Joerg Wallaschek

Design and experimental investigations of high power piezoelectric transducers for a novel squeeze film journal bearing

A novel active squeeze film journal air bearing actuated by high power piezoelectric transducers is presented. The proposed bearing uses in-air squeeze film levitation to suspend the rotating spindle without contact. Unlike conventional journal bearings, the presented bearing journal is formed by multiple independently vibrating surfaces driven individually by piezoelectric transducers. Langevin type piezoelectric transducers with a special radiation surface are developed. Detailed design procedures to develop the ultrasonic transducers are presented. A complete spindle-bearing system is constructed to test the proposed squeeze film bearing. Load carrying forces are measured at different vibration amplitude and compared with the calculated results. The proposed squeeze film journal bearing is operated in ultrasonic frequency range. The achieved load capacity is about 50N, which is five times of the load capacity achieved by the previous squeeze film bearings reported in the literatures.

Design and modeling of a novel squeeze film journal bearing

A novel squeeze film journal bearing actuated by high power piezoelectric transducers is presented. The proposed bearing uses in-air squeeze film levitation to suspend the rotating spindle without contact. Unlike conventional journal bearings, the presented bearing journal is formed by multiple independently vibrating surfaces driven individually by piezoelectric transducers. Langevin type piezoelectric transducers with a special radiation surface are developed. The design of the bearing and the theoretical calculation of the levitation force are presented in this paper. Influences of critical parameters are examined using the proposed model. Load carrying forces are measured at different vibration amplitude and compared with the calculated results. The proposed squeeze film journal bearing is operated in ultrasonic frequency range. The achieved load capacity is about 50N, which is five times of the load capacity achieved by the previous squeeze film bearings reported in the literature.

Investigation of Alternate Mistuned Turbine Blades Non-Linear Coupled by Underplatform Dampers

Freestanding turbine blades have typically low structural damping and thus require additional friction damping devices, such as underplatform dampers. The friction coupling between neighboring blades reduces response amplitude and increases resonance frequency. Along with forced response excitation large blades, especially of last stage, could be excited by fluid structural interaction (flutter). To prevent such excitation alternate mistuned blade patterns are beneficial disturbing traveling waves in the stage.In this paper the influence of alternate mistuning is investigated with a simplified oscillator chain as well as a bladed disk assembly coupled by frictional contacts. It is pointed out that the performance of friction coupling can be improved by alternate mistuning as long as the engine order of the excitation is below quarter of the number of blades. Alternate mistuning causes a mode coupling between two nodal diameter vibration mode shapes allowing for energy transfer. The in-house developed software code DATAR is enhanced and alternate mistuning can be applied to the blades as well as to the damping elements. For validation the DATAR code was applied to an alternate mistuned last stage blade of a Siemens gas turbine and compared with available field engine measurement.Copyright

Amplitude Modulation of Nonlinear Piezoelectric Transducers for Ultrasonic Levitation

Ultrasonic levitation bearings are a new kind of bearing system that offers the advantages of air-bearings combined with dynamically adjustable supporting force without the need for pressurized air. Such systems are typically driven close or in their resonance frequency, due to energetic reasons. In this contribution the two possible driving methods, namely forced- and self-excitation, are compared in sense of their transient amplitude behavior in the presence of nonlinearities. It is known that during amplitude changes at high vibration amplitudes the system’s resonance frequency varies with Duffing-characteristic due to the nonlinear stiffness of piezoelectric material. It will be shown that self-excitation is the preferable driving method in sense of obtaining a high bandwidth of amplitude.Copyright

Experimental investigations of ultrasonic levitation in a machine tool spindle system

An investigation into a non-contact bearing for machine tool spindle system that functions on the squeeze film ultrasonic levitation is conducted in this paper. Squeeze film type ultrasonic levitation is investigated experimentally and compared with the performance predicted by the theoretical models. A prototype non-contact journal bearing is developed for suspension of a solid steel spindle with diameter of 50 mm. The maximum load capacity of 51 N (6.37 N/cm2) is obtained which is considerably larger than the previously reported squeeze film bearings whose load capacities are usually within a few Newton (less than 1 N/cm2).

Dynamic Behavior of a Mistuned Air Turbine: Comparison Between Simulations and Measurements

Due to manufacturing tolerances, wear during operation or regeneration processes like maintenance operation, the structural properties of turbine blades deviate from design condition to reference blades. This deviation usually causes higher vibration amplitudes and as a consequence a lower service life expectation. Many different calculation methods can be used to simulate these increased amplitudes of mistuned blades. The major resulting problem is on the one hand to capture the occurring deviation of the eigenfrequencies from the reference blade and on the other hand to incorporate these real deviations in simulations. Solving these problems with a simplified experimental setup will make it possible to predict the maximum amplitude and to avoid costly experiments in a rotating turbine.The aim of the paper is to verify a simulation of the vibration amplitude by experiments using a reduction method to calculate a mistuned system in reasonable time. The results of the chosen simulation are compared to experiments in a rotating turbine.To reduce the number of degrees of freedom of the full finite-element model and the computational effort, a multi-step reduction method is used. In the simulation, the centrifugal force, the structural damping, the steady static pressure on the blades, and the mistuning are considered. To find the occurring deviations of each manufactured blade, an experimental modal analysis is performed for every single blade in a non-rotating setup with the eigenfrequencies of every single blade as an output. The single-stage results of the simulation are subsequently compared to experiments in a 5-stage air turbine in which the vibration amplitudes and the eigenfrequencies of every blade in the last rotor blade row are measured by a tip-timing system.Copyright

Influential Parameters on Structural Damping Values of Turbine Blades

In turbomachinery blading the avoidance of High Cycle Fatigue (HCF) failures is of great importance. The prediction and reduction of vibration amplitudes is of primary objective in context of HCF risk reduction. For this reason the quantification of the mechanical damping is of essential relevance.During the last decades the research has been focused on the usage of nonlinear calculation tools to predict vibration amplitudes of blades. These calculations require the specification of contact parameters as well as of material damping values. Especially for weakly damped systems like turbine blades, it is necessary to specify adequate structural damping values which determine the accuracy of the calculated transfer function. However, many researchers use uncertain structural damping values to calculate transfer functions.In this paper, an experimental setup for specimen specific damping determination in a vacuum chamber is presented. Three different clamping mechanisms, as well as the mechanism for specimen excitation, are introduced. A suspended-like specimen clamping, where the specimens are clamped in their nodes of vibration, is described first. To analyze potential influence of the clamping procedure, a comparison is given to specimens clamped on one side (cantilever beams), where the magnitude of the clamping force is chosen in a way that the friction loss is minimized. To allow an application of static stresses the specimens are clamped on both sides in the third approach. The excitation of the analyzed specimens is performed with the help of a voice-coil actuator. Damping values are determined by analyzing the decay curve, which is measured with a laser doppler vibrometer. Further experimental results show the influence of ambient pressure, frequency, amplitude, geometry, mode shape, static stress and clamping mechanism on the specimen specific damping value.Copyright

Dynamics of Bladed Disks With Frictional Coupling and Alternate Mistuning Pattern

In the field of turbo machinery design frictional coupling has been found to be a low cost method to increase the mechanical damping of bladed disks. Underplatform dampers (UPD’s) are commonly used which are metal devices pressed against the blades by centrifugal forces. The main task is to find the optimum value of the contact normal force to maximize energy dissipation. This optimum strongly depends on the excitation of the structure. Traveling waves are excited by engine order excitation and flutter. Flutter caused by fluid structure interaction can be reduced by intentional mistuning of the bladed disk whereas forced response levels will be typically increased by mistuning. A compromise is alternate mistuning.The present paper deals with the influence of alternate mistuning on frictional coupling of blisks. Firstly, the dynamics of a tuned blisk are explained with a simplified lumped mass cyclic oscillator model. It is pointed out that eigenfrequencies of traveling waves around the blisk are influenced by structural coupling. Alternate mistuning leads to mode coupling with the possibility of energy transfer. The performance of friction coupling strongly depends on the nodal diameter mode shape of vibration which is stated analytically for pure Coulomb sliding contact. Following this, a simplified blisk model with underplatform dampers is developed to analyze alternate mistuning and frictional coupling. The simulation results show a significant influence of the mistuning on the damping performance.Copyright

Reduced Order Modeling of Mistuned Bladed Disks considering Aerodynamic Coupling and Mode Family Interaction

A substructure-based reduced order model for the numerical prediction of the mistuned dynamics of bladed disks is presented. The structural mistuning is introduced to the tuned disk by blade-to-blade variations of the natural frequencies. Aeroelastic influence coeffi-cients provide aerodynamic inter-blade and inter-modal coupling via the fluid flow. The accuracy and efficiency of the reduced modeling approach are highlighted by a compar-ison with conventional FEA simulations and unsteady CFD results. In total, the model reduction provides a computational saving of up to 90% while predicting the amplitudes of forced vibrations within a tolerance of 0.7%. The proposed modeling technique is used to analyze the forced response and the aeroelastic stability of an axial compressor blisk. This exemplary study reveals an attenuation of the mistuned response due to an increase in aerodynamic damping. The intentionally provoked interaction of two mode families illustrates the significance of the inter-modal aerodynamic coupling.

Alternate Mistuning of Turbine Bladings Coupled by Underplatform Dampers

In turbo machinery design it is important to avoid vibrations that can destroy the turbine in the last resort. The rotating structure is exposed to periodic excitation forces. Two main types of periodic excitation can be distinguished. Flutter is the effect when mass flow forces couple with a natural vibration mode. The result is a negative damping coefficient and amplitudes will rise up to malfunction of the structure. The engine order excitation is a periodic excitation where the force signal is directly related to the speed of the rotor. A forced response calculation gives information about the blade vibration. Nonlinear coupling, i.e. friction coupling, between blades is used to increase damping of the bladed disk. Dynamic analysis of turbine blades with nonlinear coupling is a complex task and computer simulations are inevitable. Various techniques have been developed to reduce computational effort. The cyclic symmetry approach assumes each blade around the disk to be identical. Thus only one sector of the disk is sufficient to compute the steady state solution of the whole turbine blading. However, it has been observed that mistuning of blades reduces the flutter instability. On the other hand statistical mistuning can lead to dangerously high forced response amplitudes due to mode localization. A compromise is intentional mistuning. The simplest approach is alternate mistuning with every other blade exhibiting identical mechanical properties. This work explains in detail how a turbine bladed disk can be modeled when alternate mistuning is applied intentionally. Cyclic symmetry is used and each sector comprises two blades. This untypical choice of the sector size has significant impact on results of a cyclic modal analysis. Simulation results show the influence of alternate mistuned turbine bladings which are coupled by underplatform damper elements.Copyright