Adam C. Wroblewski

Condition monitoring of rotor using active magnetic actuator

It has been widely recognized that the changes in the dynamic response of a rotor could be utilized for general fault detection and monitoring. Current methods rely on the monitoring of synchronous response of the machine during its transient or normal operation. Very little progress has been made in developing robust techniques to detect subtle changes in machine condition caused by rotor cracks. It has been demonstrated that the crack-induced changes in the rotor dynamic behavior produce unique vibration signatures. When the harmonic excitation force is applied to the cracked rotor system, nonlinear resonances occur due to the nonlinear parametric excitation characteristics of the crack. These resonances are the result of the coexistence of a parametric excitation term and different frequencies present in the system, namely critical speed, the synchronous frequency, and excitation frequency from the externally applied perturbation signals. This paper presents the application of this approach on an experimental test rig. The simulation and experimental study for the given rig configuration, along with the application of active magnetic bearings as a force actuator, are presented.

Rotor Model Updating and Validation for an Active Magnetic Bearing Based High-Speed Machining Spindle

This paper presents an experimentally driven model updating approach to address the dynamic inaccuracy of the nominal finite element (FE) rotor model of a machining spindle supported on active magnetic bearings. Modeling error is minimized through the application of a numerical optimization algorithm to adjust appropriately selected FE model parameters. Minimizing the error of both resonance and antiresonance frequencies simultaneously accounts for rotor natural frequencies as well as for their mode shapes. Antiresonance frequencies, which are shown to heavily influence the model’s dynamic properties, are commonly disregarded in structural modeling. Evaluation of the updated rotor model is performed through comparison of transfer functions measured at the cutting tool plane, which are independent of the experimental transfer function data used in model updating procedures. Final model validation is carried out with successful implementation of robust controller, which substantiates the effectiveness of the model updating methodology for model correction.

Application of Pinniped Vibrissae to Aeropropulsion

Vibrissae (whiskers) of Phoca Vitulina (Harbor Seal) and Mirounga Angustirostris (Elephant Seal) possess undulations along their length. Harbor Seal Vibrissae have been shown to reduce vortex induced vibrations and reduce drag compared to appropriately scaled cylinders and ellipses. Samples of Harbor Seal vibrissae, Elephant Seal vibrissae and California Sea Lion vibrissae were collected from the Marine Mammal Center in California. CT scanning, microscopy and 3D scanning techniques were utilized to characterize the whiskers. Leading edge parameters from the whiskers were used to create a 3D profile based on a modern power turbine blade. The NASA SW-2 cascade wind tunnel facility was used to perform hotwire surveys and pitot surveys in the wake of the ‘Seal Blades’ to provide validation of Computational Fluid Dynamics simulations. Computational Fluid Dynamics simulations were used to study the effect of incidence angles from −37 to +10 degrees on the aerodynamic performance of the Seal blade. The tests and simulations were conducted at a Reynolds number of 100,000 based on inlet conditions and blade axial chord. The Seal blades showed consistent performance improvements over the baseline configuration. It was determined that a fuel burn reduction of approximately 5% could be achieved for a fixed wing aircraft.Copyright

Numerical Study of a Thermal Energy Storage Device Utilizing Graphite Foam Infiltrated with a Phase Change Material

Phase change materials (PCM) utilized for energy storage have notoriously low thermal conductivities. As a result, systems based only on a PCM have large internal thermal gradients and slow reaction times making them impractical for most applications. To overcome these issues, various approaches have been utilized to increase the conductivity of the PCM systems. One approach includes the utilization of porous, high thermal conductivity graphite foam infiltrated with a PCM. Here, a numerical approach was employed in order to study the graphite foam/PCM thermal energy storage system (TES). The numerical model was constructed to emulate an experimental set-up allowing for comparisons between the two. The numerical simulation results exhibited accurate time-dependent temperatures at various locations as well as a history of the melt-front’s progression when compared to the experimental data. Due to the model’s successful capture of the transient response of the TES, it is feasible to employ the numerical procedure for designing subsequent thermal energy storage systems.

Assessment of NDE methods for detecting cracks and damage in environmental barrier coated CMC tested under tension

For validating physics based analytical models predicting spallation life of environmental barrier coating (EBC) on fiber reinforced ceramic matrix composites, the fracture strength of EBC and kinetics of crack growth in EBC layers need to be experimentally determined under engine operating conditions. In this study, a multi layered barium strontium aluminum silicate (BSAS) based EBC-coated, melt infiltrated silicon carbide fiber reinforced silicon carbide matrix composite (MI SiC/SiC) specimen was tensile tested at room temperature. Multiple tests were performed on a single specimen with increasing predetermined stress levels until final failure. During loading, the damage occurring in the EBC was monitored by digital image correlation (DIC). After unloading from the predetermined stress levels, the specimen was examined by optical microscopy and computed tomography (CT). Results indicate both optical microscopy and CT could not resolve the primary or secondary cracks developed during tensile loading until failure. On the other hand, DIC did show formation of a primary crack at ~ 50% of the ultimate tensile strength and this crack grew with increasing stress and eventually led to final failure of the specimen. Although some secondary cracks were seen in the DIC strain plots prior to final failure, the existence of these cracks were not confirmed by other methods. By using a higher resolution camera, it is possible to improve the capability of DIC in resolving secondary cracks and damage in coated specimen tested at room temperature, but use of DIC at high temperature requires significant development. Based on the current data, it appears that both optical microscopy and CT do not offer any hope for detecting crack initiation or determining crack growth in EBC coated CMC tested at room or high temperatures after the specimen has been unloaded. Other methods such as, thermography and optical/SEM of the polished cross section of EBC coated CMC specimens stressed to predetermined levels and cycled to certain time at a given stress need to be explored.

Using a general purpose finite element approach to attain higher fidelity rotordynamic analyses

By utilizing a general purpose finite element (FE) code, the dynamic response of a rotor system was numerically studied in order to assess physical effects that are typically not taken into account using traditional rotordynamic codes. This included the allowance for disk flexibility as well as conducting a simultaneous heat transfer analysis that resulted in varying temperatures in the axial and radial directions. The numerical study utilized a generic, multi-disk model with a flexible hollow shaft. The Campbell diagrams and the mode shapes showed that neglecting any of the additional influences may cause errors regarding the predicted rotor dynamic response. By increasing the fidelity of the rotor model and accounting for the various effects, the slight signal modifications due to damage can be more easily recognized allowing for increased accuracy during rotor health monitoring.

Utilizing a general purpose finite element approach for assessing the rotordynamic response of a flexible disk/shaft system

With continual improvement in computing power and software codes that simulate multiple physical effects, complex analyses can be performed that allow for more accurate modeling of real world systems. Here, a general purpose finite element (FE) code was utilized to conduct a rotordynamic assessment of a rotor system containing a flexible disk. Typically, specialized rotordynamic software packages make numerous assumptions to simplify the various types of rotor response calculations. Disks, for example, are commonly assumed rigid and are represented by lumped masses or discrete beam elements. Such idealizations may cause inaccuracies when calculating critical speeds for rotor systems that involve a relatively flexible disk. By utilizing a general purpose FE approach, where multiple rotational effects are considered, a more accurate model can be developed that includes the dynamic contributions of a flexible disk. This paper illustrates the rotordynamic analysis of a generic, yet realistic, compressor with a shrouded impeller model, without extensive geometric simplification. Furthermore, through the utilization of the fully featured geometry, several dynamic effects are demonstrated to have a significant influence on the rotor system’s Campbell diagram. The dynamic effects investigated include disk flexibility, stress stiffening, and spin softening. It is shown that neglecting any of these may cause significant errors regarding the rotordynamic analysis predictions.

Numerical study of structural change estimation in a rotor system based on changes in resonance and antiresonance frequencies

A structural change quantification methodology is explored in which the magnitude and location of a structural alteration is identified in a rotor system. The proposed structural alterations may be interpreted as physical damage to a structure, in efforts of advancing structural health monitoring activities. The structural change quantification strategy involves the use of resonance and antiresonance frequencies which are collected from several transfer functions calculated from a finite element rotor model. These values are collected and included in an objective function which outputs an error value that is subsequently minimized. The resulting objective contains sufficient information to identify the dynamic characteristics of the rotor in both the frequency and spatial domains. A finite element model with carefully selected tunable parameters is iteratively adjusted using a numerical optimization algorithm to determine the source of the structural change. The numerical studies presented in this work utilize a generic rotor model with features such as a hollow shaft, two ball bearings, several disks, and multiple material layers. The method used for structural excitation is assumed to utilize magnetic actuators for nonintrusive operations. First, the investigations optimize the objective function using a hybrid optimization approach which applies both the NSGA-II genetic and the Nelder-Mead optimization algorithms. The objective function is optimized to maximize the sensitivity of the rotor’s finite elements to detect structural change. Second, a simulated local structural change is implemented in which the detection methodology is employed to locate. An investigation of the effect of error in the simulated data on the prediction’s accuracy is addressed.

Structural Change Quantification in Rotor Systems Based on Measured Resonance and Antiresonance Frequencies

A structural change quantification methodology is proposed in which the magnitude and location of a structural alteration is identified experimentally in a rotor system. The resonance and antiresonance frequencies are captured from multiple frequency response functions and are compared with baseline data to extract frequency shifts due to these features. The resulting expression contains sufficient information to identify the dynamic characteristics of the rotor in both the frequency and spatial domains. A finite element model with carefully selected tunable parameters is iteratively adjusted using a numerical optimization algorithm to determine the source of the structural change. The methodology is experimentally demonstrated on a test rig with a laterally damaged rotor and the frequency response functions are acquired through utilization of magnetic actuators positioned near the ball bearings.Copyright

High-speed AMB machining spindle model updating and model validation

High-Speed Machining (HSM) spindles equipped with Active Magnetic Bearings (AMBs) have been envisioned to be capable of automated self-identification and self-optimization in efforts to accurately calculate parameters for stable high-speed machining operation. With this in mind, this work presents rotor model development accompanied by automated model-updating methodology followed by updated model validation. The model updating methodology is developed to address the dynamic inaccuracies of the nominal open-loop plant model when compared with experimental open-loop transfer function data obtained by the built in AMB sensors. The nominal open-loop model is altered by utilizing an unconstrained optimization algorithm to adjust only parameters that are a result of engineering assumptions and simplifications, in this case Youngs modulus of selected finite elements. Minimizing the error of both resonance and anti-resonance frequencies simultaneously (between model and experimental data) takes into account rotor natural frequencies and mode shape information. To verify the predictive ability of the updated rotor model, its performance is assessed at the tool location which is independent of the experimental transfer function data used in model updating procedures. Verification of the updated model is carried out with complementary temporal and spatial response comparisons substantiating that the updating methodology is effective for derivation of open-loop models for predictive use.