the rubbing fault experiments, the simulation results from the rotorball bearings-support-stator coupling model are compared with the experimental results, and the consistency of the results show fully the effectiveness of the new rotor-ball bearings-support-stator coupling model with rubbing fault. [DOI: 10.1115/1.2940355]

Autobalancers for rotor/bearing systems are passive devices consisting of eccentrically mounted balance masses that freely revolve around the rotor's axis of rotation. At certain supercritical speeds, the balancer mass positions naturally adjust to cancel the rotor imbalance. This automatic-balancing phenomena occurs as a result of nonlinear dynamic interaction between the balancer masses and the rotor's lateral vibration. Previous studies have found that autobalancers can effectively compensate for mass imbalance in planar rigid rotors such as hard-disk drives and flywheels, however, their use in bladed-disk and turbomachinery applications has not been previously considered. This study explores the dynamics and stability of a flexible bladed-disk/rotor-bearing system equipped with a dual-ball automatic-balancing device. It is found that the autobalancer effectively compensates for both mass and aerodynamic imbalances produced by a bladed-loss condition over a wide revolutions/minute range at speeds above the first lateral natural frequency. It is also shown that for stable automatic balancing to occur, the ratio of automatic balancer damping to the blade aerodynamic drag coefficient must be above some critical value. The analysis demonstrates that the automatic balancer is able to simultaneously reduce both bearing loads and blade deflections for a simulated blade-loss event. DOI: 10.2514/1.43832

This paper presents a simulation to actively control transverse vibration of a flexible rotor shaft system mounted on a moving vehicle (e.g., a ship or an aircraft). Discretizing the rotor continuum with beam finite elements, equations of motion are written with respect to a noninertial frame attached to the frame of the moving vehicle. Such equations are linear but contain time-varying parametric terms due to the motion of the carrier vehicle. Through numerical simulation, it is shown that the transverse response in the bending vibration of the rotor shaft relative to the supporting structure is significantly influenced by the inertia force, as well as the parametric excitations due to vehicle motion. A control strategy is proposed using an electromagnetic actuator placed at a suitable location along the span of the rotor, and it is found extremely useful in reducing the vibration of the rotor and improving its stability. Examples are given in support by studying both the uncontrolled and the controlled motion of a rotor shaft system carried by an aircraft undergoing two different kinds of maneuvers. DOI: 10.2514/1.43378

One type of test performed for evaluating bearings for application into turbomachinery is the synchronous bearing response to rotor imbalance. This paper presents rotordynamic tests on a rotor system using a 70 mm diameter damped gas bearing reaching ultra-high speeds of 50,O00 rpm. The main objective of the study was to experimentally evaluate the ability of the damped gas bearing to withstand large rotor excursions and provide adequate damping through critical speed transitions. Two critical speeds were excited through varying amounts and configurations of rotor imbalance while measuring the synchronous rotordynamic response at two different axial locations. The results indicated a well-damped rotor system and demonstrated the ability of the gas bearing to safely withstand rotor vibration levels while subjected to severe imbalance loading. Also, a waterfall plot was used to verify ultra-high-speed stability of the rotor system throughout the speed range of the test vehicle. In addition to the experimental tests, a rotordynamic computer model was developed for the rotor-bearing system. Using the amplitude/frequency dependent stiffness and damping coefficients for the ball bearing support and the damped gas-bearing support, a pseudononlinear rotordynamic response to imbalance was performed and compared with the experiments. [DOI: 10.1115/1.3157097]

A stochastic hub-height wind speed estimation algorithm for wind turbines, which requires only the rotor angular velocity measurement in the presence of observation noise is proposed. It is assumed that the power curve and the total inertia of the rotor are known a priori. We use the maximum likelihood estimator to identify the hub-height wind speed, which is formulated as one of the state variables in the nonlinear model. The theory and algorithm are verified by both simulated and experimental data in the presence of measurement noise. The effects of the initial guess, the noise variance, and the time history are discussed. [DOI: 10.1115/1.4000604]

Unstable vibration occurs in the vicinities of the major critical speeds of asymmetrical shaft and rotor systems. It occurs also in a wide rotational speed range higher than the major critical speed of a shaft with a hollow disk partially filled with liquid. The occurrence of the unstable vibrations is a serious problem because the amplitude increases exponentially, and finally, the system is destroyed. The active vibration control can suppress unstable vibrations but the method is generally complicated and costly. No simple effective method to suppress unstable vibrations has been developed yet. In the previous paper, the authors proposed a simple method by utilizing discontinuous spring characteristics, which can suppress steady-state resonances. This paper shows that this method is also effective to suppress unstable vibrations. By using this method, the unstable vibrations can be changed into almost periodic motions, and the amplitudes are suppressed to the desired small level even in an unstable range. The validity of the proposed method is also verified by experiments. [DOI: 10.1115/1.4000842]

In this study an original analysis of the geometric design of a particular internal rotary machine and an investigation on its theoretical performances are presented. The first part of the work is focused on the geometry of the machine, characterized by more complex rotor profiles compared with the classical epitrochoidal pumps. Suitable parameters are pointed out to completely define the geometry of the machine. In the second part of the study, proper indices are set and evaluated as functions of the design parameters, in order to evaluate the performance of the machine. Finally a specific configuration of the considered pump is compared with the conventional trochoidal pumps in terms of flowrate indices. The analysis performed allows one to highlight the basic advantages and drawbacks of this original design in comparison with the classical trochoidal profiles and can be used as a basis for CAD/CAE (Computer-Aided Design and Engineering) or experimental studies. [DOI: 10.1115/1.4000698] Keywords: internal rotary pump, gerotor

Active magnetic bearing (AMB) becomes widely used in various kinds of rotating machinery. However, as the magnetic force is nonlinear, nonlinear phenomena may occur when the rotating speed becomes high and delays of electric current or magnetic flux in the AMB relatively increase. In this paper, the magnetic force in the AMB is modeled by considering both the second-order delay of the electric current and the first-order delay of the magnetic flux. The magnetic flux in the AMB is represented by a power series function of the electric current and shaft displacement, and its appropriate representation for AMB is discussed. Furthermore, by using them, the nonlinear theoretical analysis of the rigid rotor system supported by the AMB is demonstrated. The effects of the delays and other AMB parameters on the nonlinear phenomena are clarified theoretically, and they are confirmed experimentally. [DOI: 10.1115/1.3172139]

In this paper, a new rotor-ball bearing-stator coupling system dynamics model is established for simulating the practical whole aero-engine vibration. The main characteristics of the new model are as follows: (1) the coupling effect between rotor, ball bearing, and stator is fully considered; (2) the elastic support and the squeeze film damper are considered; (3) the rotor is considered as an Euler free beam of equal-section model, and its vibration is analyzed through truncating limited modes; (4) nonlinear factors of ball bearing such as the clearance of bearing, nonlinear Hertzian contact force, and the varying compliance vibration are modeled; and (5) rubbing fault between rotor and stator is considered. The Zhai method, which is a new explicit fast numerical integration method, is employed to obtain system's responses, and the whole aero-engine vibration characteristics are studied. Finally, aero-engine tester including casing is established to carry out rubbing fault experiment, the simulation results from rotor-ball bearing-stator coupling model are compared with the experiment results, and the correctness of the new model is verified to some extent. [DOI: 10.1115/1.4000475]

An efficient methodology for predicting the nonlinear forced vibration response of a turbine engine rotor with a cracked blade is presented and used to investigate the effects of the damage on the forced response. The influence of small random blade-to-blade differences (mistuning) and rotation on the forced response are also considered. Starting with a finite element model, a hybrid-interface method of component mode synthesis (CMS) is employed to generate a reduced-order model (ROM). The crack surfaces are retained as physical degrees of freedom in the ROM so that the forces due to contact in three-dimensional space can be properly calculated. The resulting nonlinear equations of steady-state motion are solved by applying an alternating frequency/time-domain method, which is much more computationally efficient than traditional time integration. Using this reduced-order modeling and analysis framework, the effects of the cracked blade on the system response of an example rotor are investigated for various mistuning levels and rotation speeds. First, the advantages of the selected hybrid-interface CMS method are discussed and demonstrated. Then, the resonant frequency shift associated with the stiffness loss due to the crack and the vibration localization about the cracked blade are thoroughly investigated. In addition, the results of the nonlinear ROMs are compared with those obtained with linear ROMs, as well as blade-alone ROMs. It is shown that several key system vibration characteristics are not captured by the simpler models, but that some insight into the system response can be gained from the blade-alone response predictions. Furthermore, it is demonstrated that while the effects of the crack often appear similar to those of mistuning, the effects of mistuning and damage can be distinguished by observing and comparing the response across multiple families of system modes. [DOI: 10.111511.4000458]

An unbalanced rotor dynamic model supported on ball bearings is established. In the model, three nonlinear factors of ball bearing are considered, namely, the clearance of bearing, nonlinear Hertzian contact force between balls and races, and the varying compliance vibrations because of periodical change in contact position between balls and races. The numerical integration method is used to obtain the nonlinear dynamic responses; the effects of the rotating speed and the bearing clearance on dynamic responses are analyzed; and the bifurcation plots, the phase plane plots, the frequency spectra, and the Poincare maps are used to carry out the analyses of bifurcation and chaotic motion. Period doubling, quasiperiod loop breaking, and mechanism of intermittency are observed as the routes to chaos. [DOI: 10.1115/1.3142883]

The aim of this paper is to investigate the potential of a self-sensing strategy in the case of an electromagnetic damper for the vibration control of flexible structures and rotors. The study has been performed in the case of a single degree of freedom mechanical oscillator actuated by a couple of electromagnets. The self-sensing system is based on a Luenberger observer. Two sets of parameters have been used: nominal ones (based on simplifications on the actuator model) and identified ones. In the latter case, the parameters of the electromechanical model used in the observer are identified starting from the open-loop system response. The observed states are used to close a state-feedback loop with the objective of increasing the damping of the system. The results show that the damping performance are good in both cases, although much better in the second one. Furthermore, the good correlation between the closed-loop model response and the experimental results validates the modeling, the identification procedure, the control design, and its implementation. The paper concludes on a sensitivity analysis, in which the influence of the model parameters on the closed-loop response is shown. [DOI: 10.1115/1.4000069]

Hydrodynamic flexure pivot tilting pad gas bearings (FPTPGBs) can enable successful operation of oil-free microturbomachinery and FPTPGBs with radially compliant pads permit rotor centrifugal and/or thermal growth to exceed original bearing clearances and achieve higher speeds. This work presents the experimental and analytical study of such bearings and the application of dampers behind the pad radial compliance structure. A time domain orbit simulation method was implemented as the primary analysis tool to predict the rotor-bearing response to imbalance, the presence and location of critical speeds, etc., and to compare with test results. Experiments demonstrate the stable hydrodynamic operation of FPTPGBs with an ~28.6 ram, 0.8 kg rotor above 120 krpm, for the first time. The rotor-bearing system was intentionally destabilized in tests by increasing bearing clearances and the viscoelastic dampers added behind the FPTPGB pads delayed the onset of subsynchronous vibrations (from 43 krpm without damper to above 50 krpm with damper). Midrange subsynchronous vibrations of the destabilized system initiated at ~20 krpm were suppressed by ~25 krpm due to the stabilizing effect of rotor centrifugal growth. The viscoelastic dampers had a negligible effect on suppressing these midrange subsynchronous vibrations in experiments, but this was not demonstrated in simulations, presumed to be due to much lower stiffness contribution of the damper at lower frequencies. [DOI: 10.1115/1.3201831] Keywords: tilting pad gas bearing, damper, rotordynamics, compliant pads

This paper is to study how the vibration modes of a cyclic symmetric rotor evolve when it is assembled to a flexible housing via multiple bearing supports. Prior to assembly, the vibration modes of the rotor are classified as 'balanced modes' and 'unbalanced modes.' Balanced modes are those modes whose natural frequencies and mode shapes remain unchanged after the rotor is assembled to the housing via bearings. Otherwise, the vibration modes are classified as unbalanced modes. By applying fundamental theorems of continuum mechanics, we conclude that balanced modes will present vanishing inertia forces and moments as they vibrate. Since each vibration mode of a cyclic symmetric rotor can be characterized in terms of a phase index (Chang and Wickert, 'Response of Modulated Doublet Modes to Travelling Wave Excitation,' J. Sound Vib., 242, pp. 69-83; Chang and Wickert, 2002, 'Measurement and Analysis of Modulated Doublet Mode Response in Mock Bladed Disks, '' J. Sound Vib., 250, pp. 379-400; Kim and Shen, 2009, 'Ground-Based Vibration Response of a Spinning Cyclic Symmetric Rotor With Gyroscopic and Centrifugal Softening Effects,' ASME J. Vibr. Acoust. (in press)), the criterion of vanishing inertia forces and moments implies that the phase index by itself can uniquely determine whether or not a vibration mode is a balanced mode as follows. Let N be the order of cyclic symmetry of the rotor and n be the phase index of a vibration mode. Vanishing inertia forces and moments indicate that a vibration mode will be a balanced mode if n [not equal to] 1, N - 1, N. When n = N, the vibration mode will be balanced if its leading Fourier coefficient vanishes. To validate the mathematical predictions, modal testing was conducted on a disk with four pairs of brackets mounted on an air-bearing spindle and a fluid-dynamic bearing spindle at various spin speeds. Measured Campbell diagrams agree well with the theoretical predictions. [DOI: 10.1115/1.3147167]

Resonance vibrations (critical speeds) play a significant role in rotor vibration control. Active vibration control methods for rotors are studied to develop solutions to enhance machines' dynamic behavior, durability, and operating range. This paper reports rotor vibration attenuation with a supplementary electromagnetic actuator located outside the rotor bearing span. Feedback and feedforward control system design are shown, and comparative experiments on two active vibration control methods for mass unbalance compensation are reported. The methods compared are adaptive FIR filter with the least mean squares (LMS) algorithm and convergent control (CC) method with a frequency-domain adaptation algorithm. The methods were experimentally validated on the rotor test rig (rotor weight 2.7 kg, length 560 mm, and first critical speed about 50 Hz). The feedback system provided wideband damping in the sub- and supercritical regions. The feedforward systems attenuated vibratory responses at the speed of rotation and its harmonic. The attenuation achieved was about 20 dB depending on the rotor speed. Also, discrete-time CC algorithm is shown to have a feedback equivalent circuit. The significance of feedback control lies in making the system phase-characteristics sufficiently smooth for feedforward control methods. Then, feedforward algorithms provided a good vibration damping performance over the operating range. CC was found to be a more effective and simpler algorithm for the purpose than the adaptive FIR filter with the LMS algorithm. The equivalent feedback circuit derived for CC, and systems similar to CC, facilitates their stability and robustness analysis. [DOI: 10.1115/1.3155015] Keywords: rotor, unbalance, vibration, active control, feedforward, adaptive

In this paper, the nonlinear response of a rotor system containing a transverse crack is analyzed experimentally in order to propose a nondestructive detection of cracks in the rotor. More particularly, the evolutions of the n X superharmonic frequency components at the various subcritical resonant peaks and the decrease in the subcritical resonant speeds are investigated ,for various crack depths. The experimental results that are presented in this study confirm the theoretical analysis of many researchers and provide a possible basis for an on-line monitoring system. [DOI: 10.1115/1.3086928]

The rotor-active magnetic bearing system subjected to a periodically time-varying stiffness having quadratic and cubic nonlinearities is studied and solved. The multiple time scale technique is applied to solve the nonlinear differential equations governing the system up to the second order approximation. All possible resonance cases are deduced at this approximation and some of them are confirmed by applying the Rung--Kutta method. The main attention is focused on the stability of the steady-state solution near the simultaneous principal resonance and the effects of different parameters on the steady-state response. A comparison is made with the available published work. [DOI: 10.1115/1.3085884] Keywords: rotor-active magnetic bearing, time-varying stiffness, multi-external excitations, stability, multiple-valued solutions, jump phenomenon

In rotating machinery, rotor unbalance causes many resonances at critical speeds corresponding to different modes. In this paper, a vibration control method for rotor systems utilizing disturbance observer is proposed. The nonlinear terms, unbalance, parameter variations, and uncertain terms of a rotor system are lumped into a disturbance term, and this term is canceled by using disturbance observer. As a result, the vibrations are suppressed to small amplitudes all over the rotational speed range. Simultaneously, unbalance of the first mode is estimated from the information of control force of disturbance observer. Moreover, the effects of parameter errors of the control system are also investigated. The validity of the proposed method is verified through numerical simulations and experiments. [DOI: 10.1115/1.3085886]

This study focuses on the vibration of the rotor supported by a repulsive-type passive magnetic bearing. It evaluates the restoring force of a repulsive magnetic bearing numerically by considering the effects of magnetic anisotropies of the inner and the outer magnets. The study mainly investigates the occurrences of linear and nonlinear parametric characteristics, and the excitation forces for each pattern of magnetic anisotropies. Moreover, it clarifies the effects of each magnetic anisotropy pattern on the occurrences of various resonance phenomena theoretically and experimentally. [DOI: 10.1115/1.3085887] Keywords: repulsive magnetic bearing, permanent magnet, nonlinear vibration

The torque produced by a synchronous reluctance machine (including the permanent-magnet-assisted machine) is studied analytically, with the aim of pointing out the effect of the position of the flux barriers on the torque ripple. It is verified that the position of the flux-barrier ends highly influences the torque waveform. With the aim of reducing torque harmonic contents, a new strategy is proposed based on the choice of couples of flux barriers of different shapes. The flux-barrier geometry is chosen so as to obtain a compensation between the torque harmonics produced by each couple. Experimental results on two prototypes confirm the analytical prediction. Index Terms--Low torque ripple, permanent-magnet (PM)-assisted synchronous reluctance machine, synchronous reluctance machine.

This paper presents a new permanent-magnet (PM) brushless machine for wind power generation. This machine adopts an outer-rotor topology, aiming at capturing wind power directly. In order to achieve high power density, a high-speed PM brushless generator is artfully integrated with a coaxial magnetic gear. The design details, with emphasis on the special constraints of wind power generation, are elaborated. By using the time-stepping finite element method, the static characteristics as well as no-load and on-load operations are simulated. A prototype is also built for experimentation. Both simulation and experimental results are given to verify the validity of the proposed machine. Finally, a quantitative comparison is made to justify that the proposed machine is of smaller size, lighter weight, and lower cost than its counterparts. Index Terms--Finite-element method (FEM), magnetic gear, outer rotor, permanent-magnet (PM) machine, wind power generation.

This paper is to study ground-based vibration response of a spinning, cyclic, symmetric rotor through a theoretical analysis and an experimental study. The theoretical analysis consists of three steps. The first step is to analyze the vibration characteristics of a stationary, cyclic, symmetric rotor with N identical substructures. For each vibration mode, we identify a phase index n and derive a Fourier expansion of the mode shape in terms of the phase index n. The second step is to predict the rotor-based vibration response of the spinning, cyclic, symmetric rotor based on the Fourier expansion of the mode shapes and the phase indices. The rotor-based formulation includes gyroscopic and centrifugal softening terms. Moreover, rotor-based response of repeated modes and distinct modes is obtained analytically. The third step is to transform the rotor-based response to ground-based response using the Fourier expansion of the stationary mode shapes. The theoretical analysis leads to the following conclusions. First, gyroscopic effects have no significant effects on distinct modes. Second, the presence of gyroscopic and centrifugal softening effects causes the repeated modes to split into two modes with distinct frequencies [[omega].sub.1] and [[omega].sub.2] in the rotor-based coordinates. Third, the transformation to ground-based observers leads to primary and secondary frequency components. In general, the ground-based response presents frequency branches in the Campbell diagram at [[omega].sub.1] [+ or -] k[[omega].sub.3] and [[omega].sub.2] [+ or -] k[[omega].sub.3], where k is phase index n plus an integer multiple of cyclic symmetry N. When the gyroscopic effect is significantly greater than the centrifugal softening effect, two of the four frequency branches vanish. The remaining frequency branches take the form of either [[omega].sub.1] + k [[omega].sub.3] and [[omega].sub.2] - k[[omega].sub.3] or [[omega].sub.1] - k[[omega].sub.3] and [[omega].sub.2] + k[[omega].sub.1]. To verify these predictions, we also conduct a modal testing on a spinning disk carrying four pairs of brackets evenly spaced in the circumferential direction with ground-based excitations and responses. The disk-bracket system is mounted on a high-speed, air-bearing spindle. An automatic hammer excites the spinning disk-bracket system and a laser Doppler vibrometer measures its vibration response. A spectrum analyzer processes the hammer excitation force and the vibrometer measurements to obtain waterfall plots at various spin speeds. The measured primary and secondary frequency branches from the waterfall plots agree well with those predicted analytically. [DOI: 10.1115/1.3025847]

Rotor imbalances of a wind turbine can cause severe damage of the turbine or its components and thus reduce the lifespan and security of the turbine significantly. At present, balancing of the rotor is a time consuming and expensive process due to the necessity of mounting test weights to measure a reference imbalance state. We describe a new method for the detection and reconstruction of imbalances in the rotor of a wind turbine avoiding test weight measurements. The method is based on a wind turbine model, which is derived by the finite element method. In some respect, the model information replaces the information of a reference imbalance state. A mathematical equation linking imbalances and the resulting vibrations is derived using the model. Thus the inverse problem of computing an imbalance from vibration measured at the nacelle is solvable with the usual techniques for ill-posed problems. We show that our model for a wind turbine can be used to predict the vibrations for a given imbalance distribution. In particular, it can be used to reconstruct the imbalance distribution of a wind turbine from noisy measurements in real time, which is verified both for artificial and real data. Also, an optimization strategy is presented in order to adapt the model to the wind turbine at hand. The new method requires a simple model of a wind turbine under consideration but reduces the measuring effort for the computation of balancing weights. It can be implemented into a condition monitoring system (CMS). For the first time, there can be not only an alarm generation but also the actual imbalance and balancing weights and positions can be computed directly from the observed CMS data. [DOI: 10.1115/1.3028042]

This paper presents the common failure mechanisms of high temperature rotors and the engineering approaches to their remnant life prediction. In fatigue crack growth at the rotor bore, cracks from original forging defects or induced during long service life may grow under cyclic loading to its critical size causing fast fracture. In fatigue-creep interaction at the shaft surface, high tensile residual stress relaxation under high operating temperature causes creep crack initiation. The cracks may then grow under the combination of cyclic loading and high operating temperature. Remnant creep life at the center of the rotor is based on the time while accumulated creep strain reaches its threshold level. Creep rupture could occur at other locations such as the outside surface of the shaft at disks/shaft radii or blade fixings. Finite element analyses were carried out to analyze stresses, temperature transients, creep strain, and reference stress for creep rupture. Fracture mechanics analyses with R5 and R6 approaches were used to estimate the crack initiation and growth rates, the critical crack sizes, and the type of the failure. Appropriate Paris law and Norton creep laws were used for fatigue and creep crack growth. Depending on the failure mechanism, a rotor's remnant life is defined in terms of allowable starts and operating hours. [DOI: 10.1115/1.3006893]

Rotordynamic data are presented for a rocker-pivot tilting-pad bearing in the load-between-pad configuration for unit loads over the range 345-3101 kPa and speeds over the range 4000-13,000 rpm. The bearing was directly lubricated through a leading-edge groove with the following specifications: Five pads, 0.282 preload, 60% offset, 57.87 deg pad arc angle, 101.587 mm (3.9995 in.) rotor diameter, 0.1575 mm (0.0062 in.) diametral clearance, and 60.325 mm (2.375 in.) pad length. Dynamic tests were performed over a range of frequencies to investigate frequency effects on the dynamic stiffness coefficients. Under most test conditions, the direct real parts of the dynamic stiffnesses could be approximated as quadratic functions of the excitation frequency and accounted for with the addition of an added-mass matrix to the conventional [K][C] matrix model to produce a frequency-independent [K][C][M] model. Measured added-mass terms in the loaded direction approached 60 kg. At low speeds, 'hardening' direct dynamic stiffness coefficients that increased with increasing frequency were obtained, which produced negative added-mass terms. No frequency dependency was obtained for the direct damping coefficients. The dynamic experimental results were compared to predictions from a bulk-flow computational fluid dynamics analysis. The static load direction in the tests was y. The direct stiffness coefficients [K.sub.xx] and [K.sub.yy] were slightly overpredicted. Measured direct damping coefficients [C.sub.xx] and [C.sub.yy] were insensitive to changes in either the load or speed in contrast to predictions of marked [C.sub.yy] sensitivity for changes in the load. Only at the highest test speed of 13,000 rpm were the direct damping coefficients adequately predicted. Measurable cross-coupled stiffness coefficients were obtained for the bearings with [K.sub.xy] and [K.sub.yx] being approximately equal in magnitude but opposite in sign--clearly destabilizing. However, the whirl frequency ratio was found to be zero at all test conditions indicating infinite stability for the bearing. [DOI: 10.1115/1.2967728]

Rotor stability is most commonly estimated using methods derived from a simple single degree of freedom system. When the modes of more complex systems, such as rotors, are closely spaced, we demonstrate that such methods can yield very poor estimates of the modal stability (damping ratio). Multiple output backward autoregression (MOBAR) is proposed as an alternative approach and is demonstrated to yield reasonably accurate estimates of modal damping even when modes are closely spaced. The performance of the MOBAR approach is then examined on an experimental rotor in tilt-pad bearings, demonstrating good performance in a realistic measurement setting. [DOI: 10.1115/1.2967484]

This paper presents a case study of a design for a complete microair vehicle thruster. Fixed-pitch small-scale rotors, brushless motors, lithium-polymer cells, and embedded control are combined to produce a mechanically simple, high-performance thruster with potentially high reliability. The custom rotor design requires a balance between manufacturing simplicity and rigidity of a blade versus its aerodynamic performance. An iterative steady-state aeroelastic simulator is used for holistic blade design. The aerodynamic load disturbances of the rotor-motor system in normal conditions are experimentally characterized. The motors require fast dynamic response for authoritative vehicle flight control. We detail a dynamic compensator that achieves satisfactory closed-loop response time. The experimental rotor-motor plant displayed satisfactory thrust performance and dynamic response. DOI: 10.1061/(ASCE)0893-1321(2009)22:1(85) CE Database subject headings: Aerodynamics; Robotics; Dynamic response.

Measured rotordynamic force coefficients (stiffness, damping, and added mass) and static characteristics (eccentricity and attitude angle) of a pressure-dam bearing are presented and compared to predictions from a Reynolds-equation model, using an isothermal and isoviscous laminar analysis. The bearing's groove dimensions are close to the optimum predictions of Nicholas and Allaire (1980, 'Analysis of Step Journal Bearings-Infinite Length and Stability,' ASLE Trans., 22, pp. 197-207) and are consistent with current field applications. Test conditions include four shaft speeds (4000 rpm, 6000 rpm, 8000 rpm, and 10000 rpm) and bearing unit loads from 0 kPa to 1034 kPa (150 psi). Laminar flow was produced for all test conditions. A finite-element algorithm was used to generate solutions to the Reynolds-equation model Excellent agreement was found between predictions and measurements for the eccentricity ratio and attitude angles. Predictions of stiffness and damping coefficients are in reasonable agreement with measurements. However, experimental results show that the bearing has significant added mass of about 60 kg at no-load conditions, versus zero mass for predictions from the Reynolds-equation model and 40 kg using Reinhardt and Lund's (1975, 'The Influence of Fluid Inertia on the Dynamic Properties of Journal Bearings,' ASME J. Lubr. Technol., 97, pp. 159-167) extended Reynolds-equation model for a plain journal bearing. The added mass quickly drops to zero as the load increases. Measured results also show a whirl frequency ratio near 0.36 at no-load conditions; however, a zero whirl frequency ratio was obtained at all loaded conditions, indicating an inherently stable bearing from a rotordynamics viewpoint. [DOI: 10.1115/1.2906173]

Seeking relevant criteria for testing the quality of turbulence models, the scale of turbulence and the gust factor have been estimated from data and compared with predictions from first-order models of these two quantities. It is found that the mean of the measured length scales is approximately 10% smaller than the IEC model for wind turbine hub height levels. The mean is only marginally dependent on trends in time series. It is also found that the coefficient of variation of the measured length scales is about 50%. 3 s and 10 s preaveraging of wind speed data are relevant for megawatt-size wind turbines when seeking wind characteristics that correspond to one blade and the entire rotor, respectively. For heights exceeding 50-60 m, the gust factor increases with wind speed. For heights larger than 60-80 m, present assumptions on the value of the gust factor are significantly conservative, both for 3 s and 10 s preaverages. The usually applied value of [k.sub.p] [approximately equal to] 3 should probably be reduced. [DOI: 10.1115/1.2931511]

To reduce the level of uncertainty associated with current rotor aerodynamics codes, improved understanding of rotor aerodynamics is required. Wind tunnel measurements on model rotors contribute to advancing our knowledge on rotor aerodynamics. The combined recording of blade loads and rotor wake is desired, because of the coupled blade and wake aerodynamics. In general, however, the small size of model rotors prohibits detailed blade load measurements; only the rotor wake is recorded. To estimate the experimental blade flow conditions, a measurement analysis tool is developed: the inverse vortex wake model. The rotor wake is approximated by a lifting line model, using rotor wake measurements to reconstruct the vortex wake. Conservation of circulation, combined with the Biot-Savart law, allows the induced velocity to be expressed in terms of the bound circulation. The unknown bound circulation can be solved for, since the velocity is known from rotor wake measurements. The inverse vortex wake model is subsequently applied to measurements on the near wake of a model rotor subject to both axial and yawed flow conditions, performed at a TUDelft open jet wind tunnel. The inverse vortex wake model estimates the unsteady experimental blade flow conditions and loads that otherwise would have remained obscured. [DOI: 10.1115/1.2931508]

The work detailed in this paper is focused on updating and refining a coning rotor wind turbine concept. The coning rotor combines the load shedding properties of flapping hinges with gross change in rotor area, via large coning angles, to affect increased energy capture at nominally constant system cost. Previous studies have indicated that the large cost of energy reductions is possible, compared to the state-of-the-art machines then, particularly for abundant but presently uneconomic low-wind sites. Almost ten years later, the fundamentals of the design remain sound, but bear reevaluation relative to current machines, both exploiting modern power electronics and control technology. The coning rotor was never optimized in its own right, so an integrated design tool suitable for human and computer refinement of the design has been developed. Incorporated into the tool is a corrected blade element momentum method that more properly accounts for coned rotor aerodynamics. A discussion of the development of coning rotors is presented, along with a comparison to present operational strategies. Results obtained for nondimensionalized rotors and specific machine optimization studies are presented, followed by a discussion of further issues to be addressed. [DOI: 10.1115/1.2931497]