Alexander A. Kaszynski

Automated Finite Element Model Mesh Updating Scheme Applicable to Mistuning Analysis

Advancement of optical geometric measurement hardware has enabled the construction of accurate 3D tessellated models for a wide range of turbomachinery components. These tessellated models can be reverse-engineered into computer-aided design (CAD) models and input into grid generation software for finite element analyses. However, generating a CAD model from scan data is a time consuming and cumbersome process requiring significant user-involvement for even a single model. While it is possible to generate finite element models (FEMs) directly from tessellated data, current direct-grid methods produce unstructured grids that can introduce fictitious, numerical mistuning in these models, obscuring geometric mistuning. Nonetheless, as-measured scan data captured in a structured grid is essential for accurate geometric mistuning analyses, provided the tessellated scan data can be rapidly and accurately transformed into a FEM. This paper outlines and demonstrates an approach for rapidly generating structured FEMs for a population of integrally bladed rotors (IBRs) without requiring the arduous task of generating a CAD model for each as-measured IBR. This is accomplished by morphing the structured mesh of a nominal model to the tessellated data set collected from an optical scanner. It is shown that the fidelity and structure of these FEMs can be utilized for accurate mistuning analyses.© 2014 ASME

Uncertainties of an Automated Optical 3D Geometry Measurement, Modeling, and Analysis Process for Mistuned IBR Reverse Engineering

An automated reverse engineering process is developed that uses a structured light optical measurement system to collect dense point cloud geometry representations. The modeling process is automated through integration of software for point cloud processing, reverse engineering, solid model creation, grid generation, and structural solution. Process uncertainties are quantified on a calibration block and demonstrated on an academic transonic integrally bladed rotor. These uncertainties are propagated through physics-based models to assess impacts on predicted modal and mistuned forced response. Process details are discussed and recommendations made on reducing uncertainty. Reverse engineered parts averaged a deviation of 0.0002 in. (5 μm) which did not significantly impact low and mid-range frequency responses. High frequency modes were found to be sensitive to these uncertainties demonstrating the need for future refinement of reverse engineering processes.© 2013 ASME

Accurate Blade Tip Timing Limits Through Geometry Mistuning Modeling

Blade tip timing (BTT) is a commonly used non-intrusive stress measurement system to estimate the operational stresses within an engine’s rotors without the costly installation of strain gauges that can add additional stiffness to the rotor. BTT systems are now standard on many engine tests and ensure safe operations by avoiding running near maximum rotor stress limits. Since these systems measure blade time of arrival (TOA), processes are applied to first convert this data to displacement and then to stress. This effort focuses on the conversion of displacement to stress where the the classic approach utilizes nominal geometry obtained from an “as-designed” nominal model and creates computes the mode shapes using finite element analysis (FEA). The predicted mode shapes of the cyclic analysis reveal the relationship between maximal blade stress and tip displacement for a given nominally designed rotor. However, manufactured rotors deviate from nominal design due to inherent variability in the machining procedures. It is now possible through high fidelity optical geometry collection systems to obtain more accurate BTT limits using measured IBR geometry from as-manufactured rotors. It will be shown that due to the high variability of blade-to-blade geometry obtained from an optically scanned rotor that the BTT limits can vary significantly between blades. A method is also developed that allows comparisons between cyclic sector and full rotor FEA. This research suggests to optimize BTT probe placement not only to measure the maximum expected deflection given blade tip mode shapes, but also to account to for blade to blade geometric variation.Copyright

Harmonic Convergence Estimation Through Strain Energy Superconvergence

Grid convergence in finite element analysis, despite a wide variety of tools available to date, remains an elusive and challenging task. Due to the complex and time consuming process of remeshing and solving the finite element model (FEM), convergence studies can be part of the most arduous portion of the modeling process and can even be impossible with FEMs unassociated with CAD. Existing a posteriori methods, such as relative error in the energy norm, provide a near arbitrary indication of the model convergence for eigenfrequencies. This paper proposes a new approach to evaluate the harmonic convergence of an existing model without conducting a convergence study. Strain energy superconvergence takes advantage of superconvergence points within a FEM and accurately recovers the strain energy within the model using polyharmonic splines, thus providing a more accurate estimate of the system’s eigenfrequencies without modification of the FEM. Accurate eigenfrequencies are critical for designing for airfoil resonance avoidance and mistuned rotor response prediction. Traditional error estimation strategies fail to capture harmonic convergence as effectively as SES, potentially leading to a less accurate airfoil resonance and rotor mistuning prediction.Copyright

Experimental Validation of a Mesh Quality Optimized Morphed Geometric Mistuning Model

High cycle fatigue due to mode localization caused by geometric and material mistuning is one of the leading failure risks of integrally bladed rotors (IBRs). Due to the computational analysis cost of full wheel models, IBR mistuned response amplifications are often modeled with reduced order models (ROMs). However, many developed ROMs are based on nominal mode assumptions that do not consider mode shape variations that have been shown to impact predicted mistuned response. Geometrically mistuned finite element models (FEMs) do account for mode shape variations but are notoriously difficult to construct and analyze. Recent advancements in optical scanning have enabled the rapid acquisition of highly accurate dense point clouds representative of manufactured hardware. Previous research pioneered a novel method to automatically and robustly construct an FEM directly from tessellated scan data, this research adds new mesh quality verification algorithms and experimentally validates this algorithm using results from traveling wave excitation. Sensitivity to mesh and point cloud density are also assessed to determine a best practice for creation of the as manufactured mistuned rotor model.Copyright

Experimental Validation of an Optically Measured Geometric Mistuning Model Using a System ID Approach

This work validates a digital replica of a mistuned rotor created from a point cloud produced from a structured light topological measurement system. The measurement system accurately captures the manufacturing deviations of each airfoil and allows prediction of the mistuned dynamic response of the rotor subject to harmonic excitation. A reverse engineering process is used to first create a nominal finite element model of the point cloud and subsequently transform each airfoil geometry to the measured point cloud. Model results are compared to a traveling wave excitation vibration test that approximates engine harmonic loading using phase shifted magnetic actuators. Uncertainties are assessed in point cloud collection, reverse engineering process, and the experiment. Results show minimal variation in the model predictions and experimental measurements. Comparison between analysis and test show excellent agreement, validating the process to create digital replicas that accurately predict complex dynamic behavior of rotor mistuned response.

Validation of Geometric Mistuning Reduced-Order Models for Single and Dual Flow-Path Integrally Bladed Rotors

This efforts validates the utilization of tuned secondary modal reductions in building approximate, highly reduced-order models of geometrically mistuned single and dual flowpath integrally bladed rotors (IBRs). The blades of each IBR are geometrically mistuned according to physical measurements taken from industrial hardware. A probabilistic study is conducted on a population of single and dual flow-path IBRs with random blade geometry perturbations; the mistuned forced response levels are calculated from different fidelity models, including: full finite element models, lower fidelity Craig-Bampton component mode synthesis models, and the lowest fidelity models reduced with tuned-secondary modal reductions. Statistical comparisons of the mistuned forced response distributions are carried out to show any differences in the lowest fidelity models’ ability to capture the forced response levels of a fleet of single and dual flow-path IBRs. Results indicate the lowest fidelity, highly-reduced models provide accurate forced response predictions for a population of rotors. Lastly, correlations between mistuned response prediction error for the single flow-path IBR and error metrics composed of secondary modes used in the lowfidelity models are investigated to determine if there are linkages between model accuracies and secondary modes used in model creation.