Oral Mehmed

Experimental Investigation of Mode Localization and Forced Response Amplitude Magnification for a Mistuned Bladed Disk

The results of an experimental investigation on the effects of random blade mistuning on the forced dynamic response of bladed disks are reported. The primary aim of the experiment is to gain understanding of the phenomena of mode localization and forced response blade amplitude magnification in bladed disks. A stationary, nominally periodic, 12-bladed disk with simple geometry is subjected to a traveling-wave out-of-plane engine order excitation delivered via phase-shifted control signals sent to piezoelectric actuators mounted on the blades. The bladed disk is then mistuned by the addition of small. unequal weights to the blade tips, and it is again subjected to a traveling wave excitation. The experimental data is used to verify analytical predictions about the occurrence of localized mode shapes, increases in forced response amplitude, and changes in resonant frequency due to the presence of mistuning. Very good agreement between experimental measurements and finite element analysis is obtained. The out-of-plane response is compared and contrasted with the previously reported in-plane mode localization behavior of the same test specimen. This work also represents an important extension of previous experimental study by investigating a frequency regime in which modal density is lower but disk-blade interaction is significantly greater.

On a Self-Tuning Impact Vibration Damper for Rotating Turbomachinery

A self-tuning impact damper is investigated analytically and experimentally as a device to inhibit vibration and increase the fatigue life of rotating components in turbomachinery. High centrifugal loads in rotors can inactivate traditional impact dampers because of friction or misalignment of the damper in the g-field. Giving an impact damper characteristics of an acceleration tuned-mass damper enables the resulting device to maintain damper mass motion and effectiveness during high-g loading. Experimental results presented here verify that this self-tuning impact damper can be designed to follow an engine order line. damping rotor component resonance crossings.

Calculation and Correlation of the Unsteady Flowfield in a High Pressure Turbine

Forced vibrations in turbomachinery components can cause blades to crack or fail due to high-cycle fatigue. Such forced response problems will become more pronounced in newer engines with higher pressure ratios and smaller axial gap between blade rows. An accurate numerical prediction of the unsteady aerodynamics phenomena that cause resonant forced vibrations is increasingly important to designers. Validation of the computational fluid dynamics (CFD) codes used to model the unsteady aerodynamic excitations is necessary before these codes can be used with confidence. Recently published benchmark data, including unsteady pressures and vibratory strains, for a high-pressure turbine stage makes such code validation possible. In the present work, a three dimensional, unsteady, multi blade-row, Reynolds-Averaged Navier Stokes code is applied to a turbine stage that was recently tested in a short duration test facility. Two configurations with three operating conditions corresponding to modes 2, 3, and 4 crossings on the Campbell diagram are analyzed. Unsteady pressures on the rotor surface are compared with data.Copyright

Fully Suspended, Five-Axis, Three-Magnetic- Bearing Dynamic Spin Rig With Forced Excitation

A significant advancement in the dynamic spin rig (DSR), i.e., the five-axis, three-magnetic-bearing DSR, is used to perform vibration tests of turbomachinery blades and components under rotating and non-rotating conditions in a vacuum. The rig has three magnetic bearings as its critical components: two heteropolar radial active magnetic bearings and a magnetic thrust bearing. The bearing configuration allows full vertical rotor magnetic suspension along with a feedforward control feature, which enables the excitation of various modes of vibration in the bladed disk test articles. The theoretical, mechanical, electrical, and electronic aspects of the rig are discussed. Also presented are the forced-excitation results of a fully levitated, rotating and non-rotating, unbladed rotor and a fully levitated, rotating and non-rotating, bladed rotor in which a pair of blades were arranged 180° apart from each other. These tests include the “bounce” mode excitation of the rotor in which the rotor was excited at the blade natural frequency of 144 Hz. The rotor natural mode frequency of 355 Hz was discerned from the plot of acceleration versus frequency. For non-rotating blades, a blade-tip excitation amplitude of approximately 100 g A−1 was achieved at the first-bending critical (≈144 Hz) and at the first-torsional and second-bending blade modes. A blade-tip displacement of 1.778×10−3m (70 mils) was achieved at the first-bending critical by exciting the blades at a forced-excitation phase angle of 90° relative to the vertical plane containing the blades while simultaneously rotating the shaft at 3000 rpm.

A MAGNETIC SUSPENSION AND EXCITATION SYSTEM FOR SPIN VIBRATION TESTING OF TURBOMACHINERY BLADES

Dexter Johnson, Gerald V. Brown, and Oral MehmedLewis Research Center, Cleveland, OhioPrepared for the39th Structures, Structural Dynamics and Materials Conferencesponsored by AIAA, ASME, ASCE, AES, and ASCLong Beach, California, April 20-23, 1998National Aeronautics andSpace AdministrationLewis Research Center

Optical Measurements of Unducted Fan Flutter

The paper describes a nonintrusive optical method for measuring flutter vibrations in unducted fan or propeller rotors and provides detailed spectral results for two flutter modes of a scaled unducted fan. The measurements were obtained in a high-speed wind tunnel. A single-rotor and a dual-rotor counterrotating configuration of the model were tested; however, only the forward rotor of the counterrotating configuration fluttered. Conventional strain gages were used to obtain flutter frequency; optical data provided complete phase results and an indication of the flutter mode shape through the ratio of the leading- to trailing-edge flutter amplitudes near the blade tip. In the transonic regime the flutter exhibited some features that are usually associated with nonlinear vibrations. Experimental mode shape and frequencies were compared with calculated values that included centrifugal effects.Copyright

FLUTTER ANALYSIS OF PROPFANS USING A THREE-DIMENSIONAL EULER SOLVER

A three-dimensional aeroelastic solver is developed and applied to investigate the flutter of a propfan. The unsteady airloads are obtained by solving three-dimensional Euler equations. An implicit-explicit hybrid scheme is used to reduce computational time for the solution of Euler equations. A finite element model is used to obtain structural properties, and a deforming grid technique is used to simulate elastic deformations of the blade. The aeroelastic equations are formulated in normal mode form and are solved for flutter using both the time and frequency domain methods. The analysis is used to calculate the dynamic aeroelastic stability of a single-rotation propfan model for which classical flutter was measured at subsonic relative flows. Comparisons with the experimental data and other calculated results are given.

APPLE - An aeroelastic analysis system for turbomachines and propfans

This paper reviews aeroelastic analysis methods for propulsion elements (advanced propellers, compressors and turbines) being developed and used at NASA Lewis Research Center. These aeroelastic models include both structural and aerodynamic components. The structural models include the typical section model, the beam model with and without disk flexibility, and the finite element blade model with plate bending elements. The aerodynamic models are based on the solution of equations ranging from the two-dimensional linear potential equation for a cascade to the three-dimensional Euler equations for multi-blade configurations. Typical results are presented for each aeroelastic model. Suggestions for further research are indicated. All the available aeroelastic models and analysis methods are being incorporated into a unified computer program named APPLE (Aeroelasticity Program for Propulsion at LEwis).

Development of a turbomachinery aeroelastic code based on a 3D linearized Euler solver

This paper describes the development of LINFLUXAE, a turbomachinery aeroelastic code based on the linearized unsteady 3D Euler solver, LINFLUX. The steady solution, required by LINFLUX is obtained from the nonlinear Euler/Navier Stokes solver TURBO-AE. The paper briefly describes the salient features of TURBO-AE, LINFLUX and the details of the aeroelastic extension. The aeroelastic formulation is based on a modal approach. The unsteady aerodynamic forces required for flutter and forced response are obtained by running LINFLUX for each mode for each interblade phase angle and for a given frequency. An eigenvalue formulation is used for flutter analysis. The forced response is calculated from the modal summation of the generalized displacements. The NASA / GE Energy Efficient Engine (Ecubed) fan configuration is chosen as a test case and the results are compared with those obtained from TURBO-AE and ASTROP2.

Experimental investigation of counter-rotating propfan flutter at cruise conditions

This article presents wind-tunnel experimental flutter results, at transonic relative flows, for a 0.62-m-diam composite propfan model. A blade row that fluttered was tested alone, and with a stable aft counter-rotating blade row. The major objectives of the experiment were to study the effect of the second blade row on the row in flutter, and to investigate the flutter. Results show that the second row had a small stabilizing effect. Two distinct flutter modes were found within the operating regime of the rotor; both apparently single-degree-offreedom instabilities, associated respectively with the first and second natural blade modes. For both flutter modes, flutter boundary, frequency, nodal diameter, and blade displacement data are given. The blade displacement data, obtained with an optical method, gives an indication of the flutter mode shape at a span near the blade tip.