Gas foil bearings (GFBs) operating at high temperature rely on thermal management procedures that supply needed cooling flow streams to keep the bearing and rotor from overheating. Poor thermal management not only makes systems inefficient and costly to operate but could also cause bearing seizure and premature system destruction. To date, most of thermal management strategies rely on empirically based "make-and-break" techniques which are often inefficient. This dissertation presents comprehensive measurements of bearing temperatures and shaft dynamics conducted on a hollow rotor supported on two first generation GFBs. The hollow rotor (1.36 kg, 36.51 mm OD and 17.9 mm ID) is heated from inside to reach an outer surface temperature of 120 degrees C. Experiments are conducted with rotor speeds to 30 krpm and with forced streams of air cooling the bearings and rotor. Air pressurization in an enclosure at the rotor mid span forces cooling air through the test GFBs. The cooling effect of the forced external flows is most distinct when the rotor is hottest and operating at the highest speed. The temperature drop per unit cooling flow rate significantly decreases as the cooling flow rate increases. Further measurements at thermal steady state conditions and at constant rotor speeds show that the cooling flows do not affect the amplitude and frequency contents of the rotor motions. Other tests while the rotor decelerates from 30 krpm to rest show that the test system (rigid-mode) critical speeds and modal damping ratio remain nearly invariant for operation with increasing rotor temperatures and with increasing cooling flow rates. Computational model predictions reproduce with accuracy the test data. The work adds to the body of knowledge on GFB performance and operation and provides empirically derived guidance for successful integration of rotor-GFB systems.

Gas foil bearings (GFB) appear to satisfy most requirements for oil-free turbomachinery, i.e. relatively simple in construction, ensuring low drag friction and reliable high speed operation. However, GFBs have a limited load capacity and minimal amounts of damping. A test rig for the rotordynamic evaluation of gas foil bearings was constructed. A DC router motor, 25 krpm max speed, drives a 1.02 kg hollow rotor supported on two bump-type foil gas bearings (L = D = 38.10 mm). Measurements of the test rotor dynamic response were conducted for increasing mass imbalance conditions. Typical waterfalls of rotor coast down response from 25 krpm to rest evidence the onset and disappearance of severe subsynchronous motions with whirl frequencies at ~ 50% of rotor speed, roughly coinciding with the (rigid mode) natural frequencies of the rotor-bearing system. The amplitudes of motion, synchronous and subsynchronous, increase (non) linearly with respect to the imbalance displacements. The rotor motions are rather large; yet, the foil bearings, by virtue of their inherent flexibility, prevented the catastrophic failure of the test rotor. Tests at the top shaft speed, 25 krpm, did not excite subsynchronous motions. In the experiments, the subsynchronous motion speed range is well confined to shaft speeds ranging from 22 krpm to 12 krpm. The experimental results show the severity of subsynchronous motions is related to the amount of imbalance in the rotor. Surprisingly enough, external air pressurization on one side of the foil bearings acted to reduce the amplitudes of motion while the rotor crossed its critical speeds. An air-film hovering effect may have enhanced the sliding of the bumps thus increasing the bearings’ damping action. The tests also demonstrate that increasing the gas feed pressure ameliorates the amplitudes of subsynchronous motions due to the axial flow retarding the circumferential flow velocity development. A finite element rotordynamic analysis models the test rotor and uses predicted bearing force coefficients from the static equilibrium GFB load analysis. The rotordynamic analysis predicts critical speeds at ~8 krpm and ~9 krpm, which correlate well with test critical speeds. Predictions of rotordynamic stability are calculated for the test speed range (0 to 25 krpm), showing unstable operation for the rotor/bearing system starting at 12 krpm and higher. Predictions and experimental results show good agreement in terms of critical speed correlation, and moderate displacement amplitude discrepancies for some imbalance conditions. Post-test inspection of the rotor evidenced sustained wear at the locations in contact with the bearings' axial edges. However, the foil bearings are almost in pristine condition; except for top foil coating wear at the bearing edges and along the direction of applied static load.

No description available.

While most propulsion systems which drive aquatic and aerial vehicles today are based on rotating blades or foils, there has recently been renewed interest in the use of oscillating foils for this purpose, similar to the fins or wings of biological swimmers and flyers. These propulsion systems offer the potential to achieve a much higher degree of manoeuvrability than what is possible with current man-made propulsion systems. There has been extensive research both on the theoretical aspects of oscillating-foil propulsion and the implementation of oscillating foils in practical vehicles, but the current understanding of the physics of oscillating foils is incomplete. In particular, questions remain about the selection of the appropriate structural properties for the use of flexible oscillating foils which, under suitable conditions, have been demonstrated to achieve better propulsive performance than rigid foils. This thesis investigates the effect of the foil inertia, stiffness, resonant frequency and oscillation kinematics on the thrust generation and efficiency of a flexible oscillating-foil propulsion system. The study is based on experimental measurements made by recording the applied forces while driving foil models submerged in a water tunnel in an oscillating motion using servo-motors. The design of the models allowed for the construction of foils with various levels of stiffness and inertia. High-speed photography was also used to observe the dynamic deformation of the flexible foils. The results show that the frequency ratio, or ratio of oscillation frequency to resonant frequency, is one of the main parameters which determines the propulsive efficiency since the phase of the deformation and overall amplitude of the motion of the bending foil depend on this ratio. When comparing foils of equivalent resonant frequency, heavier and stiffer foils were found to achieve greater thrust production than lighter and more flexible foils but the efficiency of each design was comparable. Through the development of a semi-empirical model of the foil structure, it was shown that the heavier foils have a lower damping ratio which allows for greater amplification of the input motion by the foil deformation. It is expected that the greater motion amplitude in turn leads to the improved propulsive performance. Changing the Reynolds number of the flow over the foils was found to have little effect on the relation between structural properties and propulsive performance. Conversely, increasing the amplitude of the driven oscillating motion was found to reduce the differences in performance between the various structural designs and also caused the peak efficiency to be achieved at lower frequency ratios. The semi-empirical model predicted a corresponding shift in the frequency ratio which results in the maximum amplification of the input motion and also predicted more rapid development of a phase lag between the deformation and the actuating motion at low frequency ratios. The shift in the location of the peak efficiency was attributed to these changes in the structural dynamics. When considering the form of the oscillating motion, foils driven in combined active rotation and translation motions were found to achieve greater efficiency but lower thrust production than foils which were driven in translation only. The peak efficiencies achieved by the different structural designs relative to each other also changed considerably when comparing the results of the combined motion trials to the translation-only cases. To complete the discussion of the results, the implications of all of these findings for the design of practical propulsion systems are examined. / Graduate / 0548

The recently-developed simultaneous solution scheme for solving nonlinear rotordynamic systems running on foil-air bearings (FABs) has overcome the practice of decoupling the air film, foil and rotor equations that has been typically followed to reduce computational burden at the expense of accuracy. However, the published works using the simultaneous solution technique have been limited to a simple bump foil model in which the individual bumps were modelled as independent spring-damper (ISD) subsystems. The overall aim of this thesis is to present methods that enable more realistic FAB models to be integrated into the simultaneous solution scheme, without compromising its efficiency. Two such alternative approaches are presented: (1) the full foil structure modal model (FFSMM) of the bump foil structure; (2) non-parametric system identification of the entire FAB i.e. foil and air film. The FFSMM provides a more realistic model of the bump foil structure since it considers the interaction between the bumps and foil inertia. Although the foil damping is still assumed to be linear, the model presented is adaptable to nonlinear friction forces. The dynamics of the bump foil structure are studied by finite element methods and experimentally validated using a purpose-made corrugated foil structure. The FE result shows that the effect of bump interaction increases the effective stiffness of the FAB. Foil inertia is not important for the range of speeds considered in the thesis, but the experimentally validated fundamental foil resonance of around 2 kHz is within the operating speed range of high-speed turbomachinery. The FFSMM can take into account the curvature of the bearing sleeve, but the effect of this feature is proven to be negligible for the size of bearing used in the study. The FFSMM simulation results are correlated against ISD model results and published experimental maximum film thickness and locus of the journal response. The results of the FFSMM were then compared against experimental results under unbalance response conditions measured from a purpose-built test rig. The rotor was mathematically modelled using rigid body equations of motion, which were validated by modal analysis. The unbalance rotor response results obtained from the FFSMM and experiment both show that the sub-synchronous motion is not only mainly influenced by the increment of unbalance mass, but, to a greater extent, the increment of rotor speed. The findings show good agreement between the model and experimental results. This thesis also presents the non-parametric system identification of a FAB, which is also adaptable to the simultaneous solution scheme. This work is motivated by two advantages: (a) it removes computational limitations by replacing the whole bearing equations by a displacement/force relationship, where the air film effect is taken into account; (b) it can capture complications that cannot be easily modelled, if the identification is based on empirical data. A Recurrent Neural Network (RNN) is trained to identify the full numerical model of a FAB over a wide range of speeds. The identified model of the FAB is adapted into the frequency domain rotor-dynamic simultaneous solution technique by using harmonic balance (HB) methods, thus directly producing the steady-state orbit response. Excellent correlation is demonstrated between the identification technique and the full numerical model under two validation processes: (i) using different sets of input/output data; (ii) the application of the identified RNN-FAB model to HB analysis in lieu of the full numerical model of the FAB.

No description available.

No description available.

This thesis is a study of the size effect. Improvements on both theoretical work and experimental design are involved in this thesis. The theoretical section focuses on the grain size effect, while the experimental section is related to the micro-foil bending test. Both classic experimental data and theories for the Hall-Petch relationship are reviewed comprehensively. The fitting of the datasets show that the inverse square-root dependence and simple inverse expressions are equally good. The fully Bayesian analysis strongly suggests that the latter is correct. Since the physical mechanism underlying the simple inverse dependence is a general size effect, the precise description of the Hall-Petch effect is that it is a manifestation of the general size effect, instead of having its own special character. Improvements on the classic Stolken and Evans' micro-foil bending experiments are also carried out in this thesis. The smart design of the new equipment eliminates the big risk of error in the classic experiment. By using the new device, precise datasets from the elastic region through the yield point and to high plastic strain area can be obtained. The initial results correspond well with the old published data.

Metallic honeycomb structures are being studied for use as thermal protection systems for hypersonic vehicles and as structural panels in other aerospace applications. One potential concern is the growth of fatigue cracks in the thin face-sheets used for these structures. To address this concern, the fatigue behavior of thin aluminum base alloy sheets ranging from 30 m to 250 m in thickness was investigated. The effect of material roll direction was also considered at 30 m. In all cases, the fatigue crack growth rates were found to be one to two orders of magnitude higher than that of the same material of greater thickness. In addition to data for fatigue crack growth rate, data are also presented for the effect of thickness on the fracture toughness of these materials.

During foil rolling, back-up and work rolls undergo elastic deformation resulted from the rolling reaction force, which results in non-uniform thickness distribution in the width direction, even causes waves and fracture in the rolled foils. This paper aims to propose a mathematical model for a four-high mill to analyze the elastic deformation of the rolls and discuss the relationship between axial defection of the back-up and work rolls and the rolling conditions, from which the thickness distribution of the product is then predicted. The finite element simulation is also used to analyze the rolling force and roll¡¦s elastic deformation of a four-high mill. From the predicted foil shape, the roll profiles are designed. The mathematical model is validated by comparing the analytical thickness distribution with experiment values. Rolling pass schedules are also designed. From the arrangerement of reductions and heat treatment, experimental results of stainless steels foils with 80£gm thick and 2£gm variation, pure copper foils with 20£gm thick and 2£gm variation, and aluminum foils with 15£gm thick and 3£gm variation are successfully obtained. A rolling technology for foil rolling is developed.